Non-majors A&P

Human A&P · Nervous System

Non-majors A&P · Nervous System

Nervous System: Physiology

How the nervous system works: the charged resting neuron, the action potential, how signals add up, the synapse and its messengers, and the autonomic system that runs the body automatically.

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Part 1

The Nerve Signal

How a neuron fires and carries an impulse.

Overview

How neurons send a signal

Two steps
  • A nerve signal travels in two steps.
  • 1. Electrical: an impulse races down the axon.
  • 2. Chemical: at the end, the neuron releases a chemical to pass the signal to the next cell.
Electrical down the axon, chemical across the gap.

At rest

The resting membrane potential

Charged and ready
  • A resting neuron is polarized: about -70 mV, negative inside compared to outside.
  • The sodium-potassium pump keeps sodium (Na+) high outside and potassium (K+) high inside.
  • That stored charge is like a loaded spring, ready to fire.
At rest the inside is negative and charged, waiting for a stimulus.
Resting membrane potential measured at about -70 mV

Resting potential · OpenStax, CC BY

The impulse

The action potential

All-or-none
  • A stimulus that reaches threshold (about -55 mV) triggers the impulse; it is all-or-none.
  • Depolarization: Na+ rushes in and the inside flips positive (up to +30 mV).
  • Repolarization: K+ flows out and the inside returns negative; then a brief hyperpolarization.
  • The pump restores rest. Myelin speeds it, letting the impulse jump between gaps (saltatory conduction).
Reach threshold and it fires fully: Na+ in to depolarize, K+ out to repolarize.
Action potential graph: threshold, depolarization, repolarization, hyperpolarization

Action potential · OpenStax, CC BY

Adding up

Graded potentials and summation

Deciding to fire
  • Small inputs cause graded potentials, little local changes that fade with distance.
  • Excitatory inputs (EPSPs) push toward threshold; inhibitory inputs (IPSPs) push away.
  • They add up (summation) at the axon hillock; if the total reaches threshold, an action potential fires.
Many small signals are tallied at the axon hillock; cross threshold and the neuron fires.
Summation of excitatory and inhibitory postsynaptic potentials at the axon hillock

Summation · OpenStax, CC BY

Part 2

The Synapse and Beyond

Passing the signal, and running the body automatically.

The synapse

The synapse and neurotransmitters

Chemical signal
  • The impulse reaches the axon terminal and opens calcium channels.
  • Calcium triggers synaptic vesicles to release neurotransmitter into the synaptic cleft.
  • The neurotransmitter crosses and binds receptors on the next cell, passing the signal on (or telling it to stop).
Electrical becomes chemical: calcium in, vesicles release, neurotransmitter crosses to the next cell.
A synapse: axon terminal, vesicles, neurotransmitters crossing the cleft to receptors

The synapse · OpenStax, CC BY

Messengers

Types of neurotransmitters

Excite or inhibit
  • Excitatory transmitters push the next cell toward firing; inhibitory ones push it away.
  • Acetylcholine: muscle signals and memory. Dopamine: reward and movement.
  • Serotonin: mood and sleep. GABA: the brain's main inhibitory brake.
Same machinery, different messenger: some transmitters excite, some inhibit.

Automatic

The autonomic nervous system

Involuntary control
  • The autonomic nervous system runs involuntary organs: heart, lungs, gut, glands.
  • Sympathetic (fight or flight) uses mostly norepinephrine.
  • Parasympathetic (rest and digest) uses acetylcholine.
  • The two balance each other to keep the body steady.
Sympathetic revs you up (norepinephrine); parasympathetic calms you (acetylcholine).

Wrap-up

Key takeaways

Review
  • A resting neuron is polarized (about -70 mV); the sodium-potassium pump maintains it.
  • An action potential is all-or-none: at threshold Na+ enters (depolarize), then K+ leaves (repolarize); myelin speeds it.
  • Graded potentials summate at the axon hillock to decide whether the neuron fires.
  • At the synapse, calcium triggers neurotransmitter release; transmitters either excite or inhibit the next cell.
  • The autonomic system runs organs automatically: sympathetic (norepinephrine) versus parasympathetic (acetylcholine).

Dr. Sharilyn Rennie · Focus: Human Anatomy · Muscle Tissue & Microanatomy. Need a PDF? Use Print / Save PDF above, or download the accessible PDF from Canvas.

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