1. Neurons, Synapses, and Signaling
CHAPTER 48

2. Neurons
Neurons are nerve cells that transfer information within the body.
Your brain is made of neurons, so are the ganglia of simpler animals.

3. 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.

4. 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.

5. 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.

6. 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

7. 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.

8. 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.

9. 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

10. 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 dopamine
Nitric oxide (NO): muscle contraction

11. 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

12. 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 + – – +

13. Stimulus – + A stimulus is when the nerve is stimulated.
The neuron becomes positive inside and negative outside as Na+ ions move inside. – +
Na+

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

15. The nerve impulse travels
A second wave travels down the neuron, this time K+ ions move out to reverse the charge. + –
Na+ K+
wave 

16. Re-set + – After firing, a neuron has to re-set itself
Na+ 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+ K+
wave 

17. Action Potential Graph
Resting potential
Stimulus reaches threshold potential
Depolarization Na+ channels open; K+ channels closed
Na+ channels close; K+ channels open
Repolarization reset charge gradient
Undershoot K+ channels close slowly
–70 mV
–60 mV
–80 mV
–50 mV
–40 mV
–30 mV
–20 mV
–10 mV
0 mV
10 mV
Depolarization
Na+ flows in
20 mV
30 mV
40 mV
Repolarization
K+ flows out
Threshold
Hyperpolarization
(undershoot)
Resting potential
Resting 1 2 3 4 5 6
Membrane potential