Physiology- Motor Nervous System

corticospinal or pyramidal tract

Controls contralateral distal limbs and digits motor activity Coordinates complex motor programs

brainstem pathway

Generates rhythmic CPG Controls ipsilateral postural and proximal limb muscle activity Acts as feed forward for muscle tone and postural adjustments

corticobulbar tract

controls ipsilateral movement of head, face, neck, and bilateral control of facial muscles

Tom presents to his physician with progressive weakness in his left leg and muscle atrophy in his left calf. He is diagnosed with a motor neuron disease. A. Is this an upper or lower motor neuron disease? B. Would you expect a decrease in the # of axons in the dorsal or ventral root?

(A) Lower and (b) Ventral

2. Charlie is a 16-year old soccer player who presented to his physician with progressive weakness in his right leg and muscle atrophy in his right calf. He is diagnosed with a motor neuron disease. A. Is this an upper or lower motor neuron disease? B. What area of the spinal cord would you expect a loss of neurons (apoptosis)? C. Would you expect a decrease in the number of axons in the dorsal or ventral root?

(A) Lower motor neuron disease. (B) Ventral horn of the spinal cord ( C).Yes, a decrease in the number of axons in the ventral root.

5. Explain the physiologic role of the following structures. Which receives input from the gamma motor neuron? A. muscle spindles B. intrafusal fibers C. Golgi tendon organs

(A) muscle spindle- sensory receptor located within the muscle fiber relays information re muscle length and rate of change in length of the muscle fiber. (B). intrafusal fibers- muscle fibers that act as sensory receptors not as force generators; receives input from the gamma motor neurons (C). Golgi tendon organs- stretch receptor located at tendon transmits information re muscle tension at the tendon/muscle junction

13. Cortical control of spinal motor neurons: A. is mediated by lateral and ventral corticospinal tracts. B. activates the withdrawal reflex of a limb in response to pain. C. generates the motor output used in walking. D. A, B and C

A

7. In the muscle spindle reflex, stretching of the biceps muscle leads to: A. contraction of the biceps and relaxation of the triceps B. relaxation of the biceps and contraction of the triceps C. contraction of the biceps and contraction of the triceps D. relaxation of the biceps and relaxation of the triceps

A

During active contraction of extrafusal muscle fibers, the gamma motor neurons function to do what? A) Increase sensitivity of afferent fibers to sudden stretch B) Increase tension developed by extrafusal muscle fibers C) Decrease summation of motor units D) Decrease magnitude of stretch reflex

A

In the muscle spindle reflex, stretching of biceps muscle leads to which of the following? A) Contraction of biceps and relax triceps B) Relax biceps, contract triceps C) Relax biceps, relax triceps D) Contract biceps, contract triceps

A

Which of the following receives input from the gamma motor neuron? A) Intrafusal fibers B) Golgi tendon organs C) Extrafusal fibers D) Muscle spindles

A

30 yr old woman presents with difficulty chewing because jaw muscles become tried. Diagnosed with an autoimmune disease (myasthenia gravis) in which antibody blocks Ach receptors at neuromuscular junction. She is prescribed an inhibitor of Ach-esterase, What affect does this have at the neuromuscular junction. Choose all that apply A) Prolongs each excitatory postsynaptic potential (EPP) B) Increases twitch summation C) Decreases voltage gated Na channel threshold

A and B

Alpha motor neuron

ALWAYS excitatory The effector Muscle cant contract without Efferent only can control from 3-1000 muscle fibers (cells) all will be in the same muscle Soma/nucleus is in spinal cord, axon from ventral horn out ventral root Sensory input (pain) enters dorsal root of cord, synapses with interneuron. Interneuron activates alpha motor neuron which exits the ventral horn to activate skeletal muscle

14. A single somatic motor neuron usually innervates: A. many muscle fibers from different muscles B. many fibers in the same muscle C. only a single muscle fiber D. all fibers in a muscle

B

16. A single action potential in an alpha motor neuron causes which of the following? A. summed tetanus B. single twitch C. end plate potential (EPP) D. B and C

B

22 yr old PA student c/o weakness and pain in L hand. Weakness occurs on palmar side of thumb and wrist. Pain is burning sensation that decreases with moderate movement and stretching of wrist. Dx with carpal tunnel syndrome. Simultaneoud appearance of pain and motor deficit limited to specific region suggests damage to which? A) Gamma motor neuron B) Spinal nerve C) Alpha motor neuron D) Unmyelinated C fiber

