A&P Exam 2 (Nervous System, Neurophysiology, & Synapses)

Ependymal Cells

Function: line ventricles (brain) and central canal (spinal cord); assist in producing, circulating, and monitoring cerebrospinal fluid

What are axons?

Long cytoplasmic processes capable of propagating electrical impulses known as action potentials.

What are dendrites?

Slender, sensitive processes that extend out from the cell body that play a key role in intercellular communication.

Electrochemical gradient

The sum of the chemical and electrical forces acting on ions across the plasma membrane

Spatial Summation

When simultaneous stimuli applied at different locations have a cumulative effect on the membrane potential

Hyperpolarization

makes the cell more negative if it overshoots -70mV

What is the "all or none" rule?

If the threshold is met or exceeded, the neuron generates complete action potential. If the threshold is not reached nothing happens.

Hyperkalemia is an excess of potassium in the extracellular fluid. What effect would this have on the resting membrane potentials of the nervous system and on neural excitability?

Such a change in the K+ gradient across the plasma membrane results in less net diffusion of K+ out of neurons. The K+ concentration in the cytoplasm increases, partially depolarizing the membrane and making neurons more excitable.

Repolarization

Take mV from more + to more -

Depolarization

Takes mV from - to more +

What is presynaptic facilitation?

the process in which one neuron enhances the effect of another

Excitatory Cholinergic Synapse Steps

1. Action potential arrives at the synaptic knob; voltage-regulated Ca2+ channels in the synaptic membrane open 2. Ca2+ enters synaptic knob and triggers exocytosis of ACh 3. ACh diffuses across the cleft and binds chemically (ligand)-regulated Na+ channels; channels open allowing Na+ in = depolarize postsynaptic membrane 20ms 4. Influx of Na+ produces a local potential that carries to the axon hillock and if strong enough will generate an action potential

Describe the steps that a local potential undergoes.

1. Ligand binds to receptor 2. Channel opens 3. Na+ rushes into the dendrite 4. Depolarization occurs 5. Na+ diffuses into the cytoplasm 6. Na+ diffusion produces a current along the dendrite and soma towards the axon hillock 7. If enough of a current it will reach the threshold and cause an action potential

Describe the steps that an action potential undergoes.

1. Local potential at axon hillock increases until it rises to threshold 2. Neuron produces an action potential; voltage-regulated Na+ channels open; more and more Na+ gates open as Na+ enters the cell; K+ gates open more slowly when threshold is reached (rapid depolarization) 3. When 0mV is reached/passed, Na+ gates are closed; voltage peaks at approx. +35mV (0mV in some, +50mV in others) 4. K+ gates now fully open; K+ leaves the cell repolarizing the membrane; causing shift back to negative inside and positive outside 5. K+ channels remain open a little longer than the Na+ channels and more K+ leaves than Na+ came in causing a 1 or 2 mV overshot or hyperpolarization

Describe the five steps that occur between the arrival of an action potential at the synaptic knob and the beginning of a new action potential in the postsynaptic neuron.

1. The action potential reaches the axon terminal and depolarization of the membrane causes the voltage gated calcium channels to open. 2. The high concentrations of intracellular calcium causes synaptic vesicles to fuse with the membrane 3. Neurotransmitters are released by exocytosis 4. Neurotransmitters diffuse through the synaptic cleft, and acts on the post synaptic membrane. 5. Then EPSP is generated. (Or if the released neurotransmitter is inhibitory and causes hyperpolarization of the post synaptic membrane, then IPSP is generated.

Cessation of Synaptic Signal Steps

1. The presynaptic cell stops releasing the neurotransmitter 2. The neurotransmitter is released from the membrane channel on the postsynaptic membrane and diffuses into the extracellular fluid (astrocytes may absorb it and return it to the neurons, synaptic knob may reabsorb the neurotransmitter by endocytosis then break it down with an enzyme called monoamine oxidase (MAO), or some neurotransmitters, like ACh, are degraded in the synaptic cleft)

Dopamine

A CNS neurotransmitter released in many areas of the brain. May have either an inhibitory or excitatory effect.

Serotonin

A CNS neurotransmitter responsible for attention and emotional states.

What is a ganglion?

A collection of neuron cell bodies in the PNS

What is the functional difference between a dendrite and an axon?

