Neuroscience: Neuron Functions and Properties

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Questions and Answers

Which statement regarding neurons is accurate?

Neurons can reproduce via mitosis.

Neurons require a low amount of oxygen for functioning.

Neurons cannot regenerate cell processes under any circumstances.

Neurons can regenerate cell processes if the cell body is intact. (correct)

What is the primary function of dendrites?

Produce myelin sheath.

Support neuronal function.

Receive signals from other neurons or the body. (correct)

Transmit signals to other neurons.

What distinguishes afferent nerve fibers from efferent nerve fibers?

Afferent fibers are typically sensory and conduct impulses to the CNS. (correct)

Efferent fibers are typically sensory in nature.

Afferent fibers conduct impulses from the CNS to the periphery.

Efferent fibers only exist in the spinal cord.

How does myelination affect nerve impulse conduction?

Myelinated fibers increase the speed of conduction of nerve impulses. Signup and view all the answers

What role do oligodendrocytes play in the nervous system?

They produce myelin in the brain and spinal cord. Signup and view all the answers

Which of the following describes a mixed nerve?

Contains a mixture of sensory and motor nerve fibers. Signup and view all the answers

Which structure is primarily responsible for the white appearance of white matter?

The myelin sheath covering axons. Signup and view all the answers

What characterizes the somatic nervous system?

It involves both motor and sensory nerves and is conscious in nature. Signup and view all the answers

Which ions are primarily involved in maintaining the resting membrane potential of a neuron?

Sodium and potassium ions Signup and view all the answers

What occurs during depolarization of a neuron?

Sodium channels open and sodium enters the cell Signup and view all the answers

What is the net charge of the resting membrane potential in a neuron?

-70 mV Signup and view all the answers

Which process follows depolarization when a neuron returns to its resting state?

Repolarization Signup and view all the answers

What role does the sodium-potassium ATPase pump play in neuronal function?

It maintains the polarization of the neuron Signup and view all the answers

During repolarization, which ions diffuse out of the neuron?

Potassium ions Signup and view all the answers

What is a characteristic of a neuron in its resting state?

The neuron is polarized with a negative inside charge Signup and view all the answers

What initiates the process of depolarization in a neuron?

The opening of sodium channels due to stimulation Signup and view all the answers

What is the primary effect of gabapentin on neurotransmission?

Increases effect of GABA in the brain Signup and view all the answers

What enzymes are responsible for breaking down norepinephrine?

Monoamine oxidase and COMT Signup and view all the answers

Which treatment prolongs the action of acetylcholine in cases of myasthenia gravis?

Pyridostigmine bromide Signup and view all the answers

What condition results from the autoimmune destruction of acetylcholine receptors?

Myasthenia gravis Signup and view all the answers

What occurs when acetylcholinesterase is inactivated by organophosphates?

Prolonged action of acetylcholine at the synapse Signup and view all the answers

What restores the order of sodium and potassium ions after repolarization?

Na-K ATPase pump Signup and view all the answers

What determines whether depolarization will occur following a stimulus?

The intensity of the stimulus Signup and view all the answers

What occurs when a successful stimulus opens sufficient Na ion channels?

Adjacent ion channels open Signup and view all the answers

What is the phenomenon where conduction of action potential jumps between nodes?

Saltatory Conduction Signup and view all the answers

What phase occurs when the nerve is unable to create another impulse?

Refractory period Signup and view all the answers

What happens during the relative refractory period of a neuron?

Only a strong stimulus can cause depolarization Signup and view all the answers

What is the structure at the end of a presynaptic neuron involved in neurotransmitter release?

Telodendron Signup and view all the answers

What is the impact of multiple sclerosis on neurons?

Destruction of myelin sheaths Signup and view all the answers

What principle states that a strong and weak stimulus produce the same impulse once the threshold is reached?

All-or-Nothing Principle Signup and view all the answers

Which ions' channels are open during the refractory period?

Both Na and K channels Signup and view all the answers

What occurs immediately after depolarization reaches the synaptic knob?

Calcium channels open Signup and view all the answers

Which of the following neurotransmitters is both excitatory and inhibitory?

Acetylcholine Signup and view all the answers

What is the primary action of inhibitory neurotransmitters like GABA?

