HCS216 Physiology Lab

In *motor neurons*, *conduction velocities* can be measured by...

...*stimulating the nerve sequentially in two places*, and recording the two *times it takes to initiate the muscle contraction*

The cell uses energy from adenosine 5'-triphosphate (ATP) to power *Na+/K+ ATPase pumps* in the cell membrane. These pumps move..

...*three Na+ out* of the cell and *two K+ into* the cell with each cycle to maintain an uneven distribution of ions

For effective stimulation to occur the two pads of the stimulating bar electrode should be aligned along the arm's length...

...If the stimulus status light changes in color from green to yellow, you will need to put more electrode paste on the pads.

One peripheral nerve typically contains...

...a mixture of sensory, motor, and autonomic nerve fibers. Individual nerve fibers vary in myelination, diameter, excitability, threshold, and speed of conduction

Spinal nerves...

...are the lower motor neurons. The spinal nerves branch out between each vertebrae into the periphery. They also descend from the end of the spinal cord to exit at each level of the vertebral column.

Reflex activity can produce a response of which we are either..

...aware (somatic nervous system) or unaware autonomic nervous system)

As a wave of depolarization travels along a nerve, external electrodes record the change in polarity of the wave as it passes beneath them. This gives the CAP its...

...biphasic waveform

Axons can..

...experimentally pass action potentials in either direction

Myelination contributes more to...

...increasing conduction velocity than diameter The plasma membrane has a relatively high membrane resistance. However, current is still lost across this membrane through ion channels in the membrane. Myelin sheaths that surround the axon act as insulation, which helps to increase membrane resistance. This decreases leakage of current across the membrane, and increases conduction speed

Electrical impulses travel in...

...one direction along sensory neurons. That is, impulses travel from the sensory receptor to the spinal cord

The number of digital samples depends in the...

...sampling rate

Do not apply too much electrode paste as this will...

...short circuit the current

Since we are measuring brief changes in diameter of the finger due to pressure wave rather than absolute arterial diameter...

...there are no units associated with this measurement

The CAP signal that is recorded using external electrodes is NOT related...

...to the action potential signal that is recorded from a single axon using intracellular electrodes

The nerves in the spinal cord are...

...upper motor neurons

Cell bodies of the efferent nerve are in the...

...ventral horn

Which of the statements regarding the Na+/K+-ATPase are false?

It is only active following an action potential. The correct answer is that it is active at all times

What is one defining property of a voluntary action that differentiates it from a reflex?

It is performed by choice.

Which of the following is characteristic of a reflex?

It is reproducible.

Which of these statements apply to large diameter axons and which apply to myelinated axons?

*LARGE DIAMETER AXON*: 1. Decreased membrane resistance 2. Lower internal resistance to current flow 3. Lower threshold for stimulation *MYELINATED AXON*: 1. Increased membrane resistance 2. Reduced electrical leakage across the membrane

Why is it harder to fire a second action potential in the relative refractory period?

*Many voltage-gated Na+ channels are inactivated and the cell is permeable to K+*

Motor or Efferent division

*Motor nerves carry information away from the CNS to skeletal muscle fibers* to bring about an appropriate response to the stimulus.

Oligodendrocytes

*Myelin-producing* glial cells in the *CNS*

Schwann cells

*Myelin-producing* glial cells in the *PNS*

Sensory or Afferent division

*Sensory nerves carry information towards the CNS* (brain or spinal cord) in response to stimulation such as pain, pressure, or stretch.

What is the name given to the condition where there is damage in the upper motor neurons?

*Upper motor neuron syndrome*. There are many causes of upper motor syndrome, including: *upper spinal cord injury, brain injury (including stroke), cerebral palsy (the abnormal development or damage to the motor areas of the brain), or multiple sclerosis (a demyelinating disease that affects the myelin sheaths of nerves in the brain and spinal cord).*

Unmyelinated Axon

*Where does regeneration occur?*: Regeneration occurs continuously across the axon *Location of Na+ channels*: Along the entire length of the axon *Conduction velocity*: Slower *Membrane resistance*: Lower

Myelinated Axon

*Where does regeneration occur?*: Regeneration occurs in the nodes of Ranvier *Location of Na+ channels*: They are concentrated in the nodes of Ranvier *Conduction velocity*: Faster *Membrane resistance*: Higher

Compound action potential (CAP)

- *A sum of action potentials recorded from a whole nerve* - When a peripheral nerve is externally stimulated, the total electrical activity that results *(the CAP) can be recorded between two external electrodes*

What os the threshold potential?

- 50 mV

What is the resting membrane potential?

- 70 mV

Relative refractory period

- A stimulus of greater than normal intensity can elicit a response - The membrane is more resistant to stimulation than usual - In this period *some voltage-gated Na+ channels are still inactivated*. As a result, a stronger stimulus than usual is required to open a sufficient number of these channels for another action potential to fire. - The *cell membrane is more permeable to K+ during this period*. This *further opposes depolarization* of the membrane.

Action potential

- An event that is *specific to excitable cells like neurons or muscle fibers* - It involves a *rapid and short-lasting rise in the electrical potential, immediately followed by a fall. This is the result of the opening and closing of channels in the membrane, which acutely changes the membrane permeability to different ions*. - In nerves, action potentials *start at the dendrites and are transmitted along the nerve axon*. Therefore, *action potentials transfer info over a distance*

Propagation of action potentials

- At rest, the neuronal cell membrane is polarized with the inside negative relative to the outside - *During an action potential, the polarity is briefly reversed*. This change in polarity of the cell membrane *produces local currents that cause depolarization* of the cell membrane in the region of the action potential. - When *threshold is reached, voltage-gated Na+ channels open and an action potential is generated at the new location*. Hence, action potentials *self-propagate along an axon*.

Axon diameter

- Axons with a *larger diameter have a lower internal resistance*. As a result, conduction of an action potential is faster along axons with larger diameter - To help you understand the relationship between diameter and internal resistance, an analogy may be helpful. For example, consider the flow of water through a straw with a large or small diameter. - Flow close to the wall of the straw is very slow due to friction between the water and the wall, while flow in the center of the straw is very fast. Therefore, the larger diameter straw allows water to flow through it faster, since less water is in contact with the wall of the straw. - The same principle can be applied to the conduction of action potentials along an axon. Larger diameter axons have a greater total volume for electric currents to flow through, so these currents meet less internal (or membrane) resistance. Therefore, conduction of action potentials is faster along large diameter axons. - The plasma membrane of larger diameter axons also has a *greater number of ion channels on average compared to axons of smaller diameter*. This means these axons also have a *lower membrane resistance to current flow*, and *reach threshold for firing at lower stimulation voltages*.

What nerve fibers can be classified using letters?

- Both motor and sensory fibers - Classification of fibers using letters include: A - alpha, A - beta, A - gamma, A - delta, B, and C

Myelination

- Conduction of an action potential along a nerve fiber is greatly enhanced by myelination, which *increases the resistance to current flow across the membrane* - Sheaths of myelin are separated by *gaps called nodes of Ranvier*, which contain a high *concentration of Na+ channels* - As a result, *current flow across the membrane occurs only at the nodes of Ranvier*. Myelin *accelerates nerve conduction velocity* significantly and rates of 40-60 m/s (or even higher) can be found in human myelinated nerve fibers.

