Unit 1B: Nerve Impulses and Synapses

Nerve Impulse• A neuron’s job is to transmit a message to a muscle,

gland, or another neuron• The message travels along the length of the axon as

an electrical message – a nerve impulse

Resting potential• At rest, there is a separation of ions on either side of

the axon membrane, causing there to be a voltage differential across the membrane – Na+ outside (+)– K+ (and Neg charged proteins) inside (-)

Action Potential• When a large enough signal reaches the neuron, Na+ channels in

the membrane open allowing Na+ to rush into the axon – this causes depolarization of the charges across the membrane

• A little while later, K+ channels open, allowing K+ to rush out – this causes repolarization of the charges across the membrane

• The charge reversal stimulates the next segment of the membrane to depolarize

• Na+/K+ pumps return ions to their original locations during the refractory period.

What determines the strength of a neuron’s message?

• All or Nothing Law– If the stimulus for a

neuron is too weak, no impulse is carried.

– If a stimulus is strong enough, then an impulse is carried all the way to the end of the axon with NO loss in strength

• Rate Law– The difference between

a strong signal and a weak signal is the RATE at which the neuron fires.

– A strong impulse (ex. bright light) causes rapid axon firing, and rapid axon firing causes a strong response (ex. in a muscle)

Communication between neurons• Information needs to be passed from one neuron to

another. This happens at synapses.• When the nerve impulse reaches the terminal

buttons on the axons, chemicals known as neurotransmitters are released into synaptic cleft.

Synapse Overview• There are many different types of neurotransmitters.– Some excite the next neuron (or cell)– Some inhibit the next neuron (or cell)

• The neurotransmitters work because they have a shape the allows them to bond specifically to the receptor on the next neuron.– Other chemicals (ex. drugs, venoms, etc. also can bond to

these receptors.)

Synaptic Vesicles• Neurotransmitters are produced in the Golgi

apparatus in the cell body of the neuron and are carried by synaptic vesicles to the terminal buttons through axoplasmic transport.

• A given cell will have up to a few hundred vesicles waiting in the terminal button.

Neurotransmitter Release• When an action potential arrives at the terminal button,

vesicles to begin to fuse into the synaptic cleft, releasing neurotransmitters into the space. (video below)

Postsynaptic Response• When the neurotransmitters bind to the postsynaptic

receptors, several things can happen– It stimulates a new nerve impulse– It inhibits a new nerve impulse– It controls the production of proteins– It controls the release of hormones

Postsynaptic Response (cont)• A single neuron can receive multiple inputs– some

excitatory, some inhibitory. Whether or not new nerve impulse will be generated is a result of the interaction of all of these inputs

Completion of Synaptic Communication• Once neurotransmitters have caused a response in

the next neuron, they must leave the synaptic cleft to prevent overstimulation (or inhibition) of the next neuron.– Reuptake – neurotransmitters rapidly diffuse back across

the presynaptic membrane– Enzyme deactivation