A dendrite transmits signals into the cell body, while axons transmit signals away from the cell body.

What is cerebrospinal fluid (CSF)?

A fluid that fills the central passageways between the longitudinal axis of the spinal cord and brain, as well as surrounds the brain and spinal cord. It circulates continuously, providing a protective cushion, and transports dissolved gases, nutrients, wastes, and other materials.

What is myelin?

A membranous wrapping which serves as electrical insulation. It increases the speed at which an action potential travels along the axon.

What is the function of neuromodulators? Compare and contrast neuromodulators and neurotransmitters.

A neuromodulator is a chemical messenger that is released by the brain cells to act on neurons to modulate their response to a neurotransmitter. They are not released into the synapse but from varicosities and act over long distances.

What makes an action potential rise to +35 mV? What makes it drop again after this peak?

A neuron fires, causing the voltage-gated Na+ channels to open quickly. (K+ opens more slowly) This depolarizes the membrane, which stimulates still more voltage-gated Na+. This causes the membrane voltage to rise quickly. When rising potential passes 0mV, Na+ channels are inactivated and begin closing. It drops because the Na+ gate has closed and the K+ channels are now fully open. K+ exits the cell, and their outflow repolarizes the membrane, shifting the voltage back to negative values.

What is the axolemma?

A specialized portion of the plasma membrane that surrounds the axoplasm.

What is a synapse?

A specialized site of contact between two neurons or a neuron and an effector (gland or muscle) that allows one-way flow of neural impulses

What is action potential?

A sudden change in electrochemical force that propagates on the membrane surface of a neuron

What are ependymocytes?

A type of ependymal cell that has motile cilia that aid in the circulation of CSF.

What are graded potentials?

Aka local potentials, are changes in the membrane potential that cannot spread far from the site of stimulation

What does the PNS consist of?

All of the neural tissue outside of the CNS

What do visceral motor neurons innervate?

All peripheral effectors other than skeletal muscle. i.e., smooth muscle, cardiac muscle, glands, and adipose tissue

What are motor neurons?

Also referred to as efferent neurons, motor neurons form the efferent division of the PNS. They carry instructions from the CNS to peripheral effectors in a peripheral tissue, organ, or organ system. *Axons traveling away from the CNS are called efferent fibers

Why is a single EPSP insufficient to make a neuron fire?

Because it doesn't bring us to the threshold. You must have continued EPSP (and IPSP's) close enough in time to each other so they can accumulate and reach the threshold value.

Suppose a poison were to slow down the Na+-K+ ATPase pumps of nerve cells. How would this affect the resting membrane potentials of neurons? Would it make the neurons more excitable than normal, or make them more difficult to stimulate? Why?

Because the Na+-K+ pump removes three positive charges from the cell for every two it brings in, it contributes to the negative membrane potential. If the activity of the Na+-K+ pumps was reduced, ion leakage would make the plasma membrane less polarized and more excitable.

What are collaterals?

Branches in an axon, which enable a single neuron to communicate with several other cells.

6 Types of Neuroglia

CNS: astrocytes, oligodendrocytes, microglia, ependymal cells PNS: schwann cells, satellite cells

Chemically Gated Channel

Chemically gated channels are normally closed, but when a specific chemical bonds to it, it opens the channel and allows the ion to move through it. ex: acetylcholine

What are neuromodulators?

Chemicals (small peptides) released by the synaptic bulb along with neurotransmitters that modify the effect of the neurotransmitter

What are neurotransmitters?

Chemicals released by the axon terminal of a presynaptic cell

What are Nissl bodies?

Clusters of RER and free ribosomes in the perikaryon.

What is axoplasm?

Cytoplasm of the axon which contains neurofibrils, neurotubules, small vesicles, lysosomes, mitochondria, and enzymes.

List the four types of neural circuits and describe their similarities and differences.

Divergence: Converging Circuits: Reverberating/Oscillating: Parallel after Discharge:

What is the refractory period? What are the two phases of the refractory period?