Absolute refractory period

- Impossible to initiate another action potential - During the absolute refractory period the membrane is *completely resistant to further stimulation* - no matter how strongly the membrane is stimulated, another action potential will not fire. - This is due to the characteristics of voltage-gated Na+ channels in the membrane. The "voltage-gated" property of these channels means that they are in different states (that is, open, closed, or inactivated) at different voltages. In the absolute refractory period a* large number of these channels are voltage inactivated, and will only open again when the cell enters the relative refractory period*.

What nerve fibers can be classified using Roman numerals?

- Sensory fibers only - Classifications of fibers using Roman numerals include: Ia, Ib, II, III, and IV

Why do you think the epineurium is so much tougher than the endoneurium?

- The *epineurium* must be *tough to protect nerves from stretching, mechanical strain, and injury*. The *thickness* of epineurium can *differ along the length of the nerve*. It is *thicker near the joints* where movement may place *more mechanical strain* on the nerve. - In contrast, the *endoneurium* is a *delicate layer of connective tissue that selectively allows molecules to pass into the endoneurial fluid which lies around each nerve fiber.* The *volume of this fluid increases with nerve irritation or inflammation*. This increase in fluid shows up in MRI scans, which can be used to detect regions of nerve damage.

Resting membrane potential (RMP)

- The *potential difference* that is caused by the cell interior becoming negative compared to the cell exterior - The RMP of a healthy neuron ranges from *-70 to -80 mV* - Can be measured by inserting electrodes into the cell

Neurons/ Nerve cells

- The fundamental unit of the nervous system - Neurons are *electrically excitable cells that produce action potentials when they receive electrical or chemical stimulation* - They are specialized for the transmission of signals and they *communicate with each other through chemical synapses*

What is the threshold voltage of voltage-gated sodium and potassium channels?

-40 to -50 mV

The plasma membrane has a relatively high membrane resistance. However, current is still lost across this membrane though ion channels in the membrane...

...*Myelin sheaths that surround the axon act as insulation, which helps to increase membrane resistance*. This *decreases leakage of current* across the membrane and *increases conduction speed*.

In *sensory nerves, conduction velocity* can be measured by..

...*stimulating the median nerve in a finger and recording the sensory nerve compound action potential with electrodes over the median nerve in the wrist and elbow regions*

Conduction of an action potential along a nerve fiber is greatly enhanced by myelin..

...which helps insulate the axon and reduce loss of the electrical. Sheaths of myelin that are about 20-300 layers thick surround the axon, and are separated every 1-2 mm by gaps called nodes of Ranvier - *The presence of myelin decreases membrane capacitance and increases membrane resistance.* - The result is that much of that *change in membrane permeability (and therefore current flow) occurs at the nodes*. These nodes contain a *high concentration of Na+ channels which allow for regeneration of the action potential* as it travels along the axon. The action potential is described as *"jumping" between the nodes of Ranvier*; this process is called *saltatory conduction*

The red dot on the back of the bar electrode indicated the positive electrode. Orient the electrode...

...with the positive electrode closer to the elbow

What makes up the CNS?

1) Brain 2) Brainstem 3) Spinal cord

What makes up the PNS?

1) Ganglion (plural ganglia) 2) Motor neurons 3) Sensory neurons 4) 31 pairs of spinal nerves

Upper motor neuron lesion signs:

1) Muscle paresis or paralysis 2) Muscle tenseness (hypertonia) 3) Spasticity 4) Muscle weakness (with no muscle wasting) 5) Decreased motor control 6) Clasp-knife response 7) Exaggerated stretch reflexes 8) Muscle twitches and spasms 9) Presence of the Babinski sign

Lower motor neuron lesion signs:

1) Muscle paresis or paralysis 2) Muscle wasting 3) Reduced muscle tone (hypotonia) 4) Diminished or absent reflexes (hyporeflexia) 5) Muscle fasciculation (also known as muscle twitches)

Affects the rate of conduction

1) Myelin 2) Nerve diameter 3) Temperature

What are the two functional subdivisions of the PNS?

1) Sensory/ Afferent division 2) Motor/Efferent division

What are the three basic functions of the nervous system?

1) To sense changes internal and external to the body 2) To process (integrate) this sensory information 3) To initiate a response

Events in an Action Potential

1. *Initial depolarization* starts when *channels open in response to electrical, chemical, or mechanical stimulus. This allows local currents to flow*. For the purpose of this lesson we will refer to these as *stimulus-gated ion channels*. As these channels open, there is *a nonspecific increase in ion permeability*. However, *sodium ions are farthest from their electrochemical equilibrium (ENa; +70 mV)* so they make the *greatest contribution to the change in local current*. As a result, there is some local membrane depolarization (that is, the *membrane potential becomes slightly more positive*). If the membrane potential *reaches the threshold voltage (approximately -50mV), voltage-gated Na+ channels are activated and an action potential is initiated*. 2. The *opening of voltage-gated Na+ channels increases the membrane permeability to Na+*. The membrane potential increases (becomes more positive) quickly moving towards the equilibrium potential for Na+ (ENa; +70 mV). This stage is called *depolarization*. Note that the *peak of the action potential* only reaches about *+50 mV*. This is because *voltage-gated K+ channels are also activated by membrane depolarization during this time* 3. *Na+ channels close, but voltage-gated K+ channels remain open*. The membrane potential *falls back towards the resting membrane potential (becomes more negative)*. This stage is called *repolarization* 4. The membrane potential continues to fall, and becomes* more negative than the resting membrane potential*. This stage is called *hyperpolarization*. 5. *Voltage-gated K+ channels gradually close*. As a consequence, the K+ permeability *returns to its resting state* as does the Na+ permeability. The *Na+/K+-ATPase pump rapidly corrects for the tiny quantities of Na+ that were gained and K+* that were lost by pumping Na+ out of the cell and K+ into the cell, and *RMP is restored*.

What sequence of events must occur in order for an action potential to fire? Include threshold, and the opening and closing of Na+ and K+ channels.

1. A stimulus threshold is reached which leads to stimulus-gated channels opening and the membrane to slightly depolarize. 2. The membrane potential increases from - 70mV to - 50mV. 3. Voltage-gated Na+ channels open and the permeability of Na+ increases as a result 4. Voltage-gated K+ channels open and the permeability of K+ increases as a result 5. Voltage-gated Na+ channels close and deactivate decreasing the permeability of Na+; another action potential cannot be generated when all the voltage-gated Na+ channels are closed 6. Voltage-gated K+ channels close 7. The membrane potential returns back to resting membrane potential (-70mV)

How is the nervous system organized?

1. Central Nervous System (CNS) → Brain and spinal cord 2. Peripheral Nervous System → Sensory and motor neurons > Somatic nervous system (SNS) → Motor to skeletal muscles (efferent); Sensory from sense organs (afferent) > Autonomic nervous system (ANS) → Motor to smooth muscle (efferent); Sensory from internal receptors (afferent)

Signals you may record:

1. Electrocardiogram (ECG) 2. Electroencephalogram (EEG) 3. Electromyogram (EMG) 4. Noninvasive blood pressure

Myelinated axon

1. Fast conduction velocity 2. High membrane resistance 3. Less electrical current lost across the membrane 4. Regeneration occurs at the nodes of Ranvier 5. Saltatory conduction

Which of the statements regarding the Na+/K+-ATPase are true?

