Biology 12

150 MHR • Unit 2 Homeostasis

cord injuries. In addition, Peter Erikson from

Göteborg University in Sweden and his co-worker

Fred Gage found as many as 200 new neurons per

cubic millimetre of tissue in the brains of some

patients who had recently died of cancer. This

discovery involved using a specialized drug used

to trace the formation of new cells. The scientists

estimate that up to 1000 new neurons may be

created each day, even in the brains of people in

their 50s and 70s. These new neurons apparently

arise not by mitosis of mature neurons but from a

reserve of embryonic stem cells. These cells are

found in some parts of the brain and do not form

into specialized cells during the development of

the brain. Similar stem cells are found in bone

marrow and are responsible for the formation of

the wide variety of blood cells found in the body.

The Synapse

Neurons do not touch one another; there are tiny

gaps between them. These gaps are called

synapses. The neuron that carries the wave of

depolarization toward the synapse is called the

presynaptic neuron. The neuron that receives the

stimulus is called the postsynaptic neuron. When

a wave of depolarization reaches the end of a

presynaptic axon, it triggers the opening of special

calcium ion gates. The calcium triggers the release

by exocytosis of neurotransmitter molecules. The

neurotransmitter is then released from specialized

vacuoles called synaptic vesicles, which are

produced in the bulb-like ends of the axon. The

neurotransmitter diffuses into the gap between the

axon and dendrites of neighbouring postsynaptic

neurons, as shown in Figure 5.14. The dendrites

have specialized receptor sites and the

neurotransmitter attaches to these receptors and

excites or inhibits the neuron. The excitatory

responseinvolves the opening of sodium gates,

which triggers a wave of depolarization. The

inhibitory responsemakes the post-synaptic

neuron more negative on the inside in order to

raise the threshold of stimulus. This process is

usually accomplished by opening chloride

channels to increase the concentration of these

negative ions in the neuron.

To learn more about how the synapse works, go to your

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synaptic vesicles containing

neurotransmitters

surface of

dendrite

receptor

sites

neurotransmitters

released into synapse

synapse

synapse

axon

axon

dendrites

direction of

impulse

direction of

impulse

direction of

impulse

Figure 5.14At a synapse, neurotransmitters pour into specific sites on

nearby dendrites.