Nervous System: Nervous Tissue Anatomy & Physiology

This set of vocabulary flashcards covers the mechanisms of the nervous system, focusing on action potentials, conduction types, neurotransmitter classification, and neuronal circuits.

All-or-none law

The principle that if the threshold is reached, an action potential is generated and propagated down the axon without any loss in intensity; if not reached, no action potential occurs.

Threshold

The membrane potential, specifically $-55\,mV$, that must be reached in the initial segment to trigger the opening of voltage-gated channels and initiate an action potential.

Depolarization

The phase of the action potential characterized by the gain of positive charge ($-55\,mV \rightarrow +30\,mV$) as $Na^+$ enters the axon through voltage-gated $Na^+$ channels.

Repolarization

The return to a negative membrane potential ($+30\,mV \rightarrow -70\,mV$) as $K^+$ exits the axon through voltage-gated $K^+$ channels.

Hyperpolarization

A state where the membrane potential becomes more negative than the resting membrane potential ($-70\,mV \rightarrow -80\,mV$) because voltage-gated $K^+$ channels stay open for a longer duration.

Absolute refractory period

The period of time (about $1\,ms$) after the start of an action potential when it is impossible to fire another one because $Na^+$ channels are open or inactivated.

Relative refractory period

The period following the absolute refractory period when another action potential is possible, but requires a stronger stimulus because the cell is hyperpolarized and further from the threshold.

Continuous conduction

The sequential propagation of an action potential along the entire length of an unmyelinated axon.

Saltatory conduction

The rapid propagation of an action potential on myelinated axons where the impulse appears to jump between neurofibril nodes.

Group A nerve fibers

Large diameter, myelinated fibers with conduction velocities as fast as $150\,m/sec$, including most somatic sensory and motor neurons.

Group B and C nerve fibers

Small diameter and/or unmyelinated fibers with slower conduction velocities (Group B at $15\,m/sec$; Group C at $1\,m/sec$).

Transmissive segment

The part of the neuron (synaptic knob) where the arrival of an action potential triggers the release of neurotransmitters via exocytosis.

Biogenic amines (monoamines)

A class of neurotransmitters derived from modified amino acids, including catecholamines like dopamine and indolamines like serotonin or histamine.

Neuropeptides

Neurotransmitters consisting of amino acid chains $2$ to $40$ units long, such as beta-endorphins and substance P.

Acetylcholinesterase

The enzyme that clears Acetylcholine (ACh) from the synaptic cleft by breaking it down into acetate and choline.

Nicotinic receptors

Ionotropic receptors that bind Acetylcholine to directly open ion channels and cause an Excitatory Postsynaptic Potential (EPSP).

Muscarinic receptors

Metabotropic receptors that bind Acetylcholine to activate G proteins and second messenger systems, resulting in either an EPSP or an IPSP.

Facilitation

A form of neuromodulation that causes a greater response in the postsynaptic neuron by increasing neurotransmitter release or the number of postsynaptic receptors.

Nitric oxide

A short-lived, nonpolar gas neurotransmitter made by postsynaptic neurons that can strengthen memory in the brain or cause blood vessel dilation in the PNS.

Endocannabinoids

Small, nonpolar molecules released by postsynaptic neurons that influence appetite, memory, and presynaptic neurotransmitter release.

Converging circuit

A neuronal circuit where input from several neurons joins at a single postsynaptic neuron, such as sensory inputs leading to salivation.

Diverging circuit

A circuit that spreads information from one presynaptic neuron to several postsynaptic neurons, such as the brain controlling multiple muscles for balance.

Reverberating circuit

A circuit that uses feedback to produce repeated, cyclical activity, such as those maintaining regular breathing during sleep.

Parallel-after-discharge circuit

A circuit where input is transmitted simultaneously along several paths to a postsynaptic cell, resulting in signals arriving at various times; involved in higher-order thinking.