1. It creates a concentration gradient for Na+ and K+ 2. *It is active at all times* 3. It transports more sodium out than potassium in, in each cycle 4. It is important for maintaining RMP

Unmyelinated axon

1. Lower membrane resistance 2. Slow conduction velocity 3. Regeneration occurs continuously along the axon

When a beam of light is directed into the eye the following occurs:

1. Parasympathetic nerves activate the circular "sphincter" muscles in the iris. 2. The iris constricts which reduces the pupil aperture and the amount of light that enters the eye. 3. At the same time, the iris sphincter muscle of the other eye constricts as well.

What causes diminished knee-jerk reflex?

1. Sensory receptor damage 2. Inflammation of nerve ends It indicates *lower motor neuron lesions, which include damager to sensory receptors and nerve ends*. Decreased reflexes are *caused by disruptions of the sensory signals being sent to the motor neurons*

Simplified order of events in an action potential?

1. Stimulus-gated ion channels open, membrane depolarizes slightly 2. Membrane potential increases to -50 mV 3. Voltage-gated Na+ channels open, increased permeability to Na+ 4. Voltage-gated K+ channels open, increased permeability to K+ 5. Voltage-gated Na+ channels close and deactivate 6. Voltage-gated K+ channels close 7. Membrane potential returns to -70 mV

What causes an exaggerated knee-jerk reflex?

1. Stroke 2. Brain injury 3. Cerebral palsy 4. Upper spinal cord injury

Describe the order of physiological events that occur during the latency

1. The electrical stimulus generates an action potential in the median nerve 2. The action potential is conducted along the nerve axon to the neuromuscular junction 3. Acetylcholine (ACh) is released into the synaptic cleft 4. ACh diffuses across the synaptic cleft 5. ACh binds to "nicotinic" acetylcholine receptors on the motor endplate, leading to depolarization 6. Initiation of an action potential that spreads across the motor unit 7. The action potential stimulates release of calcium ions from the sarcoplasmic reticulum 8. Increased cellular calcium levels start the biochemical events that underlie contraction

What gives you a nice, smooth signal for ECG?

1000 samples per second (1 k/s)

What is a nice, smooth signal for a slow-changing signal such as breathing?

200 samples per second

What is the approximate conduction velocity of motor nerves?

50 m/s

Synapse

A junction between two nerve cells. It is across this junction that signals are sent from one nerve cell to another by chemical transmission

Abductor pollicis brevis

A muscle in the hand that functions as an abductor of the thumb. It is generally supplied by the median nerve

How does a nerve fiber differ from an axon?

A nerve fiber is an axon and the myelin that surrounds it

Quadriplegic/Tetraplegia

A person with quadriplegia, that is, paralysis of all four limbs

Pulse

A pressure wave created by each heartbeat following the expansion and recoil of arteries

Electrocardiogram (ECG)

A recording of the electrical activity of the heart

Digital signal

A series of discrete values

Compare the difference in signals for action potentials and CAPs.

ACTION POTENTIAL: 1) Generated spontaneously or through stimulation 2) Monophasic in shape 3) Amplitude 80 - 100 mV 4) Requires highly specialized equipment for intracellular recording COMPOUND ACTION POTENTIAL: 1) External electrical stimulation required 2) Biphasic in shale 3) Amplitude 2-4 mV 4) Requires less specialized equipment to for extracellular recording

Match the following descriptions to either afferent or efferent nerve fibers.

AFFERENT NERVE FIBERS: 1) Coordinate sensory information 2) Carry sensory information from the periphery to the brain EFFERENT NERVE FIBERS: 1) Coordinate motor activity 2) Initiate contractions and secretions

Do autonomic or somatic reflexes activate the following muscles?

AUTONOMIC: 1. Cardiac 2. Smooth SOMATIC: 1. Skeletal muscle

How is ACh cleared from the synaptic cleft?

It is cleaved by acetylcholinesterase.

Why do you think the action potential is described as "jumping" between nodes of Ranvier?

Action potential conduction along the lengths of myelin is very fast, and then slow at the nodes of Ranvier where the action potential regenerates. As a result, the action potential is described as "jumping" between nodes of Ranvier

How do action potentials travel along a nerve fiber? Include an explanation of saltatory conduction.

Along an unmyelinated axon, regeneration occurs continuously across the axon of the nerve fiber. However, for myelinated axons, where there is greater membrane resistance and decreases membrane capacitance in the myelin sheath and a concentration of voltage-gated Na+ channels in the nodes of Ranvier, the action potential "jumps" between the nodes of Ranvier, which is called saltatory conduction.

Do action potentials in the body travel in one direction only, or in both? Explain your reasoning.

Although experimentally it is shown that axons can experimentally pass action potentials in both directions, typically in the body, action potentials start in one direction. Furthermore, during the absolute refractory period, it is not possible to create another action potential as the voltage-gated Na+ channels are closed . This refractory period prevents the backward propagation of an action potential.

PowerLab

An *analog-to-digital converter*. It takes a continuous signal (for example, your temperature) which may fluctuate over time, and converts it into a digital signal

Electroencephalogram (EEG)

An amplified recording of the waves of electrical activity that sweep across the brain's surface. These waves are measured by electrodes placed on the scalp.

Which is true regarding the electrical potential called an evoked potential?

An evoked potential occurs in response to a stimulus.

Clasp-knife response

An initial high resistance to movement, rapidly followed by low resistance. The term derives its name from its similarity with the closing motion knife, or pocketknife

What does the term supramaximal stimulus refer to?

Any stimulus strength greater than that required to activate all motor units in the nerve.

Median nerve

Arises from the brachial plexus in the axilla (armpit). It travels down the upper arm on the median side of the biceps brachii, and along the middle of the lower arm. It branches to supply the lateral three fingers and the thumb The only nerve that runs under the carpal tunnel into the hand

Hyporeflexia

Below normal or absent reflexes

Considering the stimulation of a single axon, what would see for each level of stimulation?

Below threshold → No Action Potential; Subthreshold Response At threshold → Action Potential; Amplitude is 80 - 100 mV; all-or-none principle Increasing stimulus voltage *but still below the maximum response) → Action Potential; Amplitude is 80 - 100 mV At a stimulus voltage beyond that needed for maximum response → Action Potential; Amplitude is 80 - 100 mV

Considering the stimulation of a whole nerve, what would see for each level of stimulation?

Below threshold → No CAP; - At threshold → CAP; Small CAP amplitude is observed Increasing stimulus voltage *but still below the maximum response) → CAP; A gradual increase (graded) in CAP amplitude as the stimulus voltage increases At a stimulus voltage beyond that needed for → CAP; The CAP amplitude will remain the same as what is needed for the maximum response

A spinal cord injury patient has upper motor neuron lesion signs in their legs, but shows lower motor neuron lesion signs in their arms. Where in their spinal cord is the injury?

Between C6 and T2

Consensual response

Brisk, equal, and simultaneous constriction of both pupils when one eye then the other is stimulated with light

Fasicles

Bundles of muscle fibers

Which of these stimuli can trigger the blink reflex and which cannot?

CAN TRIGGER THE BLINK RESPONSE: 1. Bright light 2. Loud noise 3. Butter taste 4. Touching the cornea 5. Object speeding directly towards the eye 6. Sneezing CANNOT TRIGGER THE BLINK REFLEX: 1. Coughing

Assume that a supramaximal response is obtained at a stimulus voltage of 20 mA. What would happen to the CAP amplitude if you doubled the stimulation voltage to 40 mA?

CAP amplitude would remain the same.

What is a CAP?

CAP stands for compound action potential and is the sum of all the action potentials in a nerve. It is a recording of electrical activity in an entire nerve.