B

4. Which of the following characteristics is a feature of Golgi tendon organs but not of muscle spindles? A. sensory signals are carried by Ia afferent fibers B. contraction of the muscle increases action potential generation C. contraction of the intrafusal fibers increases action potential generation

B

6. Fred steps on a nail when walking in his backyard barefoot. Which of the following actions is NOT a component of the withdrawal reflex of the foot? (The ipsilateral side is the side receiving the painful stimulus.). A. ipsilateral flexors contract. B. ipsilateral flexors are inhibited. C. contralateral extensors contract. D. ipsilateral extensors are inhibited.

B

A single somatic motor neuron usually innervates which of the following? A) Many muscle fibers from different muscles B) Many fibers in same muscle C) Only a single muscle fiber D) All fibers in a muscle

B

Golgi tendon organ fires AP most frequently when its associated muscle is in which state? A) Stretched B) Contracted C) Relaxed

B

What is location of locomotor generators involved in walking? A) Motor cortex B) Spinal cord C) Thalamus D) Cerebellum

B

Which of following would suggest upper motor neuron lesion rather than lower motor neuron lesion? A) Pronounced muscle atrophy B) Spasticity of muscle C) Profound muscle weakness

B

Which of the following characteristics is a feature of golgi tendon organs but not muscle spindles? A) Contraction of intrafusal fibers increases generation of action potential B) Contraction of muscle increases generation of AP C) Sensory signals are carried by afferent fibers

B

What area of spinal cord would you expect a loss of neurons (apoptosis) in a patient with progressive weakness and muscle atrophy in R calf? A) Medial dorsal region B) R ventral horn C) R dorsal horn D) L ventral horn

B) R ventral horn

11. A 22 -year old PA student presents to the Duke university health clinic complaining of weakness and pain in her left hand. The weakness occurs on the palmar side of her thumb and wrist. The pain is a burning sensation that decreases with moderate movement and stretching of the wrist. She is diagnosed with carpal tunnel syndrome. The simultaneous appearance of pain and motor defect limited to a specific region suggests damage to a(n)____. A. alpha motor neuron B. spinal nerve C. gamma motor neuron D. unmyelinated C fiber

B. spinal nerve contains sensory input and motor output

12. The Golgi tendon organ fires action potentials most frequently when its associated muscle is in which state? A. relaxed B. stretched C. contracted

C

3. Which of the following is an example of a large motor unit? A. Muscles of the thumb B. Muscles mediating speech C. Axial muscles controlling posture D. Muscles mediating facial expression

C

8. Which if the following observations would suggest an upper motor neuron lesion rather than a lower motor neuron lesion? A. profound weakness B. pronounced atrophy C. spasticity

C

A patient has larger than normal patellar tendon reflex. One cause may be: A) Long duration of presynaptic inhibition B) Decreased # of voltage gated Na channels C) Ingestion of anti-Ach-esterase D) Degradation of schwann cells

C

A single AP in an alpha motor neuron causes what? A) Inhibitory end plate potential (IPP) B) Summed tetanus C) Single twitch D) Excitatory end plate potential (EPP)

C

Fred steps on nail barefoot. Which is NOT a component of the withdrawal reflex of foot? (ipsilateral side receiving painful stimulus) A) contralateral extensors contract B) ipsilateral flexors contract C) ipsilateral flexors inhibited D) ipsilateral extensors inhibited

C

Which of the following is an example of a large motor unit? A) Muscles mediating facial expression B) Muscles mediating speech C) Axial muscles controlling posture D) Muscles of the thumb

C

Would you expect a decrease in # of axons in dorsal or ventral root in a patient with progressive weakness and muscle atrophy in R calf? A) Dorsal root B) Both dorsal and ventral roots C) Ventral root

C

15. During active contraction of extrafusal muscle fibers, the gamma motor neurons function to: A. increase the tension developed by the extrafusal muscle fibers B. decrease the summation of motor units C. increase the sensitivity of the 1a afferent fibers to sudden stretch D. decrease the magnitude of the stretch reflex

C. Gamma motor neurons are co-activated with alpha motor neurons. The gamma motor neuron shortens the intrafusal fibers commensurate with the extrafusal fibers to maintain the sensitivity of the 1a stretch receptors. This increases the magnitude of the stretch reflex if evoked during contraction.

