Chapter 07

Central nervous system (CNS)

Brain and spinal cord.

Peripheral nervous system (PNS)

Nerves, ganglia, and nerve plexuses outside the CNS.

Neuron

Structural and functional unit of the nervous system; conduct impulses.

Glial cells (neuroglia)

Support neurons; do not conduct impulses; can divide.

Association neuron (interneuron)

Multipolar neuron located entirely within the CNS.

Sensory neuron (afferent neuron)

Transmits impulses from a sensory receptor into the CNS.

Motor neuron (efferent neuron)

Transmits impulses from the CNS to an effector organ (muscle or gland).

Nerve

Bundle of axons outside the CNS; may be mixed (sensory and motor).

Somatic motor nerve

Nerve that stimulates contraction of skeletal muscles.

Autonomic motor nerve

Nerve that stimulates involuntary targets (smooth/cardiac muscle; glands).

Ganglion

Clustering of neuron cell bodies outside the CNS.

Nucleus

Clustering of neuron cell bodies within the CNS.

Tract

Grouping of axons interconnecting regions of the CNS.

Nissl bodies

Rough endoplasmic reticulum in the neuron cell body.

Dendrite

Receives impulses and conducts graded potentials toward the cell body.

Axon

Conducts action potentials away from the cell body.

Axon hillock

Region where action potentials are generated; initial segment.

Myelin

Insulating sheath around axons; increases conduction speed.

Nodes of Ranvier

Gaps in the myelin sheath with concentrated ion channels.

Axon collaterals

Branches of an axon.

Anterograde transport

Movement from cell body to axon terminals; kinesin motors.

Retrograde transport

Movement from axon terminals to the cell body; dynein motors.

Schwann cells

PNS glia that form myelin around peripheral axons.

Satellite cells

PNS glia that support neuron cell bodies in ganglia.

Oligodendrocytes

CNS glia that form myelin sheaths around CNS axons.

Microglia

CNS phagocytic glia; immune defense.

Astrocytes

Glial cells regulating the extracellular environment; form BBB.

Ependymal cells

Line ventricles; produce cerebrospinal fluid (CSF).

White matter

Myelinated axons; appears white.

Gray matter

Neuron cell bodies and dendrites; lacks myelin.

Neurilemma

Schwann cell outer membrane around PNS axons; forms neurilemma.

Myelin sheath (PNS)

Schwann cell wrapping around axon; includes neurilemma; gaps are nodes of Ranvier.

Regeneration (PNS)

Repair of a severed PNS axon guided by Schwann cells and growth factors.

Neurotrophins

Growth factors promoting neuronal growth and survival (NGF, BDNF, GDNF, NT-3/4/5).

Blood-brain barrier (BBB)

Tight-junction capillaries; astrocyte influence; restricts substance entry to CNS.

Resting membrane potential

Voltage difference across the neuron's membrane at rest (~−70 mV).

Polarized

Inside of the cell is more negative than outside at rest.

Depolarization

Membrane potential becomes more positive; typically Na+ influx; excitatory.

Repolarization

Return toward resting potential after depolarization.

Hyperpolarization

Inside becomes more negative than resting potential.

Ion channels

Protein pores that allow ions to cross the membrane; include leak and gated channels.

Threshold

Membrane potential (~−55 mV) at which voltage-gated Na+ channels open.

Action potential

Rapid, all-or-none electrical impulse; Na+ in, then K+ out; peaks near +30 mV.

Absolute refractory period

Na+ channels inactivated; no new AP can be produced.

Relative refractory period

K+ channels open; a strong stimulus can trigger another AP.

All-or-None law

Once threshold is reached, AP is produced with constant amplitude and duration.

Coding for stimulus intensity

Stronger stimuli increase AP frequency and recruitment.

Cable properties

Passive electrical properties of the neuron; high internal resistance and leakage.

Unmyelinated conduction

APs propagate along the entire membrane; slower and decremental.

Saltatory conduction

AP hops between nodes of Ranvier; faster conduction in myelinated axons.

Conduction velocity

Speed of nerve impulse; affected by diameter and myelination.