Perineurium

Coarse connective tissue that bundles fibers into fascicles

Raw Input

Comes from a number of biological/physical sources, usually in the form of an analog voltage (+- 10 V). The signal has amplitude that varies continuously over time

Compound Action Potential Lab 2

Compound Action Potential Lab 2

What is the response called when light is shone into an eye and the iris of the opposite eye constricts?

Consensual response

Tonic inhibition

Constant inhibition

Miosis

Constriction of the pupil

Autonomic nervous system (involuntary nervous system)

Contains sensory and motor components just like the SNS, however, info from the CNS *travels to smooth muscle, cardiac muscle, and glands along a two-neuron efferent pathway* Bodily functions that are unconsciously controlled include *cardiac function, digestion, respiration, salivation, sweating, pupil diameter, urination, and sexual arousal*.

Categorize the following according to the effect each will have on conduction velocity in a mammalian motor nerve.

DEACREASE NERVE CONDUCTION VELOCITY: 1. Blocking sodium channels with tetrodotoxin 2. Immersion in ice-cold water 3. Loss of myelination INCREASES NERVE CONDUCTION VELOCITY: 1. Increase in nerve diameter

What can result from damage to the sympathetic supply?

Damage to the sympathetic supply can result in a *drooping upper eyelid (ptosis)* and a *constricted pupil (Horner syndrome).*

What are voltage-gated potassium channels called since they are much slower then voltage-gated sodium channels?

Delayed rectifiers

Endoneurium

Delicate connective tissue around individual nerve fibers in nerve

Epineurium

Dense connective tissue that surrounds entire nerve including fascicles and blood vessels. It is tough and more resistant to tearing

How can you use reflexes to help determine the location of a spinal cord injury?

Depending on where (upper or lower limbs) and how (exaggerated or diminished) the reflexes are abnormal or normal, can determine the location of a spinal cord injury. Exaggerated reflexes are indicative of upper motor neuron lesions while diminished reflexes are indicative of lower motor neuron lesions. For example, when there is a spinal injury between T3-L3, upper motor neuron lesion signs are present in the lower limb, however, the upper limbs are normal.

How does ion permeability change during depolarization, repolarization, and hyperpolarization?

Depolarization --> Ion permeability of first Na+ increases and then K+ increases. Repolarization --> Ion permeability of Na+ decreases Hyperpolarization --> Ion permeability of K+ decreases back down to resting value

Evoked potential or evoked CAP

Describes the electrical activity recorded after an electrical stimulus. It is recorded using surface electromyography (EMG)

What contributes most to the threshold stimulation required for a CAP in the body?

Diameter of axons in the nerve. The plasma membrane of *larger diameter axons have a greater number of ions channels on average*, compared to axons of smaller diameter. This means they have *a lower membrane resistance to current flow, and reach threshold for firing at lower stimulation voltages*

What is the response called when light is shone into an eye and the iris of that eye constricts?

Direct response

What is the physiological basis for the absolute and relative refractory periods?

During the absolute refractory period, another action potential cannot be initiated as all the voltage-gated Na+ channels are closed, so at that time period, another stimulus will not be recognized. During the relative refractory period, a stronger stimulus must be needed to generate an action potential since some of the Na+ channels are closed and the cell membrane is more permeable to K+ channels which further opposes depolarization.

Why does an action potential normally flow in only one direction along a nerve fiber?

During the absolute refractory period, voltage-sensitive Na+ channels cannot be reactivated.

Do fibers from motor neurons carry afferent or efferent information?

Efferent

Synaptic terminals

Ends of axons that form one side of the synaptic cleft; the location where neurotransmitters are stored. LOCATED IN THE *CNS*

Patellar tendon

Extends from the anterior muscle of the quadriceps femoris (thigh) to the patella 9knee cap), and then inserts into the tibia

Which statement regarding a CAP is true?

External electrodes record the change in polarity of the wave as it passes underneath them.

Where are sodium ions (Na+) largely located?

Extracellularly

Periphery reflexes

Extremely fast reflexes since they occur in the spinal cord through neural pathways called *reflex arcs*. That means that the *signals DO NOT have to TRAVEL TO AND FROM THE BRAIN IN ORDER TO INSTIGATE A RESPONSE* which accelerated the response

(T/F) Most cells (including muscle cells and neurons) have a voltage difference across their cell membranes

False, ALL CELLS have a voltage difference

What happens to the size and shape of a CAP as stimulus voltage is increased?

For CAP, the size and shape gradually increase as long it is below the maximum stimulus. Since a nerve is composed of axons with differing diameters and myelination, some of these axons reach their threshold at a lower level of stimulus compared to others, until they all reach their threshold with the maximum stimulus needed to reach the maximum CAP amplitude.

Jendrassik maneuver

For this, the volunteer should clench the teeth and interlock the fingers of both hands. As the reflex is elicited, he or she should pull hard.

Local anesthetics inhibit voltage-gated Na+ channels in the nerve membrane. What effect do you think this could have on nerve conduction?

If all the voltage-gated Na+ channels are blocked by local anesthetics, then even after depolarization, the voltage-gated Na+ channel will remain blocked, so nerve conduction will not occur as proper membrane permeability change will not occur.

Did you observe a difference between auditory and visual cues? Could this difference really be due to different processing times in the brain?

In general, the reaction times to auditory stimuli should be faster than those to visual stimuli

Somatic nervous system (voluntary nervous system)

Information travels from the periphery to the CNS along afferent sensory nerves, and *responses travel directly to skeletal muscles along effector motor nerves*

Where are potassium ions (K+) largely located?

Intracellular

Reflexes

Involuntary movements which occur immediately after a stimulus

When does the isolated stimulator become active?

It becomes active only during a recording It is switched off internally at all other times

What happens to the pupil when light is suddenly shone into the eye?

It constricts

What is the cell membrane more permeable towards?

It is *more permeable to K+* rather than Na+, so there is a greater tendency for K+ to diffuse out of the cell then for Na+ to diffuse into the cell. This *results in the cell interior becoming negative compared to the cell exterior, and this potential difference is known as the resting membrane potential (RMP) *Diffusion potential dominated by the contribution of K+ diffusion*

How do large and small diameter mammalian axons differ?

LARGE DIAMETER AXONS: 1. Contains nodes of Ranvier 2. Wrapped in layers of Schwann cell membrane 3. Fast conduction 4. Myelinated SMALL DIAMETER AXONS 1. Slow conduction 2. Unmyelinated 3. Covered by Schwann cell cytoplasm

Lab 1

Lab 1

If a lesion occurs on the *right side of the cerebral cortex*, on which side of the body will the signs be seen?

Left

How might local anesthetics block pain?

Local anesthetics will inhibit voltage-gated Na+ channels which will block the nerve conductivity in a nerve. If the nerve was a periphery nerve, the action potential will not be reached by the brain so the sensation of pain will not be recognized.

Compared to a small diameter unmyelinated axon, which statement is true for a larger diameter unmyelinated axon?

Lower internal resistance to current flow.

What region of the spine does the spinal cord end at?

Lumbar region

How does maintaining the resting membrane potential allow the continued firing of action potentials?

Maintaining the resting membrane potential (RMP) is important as the opening and closing of the voltage-gated channels of Na+ and K+ depend on specific voltages being reached. Since the action potential is dependent on the changes in the permeability to Na+ and K+ which is caused by the opening and closing of the voltage-gated channels, it is crucial for the RMP to be maintained to allow the channels to undergo the normal cycle for an action potential to generate.

