IMPULSE TRANSMISSION ACROSS A SYNAPSE

Impulses are transmitted from one neuron to another across a synapse ( a junction between two neurons). The term synapsis means connection. The synapse is esssential for homeostasis because of its ability to transmit certain impulses and inhibit others.

synaptic cleft. - an exceedingly minute gap between neurons about 200

Angstroms across

presynaptic neuron - the neuron located before a synapse

postsynaptic neuron - the neuron located after a synapse

neuromuscular junction - the area of contact (synapse) between a neuron and a muscle cell

neuroglandular junction - the area of contact (synapse) between a neuron and a glandular cell

telodendria - the branches at the end of an axon

synaptic knob - the rounded or oval expansion at the end of a telodendrium

location of synapses - a presynaptic neuron can synapse with the dendrites,

cell body or axon hillock of the postsynaptic neuron

neurotransmitter - a chemical which will carry the impulse across the

synaptic cleft

-made by the neuron, usually from amino acids

-each neuron can make only one kind of transmitter substance

-best known transmitter is acetylcholine (released by many neurons outside the CNS and by some neurons in the CNS).....excitatory

synaptic vesicle - a small membrane bound sac found in the synaptic knob

- holds neurotransmitter after it is produced

EVENTS

action potential arrives at the synaptic knob

depolarization of the membrane causes voltage sensitive calcium channels in

the membrane to open

calcium diffuses into the presnyaptic knob from the extracellular space

increase in Ca ions causes synaptic vesicles to fuse with release sites on the

presynaptic membrane

contents of vesicles (neurotransmitter molecules) released into synaptic

cleft by exocytosis

transmitter molecules diffuse across the cleft and bind to a receptor site

on the plasma membrane of the postsynaptic cell

this combination of transmitter molecule occupying a receptor site causes

Na+ channels to open (if the transmitter is acetylcholine) and initiates a

new action potential in the postsynaptic neuron

the ion channels will not close until the transmitter leaves the receptor

acetylcholine is removed from the receptor by an enzyme called

acetylcholineesterase, which transforms it into an effective substance

there are two major classes of chemical synapses

1. excitatory - postsynaptic response to the neurotransmitter is depolarization (by allowing entry of Na+ into cell) .....makes it more likely that the cell will generate an action potential

-called EPSP ... excitatory postsynaptic potential

2. inhibitory - postsynaptic response is lessen likelihood that the cell will generate an action potential. .....achieved by opening Cl- channels so that the potential Cl- enters the cell and it becomes more negative (hyperpolarized)

-called IPSP .....inhibitory postsynaptic potential

 

Inactivation of Neurotransmitters

The action of neurotransmitters can be stopped by four different mechanisms

1. Diffusion: the neurotransmitter drifts away, out of the synaptic cleft where it can no longer act on a receptor.

2. Enzymatic degradation (deactivation): a specific enzyme changes the structure of the neurotransmitter so it is not recognized by the receptor. For example, acetylcholinesterase is the enzyme that breaks acetylcholine into choline and acetate.

3. Glial cells: astrocytes remove neurotransmitters from the synaptic cleft.

4. Reuptake: the whole neurotransmitter molecule is taken back into the axon terminal that released it. This is a common way the action of norepinephrine, dopamine and serotonin is stopped...these neurotransmitters are removed from the synaptic cleft so they cannot bind to receptors.

RECEPTION AND PROCESSING

A single postsynaptic neuron can have a large number of neurons synapsing with it. Some cause excitation and some cause inhibition.

Summation will occur both temporally and spatially. Two subthreshold EPSP can cause enough change to creat an action potential if the are applied close together in either time (temporal) or location (spatial).

Also an IPSP can cancel an EPSP.

If the resulting effect is excitation, a nerve impulse will be initiated at the axon hillock and sent along the axon.

The brain is made of cells (called neurons) which communicatewith each other at places called synapses. A synapse is a functional (but not physical) contact between two neurons. There are about 100 billion neurons in the human brain and each has about 10,000 contacts with other neurons. The number of synapses in the human brain is about 10 to the 15th power. The neurons of one human cerebral cortex would reach over 250,000 miles if placed end to end. The complexity of this organ is obviously enormous!