Chapter 12 - The Nervous System: Integrative Approach (Lecture Notes Review)

A set of practice flashcards covering the major concepts from Chapter 12: nervous system structure, neuron function and structure, synapses and neurotransmitters, glial cells, myelination, axon regeneration, ion pumps/channels, membrane potentials, action potentials, and neural networks.

What is the function of receptors in the nervous system?

Detect stimuli and send sensory signals to the brain and spinal cord (collect information).

What are the two major structural divisions of the nervous system and what do they include?

CNS: brain and spinal cord; PNS: nerves and ganglia.

What are the functional divisions of the nervous system?

Sensory (afferent) nervous system detects stimuli and transmits to CNS; Motor (efferent) nervous system initiates motor output from CNS to effectors; includes Somatic (voluntary) and Autonomic (involuntary) divisions, with sympathetic and parasympathetic branches.

What are the three connective tissue wrappings of a nerve and what does each enclose?

Epineurium surrounds the entire nerve; Perineurium wraps a fascicle; Endoneurium surrounds an individual axon.

What does it mean that nerves are vascularized?

Nerves have blood vessels that branch through the epineurium and perineurium to form capillaries, allowing exchange between axons and blood.

Structural classification of nerves: what are cranial and spinal nerves?

Cranial nerves extend from the brain; spinal nerves extend from the spinal cord.

Functional classification of nerves.

Sensory (afferent) nerves carry signals to the CNS; Motor (efferent) nerves carry signals from the CNS to effectors; Mixed nerves contain both sensory and motor fibers; most named nerves are mixed; individual axons transmit only one type of information.

What is a ganglion?

A cluster of neuron cell bodies in the peripheral nervous system.

Name the general neuron characteristics.

Excitability, conductivity, secretion; longevity; amitotic.

Describe the neuron cell body (soma) and Nissl bodies.

Cell body houses the nucleus and initiates some graded potentials; contains Nissl bodies (ribosomes) for protein synthesis.

What are the key parts of a neuron’s axon and their functions?

Axon hillock; axon (with axolemma and axoplasm); axon collaterals; telodendria; synaptic knobs containing neurotransmitter vesicles; conducts action potentials and releases neurotransmitter.

What are neurofilaments (neurofibrils) and their role?

Cytoskeletal elements that provide tensile strength; aggregate to neurofibrils.

What are anterograde and retrograde transport in neurons?

Anterograde moves materials from soma toward synaptic knobs; retrograde moves used materials from axon back to the soma; can be fast or slow.

Structural neuron classifications by number of processes from the soma

Multipolar (many dendrites, one axon); Bipolar (one dendrite, one axon); Unipolar/Pseudounipolar (one process that splits into peripheral and central); Anaxonic (dendrites but no axon).

Functional classification of neurons by direction of propagation

Sensory (afferent) neurons to CNS; Motor (efferent) neurons from CNS; Interneurons (association neurons) within CNS.

What is a synapse?

A junction where a neuron connects to another neuron or an effector; two types: chemical and electrical.

What are the events of a chemical synapse?

Presynaptic terminal releases neurotransmitter into the synaptic cleft; neurotransmitter binds postsynaptic receptors; initiates a postsynaptic potential; there is a synaptic delay.

Glial cells in the CNS and PNS: which types and roles?

CNS: astrocytes, ependymal cells, microglia, oligodendrocytes; PNS: satellite cells, neurolemmocytes (Schwann cells).

Astrocyte functions

Form the blood-brain barrier; regulate interstitial fluid composition (e.g., potassium); provide structural support; guide neural development; modulate synapses.

Oligodendrocytes vs Schwann cells

Oligodendrocytes myelinate CNS axons; Schwann cells (neurolemmocytes) myelinate PNS axons and form the neurilemma.

What is myelin and what are the nodes of Ranvier?

Myelin is multiple lipid-rich layers that insulate axons; nodes of Ranvier are gaps between myelin segments where voltage-gated channels are concentrated.

How are unmyelinated axons arranged in the PNS and CNS?

PNS: axon sits in a depressed portion of a Schwann cell; CNS: unmyelinated axons are not associated with oligodendrocytes.

Clinical View: demyelinating diseases (MS vs Guillain-Barré)

MS: progressive demyelination in the CNS (oligodendrocytes attacked); Guillain-Barré syndrome: demyelination of peripheral nerves (inflammation) with potential recovery.

12.4c Myelination basics (PNS vs CNS)

In the PNS, Schwann cells form the myelin sheath and neurilemma; in the CNS, oligodendrocytes form myelin; many axons are myelinated; CNS lacks a neurilemma.