Synapse

Functional connection between a neuron and its target cell.

Presynaptic neuron

Neuron that releases neurotransmitter.

Postsynaptic neuron

Neuron that receives neurotransmitter.

Electrical synapses

Gap junctions; direct ionic flow; fast; connexin proteins.

Chemical synapses

Neurotransmitter release across the synaptic cleft.

Neurotransmitter

Chemical messenger released by the presynaptic terminal to bind receptors.

Release of neurotransmitter

AP triggers Ca2+ influx; vesicles fuse via SNARE and synaptotagmin; exocytosis.

SNARE complex

Proteins that mediate vesicle docking and fusion with the presynaptic membrane.

Synaptotagmin

Ca2+ sensor that triggers vesicle fusion.

Ligand-gated channels

Ion channels directly opened by neurotransmitter binding.

Nicotinic ACh receptor

Nicotinic receptor; ligand-gated; Na+/K+ channel; EPSP at NMJ.

Muscarinic ACh receptor

Muscarinic receptor; GPCR; can cause EPSP or IPSP depending on tissue.

Agonist

Drug or chemical that activates a receptor.

Antagonist

Drug or chemical that inhibits a receptor.

AChE (Acetylcholinesterase)

Enzyme that hydrolyzes ACh to acetate and choline for reuse.

End plate potential

Local depolarization at the motor end plate; triggers muscle AP.

Neuromuscular junction (NMJ)

Synapse between a motor neuron and a muscle fiber.

Curare

Nicotinic receptor antagonist; causes paralysis.

Botulinum toxin

Inhibits acetylcholine release at the NMJ.

Monoamines

Regulatory neurotransmitters derived from amino acids (dopamine, norepinephrine, epinephrine, serotonin, histamine).

Reuptake

Reabsorption of neurotransmitter into the presynaptic neuron.

MAO

Monoamine oxidase; enzyme that degrades monoamines.

Dopamine

Monoamine involved in movement ( Nigrostriatal) and reward (Mesolimbic).

Norepinephrine

Monoamine used in CNS and PNS; involved in arousal and sympathetic signaling.

Serotonin

Monoamine involved in mood, appetite; targeted by SSRIs.

Histamine

Monoamine involved in wakefulness and inflammatory responses.

Nigrostriatal pathway

Dopaminergic system essential for movement; implicated in Parkinson’s disease.

Mesolimbic pathway

Dopaminergic reward pathway; linked to addiction and schizophrenia.

Glutamate

Major excitatory neurotransmitter in the brain; NMDA/AMPA/Kainate receptors.

NMDA receptor

Glutamate receptor that is involved in memory; Ca2+ permeability.

AMPA receptor

Glutamate receptor; rapid excitatory transmission; works with NMDA.

GABA

Gamma-aminobutyric acid; major inhibitory neurotransmitter in brain; opens Cl− channels.

Glycine

Inhibitory amino acid neurotransmitter in the spinal cord.

Huntington disease

Neurodegenerative disease associated with loss of GABAergic neurons.

Endocannabinoids

Retrograde neurotransmitters binding to cannabinoid receptors; modulate NT release; affect learning and appetite.

Nitric oxide (NO)

Gas neurotransmitter; diffuses freely; activates cGMP; vasodilation.

Carbon monoxide (CO)

Gas neurotransmitter; activates cGMP; involved in CNS signaling.

ATP and adenosine

Purine cotransmitters; act on purinergic receptors; modulate vascular and neural activity.

Synaptic plasticity

Change in synaptic strength with learning; includes LTP and LTD.

Long-term potentiation (LTP)

Strengthening of synaptic transmission; often involves AMPA receptor insertion in hippocampus.

Long-term depression (LTD)

Long-lasting decrease in synaptic strength; involves endocannabinoids and AMPA receptor removal.

Synaptic inhibition

Inhibitory neurotransmission that hyperpolarizes postsynaptic neuron (e.g., GABA, glycine).

Presynaptic inhibition

Inhibition of neurotransmitter release from the presynaptic neuron, often via modulation of Ca2+ channels.