During an action potential and a few msec after, it is difficult or impossible to stimulate to produce another action potential. Two Phases: 1. Absolute refractory period (the membrane cannot respond to further stimulation) 2. Relative refractory period (the membrane can only respond to larger than normal stimulus)

Extracellular Fluid vs. Intracellular Fluid & the Sodium-Potassium Pump

ECF: larger concentration of Na+ outside, smaller concentration of K+ outside, proteins have a +charge ICF: smaller concentration of Na+ inside, larger concentration of K+ inside, proteins have a -charge *Electrically, potassium and sodium want to ENTER the cell. Chemically, potassium wants to LEAVE the cell, and sodium wants to ENTER the cell.

What is an excitatory postsynaptic potential (ESPS)? An inhibitory postysaptic potential (IPSP)?

EPSP: a graded depolarization caused by the arrival of a neurotransmitter at the postsynaptic membrane IPSP: a graded hyperpolarization of the postsynaptic membrane

What are the two divisions of the PNS?

Efferent and afferent. Afferent division: brings sensory information to the CNS Efferent division: carries motor commands from the CNS to muscles/glands/adipose tissue

What two types of effects can a neurotransmitter have an a postsynaptic cell?

Excitatory or inhibitory. ex: acetylcholine (ACh), norepinephrine (NE), dopamine, and serotonin

Astrocytes

Function: maintain the blood-brain barrier; provide structural support; regulate ion/nutrient/dissolved gas concentrations; absorb/recycle neurotransmitters; form scar tissue after surgery

Microglia

Function: removes cell debris/wastes/pathogens by phagocytosis

Schwann Cells

Function: surround all axons in PNS; responsible for myelination of peripheral axons; participate in repair process after injury

Satellite Cells

Function: surround neuron cell bodies in ganglia; regulate O2/CO2/nutrient/neurotransmitter levels around neurons in ganglia

Oligodendrocytes

Function: to myelinate CNS axons; provide structural framework *myelination: the production of the myelin sheath - a fatty white substance that surrounds the axon of some nerve cells; forms an electrically insulating layer

Explain why myelinated fibers conduct signals much faster than unmyelinated fibers.

Gaps in the myelin sheath (referred to as nodes of ranvier) formed between myelin sheath cells enable action potential passing down the axons. The signals jump from node to node (called saltatory conduction) causing the impulse to speed up.

GABA (Gamma-Aminobutyric Acid)

Generally has an inhibitory effect. Released in ~20% of the brain. Function still incompletely understood. In the CNS, it appears to reduce anxiety.

Compare and contrast continuous and saltatory propagation.

In saltatory propagation, the nerve impulse will jump between the spaces between the nodes of Ranvier (the pieces of insulation that surround the neuron). This is faster than continuous propagation, where the nerve impulse travels down the whole unmyelinated neuron.

Temporal Summation

Is the addition of stimuli occurring in rapid succession at a single synapse that is active repeatedly

How can a severed peripheral nerve fiber find its way back to the cells it originally innervated?

Near the injury, Schwann cells, the basal lamina, and the neurilemma form a regeneration tube. The Schwann cells produce cell-adhesion molecules and nerve growth factors that enable a neuron to regrow to its original destination. When one growth process finds its way into the tube, it grows rapidly and the other growth processes are retracted.

What does the perikaryon consist of?

Neurofilaments, neurotubules, organelles (mitochondria, free/fixed ribosomes, and rough endoplasmic reticulum (RER))

What is a synapse between a neuron and a muscle cell called?

Neuromuscular junction

Norepinephrine

Neurotransmitter that is widely distributed in the brain and in portions of the ANS. Aka noradrenaline. Has an excitatory, depolarizing effect on postsynaptic membranes.

How is myelin produced? How does myelin production in the CNS differ from that in the PNS?

Oligodendrocytes wrap their branch-like extensions around axons to create the myelin sheath. Schwann cells simply produce myelin. Myelin is produced by oligodendrocytes in the CNS, while myelin in the PNS is produced by Schwann cells.

•How (if at all) are PNS cells repaired if damaged? •How (if at all) are CNS cells repaired if damaged?

PNS: dendrites and myelinated axons can be repaired if the cell body is still intact and schwann cells are active CNS: little to no repair takes place, even if the cell body is intact

Mechanically Gated Channel

Pressure/stress/vibration is applied to the membrane, opening the channel.

What is the difference between the presynaptic membrane and the postsynaptic membrane?