Cause hyperpolarization of postsynaptic membrane Signup and view all the answers

How do excitable neurotransmitters affect the postsynaptic membrane?

They promote the postsynaptic membrane to reach the threshold Signup and view all the answers

What is a consequence of increased GABA in the brain?

Inhibition of brain activity Signup and view all the answers

Which neurotoxin inhibits glycine in the spinal cord?

Strychnine Signup and view all the answers

What physiological response is primarily related to norepinephrine?

Fight-or-flight response Signup and view all the answers

Which drug is known to increase the effects of GABA in the brain?

Valium Signup and view all the answers

What is the role of vesicles in neurotransmission?

To release neurotransmitters into the synaptic cleft Signup and view all the answers

What is a common effect of diabetes on neurotransmission?

Decreased release of neurotransmitters Signup and view all the answers

Study Notes

Neurology Microanatomy & Physiology

Neurons are the functional units of the nervous system
Neurons have high oxygen requirements
Neurons cannot reproduce via mitosis
Neurons can regenerate their processes if the cell body remains intact
Neurons receive and send impulses

Basic Neuron Structure

Cell body (soma or perikaryon)
Dendrites receive signals from other cells
Axons transmit signals to other cells
Dendrites are afferent processes; numerous and branching
Axons are efferent processes; a single extension from each neuron
A nerve is a bundle of axons

Neurons Continued

Axons are covered by a fatty sheath called myelin
Myelin gives white matter its white appearance
Oligodendrocytes are the glial cells in the brain and spinal cord, producing myelin
Schwann cells are glial cells in peripheral nerves, producing myelin

Myelin Sheath

Oligodendrocytes and Schwann cells form a tight wrap around axons
The myelin sheath lines the axon end to end
Nodes of Ranvier are small gaps between myelin segments
Myelin increases the speed of nerve impulse conduction; myelinated is faster than unmyelinated

Afferent Impulses

Afferent nerve fibers conduct impulses from the periphery to the central nervous system (CNS)
Typically sensory in nature
Efferent nerve fibers conduct impulses from the CNS to the periphery
Typically motor in nature
Nerves are bundles of afferent and efferent axons
Sensory nerves contain only sensory fibers
Motor nerves contain only motor fibers
Mixed nerves contain both sensory and motor fibers

Autonomic vs. Somatic

Somatic nervous system is conscious, involving both motor and sensory nerves and impulses to skeletal muscle
Autonomic nervous system is unconscious, involving both motor and sensory nerves and impulses to smooth, cardiac muscle, and glands

Depolarization & Repolarization

A nerve "firing" is depolarization
Repolarization is the return to the resting state

Sodium Potassium ATPase Pump

At rest, the neuron is polarized
The sodium-potassium pump maintains this polarization
2 potassium ions are pumped inside the cell from outside
3 sodium ions are pumped outside the cell from inside
The membrane is polarized because one side is more positively charged (outside) and the other more negatively charged (inside)

Resting Membrane Potential

Resting membrane potential is the difference in electrical charge across the neuron.
The resting membrane potential is -70 mV

Depolarization

Stimulation of a nerve opens sodium channels
Sodium flows into the cell by diffusion
The inside of the cell becomes positively charged (depolarization)

Repolarization

Sodium channels close, and potassium channels open
Potassium flows out of the cell by diffusion
The inside of the cell returns to its negative resting potential

Repolarization vs. Resting State

Resting membrane potential: K+ inside, Na+ outside
Repolarization: K+ outside, Na+ inside
Na+/K+ ATPase pump restores the resting state

Threshold Stimulus

A threshold of stimulus intensity is required for depolarization.
Weak stimuli may cause some sodium channels to open, but not enough for depolarization.

Wave of Depolarization

Adjacent channels open once a stimulus opens some sodium channels
A wave of depolarization spreads through the neuron
This process is called conduction of the action potential, which occurs in practical terms in a "nerve impulse."

All-or-Nothing Principle

If a stimulus reaches threshold, the impulse is generated along the axon with the same strength.
A strong or weak stimulus that reaches the threshold produces the same impulse.