9. What is the location of locomotor generators involved in walking? A. thalamus B. cerebellum C. motor cortex D spinal cord

D

Activation of basal ganglia leads to what? A) Induces rigidity B) Increases international tremor C) Produces parkinsons disease like symptoms D) Decreases muscle tone throughout body

D

The addition of a drug that inhibits Ca++ channels at neuromuscular joint will... A) Increase presynaptic Ca++ influx B) Increase postsynaptic amplitude of EPP C) Decrease amplitude of action potential D) Decrease presynaptic release of neurotransmitter

D

Which of following = tru regarding cortical control of spinal motor neurons? A) Activates withdrawal reflex of a limb in response to pain B) Generates motor output used in walking C) Activates dorsal root fibers D) Mediated by lateral and ventral corticospinal tracts

D

Why is the motor cortex map broader (less precise) than that of the sensori-motor cortex?

Flexibilty of program

1. A 30- year old woman presents with difficulty in chewing. When she eats certain foods (meats), her jaw muscles become weak and tired. After a period of rest, her jaw muscles regain their strength until she eats again. It is determined that she has myasthenia gravis. She is prescribed an inhibitor of acetylcholinesterase. A. What effect does the acetylcholinesterase have at the neuromuscular junction?

In myasthenia gravis, antibodies bind to the acetylcholine receptor reducing the number of available receptors at the neuromuscular junction. The release of acetylcholine in these patients is normal but because the receptor number is decreased the EPP is reduced and may not be sufficient to reach threshold for triggering an action potential. This reduces twitch summation causing weakness and fatigue. Treatment with acetylcholine inhibitors decreases the degradation of acetylcholine thus enhancing and prolonging each EPP, increasing twitch summation and reducing weakness.

1. A 30- year old woman presents with difficulty in chewing. When she eats certain foods (meats), her jaw muscles become weak and tired. After a period of rest, her jaw muscles regain their strength until she eats again. It is determined that she has myasthenia gravis. She is prescribed an inhibitor of acetylcholinesterase. D. What type of channel initiates the action potential?

Initiation and propagation of the action potential uses the voltage gated Na+ channel.

Degeneration in the anterior portion of the right cerebellum would affect movement in the lower limbs on the (right ipsilateral, left contralateral) side of the body.

Right ipsilateral

Explain the physiologic role of the muscle spindle:

Sensory receptor detects rate of change in length of muscle fiber

The hand occupies a larger portion of the primary motor cortex "map" than the leg. This means that the hand has (larger, smaller) motor units.

Smaller

Explain the components that mediate the flexor reflex to noxious stimuli and the cross extensor reflexes in rhythmic activity (e.g., walking).

Step on a tack Flexor muscles are activated and extensor muscles of the ipsilateral leg are inhibited move limb away. Extensor muscles are excited on contralateral leg contralateral flexor inhibited. This is also how it works while we are walking (repeating this reflex over and over). Uses interneurons to cross the midline of the spinal cord.

10. Alpha motor neurons are found in which area of the spinal cord?

The cell bodies of alpha motor neurons are in ventral horn of spinal cord; axons exit by ventral root

hypokinestic disorder

absence of movement (resting tremor, akinesia, muscle rigidity) Tonic inhibition of thalamus altered due to loss of neurons in substantia nigra resulting in decreased excitatory outflow to motor neurons by thalamus I.e. parkinsons disease- frozen

1b sensory neurons

afferent nerve that sends output from Golgi tendon organ

1a sensory neurons

afferent sensory nerve that reports stretch from the intrafusal muscle fibers.

Alpha motor neurons = found in which area of spinal cord?

cell bodies in ventral horn of spinal cord, axons exit by ventral root

neuromuscular junction

chemical synapse formed by the contact between a motor neuron and a muscle fiber. It is at the neuromuscular junction that a motor neuron is able to transmit a signal to the muscle fiber, causing muscle contraction.

mediates feed-forward and feed-back control

control of posture • Feedforward is pre-programmed and typically preceds onset of movement • Feedback is initiated by sensory inputs that detect instability (feedforward) brainstem command --> limb movements and postural adjustments --> postural instability (error) --> feed back to postural adjustments (cerebellum)--> reticulospinal pathway to brainstem

brain stem motor centers

control spinal CPG without input from motor cortex

upper motor neuron syndrome

damage to primary cortex or corticospinal tracts, weakness, spasticity (increased muscle tone, hyperactive stretch reflexes, spasms), loss of fine voluntary movement.