Conditions for regeneration of PNS axons

Neuron cell body intact and enough neurilemma remains; damage less extensive and shorter distance to the innervated structure increases success.

Steps of PNS axon regeneration

Proximal axon seals and swells; distal segment degenerates (Wallerian degeneration); neurilemma and endoneurium form a regeneration tube; axon regenerates guided by growth factors; reinnervation of the original effector or receptor.

Why CNS axon regeneration is limited

Oligodendrocytes secrete growth-inhibiting molecules; many axons in CNS cannot regrow; there are also glial scars from astrocytes and connective tissue.

12.6a Pumps and channels: basic roles

Pumps maintain concentration gradients (e.g., Na+/K+, Ca2+ pumps); channels (leak, chemically gated, voltage-gated; modality gated) regulate ion flow.

Three states of voltage-gated Na+ channels

Resting state: activation gate closed, inactivation gate open; Activation state: activation gate open, inactivation gate open; Inactivation state: activation gate open, inactivation gate closed.

Modality gated channels

Normally closed; open in response to a specific sensory stimulus (temperature, pressure, light); found in sensory neuron membranes.

Distribution of channels in neuron segments

Receptive segment: chemically gated channels; Initial segment (axon hillock): voltage-gated Na+ and K+ channels; Conductive segment (axon): voltage-gated Na+ and K+ channels; Transmissive segment (synaptic knob): voltage-gated Ca2+ channels and Ca2+ pumps.

What is resting membrane potential (RMP) and its typical value?

An electrical charge difference across the membrane; typically about −70 mV.

What establishes the resting membrane potential (RMP)?

Uneven ion distribution due to pumps and leak channels; especially K+ diffusing out; Na+ leak in; Na+/K+ pumps maintain gradients.

Ohm’s Law and neurons

Current I = voltage V divided by resistance R (I = V/R); in neurons, current is ion flow; voltage arises from ion gradients; resistance changes with gated channels.

Graded potentials vs action potentials

Graded potentials: small, local, varied in magnitude, occur in receptive segment via chemically gated channels; Action potentials: all-or-none, occur in conductive segment via voltage-gated channels and propagate along the axon.

EPSP vs IPSP

EPSP: depolarization caused by Na+ entry; IPSP: hyperpolarization caused by K+ exit or Cl− entry.

Threshold and initiation of an action potential at the axon hillock

Threshold is typically around −55 mV; when summated graded potentials reach threshold, voltage-gated channels open and an action potential is generated.

Conductive segment: propagation of an action potential

Depolarization via Na+ influx followed by repolarization via K+ efflux; the impulse propagates along the axon toward the synaptic knob.

Refractory periods

Absolute refractory period: ~1 ms; no new AP can be generated; Relative refractory period follows; a new AP is possible with a greater stimulus.

Continuous vs saltatory conduction

Continuous conduction occurs on unmyelinated axons; saltatory conduction occurs on myelinated axons, with APs at nodes of Ranvier and faster propagation.

Transmissive segment: neurotransmitter release at the synaptic knob

AP opens voltage-gated Ca2+ channels; Ca2+ triggers exocytosis of neurotransmitter; neurotransmitter binding to receptors on the postsynaptic cell; transmitter frequency affects release.

Nerve fiber groups and velocity of conduction

Group A: up to ~150 m/s (large, myelinated, e.g., somatic neurons); Group B: ~15 m/s; Group C: ~1 m/s (small or unmyelinated).

Classification of neurotransmitters by structure

Four main classes: Acetylcholine; Biogenic amines; Amino acids; Neuropeptides.

Neurotransmitter function: excitatory vs inhibitory and direct vs indirect

Excitatory transmitters cause EPSPs; inhibitory transmitters cause IPSPs; Direct transmitters bind to ion channels (fast); Indirect transmitters use G proteins/second messengers (modulatory).

Acetylcholine features and receptors

ACh is released at neuromuscular junctions; broken down by acetylcholinesterase; nicotinic (ionotropic) receptors cause EPSP; muscarinic (metabotropic) receptors can cause EPSP or IPSP.

Neuromodulation examples

Nitric oxide can act as a neuromodulator affecting presynaptic release; endocannabinoids influence memory and neurotransmitter release.

Neuronal pools: four circuit types and their function

Converging: inputs converge on a single neuron; Diverging: one neuron sends to many; Reverberating: feedback loop enabling sustained activity; Parallel-after-discharge: multiple paths to a common output, contributing to higher-order processing.