Refractory Period

This is the time when a neuron cannot produce another impulse
Sodium and potassium channels are temporarily closed/open during this period
Relative refractory period happens when cells may produce a depolarization response but only with a stronger stimulus.

Saltatory Conduction

Myelin sheaths increase impulse speed, preventing sodium flow along the neuron, except at the Nodes of Ranvier.
Impulse "jumps" from node to node.

Multiple Sclerosis

An autoimmune disease that damages Schwann cells, destroying myelin sheaths.
Reduces ability to quickly conduct nerve impulses
Sensory and motor deficits

How Neurons Communicate: The Synapse

The synapse is the junction between two neurons or a neuron and target cell; a gap (synaptic cleft)
There is no physical contact between the cells
Telodendron is a branched structure at the end of the pre-synaptic neuron; synaptic end bulbs or knobs
Mitochondria supports the activity.
Synaptic vesicles hold neurotransmitters

Synaptic Transmission

Depolarization reaches the synaptic knob
Calcium channels open, allowing extracellular calcium to enter the synaptic knob.
Vesicles fuse with the membrane; releasing neurotransmitters into the synaptic cleft
Neurotransmitters diffuse across the cleft

Postsynaptic Membrane

Neurotransmitters bind to receptors on the postsynaptic membrane
This binding results in a target cell response, such as the generation of a new action potential

Excitatory & Inhibitory Neurotransmitters

Two types of neurotransmitters: Excitatory neurotransmitters typically increase sodium influx to move the postsynaptic membrane closer to threshold.
Inhibitory neurotransmitters usually hyperpolarize, changing the cell to a more negatively charged state, making it harder to produce another action potential.

Neurotransmitters

Some neurotransmitters are both excitatory and inhibitory, depending upon the cell they act on.
Acetylcholine (ACh) is either excitatory or inhibitory.
Catecholamines (e.g., norepinephrine, dopamine, epinephrine) are often involved in the "fight-or-flight" response.

Catecholamines

Norepinephrine is involved in the flight-or-flight response.
Epinephrine is also associated with flight-or-flight.
Dopamine is involved in brain function, autonomic functions, and muscle control—it’s associated with Parkinson's.

GABA & Glycine

GABA (gamma-aminobutyric acid) is an inhibitory neurotransmitter found in the brain.
Some drugs increase GABA activity or inhibit brain activity.
Glycine is primarily found in the spinal cord and is an inhibitory neurotransmitter.

Clinical Case: Metaldehyde Toxicity

Slug bait toxicity resulting in a "shake-and-bake" effect.
Pathophysiology: reduces inhibitory neurotransmitter (GABA) and lowers seizure threshold.
Clinical signs: Severe muscle tremors, hyperesthesia, tachycardia, and hyperthermia.

Clinical Case: Other Neurotoxins

Pesticides (e.g., strychnine, permethrin, bromethalin) can affect neurons.
Strychnine inhibits glycine.
Permethrin affects sodium channels (particularly in cats).
Bromethalin causes demyelination and cerebral edema (often in rats)

Clinical Case: Gabapentin

Extremely fractious or nervous cats can be calmer, with sedation, with Gabapentin.
A drug that increases GABA activity.

Stopping and Recycling Neurotransmitters

Neurotransmitters are removed from the synaptic cleft by enzymes.
Acetylcholine is broken down by acetylcholinesterase.
Organophosphates are common agricultural pesticides that inhibit the activity of acetylcholinesterase, causing an excess of acetylcholine in the synaptic cleft.

Recycling Neurotransmitters

Some neurotransmitters, like norepinephrine, are broken down using enzymes such as MAO (monoamine oxidase) in the synaptic knob and COMT (catechol-O-methyltransferase) in the synaptic cleft.
Some antidepressants work by inhibiting these enzymes.

Clinical Case: Myasthenia Gravis

This is an autoimmune disease resulting in muscle weakness.
Destruction of acetylcholine receptors, leading to decreased muscle strength.
Diagnosed through testing (ACh antibody test and Tensilon test)
Treated with drugs that prolong acetylcholine action in the synaptic cleft

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Test your knowledge on the structure and function of neurons with this quiz. Explore essential concepts like dendrites, myelination, and the roles of various ions in neuronal activity. This quiz is perfect for students studying neuroscience and physiology.