Why are compound action potentials easier to record than action potentials?

Measuring *action potentials from single axons requires specialized equipment*. It is much easier to measure the activity of *whole nerves using surface electrodes*. This gives the summed activity of all the action potentials in the nerve. The electrical potentials recorded in this way are known as compound action potentials (CAPs).

Does the concentration of sodium and potassium ions in the cell change much during an action potential?

NO! Changes in membrane potential are a result of an increase in Na+ permeability, followed by an increase in K+ permeability. However only tiny amounts of these ions actually cross the membrane, so intracellular Na+ and K+ concentrations are not altered measurably by this. The idea of ions rushing in and out of the cells is a common misunderstanding about action potentials.

When the sole of the foot is stroked, what are normal responses and what are abnormal responses?

NORMAL RESPONSE: 1. An adult's big toe flexes downwards 2. A newborn's big toe extends upwards ABNORMAL RESPONSE: 1. An adult's big toe extends upwards

Does not affect the rate of conduction

Nerve length

Can you tell from the records above, if the stimulus in record 7 is supramaximal?

No, you cannot, you would need a stimulus with greater strength than record 7 to see. If there is a greater increase in the size of the response, then it is below the maximum level of stimulus. If the size of the stimulus does not increase, then more stimulus tests with strength below record 7 need to be tested to make sure it is not the maximum value before confirming it is supramaximal.

A 37-year-old man is admitted to the emergency room after a car accident. The computed tomography (CT) scan shows damage to the spinal column at the level of T8. Would you expect this man to have damage to the phrenic nerve resulting in breathing difficulties?

No. the phrenic nerve arises from C3 - C5 in the neck, therefore, damage to T8 will not affect it

How tight should the Velcro strap be?

Not too loose and not too tight • If the strap is too loose, the signal will be weak, intermittent, or noisy. • If the strap is too tight, reduced blood flow to the finger may weaken the signal, and could cause discomfort.

What does OOC stand for?

Our of Compliance

Do the parasympathetic or sympathetic nerves trigger pupil constriction?

Parasympathetic nerves

Do the parasympathetic or sympathetic nerves mediate accommodation to a near object?

Parasympathetic nerves (pupils constrict)

Paresis

Partial paralysis

Peripheral Nerve Lab 2

Peripheral Nerve Lab 2

What are the possible consequences of this injury for Mason?

Possible consequences of this injury are exaggerated reflexes in the lower limb, as well as the Babinski sign. However, there will be no impact on the reflexes in the upper limb. He will have normal function in the upper half of his body but not the lower half of his body, which depending on the severity of the lesion, could prevent him from walking properly ever again. This could end his career in construction.

Which of the following factors could decrease an individual's reaction time?

Practice

Pre-Lab 1

Pre-Lab 1

What is the process of action potential conduction along the axon called?

Propagation

Saltatory conduction

Rapid transmission of a nerve impulse along an axon, resulting from the action potential *jumping from one node of Ranvier to another*, skipping the myelin-sheathed regions of membrane.

Electromyogram (EMG)

Record of the (intrinsic) electrical activity in a (skeletal) muscle

Compared to a myelinated axon of the same diameter, which statement is true regarding an axon with more myelin?

Reduced electrical leakage across the membrane.

Reflexes Lesson 3

Reflexes Lesson 3

What are the two structural subdivisions of the PNS?

SNS and ANS

Which of the following are characteristic of a stretch reflex and which are characteristic of a withdrawal reflex?

STRETCH REFLEX: 1. Initiated by stretching of the muscle 2. Monosynaptic 3. An example is the knee-jerk reflex 4. Fastest of all spinal reflexes WITHDRAWAL REFLEX: 1. Initiated by painful stimulus 2. Polysynaptic 3. An example is the crossed-extensor reflex

Transducer

Sensors that convert biological signals into electrical analog signals Ex. Pulse transducer

Which of these choices provides a logical description of the stages in a reflex arc?

Sensory neuron, spinal cord, (interneuron), motor neuron, muscle.

Sensory nerve endings

Sensory receptors stimuli such as touch, temperature, pain, and pressure. LOACTED IN THE *PERIPHERY*

So far, Mason's reflex tests have shown an exaggerated knee-jerk reflex and the Babinski sign. However, the response was normal when the doctor tested his elbow (triceps) reflex. In which area of the spinal cord does the doctor narrow down Mason's spinal injury to?

T3-L3

When the doctor uses a tendon hammer to tap just below Mason's knees, his reflex response is unusually pronounced. What can the doctor imply about Mason's back injury?

Since it is unusually pronounced, it means there was damage to the upper motor neurons as there is an exaggeration of a reflex (knee-jerk reflex) that involves the lower motor neuron. The upper motor neuron lesions may be caused by a stroke, brain injury, cerebral palsy, or upper spinal cord injury.

What does the effect of the Jendrassik maneuver indicate about the neural pathways involved in simple reflexes?

Since the magnitude is suppose to be greater, the Jendrassik maneuver decreases some inhibitory pathways, causing a more exaggerated knee-jerk response.

A patient who is brain dead may exhibit brief movement when their hand or foot is touched. Why is this?

Spinal cord reflexes may still be able to occur.

The phrenic nerve (which innervates the diaphragm) projects from C3-C5. What are the implications of injury to this part of the spinal cord?

Spinal injuries in this region can cause paralysis of the diaphragm. If this occurs, these patients will require a ventilator in order to breathe.

Muscle twitches

Spontaneous contractions of groups of muscle fibers

What are the differences between stretch reflexes and flexion reflexes?

Stretch stretches are initiated by stretching of the muscle, are monosynaptic, the fastest of all spinal reflexes, and an example includes the knee-jerk reflex. Flexion reflexes are initiated by painful stimulus, are polysynaptic, and an example includes crossed-extensor reflex.

Ulnar nerve

Supplies the little finger

Do the parasympathetic or sympathetic nerves mediate accommodation to a distant object?

Sympathetic nerves (pupils dilate)

Which statements regarding peripheral nerves are true and which are false?

TRUE: 1. Afferent and efferent are functional divisions of the PNS 2. Cell bodies of afferent nerve fiber are in the dorsal root ganglia 3. The SNS contains both sensory and motor fibers FALSE: 1. Cell bodies of efferent nerve fibers are in the dorsal root ganglia 2. The ANS is part of the SNS 3. The SNS and ANS are functional divisions of the PNS

Which of the statements regarding the "all or none" property of action potentials are true and which are false?

TRUE: 1. An action potential will always fire once the threshold is met 2. *All action potentials are the same size* FALSE: 1. Action potentials vary amplitude depending on the stimulus strength

How does the evoked CAP in the muscle relate to the stimulus voltage that is delivered to the nerve?

TRUE: 1. As the size of the stimulus voltage is increased, more motor units are recruited 2. At maximum amplitude, all the motor units have been stimulated by the nerve stimulation 3. The CAP amplitude recorded at the muscle will increase to a max value FALSE: 1. The CAP amplitude can increase past maximum value if a strong enough stimulus voltage is applied

Which of these statements about nerve conduction velocity are true and which are false?