Upper motor neurons

descending systems in the motor cortex - planning initiating, directing Basal ganglia- proper info of mov't and cerebellum- corrects motor errors --> descending systems (upper motor neurons)--> motor cortex (planning/initiating/directing voluntary mov't) and brainstem centers (basic mov't and posture control) --> spinal cord and brainstem circuits

hyperkinetic disorder

excessive and abnormal movement (writhing, tremor at rest) Tonic inhibition of thalamus is altered resulting in increased excitatory outflow to motor neurons by thalamus i.e. hunington's disease

alpha motor neurons

innervate extrafusal muscle fibers to generate force for posture and movement, efferent coactivated with gamma motor neurons in contraction, efferent

gamma motor neurons

innervate intrafusal muscle fibers (muscle spindle). Muscle spindle shortens concurrent with shortening of the extrafusal muscle fibers. This enables the muscle spindle to correctly "sense" fiber length and stretch, efferent

upper motor neurons in cerebral cortex

lateral white matter of spinal cord corticospinal or pyramidal tract lower motor neurons in lateral ventral horn skilled movements, distal limbs

A 16 yr old soccer player presents with progressive weakness in R leg and muscle atrophy n R calf. Dx with motor neuron disease. Is this upper or lower motor neuron disease?

lower

upper motor neurons in brainstem

medial white matter of spinal cord vestibulospinal and reticular tracts lower motor neurons in medial ventral horn --> posture and balance proximal limbs

Intrafusal muscle fibers

modified muscle fibers within a muscle spindle - innervated by gamma motor neurons. Enables responding to stretch and length (efferent).

Lower motor neurons

motor unit in spinal cord. Primary is here. (secondary is brainstem and cerebellum, tertiary is cerebral cortex) spinal cord and brainstem circuits:: local circut neurons (reflex coordination)--> motor neuron pools (lower motor neurons) --> skeletal muscles

1. A 30- year old woman presents with difficulty in chewing. When she eats certain foods (meats), her jaw muscles become weak and tired. After a period of rest, her jaw muscles regain their strength until she eats again. It is determined that she has myasthenia gravis. She is prescribed an inhibitor of acetylcholinesterase. C. What ion channel is gated by acetylcholine?

nicotinic Ach receptor is a ligand gated Na+ ion channel. Activation of the channel permits Na+ ion to enter the cell and K+ to exit. Entry of Na+ will depolarize the membrane locally.

Spinal Central Pattern Generators (CPG)

o Definition: neural circuits that produce timing and coordination of movement independent of sensory input o Characterized by: Oscillatory: i.e. normal walking has stance phase and swing phase Flexible: i.e. change walking to running by decreasing stance phase and sequence of limb movements in quads o Modulated by brain stem - brain stem can adjust CPG in response to sensory feedback (smooth gait) i.e. brain stem coordinates CPG governing limbs to walk forward or backward

upper motor neurons and cerebellum

o Modifies upper motor neuron activity to provide smooth well coordinated movements = feed forward o Detects "motor error" btwn intended and actual movement and "corrects" this error by influencing upper motor neuron output to antagonistic skeletal muscles o Corrections can be made during a movement or as "motor learning" when correction is being stored (motor memory) = feedback o Damage causes persistent errors in executing movement (cerebellar ataxia = jerky imprecise movements, note these movement errors are always on the same side of the body as the damage to the cerebellum o Cerebellum doesn't initiate movement but instead facilitates coordinated movement

withdrawal reflex

o Reflex circuitry mediates withdrawal of limb from painful sensory stimulus (flexor reflex) o crossed extensor reflex provides postural suppot during withdrawal of affected limb o pain input to signal cord excites/inhibits reciprocal muscles on injured side of the body (causing withdrawal reflex) and on opposite side of the body to support body weight

locomotion

o Uses reciprocal innervation and withdrawal reflex. Reciprocal swing phase of a limb (forward flex) and stand phase (backward extension) is used in walking. o Repeated spinal reflex

