Fundamentals of the Nervous System – Lecture 1 Review

What are the three basic functions performed by every nervous system?

Sensory input, integration, and motor output.

Which organs or tissues do motor neurons typically stimulate and what are they collectively called?

Muscles or glands; they are called effector organs.

Name the four major anatomical parts of a typical neuron.

Cell body (soma), dendrites, axon, and axon terminals.

Which parts of a neuron make up the "input zone" and why is their surface area important?

The dendrites and cell body; their extensive surface area increases the number of synaptic inputs they can receive.

Where are most neurotransmitters synthesized and in what organelle does this occur?

In the cell body, primarily on the rough endoplasmic reticulum (RER).

By what means are newly made neurotransmitters transported from the cell body to the axon terminal?

They are packaged into synaptic vesicles and carried along cytoskeletal tracks by motor proteins.

What is the name of the tiny gap that neurotransmitters must diffuse across during chemical synaptic transmission?

The synaptic cleft (or synaptic gap).

Define membrane potential.

The voltage (charge difference) that exists across a cell’s plasma membrane due to unequal distribution of ions.

What is a typical resting membrane potential for a motor neuron?

Approximately –70 mV (millivolts).

Which two ions are most responsible for establishing the resting membrane potential in neurons?

Sodium (Na⁺) and potassium (K⁺).

Where is the concentration of Na⁺ highest when a neuron is at rest: inside or outside the cell?

Outside the cell.

Where is the concentration of K⁺ highest when a neuron is at rest: inside or outside the cell?

Inside the cell.

How many Na⁺ and K⁺ ions does the Na⁺/K⁺-ATPase pump move during one cycle, and in which directions?

It pumps 3 Na⁺ ions out of the cell and 2 K⁺ ions into the cell.

Why does the Na⁺/K⁺ pump make the interior of the cell more negative?

Because it exports more positive charges (3 Na⁺) than it imports (2 K⁺), resulting in a net loss of positive charge inside.

What type of membrane channels are always open and allow passive ion flow?

Leak (passive) channels.

Which leak channels are far more numerous in neurons: Na⁺ or K⁺, and what is the consequence?

K⁺ leak channels; their abundance lets more positive charge leave the cell, contributing to the negative resting potential.

Differentiate ligand-gated channels from voltage-gated channels.

Ligand-gated channels open when a chemical messenger binds; voltage-gated channels open in response to a change in membrane potential.

What two gates are found in a voltage-gated Na⁺ channel and what is the functional significance?

An activation gate that opens quickly with depolarization and an inactivation gate that closes shortly afterward, creating the refractory period and ensuring one-way propagation.

During which phase of the action potential do voltage-gated Na⁺ channels open?

During depolarization.

Which ion’s efflux is chiefly responsible for the repolarization phase of an action potential?

Potassium (K⁺) leaving the cell through voltage-gated K⁺ channels.

What term describes the brief period when the membrane potential becomes more negative than the original resting value?

Hyperpolarization (the after-hyperpolarization).

What is the absolute refractory period?

The time during an action potential when a second action potential cannot be initiated because voltage-gated Na⁺ channels are either open or inactivated.

What is the relative refractory period?

The interval after the absolute refractory period when a stronger-than-normal stimulus can trigger a second action potential because some Na⁺ channels have reset but K⁺ efflux is still occurring.

Where on a neuron are action potentials typically initiated and why?

At the axon hillock, because that region has the highest density of voltage-gated Na⁺ channels and integrates all incoming graded potentials.

Compare graded potentials with action potentials in terms of distance and signal strength.

Graded potentials are short-distance, decremental signals whose strength diminishes with distance; action potentials are long-distance, non-decremental, all-or-nothing signals.

Which type of membrane channels generate graded potentials on dendrites and cell bodies?

Ligand-gated channels (often Na⁺ channels).

Why do action potentials normally travel in only one direction down an axon?

Because regions just behind the advancing AP are in the refractory period and cannot be re-excited immediately.

What is myelin and what is its primary effect on conduction velocity?

A lipid-rich insulating sheath produced by glial cells; it greatly increases the speed of action potential conduction.

What are the gaps between adjacent myelin segments called, and what occurs there?

Nodes of Ranvier; they contain concentrated voltage-gated channels where action potentials are regenerated, enabling saltatory conduction.

Name the glial cell that forms myelin in the peripheral nervous system.

Schwann cell.

What term describes the "jumping" of an action potential from node to node along a myelinated axon?

Saltatory conduction.

Rank the fiber types A, B, and C in order of conduction speed from fastest to slowest.

Type A (large, heavily myelinated) > Type B (moderately myelinated) > Type C (unmyelinated, smallest diameter).

Which ion enters the axon terminal through voltage-gated channels to trigger neurotransmitter release?

Calcium (Ca²⁺).

Describe the sequence of events that converts an arriving action potential into neurotransmitter release.

AP depolarizes terminal → opens voltage-gated Ca²⁺ channels → Ca²⁺ influx → Ca²⁺ triggers synaptic vesicle fusion with membrane → exocytosis of neurotransmitter into synaptic cleft.

Why can the same neurotransmitter be excitatory at one synapse and inhibitory at another?

Because the effect depends on the type of receptor it binds to on the postsynaptic cell.

What common neurotransmitter excites skeletal muscle but inhibits cardiac muscle?

Acetylcholine (ACh).

What two properties of potassium channels (relative to sodium channels) are essential for producing a normal action potential?

They open more slowly and close more slowly than voltage-gated Na⁺ channels.

What is the threshold potential and why is it important?

The membrane voltage (usually about –55 mV) at which enough voltage-gated Na⁺ channels open to trigger an all-or-nothing action potential.

In simple terms, what does the term "depolarization" mean?

A decrease in membrane potential (inside becomes less negative or moves toward zero).

During what phase of an AP are voltage-gated Na⁺ channels inactivated but voltage-gated K⁺ channels fully open?

The repolarization phase.

Which cellular mechanisms restore ion distributions after an action potential?

Na⁺/K⁺-ATPase pumps and passive K⁺ and Na⁺ leak channels during the recovery phase.