Cells of the Nervous System; Neurotransmitters at Synapses

Neurons carry electrical impulses

Neuron Structure:

Axon: carries nerve impulses away from a cell body which is insulated by myelin

Dendrites: receive nerve impulses and pass them toward a cell body

A myelin sheath increases the speed of the nerve impulses along the neuron

Certain diseases, like polio, cause the myelin sheath to be destroyed which damages motor neurons

Myelin sheath is produced by glial cells

Glial cells maintain homeostasis, for myelin, provide support and protection for neurons and remove debris by phagocytosis

Synapses

A synapse is a small gap where two neurons meet

Each neuron's membrane is separated by a narrow space called a 'synaptic celft'

Transmission of nerve impulses

3. Neurotransmitters diffuse across the synaptic cleft and bind with receptor molecules on the membrane of the second neuron

4. Receptors bind to specific neurotransmitters

2. When the nerve impulse reaches the dendrites, neurotransmitters are released

5. Neurotransmitter binding to receptors stimulates second neuron to transmit an electrical impulse along its axon.

1. Nerve impulse travels along the axon

Neurotransmitters are stored inside vesicles, found in the presynaptic neuron

During impulses transmitter molecules are rapidly removed from the synaptic cleft

Re-uptake - neurotransmitter is reabsorbed

Enzyme degradation - broken down and resynthesised as neurotransmitter

Excitatory and Inhibitory Signals

excitatory increases contraction of muscle fibres

Inhibitory decreases contraction of muscles fibres

Synapses can filter out weak stimuli arising from insufficient secretion of neurotransmitters

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Neurotransmitter Effects on Mood & Behaviour

Endorphins

Stimulate neurons involved in reducing the intensity of pain

Increased levels are involved with feelings of pleasure (prolonged exercise, eating, sex, etc)

Endorphin production increases in response to severe injury, prolonged and continuous exercise, stress & certain foods

Dopamine induces feelings of pleasure and reinforced particular behaviour in the reward pathway

Neurotransmitter-related Disorders / Treatment

Alzheimer's disease is related to levels of acetlycholine

Parkinson's disease consists of low dopamine levels which the part of the brain controlling movement

Treatments:

Agonist

Antagonist

Mimic neurotransmitter effect where they bind to and stimulate receptors (e.g. morphine)

Block neurotransmitters from binding to and stimulating receptors (e.g. antihistamines)

Other drugs inhibit the enzymes which degrade neurotransmitters or inhibit re-uptake

Recreational Drugs

Mode of Action

Can mimic neurotransmitters

Changes in neurochemistry, like alter mood, cognition, perception and behaviour

Many affect neurotransmission in the reward circuit in the brain

Drug Addiction / Tolerance

Sensitisation is the increase in number and sensitivity of
neurotransmitter receptors - anagonsits - addiction

Desensitisation is the decrease in sensitivity and neurotransmitter receptors - agonist - tolerance