TRUE: 1. Conduction velocity can be measured separately for motor and sensory neurons 2. Different diameters axons exhibit different conduction velocities 3. *Normal conduction velocity in motor nerves is approximately 50-60 m/s* FALSE: 1. Sensory nerve conduction velocities are too fast to be recorded 2. Normal conduction velocity in motor nerves is approximately 20 -100 m/s 3. Sensory nerve potentials are much large than evoked potentials

Which statements regarding nerve fibers in peripheral nerves are true and which are false?

TRUE: 1. Fibers that respond to pain tend to be smaller in diameter 2. Smaller axons are not myelinated FALSE: 1. Large diameter fiber conduct impulses slower than those of small diameter

Which statement regarding RMP are true and which are false?

TRUE: 1. Na+/K+ ATPase maintains an uneven distribution of cations across the membrane 2. The cell interior is more negative than the cell exterior FALSE: 1. The membrane is more permeable to Na+ than to K+ 2. The RMP is -50 mV

Which statements regarding peripheral nerves are true and which are false (continued)?

TRUE: 1. Nerve fibers are grouped into fascicles 2. The endoneurium acts as a "filter" controlling the substances that have access to the fluid surrounding the nerve fiber 3. A single nerve may contain a mix of sensory, motor, and autonomic nerve fibers FALSE: 1. All axons are wrapped in myelin sheath 2. Each nerve fiber is surrounded by perineurium 3. Fascicles are surrounded by endoneurium

Which of the following regarding refractory periods are true and which are false?

TRUE: 1. The refractory period lasts until the membrane potential returns to RMP 2. *The start of an action potential is the start of the refractory period* FALSE: 1. It is impossible for another action potential to fire until RMP is restored

Which statements regarding CAPs are true and which are false?

TRUE: 1. CAP amplitudes initially increases with increased stimulus voltage 2. The CAP recorded represents the sum of action potentials from axons that have fired at a given stimulation voltage 3. The size and shape of a CAP is determined by the number of axons stimulated at a given voltage FALSE: 1. All fibers have the same threshold potential 2. Action potentials in small diameter contribute more to CAP amplitude at lower stimulation voltages 3. As the stimulus strength is increased, the shape of the CAP remains the same

The stretch reflex is a protective measure for the muscles. What does it prevent?

Tearing

Only one eye has to be stimulated in order for the reflex to close both eyelids. What is this phenomenon called?

The *response elicited* in the *opposite eye to the stimulus* is called a *consensual response*. (The *response elicited in the same eye as the stimulus* is called a *direct response*.)

What is the Babinski sign, and what is it indicative of?

The Babinski sign is where the big toe curls upward in response to a stimulus on the sole of the feet; an abnormal plantar reflex where the big toe curls flex upward rather than downward. This is abnormal if it is occurring after 2 years of age and is indicative of an upper motor neuron lesion (damage to the spinal cord or the brain) since there is no inhibition of the local dorsiflexion reflex by the frontal lobes.

How does diameter and myelin affect action potential propagation?

The greater the diameter and myelin, the faster the action potential propagation is. The *diameter decreases* the *internal resistance* while the *myelin decreases membrane capacitance* and *increases membrane resistance*.

Peripheral reflexes are controlled by a reflex arc in the spinal cord. In addition, signals are also still sent to the brain. Why is this?

The brain receives sensory signals at the same time as a reflex response occurs. This allows for *conscious awareness of the stimulus and for wider responses to an event*. For example, if you accidentally touch a hot dish that has just come out of the oven, you will automatically withdraw your hand (reflex arc in the spinal cord), and register the pain as a burning sensation (processing in the brain).

Peripheral nerves

The bundles where somatic nerve fibers running to and from the periphery are collected together

Refractory period

The cell may either be unable to fire a second action potential (absolute refractory period), or be more resistant to firing a second action potential (relative refractory period)

What signal does the pulse transducer "transduce" into an electrical signal?

The change in pressure from pulsatile blood flow from the arteries

Spasticity

The combination of paralysis and hypertonia. It is the inability to move muscles which are tense and contracted (often painfully)

What does it mean if the Out of Compliance (OOC) flashes yellow?

The current is not flowing properly

What region of the spine does the spinal nerve extend from?

The end of the spinal cord into the lower regions

What are some processes that are responsible for the time delay between Ian's thought and his action?

The firing pattern needs time to travel through the cables and the nerves (conduction velocity), the software needs time to decode the firing pattern, and the time it takes for the brain to initiate the firing pattern.

Latency

The time elapsed from the *beginning of the stimulus pulse to the beginning of the evoked response*; DELAY

Look at the latency for the median nerve at the wrist and at the elbow. How do these latencies differ? Why do you suppose this is?

The latency for the elbow should be higher since it is further from the muscle, so it takes longer for the impulse to travel, which would make sense

Based on your observations, what is the apparent biological advantage of the pupillary light reflex?

The main function of this reflex is to protect the retina from damage that can occur due to intense light

Parasympathetic nervous system

The parasympathetic nervous system is generally associated with "rest and digest" activities (for example, pupil constriction, increased salivation, lacrimation (tears), urination, digestion, defecation, and sexual arousal). *Together with the sympathetic nervous system, the parasympathetic nervous system forms the autonomic nervous system*.

Based on what you have seen today, why do you think it would be advisable to record the heart rate for 60 seconds rather than for a shorter duration (for example, 15 or 30 seconds)?

The person may have an irregular heartbeat, which could be missed if recording for only 15 or 30 seconds. Recording for 60 seconds gives you a more accurate estimation of heart rate, which is necessary if the person has an irregular heart rate.

Describe the pupillary light reflex and its function.

The pupillary light reflex causes constriction or dilation of the pupil, depending on the change of the intensity of the light. It's function is to control how much light enters the eye.

Nerve conduction velocity

The rate at which an action potential travels along a nerve. You can measure this by stimulating the nerve supramaximally in two places, and recording the different times it takes to reach a measuring electrode.

Membrane resistance (Rm)

The resistance to current flow across the membrane

Internal resistance (Rin)

The resistance to current flow along the length of an axon

Direct response

The response elicited in the same eye as the stimulus

What happens to the size and shape of an action potential as stimulus voltage is increased?

The size and shape of an action potential do not change due to the all or none principle as the same size and same will be fired when the threshold potential is reached. For CAP, however, the action potential increases as long it is below the maximum stimulus level needed to reach the maximum CAP amplitude.

Aperature

The size of the pupil, the hole through which light enters

Spinal cord segments

The spinal cord can be divided into four regions: 1) Cervical (8) 2) Thoracic (12) 3) Lumbar (5) 4) Sacral (5) 5) Coccygeal (1) Overall, the human spinal cord is divided into 31 segments

Which statement regarding the stimulus artifact in a CAP is true?

The stimulus artifact is a result of current that spreads passively from the stimulating to recording electrodes. The stimulus artifact occurs when a nerve is electrically stimulated using external stimulating electrodes. The current from this stimulation spreads passively from the stimulating electrodes and is detected by the recording electrodes. It is referred to as an artifact because it results from HUMAN INTERVENTION, and NOT FROM THE NERVE ITSELF

What is the amplitude and shape of a CAP determined by?

The stimulus voltage and the diameter of axons in the nerve A CAP recording represents the sum of all action potentials that have fired at a given stimulus voltage. At low stimulus voltages, a CAP may be smaller in amplitude because only a few axons are firing. As the strength of stimulation increases, more and more axons reach the threshold for firing. Therefore, the CAP will increase in amplitude up until a maximum value when all axons in the nerve are firing.

