BIOFOUND 5.8 Nerve Signaling pt 3

Myelinated axon

An axon wrapped in insulating myelin sheath; enables fast saltatory conduction by allowing action potentials to "jump" between nodes of Ranvier.

Unmyelinated axon

An axon without a myelin sheath conducts action potentials slowly via continuous conduction.

Action potential conduction

Action potentials move in one direction—from the initial segment (axon hillock) to the axon terminal—because sodium channels behind the signal are inactivated.

Diffusion of sodium ions in conduction

Sodium ions entering during an action potential spread locally inside the axon, depolarizing adjacent membrane segments and triggering new action potentials.

Why action potentials are unidirectional

Sodium channels become inactivated immediately after opening, preventing the signal from traveling backward.

Continuous conduction

Slow propagation of action potentials along unmyelinated axons by depolarizing every segment of the membrane.

Saltatory conduction

Fast propagation of action potentials along myelinated axons by jumping from one node of Ranvier to the next.

How saltatory conduction works

Myelin insulates sections of the axon, and action potentials only occur at gaps (nodes of Ranvier), where voltage-gated channels cluster.

Speed difference in conduction

Saltatory conduction is faster than continuous conduction because the signal skips over myelinated regions instead of activating every segment.

Synaptic vesicles

Small sacs in the axon terminal that store neurotransmitters and release them into the synapse during nerve signaling.

Neurotransmitters

Chemical messengers released by neurons at synapses to transmit signals to another neuron, muscle, or gland.

Step one of nerve signaling at a synapse

Action potential reaches axon terminal.

Step two of nerve signaling at a synapse

Calcium channels open and Ca²⁺ enters.

Step three of nerve signaling at a synapse

Synaptic vesicles fuse with membrane.

Step four of nerve signaling at a synapse

Neurotransmitters are released into synaptic cleft.

Step five of nerve signaling at a synapse

Neurotransmitters bind to receptors on post-synaptic cell.

Step six of nerve signaling at a synapse

Post-synaptic cell responds (e.g. electrical signal, muscle contraction, gland secretion).

Post-synaptic responses

Can include generating a new action potential (in nerves), triggering muscle contraction (in muscles), or activating secretion (in glands), depending on cell type and neurotransmitter used.