Synapses

Unit 4.

Neurons, Hormones, and the Brain (cont'd)

Synapses:

Neurons work by means of electricity. When a nerve cell is active, it sends a pulse of electricity (nerve impulse) down the axon to the next cell in line. After passing down the axon, the nerve impulse reaches the swollen, knob-like end of the axon; this area is referred to as the axon terminal. The axon terminal sits close to the membrane of the next cell in line (the postsynaptic cell), but does not actually touch it. The space betwen the axon terminal and the membrane of the postsynaptic cell is called the synaptic gap. This area, where an axon terminal ends close to the cell membrane of another neuron, is called a synapse.

synapse = axon terminal + synaptic gap + membrane of postsynaptic cell

axon terminal = the knobby, swollen end of an axon

synaptic gap = the space between the axon terminal and the membrane of the next cell in line

postsynaptic membrane = the membrane of the next cell in line

Nerve impulses, which are electrical, do not jump across the synaptic gap at synapses. Instead, the arrival of a nerve impulse at the axon terminal triggers the release of chemicals called neurotransmitters from the axon terminal into the synaptic gap. The neurotransmitter molecules diffuse across the gap to the membrane of the postsynaptic cell, in effect serving as chemical messengers. There are molecules called receptors that stick off the membrane of the postsynaptic cells. In order to have any effect on the postsynaptic cell, a neurotransmitter molecule must fit onto a receptor. There are specific receptors for specific neurotransmitters, and the neurotransmitter molecule must fit onto the receptor precisely, as a key fits into a lock. When a neurotransmitter successfully fits ont a receptor, it causes changes in the postsynaptic cell, making it more (or less) likely to fire off a nerve impulse of its own.

neurotransmitters - chemicals released from the axon terminal that diffuse across the synaptic gap to carry the "message" to the postsynaptic cell

receptors - molecules that stick off of the postsynaptic membrane. Each receptor has a specific shape. In order for a neurotransmitter to have any effect on a postsynaptic cell, it must be able to fit onto a receptor, as a key fits into a lock.

The image below is a static drawing of a synapse.

**Static drawing of a synapse**

Below is an animated drawing of neurotransmitters being released from the axon terminal and binding to receptors on the post-synaptic cell.

**Animated drawing of a synapse**

Check out the following Web pages for more information about neurons (nerve cells), their parts, and how they communicate.

How do Nerve Cells Communicate? - information prepared by the Society for Neuroscience. http://www.sfn.org/backgrounders/communication.html

Making Connections - The Synapse - clear, comprehensible, explanation of how synapses work, with nice illustrations, prepared by Eric Chudler http://faculty.washington.edu/chudler/synapse.html

Neural Processes - excellent interactive animated tutorial http://psych.hanover.edu/Krantz/neurotut.html

Having too much or too little of a particular neurotransmitter is associated with a number of psychological disorders. Some of these are summarized in the table below.

**Disorders associated with having too much or too little neurotransmitter**
Neurotransmitter	Amount of Neurotransmitter	Associated Psychological Disorders
dopamine	too much	schizophrenia
dopamine	too little	Parkinson's disease
serotonin	too little	depression obsessive compulsive disorder
norepinephrine	too little	depression
acetylcholine	too little	Alzheimer's disease

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