Sympathetic nervous system

The sympathetic nervous system is generally associated with the "fight or flight" response which causes pupil dilation, increased heart rate, increased blood pressure, and increased sweating. Together with the parasympathetic nervous system, the sympathetic nervous system forms the autonomic nervous system.

What are the divisions of the nervous system and the general physiological roles of the nervous system? Include CNS and PNS, SNS and ANS.

The three basic functions of the nervous system include (1) To sense changes within and external to the body (2) To process sensory information from the environment and (3) To create a response. The divisions of the nervous system are the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS included the spinal cord and the brain while the PNS included the rest of the nervous system elsewhere in the body. The PNS is structurally subdivided into the somatic nervous system (SNS; also known as the voluntary nervous system) and the autonomic nervous system (ANS; also known as the involuntary nervous system). The SNS is the subdivided nervous system that we have control over such as the motor neurons that control skeletal muscles, while the ANS is involuntary so we cannot control it, such as smooth muscle, cardiac muscle of the heart, and controlled exocrine gland secretions through a two-neuron efferent pathway. In the SNS, information from the CNS travels directly to the skeletal muscle through effector motor neurons.

What are the two main properties of nerve fibers, and how does each property affect current flow and conduction velocity?

The two main properties of nerve fibers is: 1) Myelination → The more myelination there is, the faster the conduction velocity, as there is more membrane resistance, and the insulation reduces the electrical leakage 2) Diameter → The greater the diameter, the greater the current flow is due to less internal resistance (cause of a greater total volume for electrical currents to go through) and a greater amount of ion channels usually compared to axons with a smaller diameter

Briefly describe the evolutionary advantage of a withdrawal reflex.

The withdrawal reflex is used to warn us of danger and to protect us of any incoming harmful stimuli such as fire.

Why is the Babinski sign normally seen in infants under 1-2 years of age?

Their corticospinal tract has not yet fully myelinated

Why would there be a lag between the raw data signal and the channel calculation signal that you see on screen?

There is some processing time required to perform a channel calculation on the raw data, so there will always be a lag between seeing the raw data and the channel calculation.

Which is true in an evoked EMG?

There is synchronous excitation of the muscles supplied by the stimulated nerve.

Even with the neural implant, Ian has no sensation in his hands. If he was able to receive sensory feedback from his hand (for example, touch), how would this help him produce movements more effectively?

There would be a loop between his sensory feedback, brain, and neural implant. This will give the brain feedback on what adjustments need to be done without the need of a visual aid.

When Mason's foot is tested for the plantar reflex his big toe extends upwards. What does this response tell us?

This is an abnormal response--known as the Babinski sign--and tells us that there is an upper motor neuron lesion (damage to the spinal cord or the brain) since there is no inhibition of the local dorsiflexion reflex by the frontal lobes.

(T/F) The somatic nervous system is also known as the voluntary nervous system.

True

Proper connection of the dry earth strap

Unshielded electrode cables should be inserted into the pin closest to the label on the Bio Amp cable. For example, the electrode cable attached to the dry earth strap should be inserted as shown in the image to the right.

Are the following signs of an upper motor neuron lesion or a lower motor neuron lesion?

Upper motor neuron lesion: 1. Babinski sign 2. Exaggerated reflexes Lower motor neuron lesions: 1. Absent reflexes 2. Diminished reflexes

What does the presence of the Babinski sign indicate?

Upper motor neuron syndrome

Which of the following statements regarding the knee-jerk reflex is false?

Visual receptors sense the hammer tap.

"All or none" events

When an action potential begins, it propagates down the length of the axon. When the action potential reaches the end of the axon, a neurotransmitter is released into the synapse.

Signal conditioning

When raw data is amplified, filtered and isolated Signal conditioners include : 1) Bio Amp 2) Spirometer Pod

Why does the doctor conclude that the damage to Mason's spinal cord is in this area?

When there is a spinal injury between T3-L3, upper motor neuron lesion signs are present in the lower limb, however, the upper limbs are normal. As a result, lower limbs will show signs of exaggerated reflexes since there is upper motor neuron damage, which is the case for Mason's knee-jerk reflex; it would also make sense that there is a normal response to the elbow (triceps) reflex since it is an upper limb. Furthermore, the Babinski sign is present as there there is no inhibition of the local dorsiflexion reflex by the frontal lobes--a consequence of upper motor neuron damage. Check Answer

Observe your data. When the volunteer performed the Jendrassik maneuver how did the knee-jerk reflex change?

When your volunteer performed the Jendrassik maneuver the *amplitude* of the EMG response (the electrical activity resulting in muscle contraction) should *increase*, while the *latency* should be *relatively unchanged*

Ventral horn

Where *cell bodies of neurons* carrying information from the *spinal cord to the skeletal muscles are located/ EFFERENT*

Dorsal root ganglia

Where the *cell bodies of sensory neurons/ AFFERENT* are located

Can you explain why there is such a large difference in recorded amplitudes for an action potential and a CAP?

While for an action potential, the recording electrode is placed directly into the cell, when recording the amplitude. For CAP, an external recording electrode is placed outside the body creating distance between CAP and the recording electrode. Furthermore, the myelin sheath has insulating properties that will prevent the passive spread of current to the electrode. As a result, the strength and quality of the amplitude is greatly diminished in CAP as it reaches the external sensor.

In the knee-jerk reflex arc, where does the synapse between a sensory neuron and a motor neuron occur?

Within the spinal cord.

Can you connect stimulating electrodes to yourself?

Yes

To test a 63-year-old man for possible nerve damage, his nerve conduction velocity was measured. Can you explain how this is done?

You would stimulate a nerve with an electrical impulse, and then record with a sensor on the muscle to check it it contract. If it does not contract, there is nerve damage. Then you repeat this on another spot on the same nerve and record the time, and use this to calculate the nerve conduction velocity. Then use the distance and time differences to calculate the velocity.

Events in a CAP

a. The wave of depolarization has not yet reached the space under the electrodes. As a result, *both electrodes record the same voltage*, and the potential difference recorded is *zero*. b. The wave of depolarization has traveled along the nerve, and is *currently in the space under the first electrode but not the second*. The space under *the first electrode is now more negative relative to that under the second electrode*. The potential difference recorded between the electrodes is shown as a *positive deflection* in the recording. c. The wave of depolarization is now *under both recording electrodes.* As a result, both electrodes *record the same voltage* and the potential difference recorded is *zero*. d. The wave of depolarization has *traveled past the first electrode, but is still under the second electrode*. The space under the *second electrode is now more negative relative to that under the first electrode*. The potential difference recorded between the electrodes is shown as a *negative deflection* in the recording. e. The wave of depolarization has *traveled past both electrodes*. As a result, both electrodes record the same voltage and the *potential difference recorded is zero*.

Spinal injury between T3 and L3

• *Upper motor neuron lesion signs are seen in the lower limbs* • *The upper limbs are normal* • *Triceps reflex*: Normal • *Knee-jerk reflex*: Exaggerated • *Babinski sign*: Present

Spinal injury between C1 and C5

• *Upper motor lesion signs are seen in both the upper and lower limbs* • *Triceps reflex*: Exaggerated • *Knee-jerk reflex*: Exaggerated • *Babinski sign*: Present

Brain and brainstem injuries

• *Upper motor neuron lesion signs are present* • *Triceps reflex*: Exaggerated • *Knee-jerk reflex*: Exaggerated • *Babinski sign*: Present

Spinal i jury between C6 and T2

• *Upper motor neuron lesions can be seen in the lower limbs* • *Lower motor neuron signs are seen in the uper limb* • *Triceps reflex*: Decreased or Absent • *Knee-jerk reflex*: Exaggerated • *Babinski sign*: Present

Knee-jerk reflex

• A *Stretch reflex* 1. *Striking the patellar ligament* just below the patella will *stretch the quadriceps muscle*. 2. *Sensory receptors* in the muscle are *stimulated* and *trigger* an *impulse* in a *sensory axon*. 3. The sensory axon *synapses directly with a motor neuron* that conducts the *impulse* to the *quadriceps*, which *triggers contraction*.

