Neurons - Nerve Cells
Our bodies are bio-electrically powered machines. The brain communicates with organs and muscles via electrical impulses, and the nerves function like biological conducting 'wires'. Sending impulses allow blood to be pumped around the body and allow movement - voluntary and involuntary. Without these electrical signals we would be dead!
A neuron (also known as a neurone or nerve cell) is an electrically excitable cell that processes and transmits information by electrical and chemical signalling.
Chemical signalling occurs via synapses, specialized connections with other cells.
Neurons connect to each other to form neural networks. Neurons are the core components of the nervous system, which includes the brain, spinal cord, and peripheral ganglia.
A number of specialized types of neurons exist: sensory neurons respond to touch, sound, light and numerous other stimuli affecting cells of the sensory organs that then send signals to the spinal cord and brain. Motor neurons receive signals from the brain and spinal cord, cause muscle contractions, and affect glands. Interneurons connect neurons to other neurons within the same region of the brain or spinal cord.
Electrical impulses originate in:
- sensory cells in the body or
- the brain or
- the sino-atrial (SA) node (in the case of the heart).
The nerve impulse travels out from the nerve cell along the axon. Eventually the impulse reaches the tip end of the axon - the axon terminal region. This is usually positioned very close to another neuron or to a muscle cell or gland. Axons do not actually make direct contact with other neurons or with large tissue. Instead, a narrow gap, 10 to 20 nanometers across, separates the axon tip and the target neuron or tissue. This junction of an axon with another cell is called a synapse. The membrane on the axon-side of the synapse is called the presynaptic membrane; the membrane on the receiving side of the synapse is called the postsynaptic membrane.
When a nerve impulse gets to the end of an axon, its message must cross the synapse if it is to continue. Messages do not "jump" across synapses. Instead, they are carried across by chemical messengers called neurotransmitters. These chemicals are packaged in tiny sacs, or vesicles, at the tip of the axon. When a nerve impulse arrives at the tip, it causes the sacs to release their contents into the synapse. The neurotransmitters diffuse across the synapse and bind to receptors in the membrane of the cell on the other side, passing the signal to that cell by causing special ion channels in the postsynaptic membrane to open. Because these channels open when stimulated by a chemical (in this case, a neurotransmitter) they are said to be 'chemically gated''.
Why go to all this trouble? Why not just wire the neurons directly together? For the same reason that the wires of your house are not all connected but instead are separated by a host of switches. When you turn on one light switch, you don't want every light in the house to go on, the toaster to start heating, and the television to come on! If every neuron in your body were connected to every other neuron, it would be impossible to move your hand without moving every other part of your body at the same time. Synapses are the control switches of the nervous system. |
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You need to know the vocabulary related to nerve cells:
| Synapse |
A synapse is a tiny gap between nerve cells. Nerve cells communicate chemically with one another across these tiny gaps. Synapses are of the order of nanometres in width and ensure the impulses pass in one direction only (from the end of the axon to the next cell). |
| Dendrites |
Dendrites are branches of the main part of the nerve cell. They receive electrical impulses from other nerve cells. |
| Nucleus |
The main part of the nerve cell contains its nucleus. |
| Axon |
A nerve fibre, or axon, is a long (often more than 1 mtre in length), thin (a few micrometres in diameter) extension of a nerve cell (neuron), and consists of a central core of axoplasm, surrounded by a high-resistance membrane. It transmits electrical impulses to effector cells in the muscle fibres to make the fibres contract. |
| Myelin |
Myelin forms a sheath of around the axon. It insulates it from the surrounding tissue. It has gaps at intervals of about a millimetre. The gaps, referred to as nodes of Ranvier, boost the speed of the nerve impulses, making each impulse jump from gap to gap. |
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