Neuron Summary
Neurons in the spinal cord, CNS and PNS
§ A neuron is a cell that sends and receives information in the form of neural impulses (neural impulses are tiny pulses of electro chemical energy)
§ Neurons have five main parts
1. Dendrite – a specialised, short, thin and widely branching fibre that is specialised to detect and receive incoming neural information (neural impulses)
2. Soma – cell body, the section that determines whether the neuron will be activated and thus transmit (pass on) the neural stimulation to other neurons
3. Axon – a single tube like extension that transmits messages (neural impulses) from the soma to other cells in the body, including other neurons, muscles, glands. Axons vary in length, some extend over a meter from the spinal cord to the foot, and others are as short as the width of hair.
4. Axon terminals – branches protruding from the end of each axon, at the end of each terminal exists a small knob like structure called a terminal button. These buttons store chemicals called neurotransmitters which enable the transmission of a neural impulse from one neuron to the next. These chemicals are necessary as no two neurons actually touch, the neurotransmitter is released into the gap between the terminal button and the next neurons dendrite where it crosses the gap and binds to receptor sites on the dendrite of the next neuron, thus the message continues
5. Myelin sheath – a white fatty covering that helps insulate the axon from axons of other neurons, it also helps speed up the transmission of neural information
A neural impulse (or action potential as it is also called) is a combination of electrical and chemical energy (electrochemical energy) comprising the neural information which travels along the axon.
Neurons do not link together like a chain. Each neuron is separated from the next neuron by a tiny gap called a synaptic gap. The synaptic gap is about 500 times thinner than the thinnest strand of your hair. The point of communication between neurons is called the synapse.
When the neural impulse reaches the end of each axon, the synaptic knob releases chemicals called neurotransmitters
A neurotransmitter is a chemical substance that is manufactured by the neuron. It contains ions that travel across the synapse to the receptors on the dendrites of the receiving neuron. Sometime the neurotransmitter triggers or activates a neural impulse on the connecting neuron. At other times, the neurotransmitter inhibits or prevents the connecting neuron from firing.
Neuromodulators do not directly pass neural information from one neuron to another. They make neurons more or less receptive to a neurotransmitter.
Transmission of sensory and motor information
§ Sensory and motor neurons do not share the same tracts to and from the brain, they follow separate but adjacent paths
§ Sensory neurons – afferent
§ Motor neurons – efferent
§ Interneurons – connecting (enable the spinal reflex, sensory communicate with motor)
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The nervous system
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