Control & communication · Topic 7 of 18

Nervous Tissue: Neurons & Impulses

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Anatomy · nervous tissue

A neuron takes signals in through dendrites and sends them out along its axon; sodium rushing in fires the impulse, potassium leaving resets it.

01 · Neuron partsDendrites in → cell body → axon out

02 · At restResting potential ≈ −70 mV

03 · The impulseNa⁺ in depolarizes; K⁺ out repolarizes

Neuron = the nervous system's signaling cell (electrical along, chemical across)

Nervous tissue = neurons (signal) + glial cells (support)

Saltatory conduction: impulse jumps node to node — faster than unmyelinated

Na⁺/K⁺ pump (uses ATP): Na⁺ out, K⁺ in — keeps the gradients

Refractory period: no new impulse right away → signal travels one way

Neurotransmitters (ACh, dopamine, serotonin, norepinephrine) excite or inhibit

Step by step

Na⁺ rushes in = depolarization; K⁺ flows out = repolarization

Synapse: neurotransmitter crosses the cleft to the next cell's receptors

Dendrites receive → cell body (nucleus) → axon carries the impulse away must know

Myelin makers: oligodendrocytes (CNS), Schwann cells (PNS)

Myelin = fatty insulation that speeds impulses; gaps = nodes of Ranvier

Resting membrane potential ≈ −70 mV (inside negative to outside) must know

All-or-none: threshold ≈ −55 mV; a stronger stimulus = more impulses, not bigger ones

Sensory = toward the CNS; motor = out to muscles/glands; interneurons link them

Electrical synapse: ions pass directly cell to cell, no neurotransmitter

Commonly confused

	Depolarization	Repolarization
Ion that moves	Sodium (Na⁺)	Potassium (K⁺)
Direction	Into the cell	Out of the cell
Voltage	Rises from threshold (−55 mV) to about +30 mV	Falls back toward −70 mV
Channels	Voltage-gated Na⁺ channels open	Voltage-gated K⁺ channels open

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Other glia: astrocytes (support), microglia (immune), ependymal (CSF)

Hyperpolarization: brief dip below −70 mV before returning to rest

Ca²⁺ entering the axon terminal triggers neurotransmitter release

Signal stops when neurotransmitter is broken down or taken back up (reuptake)

Wider axon = faster conduction (along with myelin)

Most neurons are multipolar; bipolar in retina and nose; unipolar = sensory

Pearl

Sodium in makes the inside positive; potassium out makes it negative again.

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