Motor programs

pattern of neural activity required to properly perform the desired movement

golgi tendon organ

sensory receptor o Encapsulated afferent nerve endings located in the tendon which are in series with the extrefusal muscle fibers • Activated by contraction of the muscle and not by stretch • Afferent sensory signal is sent to spinal cord by 1B sensory neuron

muscle spindle

sensory receptor o Reports stretch (muscle length and speed of stretch) o Spindle = intrafusal muscle fibers innervated by gamma motor neurons o When the spindle is stretched the 1a afferent sensory nerve to inform the fiber informs the spinal cord of the change o Activation of the spindle leads to contraction of the stretched muscle and concurrent relaxation of the antagonistic muscle by reciprocal innervation (i.e. reciprocal inhibition)

brainstem motor programs

serve essential needs (breathing, swallowing)

Extrafusal muscle fibers

skeletal muscle fibers that for the bulk of the muscle and generate its force and movement - generated by alpha motor neurons.

1. A 30- year old woman presents with difficulty in chewing. When she eats certain foods (meats), her jaw muscles become weak and tired. After a period of rest, her jaw muscles regain their strength until she eats again. It is determined that she has myasthenia gravis. She is prescribed an inhibitor of acetylcholinesterase. B. What type of muscle is affected?

skeletal muscle.

muscle stretch reflex

stimulus --> muscle spindle --> 1a fiber afferent --> to brain, + signal to quads, inhibitory signal to hamstring (efferent) Divergence - of afferent signal in spinal cord injury, sensory to brain, activation of alpha motor neuron to extensor (quadriceps) and inhibition of alpha motor neuron to flexor (hamstrings) Gamma motor neuron co-activated to hamstrings to maintain length of the spindle

primary motor cortex

the section of the frontal lobe responsible for voluntary movement • Neurons of cortex initiate complex movement o Regions with smallest motor units = largest area in motor cortex o Map depicts contralateral movement with lower extremity depicted most medial o frontal lobe- plans and initiates motor programs, is regulated by basal ganglia and cerebellum via the thalamus and by sensory input from parietal lobe o objective = optimal task performance not stereotypical movement patterns hence "map" depicts movements not specific muscles

Describe the symptoms of upper motor syndromes

• Damage to corticospinal tracts and/or primary cortex • Weakness • Spasticity Increased muscle tone Hyperactive stretch reflexes Spasms in response to stretch o Loss of fine voluntary movement i.e. loss of independent movement of fingers

Describe the symptoms associated with the lower motor syndrome.

• Damage to spinal cord (alpha motor neuron) • Flaccid paralysis (loss of movement) or paresis (weakness) of the affected muscles • Hypoactive reflexes • Loss of muscle tone (i.e. resistance to stretch) and later atrophy of affected muscle

Skeletal muscles

• Most skeletal muscles move the skeleton o Some exceptions: esophagus, muscle of lips, middle ear • Skeletal muscle often inserts just beyond a joint - important for lever action, small degrees of shortening cause large movements • Usually have skeletal muscle on either side of a joint o Flexors close the angle o Extensors open the angle

organization of neurons in spinal cord

• Motor neurons in medial ventral horn of spinal cord (cervical and lumbar) govern postural control (back and shoulder) o Receive input from upper motor neurons and from local interneurons o Project bilaterally • Motor neurons in lateral ventral horn govern fine skilled behavior of forearm, hand, and most lateral fingers.... Extensors & flexors • Motor output exits from the ventral horn

Describe some general principles of motor control.

• Motor programs serve essential needs (i.e., locomotion, posture, breathing) and range from involuntary actions (withdrawal reflexes) to complex voluntary activities. • Motor control depends on sensory signals (stretch receptors) and reflex activity (spinal cord). Higher motor centers (brain) superimpose commands (motor programs) on spinal cord reflexes. • Motor unit = motor neuron and the muscle fibers it innervates is the basic element in motor control. • Alpha motor neuron is the final common pathway by which CNS causes muscle contraction. • Alpha motor neuron is EFFERENT ONLY pathway!

motor unit

• Motor unit = motor neuron and all muscle fibers it controls o Small motor unit has few fibers (3-10) = finer movement o Large unit = more fibers (>1000) = more tension

Explain the role of the basal ganglia and cerebellum in voluntary movement

• Regulate movement • Select which motor program in brain stem and spinal cord is initiated by regulating tonic inhibition of the thalamus • Note: loss of dopamine from basal ganglia (Subt. Migra) to the striatum increases inhibition of the thalamus by substantia nigra (Pr) and globus palladius resulting in hypokinesis