CHAPTER 48

Neurons, Synapses, and Signaling

Neurons

Neurons are nerve cells that transfer information within the body.

Your brain is made of neurons, so are the ganglia of simpler animals.

Types of Neurons

1. Sensory Neurons transmit information from eyes and other sensor that detect stimuli (light, sound, touch, heat, smell, or taste) or internal conditions (blood pressure, oxygen levels, muscle tension).

2. Interneurons make up the brain and are used for local connections. They interpret information from sensory neurons and relay it to motor neurons.

3. Motor neurons transmit signals to muscle cells, causing them to contract.

Neuron Structure

Neurons can be very long (2m in people, 30m in whales).

The cell body has a nucleus and organelles with branched dendrites coming off of it.

Dendrites receive nerve signals from other neurons.

Neuron Structure

A long axon, covered by a myelin sheath, extends from the cell body, and transmits impulses.

At the end is the synaptic terminal, where two neurons meet at a junction called the synapse.

Neuron Structure

The axon is coated with Schwann Cells that produce the myelin sheath

This insulates the axon and speeds up the signal so it can hop from node to node

Synapses

A synapse is a junction between the end of an axon of one neuron and the dendrites of another neuron.

Synapses can be chemical or electrical.

Chemical Synapses

Neurotransmitters pass information from one neuron to another across synapses.

Neurotransmitters are chemical messengers.These signals tend to be transmitted more

slowly and over short distances.

Chemical Synapses

They are triggered to release from vesicles when Ca2+ ion concentration increases (alerting the neuron that a signal is about to cross)

Can be proteins or gases

Neurotransmitters

Aceytlcholine: transmits signals to skeletal muscles

Epinephrine, adrenaline, and norepinephrine: fight or flight response, activates in times of stress to increase ATP production

Dopamine: affects sleep, mood, attention, learning (excitatory). Too little– Parkinson’s disease; too much– schizophrenia

Serotonin: opposite effects of dopamineNitric oxide (NO): muscle contraction

Electrical Synapses

Neurons have a membrane potential (voltage resulting in a difference of electrical charge) across their plasma membranes.

Electrical synapses change this potential to transmit signals.

These are generally signals that travel very fast over long distances in the body

Membrane potential

When a neuron is at rest, it’s voltage is between -60 and -80 mV.

The outside is positive, the inside is negative

– – – – – – – ––– – – – –

– – – – – – – ––– – – – –+ + + + + + + ++ + + + + + +

+ + + + + + + ++ + + + + + +

Stimulus

A stimulus is when the nerve is stimulated.The neuron becomes positive inside and

negative outside as Na+ ions move inside.

– + + + + + + ++ + + + + + +

– + + + + + + ++ + + + + + +

+ – – – – – – –– – – – – – –

+ – – – – – – –– – – – – – –Na+

Wave

The nerve impulse travels down the neuron, letting in more Na+ as it goes. This is called the action potential.

– – + + + + + +– + + + + + +

– – + + + + + +– + + + + + +

+ + – – – – – –+ – – – – – –

+ + – – – – – –+ – – – – – –Na+

wave

The nerve impulse travels

A second wave travels down the neuron, this time K+ ions move out to reverse the charge.

+ – – + + + + +– – + + + + +

+ – – + + + + +– – + + + + +

– + + – – – – –+ + – – – – –

– + + – – – – –+ + – – – – –Na+

K+

wave

Re-set

After firing, a neuron has to re-set itselfNa+ needs to move back out (3)K+ needs to move back in (2)This requires energy because it is going

against the concentration gradient-needs a pump.

+ + + + + – – ++ + + + + – –

+ + + + + – – ++ + + + + – –

– – – – – + + –– – – – – + +

– – – – – + + –– – – – – + +Na+

Na+Na+

Na+ Na+Na+

K+K+K+K+ Na+Na+Na+

Na+Na+

Na+Na+Na+

Na+Na+

Na+

K+K+K+K+

K+K+

K+ K+

wave

Action Potential Graph

1. Resting potential2. Stimulus reaches

threshold potential3. Depolarization

Na+ channels open; K+ channels closed

4. Na+ channels close; K+ channels open

5. Repolarizationreset charge gradient

6. UndershootK+ channels close slowly

–70 mV

–60 mV

–80 mV

–50 mV

–40 mV

–30 mV

–20 mV

–10 mV

0 mV

10 mVDepolarizationNa+ flows in

20 mV

30 mV

40 mV

RepolarizationK+ flows out

ThresholdHyperpolarization(undershoot)

Resting potential Resting1

2

3

4

5

6

Mem

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