Crossed extensor reflex

• A type of *withdrawal reflex* • Reflex involves *your body shifting its center of gravity to support withdrawal of the stimulated limb* • For example, if you accidentally step on a pieces of sharp broken glass, the withdrawal reflex causes you to immediately pull your foot away. At the *same time*, strength is increased in your *opposite leg* to support the sudden shift of all your body weight

Flexion Withdrawal reflex

• Also known as the *"flexion reflex"* or the *"withdrawal reflex"* • Mediates the withdrawal of a limb from a painful stimulus such as a pin prick or a flame, to protect the body from harm • It is a *polysynaptic reflex* • The *crossed extensor reflex* is a type of withdrawal reflex The steps include: 1. *Pain sensory neurons* (*nociceptors*) synapse with interneurons. 2. The interneurons synapse with motor neurons. 3. The motor neurons send impulses to the muscles which trigger withdrawal of the limb away from the stimulus.

Pupillary light reflex

• Causes the *constriction or dilation of the pupil in response to a change in the light intensity* • *Constriction*: *Parasympathetic motor nerve fibers* innervate the *iris sphincter (iris circular) muscles*. Contraction of these muscles causes *the iris to constrict* (known as *miosis*) which *reduces the aperture of the pupil* • *Dilation*: *Sympathetic motor nerve fibers* supply the *iris dilator (iris radial) muscle*. Contraction of these muscles leads to the *dilation of the pupil* ( also known as *mydriasis*). The sympathetic fibers also innervate muscles in the eyelids, which holds the upper eyelid open

Cranial nerve III (Oculomotor nerve)

• It is responsible for *controlling most of the eyes' movements, including constriction of the pupil • The oculomotor nerve arises from the anterior aspect of the midbrain (mesencephalon)

Cranial nerve VII (Facial nerve)

• Known as the *facial nerve* • Controls the *muscles of facial expression, the secretion of tears, and works to convey taste sensations from the front two-thirds of the tongue and oral cavity* • The nerve *provides the motor control of the lacrimal glands, which secrete tears*

Spinal injury between L4 and S2

• Lower motor neurons lesion signs are seen in the lower limb* • *The upper limbs are normal* • *Triceps reflex*: Normal • *Knee-jerk reflex*: Decreased or Absent • *Babinski sign: Absent

Monosynaptic reflex

• Monosynaptic means *one synapse* • A reflex pathway that consists of two neurons: > 1. One sensory neuron > 2. One motor neuron • Only the most simple reflexes such as the *kneejerk reflex*

Plantar Reflex

• Occurs when the *sole of the foot is stroked and the bug toe flexes (curls downward) and the foot inverts (arches up)* • It *depends on the full myelination of the corticospinal pathways, which does not occur until 1-2 years of age* (therefore, this reflex does not occur in infants until around 1-2 years of age). This is *because the reflex is dependent on the inhibition of dorsiflexion (big toe extends upward) by the frontal lobes*

Pupil constriction

• Parasympathetic innervation (via *cranial nerve III*) *contracts the iris sphincter muscle*, thereby constricting the pupil • Note that the pupil is simply a hole in the middle of the iris. The *iris sphincter and dilator muscles control the size of the pupil*, and therefore the *amount of light that enters through the pupil*

Polysynaptic reflexes

• Polysynaptic means *two or more synapses* • A reflex pathway that consists of at least three neurons: > 1. One sensory neuron > 2. One or more interneurons > 3. One motor neurons • *Most reflexes* are polysynaptic* as the *interneurons* connecting the sensory and motor neurons allow for *processing or inhibition of the reflex by the brain*. An example includes the *withdrawal reflex*

Stretch Reflex

• Produced in response to the stretching of the muscle and is a *monosynaptic reflex* •The central pathway for this reflex is *confined to the spinal cord, although info is passed to the brain* • *FASTEST* of all spinal reflexes • Example includes the *Knee-jerk reflex*

Unilateral oculomotor nerve damage

• Results in *the loss of both direct and consensual response in that eye* as the pupil in that eye is no longer innervated by the nerve • *Light shone into the other eye will also produce only a direct response*

Unilateral optic nerve damage

• Results in the *loss of the direct response in that eye, and loss of the consensual response in the other eye* • *Light shone into the other eye will produce the normal responses* in both eyes, as the optic nerve to this eye is intact and the oculomotor nerves to both eyes are intact

Muscle spindle

• Sensory receptors that are stimulated by changes in muscle length • The reflex sensitivity is controlled by the lengthening (relaxing) and shortening (contracting) of the muscle spindle fibers • Play an important role in *initiating the reflex*, but are also essential in *regulating the contraction of the muscle to ensure it is not overstretched* during the reflex. This is *achieved by the activation of motor neurons that resist the muscle stretch*

Babinski sign

• The name given to a plantar reflex that is abnormal when seen in those above two years of age. The Babinski sign is when the *big toe extends upwards instead of downwards* • Indicative of an *upper motor lesion (damage to the spinal cord or brain). The big toe extends upwards in these cases because the lesion has effectively removed the inhibition of the local dorsiflexion reflex by the frontal lobes*

Cranial nerve V (Trigeminal nerve)

• The nerve is responsible for *sensation in the face, motor functions (such as chewing), and the BLINK (CORNEAL) reflex* • It is the *largest of the cranial nerves* • *Each trigeminal nerve (one on each side of the pons)* has *three major branches*: the *ophthalmic nerve, the maxillary nerve, and the mandibular nerve*. These branches *converge in the trigeminal ganglion*, and from there a *single large sensory root enters the brainstem at the level of the pons* • It is the *ophthalmic nerve that senses the stimulation of the cornea, which elicits the blink reflex*

The Stretch Reflex in Action

• The stretch reflex helps us *maintain our upright posture*. For example, if we begin to lean to one side, the postural muscles on the side that you are leaning to will stretch. This stimulates stretch receptors in that muscle, which cause the postural muscles to contract and therefore corrects the posture. • However, there is *always some tonic inhibition of the lower motor neurons from the cerebral cortex*. • Therefore when *upper motor neurons or the pathways to the spinal cord above the level of the lower motor neuron are damaged*, then reflexes which involve the *lower motor neurons are exaggerated*

The accommodation-convergence reflex

• Used to *maintain focus on objects at different distances, the eyes adjust the degree to which light entering the eye is bent so that it falls in the retina* • When we view* near objects* the *eyes converge, the pupils constrict, and curvature of the lens increases* • When we view *distant objects*, vice versa occurs: the *eyes diverge, the pupil dilates, and the curvature of the lens decreases*

Blink Reflex

• When the cornea of the eye is stimulated, which causes both eyelids to shut involuntarily • The *afferent limb* of this reflex is via the *nasociliary branch of cranial nerve V (trigeminal nerve)*. The *efferent nerve* involves *cranial nerve VII (facial nerve)*