Chapter #11

What structures are in the CNS vs PNS?

CNS = brain & spinal cord

PNS = cranial nerves, spinal nerves, & ganglia.

How is the PNS divided functionally?

-Sensory (afferent) *arrive at CNS

&

-Motor (efferent) divisions. *exit from CNS

What are the two sensory divisions of the PNS & what do they sense?

1. Somatic sensory: (skin, skeletal muscles, & joints)

2. Visceral sensory: (visceral organs in ventral cavity).

What are the two motor divisions of the PNS & their effectors?

1. Somatic motor → skeletal muscle;

2. Visceral motor (autonomic NS) → visceral organs.

Name the ANS subdivisions & general effects.

Sympathetic (mostly stimulatory)

Parasympathetic (mostly inhibitory).

List the CNS neuroglia & one key function each.

-Astrocytes—exchange with capillaries, control permeability, guide neuron migration, regulate K+ & neurotransmitters;

-Microglia—immune, phagocytosis;

-Ependymal cells—ciliated lining of ventricles, help form choroid plexus/CSF;

-Oligodendrocytes—Form myelin around axons in CNS.

How do astrocytes communicate & influence neurons?

Via gap junctions using slow intracellular Ca2+ waves; release extracellular chemical signals that influence neuronal function.

What forms CSF & where?

Choroid plexus formed by ependymal cells + brain capillaries.

Which CNS glia myelinate axons?

Oligodendrocytes (wrap larger CNS axons).

List the major PNS glia & their roles.

Satellite cells—regulate environment around PNS neurons (astrocyte-like);

Schwann cells—form PNS myelin (oligodendrocyte-like).

What are the three special properties of neurons?

1. Long-lived

2. Amitotic (do not divide)

3. High metabolic rate.

What part of the neuron is the receptive region?

Dendrites & cell body (including axon hillock) with synaptic spines & membrane.

What part of the neuron conducts impulses?

Axon—begins at axon hillock/initial segment → terminal → end bulb.

Define axonal transport.

Movement of molecules/organelles along the axon

Both anterograde & retrograde.

Differentiate anterograde vs retrograde axonal transport.

Anterograde: Away from soma to terminals.

Retrograde: Back to soma (proteins, organelles, growth factors like NGF).

Who makes the myelin sheath in the PNS & what’s the outer layer called?

Schwann cells; outermost Schwann cell membrane is the neurilemma.

What are nodes of Ranvier?

Gaps between myelin segments where axon membrane is exposed.

Structural classes of neurons?

-Multipolar: (1 axon + 2 or more dendrites)

-Bipolar: (1 dendrite + 1 axon from opposite poles),

-Unipolar: (single process splits into peripheral & central branches).

Which structural type dominates the CNS?

Multipolar (~99% of CNS neurons).

Describe unipolar sensory neurons.

Cell body in PNS ganglia; peripheral end has receptor (naked or encapsulated); only the very end is dendritic—remainder behaves as axon.

Functional classes of neurons & directions of impulse flow.

-Sensory: (Toward CNS),

-Motor/efferent (Away from CNS to effectors),

Interneurons: (Between sensory & motor; ~99% of all neurons).

What carries electrical current in neurons?

Ions—especially Na+, K+, & also Ca2+.

What provides electrical resistance in neurons?

The cell (plasma) membrane.

Name the channel types in neurons.

-Leakage (always open);

-Gated—Chemically gated, voltage-gated, & Mechanically gated.

In what direction do ions move when gated channels open?

Down electrochemical gradients (chemical + electrical).

Define depolarization & its ionic basis.

Membrane potential becomes less negative

(can overshoot above 0) due to Na+ influx.

Define hyperpolarization & its ionic basis.

Membrane becomes more negative due to K+ efflux.

Where do graded potentials vs action potentials occur?

Graded: Dendrites & cell body.

Action potentials: Start at axon hillock & propagate along axon to terminals.

Which channels are key for action potentials?

-Voltage-gated Na+,

-Voltage-gated K+,

-Voltage-gated Ca2+ (notably at terminals).

Which channels mediate graded potentials?

Chemically gated Na+ & K+ channels.

Define threshold & why it matters.

Level of depolarization at the axon hillock that opens voltage-gated Na+ channels—triggers the action potential.

How is stimulus intensity encoded in the nervous system?

By the frequency of action potentials (amplitude is constant).

What two factors determine conduction velocity?

1. Axon diameter (larger = faster)

2. Degree of myelination.

What is saltatory conduction & where do AP channels concentrate?

AP “jumps” node-to-node in myelinated axons; voltage-gated channels are concentrated at nodes of Ranvier.

Nerve fiber classifications & features (Type A/B/C)?

A: Large, heavily myelinated, fastest (somatic motor/sensory to skin, skeletal muscles, & joints).

B: Medium, myelinated, & moderate speed.

C: Small, unmyelinated, & slowest.

Define presynaptic vs postsynaptic neuron.

Presynaptic: Neuron sending signal via axon terminal;

Postsynaptic: Neuron receiving on dendrite, soma, or hillock with specialized synaptic machinery.

Broad classification scheme for neurotransmitters?

By chemical structure:

-Acetylcholine; biogenic amines (catecholamines & indoleamines);

-Amino acids; peptides; purines; gases; endocannabinoids.

Which neurotransmitter is key for pain pathways?

Substance P.

Give examples of biogenic amines.

-Catecholamines

-Epinephrine,

-Norepinephrine,

-Dopamine;

-Indoleamines

-Serotonin

-Histamine.

List the main amino acid neurotransmitters.

-Glutamate

-GABA

-Glycine

-Aspartate.

List peptide neurotransmitters & functions noted.

Substance P (pain); = Neurotransmitter for pain neurons

Endogenous opioids—

Endorphin,

Dynorphin

Enkephalin.

Name purine & gaseous neurotransmitters.

Purines: ATP & adenosine.

Gases: NO, CO, H2S.

What are endocannabinoids?

Lipid-derived neurotransmitters/modulators acting at cannabinoid receptors.

How are neurotransmitters released from the presynaptic terminal?

-AP depolarizes terminal →

-Voltage-gated Ca2+ channels open →

-Ca2+ influx triggers vesicle fusion →

-NT released into synaptic cleft.

Define EPSP & its ionic basis & effect.

Excitatory postsynaptic potential: Na+ entry makes inside less negative; moves hillock toward threshold.

Define IPSP & its ionic basis & effect.

Inhibitory postsynaptic potential: K+ exit (or Cl− entry) makes inside more negative; moves hillock away from threshold.

How do EPSPs & IPSPs lead to APs?

Continuous spatial/temporal summation at the axon hillock; each time threshold is reached, an AP fires.

What is neuronal facilitation?

A neuron partially depolarized (closer to threshold) is more likely to fire—i.e., facilitated.

Define presynaptic inhibition & its mechanism.

At an axoaxonic synapse, inhibition reduces opening of voltage-gated Ca2+ channels in the presynaptic terminal → less Ca2+ influx → less NT release → smaller EPSP/IPSP in postsynaptic cell.

Name the three anatomical synapse types.

1. Axodendritic (onto dendrite),

2. Axosomatic (onto soma)

3. Axoaxonic (onto axon/terminal).