Presynaptic membranes are where neurotransmitters are released. Postsynaptic membranes bear receptors for neurotransmitters.

What are action potentials?

Propagated changes in the membrane potential that, once initiated, affect an entire excitable membrane

How do proteins and other chemicals synthesized in the soma get to the synaptic knobs? By what process can a virus that invades a peripheral nerve fiber get to the soma of that neuron?

Proteins and other chemicals are synthesized through the terminal arborization, which is an extensive complex of fine branches which ends in a synaptic knob. A virus can invade a peripheral nerve fiber via retrograde transport.

What is a receptor? What is an effector?

Receptors are sensors structures that either detect changes in the environment (internal or external) or respond to specific stimuli. Effectors are target organs (muscles/glands/adipose tissues) that respond by doing something.

3 Functional Classifications of Neurons

Sensory (afferent) neurons: transmit nerve impulses from receptors in the skin, sense organs, muscles, joints and viscera toward the CNS Motor (efferent) neurons: transmit nerve impulses from the CNS to effectors Interneurons (association neurons): distribute sensory information and coordinate motor activity; information processing; memory, planning and learning; brain and spinal cord

Explain the difference between a sensory neuron, a motor neuron, and an interneuron.

Sensory neurons: carry electrical signals (impulses) from receptors or sense organs to the CNS. They are also called afferent neurons. Motor neurons: transmit signals from the CNS to the effector cells. They are also known as efferent neurons. Interneurons: carry information between sensory and motor neurons. They are also called intermediate/relay/associative neurons.

What do somatic motor neurons innervate?

Skeletal muscles

What are somatic sensory neurons responsible for? Visceral sensory neurons?

Somatic sensory neurons monitor the outside world and our position within it. Visceral sensory neurons monitor internal conditions and the statues of other organ systems.

How does the plasma membrane at the trigger zone differ from that on the soma? How does it resemble the membrane at a node of Ranvier?

The action potentials can occur at nodes of Ranvier so the nerve signal appears as if it was jumping from node to node and at the trigger zone if an excitatory local potential arrives and is still strong enough it can open channels and generate action potential which is a rapid up-and-down shift in voltage

What is an axon hillock?

The base, or initial segment, of an axon in a multipolar neuron that joins the cell body.

What does the CNS consist of?

The brain and spinal cord

What causes K+ to diffuse out of a resting cell? What attracts it into the cell?

The electrochemical gradient is what causes K+ to diffuse out of a resting cell. Electrically, K+ wants to enter the cell. Chemically, it wants to leave. The - charge on the inside of the cell is what attracts the K+.

What are telondendria?

The main axon trunk + any collaterals in a series of fine extensions

What is axoplasmic transport?

The movement of materials between the cell body and axon terminals

What does the soma (cell body) consist of?

The nucleus, nucleolus, perikaryon (cytoplasm)

What is neural coding?

The process the nervous system uses to convert this information into meaningful patterns of action potentials

The local anesthetics lidocaine (Xylocaine) and procaine (Novocaine) prevent voltage-gated Na+ channels from opening. Explain why this would block the conduction of pain signals in a sensory nerve.

The production of action potentials involves Na+ inflow into a neuron through these voltage-gated Na+ channels. If these channels were inhibited, then no action potentials and no nerve signals could occur, so tissue damage would not trigger the conduction of pain signals in a nerve.

What is presynaptic inhibition?

The release of GABA inhibits the opening of voltage-gated calcium channels in the axon terminal

What is another name for the visceral motor nervous system? What are its two subdivisions? What are their functions?

The visceral motor nervous system is also referred to as autonomic nervous system (ANS) Its two subdivisions are the sympathetic NS and parasympathetic NS. The sympathetic NS is responsible for ~~~. The parasympathetic NS is responsible for ~~~.

What is the function of interoreceptors?

They monitor the digestive, respiratory, cardiovascular, urinary, and reproductive systems and provide sensations of distension (stretch), deep pressure, and pain.

What is the function of proprioceptors?

They monitor the position and movement of skeletal muscles and joints.

What is the function of exterecptors?

They provide information about the external environment in the form of touch, temperature, or pressure sensations and the more complex senses of taste, smell, sight, equilibrium, and hearing.

What is a synaptic bulb?

