2. Neurons and synaptic transmission

1. Structure of neuron

A neuron is specialised cell that transmits electrical impulses.

1.The cell body (soma)- process info, contains nucleus

2.Nucleus- controls cell activity(genetic material)

3.Dendrites- carry nerve impulses from neighbouring neurons towards body cell, down the length of the neuron

4.Axon- carries electrical impulse away from the cell body

5.Myelin Sheath- fatty layer that protects axon, speeds up transmission

6.Nodes of Ranvier- gap between the myelin sheath, help speed transmission (impulses have to “jump” the gap)

7.Terminal button- at the end of the axon where neurotransmitter are released (communicate with a next neurone in the neurone chain, across the synapse)

8.Synapse- gap between neurons

Types of neurone

Sensory Neuron:

-carries impulses from receptor—> CNS

-located in PNS (cell body in ganglia)

-long dendrites, short axon

Relay Neuron:

-connects neurons with CNS(middleman doing decision-making)

-found in brain and spinal cord

-short dendrites, short axon

Motor Neuron:

-carries impulses from CNS—> effector (muscles/glands)

-located in CNS( brain, spinal cord), but long axons form part in PNS

-short dendrites, long axon

The Reflect Arc

It’s a rapid, automatic response to a stimulus.

Stimulus(hammer hits the knee) → detected by sense organs in PNS →which convey message along a sensory neuron → message reaches CNS (where it connects with relay neurone) → this then transfers message to a motor neuron → carries message to effector(muscle, which causes it to contract) → response(knee has moved)

Electrical Activity in a Neuron (firing)

1) At rest, neuron is negatively charged

2) When stimulated—> becomes positively charged (depolarisation), which causes action potential(electrical impulses) to occur

3) Impulses travel along the axon toward the end of the neurone where synaptic transmission occurs

Synaptic Transmission

1.Action potential occurs- electric impulses travel along the presynaptic neuron

2.Presynaptic Neuron- reaches the terminal button

3.Synaptic vesicles- vesicles containing neurotransmitters move toward the presynaptic membrane

4.Action potential reaches vesicles- vesicles fuse with membrane and release neurotransmitters by exocytosis

5.Neurotransmitters- diffuse across the synaptic gap (cleft)

6.Binding to a postsynaptic receptor- once neurotransmitters crossed the gap, they bind to specific receptor on postsynaptic neuron (lock-and-key)

7.Conversion- that caused depolarisation of the postsynaptic neuron, generating new action potential if threshold is reached

8.Reuptake- neurotransmitters are removed by reuptake or enzymatic breakdown

One way transmission

Info only travels in one direction because:

-Neurotransmitters are released only from presynaptic neuron

-Receptors are only on postsynaptic neuron

Excitation neurotransmitters

Excitation (adrenaline)

→ they increase the positive charge of the post-synaptic neuron → this makes it MORE likely that this neuron will pass on the electrical impulse.

Inhibition neurotransmitters

Inhibition (serotonin)

→ they increase the negative charge of the post-synaptic neuron → this makes it LESS likely that this neuron will pass on the electrical impulse

Summation process

Summation decides whether the postsynaptic neuron is going to pass on the electrical impulses.

The excitatory and inhibitory effects are summed:

• if the net effect on the postsynaptic neuron is inhibitory, it is less likely to fire.

• If the net effect is excitatory then it is more likely to fire (passes on the electrical impulse down the neuron)