Tips of the presynaptic neuron that contain synaptic vesicles (which contain a neurotransmitter that will aid in signal transmission across the synaptic cleft 20-30nm in length)

What is the threshold of an axon?

Typically between -60mV and -55mV *A stimulus that shifts the resting membrane potential from -70mV to -62mV will not produce an action potential

Voltage Gated Channel

Voltage changes inside the cell cause the channel to open.

What is a neuroglandular junction?

Where a neuron controls or regulates the activity of a secretory (gland) cell

What are axon terminals? (aka synaptic terminals, synaptic knobs, and synaptic boutons)

Where the telondendria ends; they play a role in communication with other cells

At what voltage does a regulated channel a) close b) open and c) inactivate

a) -70mV b) -60mV c) +30mV

Most CNS neurons lack centrioles. This observation explains: a. Why CNS neurons grow such long axons. b. Why CNS neurons cannot divide to regenerate damaged tissue. c. The ability of neurons to generate an action potential. d. The ability of neurons to communicate with each other. e. The ability of neurons to produce a resting membrane potential.

b. Why CNS neurons cannot divide to regenerate damaged tissue

The sodium-potassium ion exchange pump: a. Must reestablish ion concentrations after each action potential. b. Transports sodium ions into the cell during depolarization. c. Transports potassium ions out of the cell during repolarization. d. Moves sodium and potassium opposite to the direction of their electrochemical gradients. e. Depends on a hydrogen gradient for energy.

d. Moves sodium and potassium opposite to the direction of their electrochemical gradients

How does blocking retrograde axoplasmic transport in an axon affect the activity of a neuron? a. The neuron becomes unable to produce neurotransmitters. b. The neuron becomes unable to produce axon potentials. c. The soma becomes unable to export products to the synaptic terminals. d. The soma becomes unable to respond to changes in the distal end of the axon. e. The neuron becomes unable to depolarize when stimulated.

d. The soma becomes unable to respond to changes in the distal end of the axon

If the sodium-potassium pumps in the plasma membrane fail to function, all of the following occur except: a. The intracellular concentration of potassium ions will increase b. The neuron will slowly depolarize c. The membrane will slowly lose its capacity to generate action potentials. d. The inside of the membrane will have a resting potential that is more positive than normal. e. The intracellular concentration of sodium ions will increase

e. The intracellular concentration of potassium ions will increase.

At the normal resting potential of a typical neuron, its sodium-potassium exchange pump exchanges: a. 1 intracelluar sodium ion for 2 extracellular potassium ions. b. 2 intracellular sodium ions for 1 extracellular potassium ion. c. 3 intracellular sodium ions for 1 extracellular potassium ion. d. 3 intracellular sodium ions for 2 extracellular potassium ions. e. 3 extracellular sodium ions for 2 intracellular potassium ions.

e. exchanges 3 extracellular sodium ions for 2 intracellular potassium ions

Characteristics of Action Potentials

•All or none rule •No signal degradation (signal remains the same over distance travelled) •Irreversible (once an action potential is started it cannot be stopped until it reaches the synapse)

How are neurons broken down?

•Cell Body (Soma) •Nissl bodies •Dendrites •Axon •Axon Hillock •Initial segment •Axoplasm •Axolemma •Axon collaterals •Synaptic terminal •Axoplasmic transport???

How is the nervous system broken up?

•Central Nervous System (CNS) •Peripheral Nervous System (PNS) *Afferent Nervous System *Efferent Nervous System -Somatic Nervous System -Autonomic Nervous System ~Parasympathetic ~Sympathetic

Characteristics of a Local Potential

•Graded (the stronger the stimulus the stronger the signal) •Decremental (the further the signal travels the weaker it becomes) •Reversible (if stimulation stops the Na-K pump quickly returns the membrane potential to -70mV) •Excitatory (depolarize) or inhibitory (hyperpolarize)

4 Types of Neurons

•Multipolar: have more than two processes; there is a single axon and multiple dendrites •Bipolar: have two processes separated by the body •Unipolar: have a single elongated process, with the cell body located off to the side. Their processes are known as afferent fibers. •Anaxonic: have more than two processes, and they are all dendrites

4 Types of Membrane Channels

•Passive/Leak •Chemically gated •Mechanically gated •Voltage gated