A neuron, also known as a neurone or nerve cell, is an electrically excitable cell that processes and transmits information through electrical and chemical signals. A chemical signal occurs via a synapse, a specialized connection 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.
A typical neuron possesses a cell body (often called the soma), dendrites, and an axon. Dendrites are thin structures that arise from the cell body, often extending for hundreds of micrometres, branching multiple times and giving rise to a complex "dendritic tree." An axon is a special cellular extension that arises from the cell body at a site called the axon hillock and travels for a distance. At the majority of synapses, signals are sent from the axon of one neuron to a dendrite of another. There are, however, many exceptions to these rules. For example, there are neurons that lack dendrites, neurons that have no axon, and synapses that connect an axon to another axon or a dendrite to another dendrite, etc.
Propagation of action potentials
All neurons are electrically excitable, maintaining voltage gradients across their membranes by means of metabolically driven ion pumps which combine with ion channels embedded in the membrane to generate intracellular-versus-extracellular concentration differences of ions such as sodium, potassium, chloride, and calcium. Changes in the cross-membrane voltage can alter the function of voltage-dependent ion channels. If the voltage changes by a large enough amount, an "all-or-nothing" electrochemical pulse called an action potential is generated which travels rapidly along the cell's axon, and activates synaptic connections with other cells when it arrives.
A nerve impulse is a self-propagating wave of electrical disturbance that travels along the surface of the axon membrane. This electrical disturbance is comprised of a temporary reversal of the electrical potential difference (not an electrical current). The axon is usually negatively charged compared to the outside of the axon; this is known as the resting potential and the value of which is usually around -65mV. When a stimulus is received, a reversal in electrical potential difference is caused, and this is known as the action potential (normally around +40mV).
Mechanism of propagation of action potentials
The inside of the axon is negatively charged compared with the outside of the axon. The change in potential difference that is needed to fire off an action potential is controlled by the movement of sodium and potassium ions (an ion is a positively or negatively charged molecule) in and out of the axon. This movement occurs via the action of ion pumps and channels. The ions cannot just diffuse in and out of the axon uncontrollably; this diffusion is prevented by a membrane around the axon. Periodically placed along the membrane are proteins that act as channels for ions to pass through. Sodium gated channels and potassium gated channels open and close to allow the ions to pass through only at specific times. Sodium-potassium pumps transport both Na+ and K+ in and out of the axon.
The inside of the axon starts at around -65mV less than the outside of the axon. An action potential is reached when the axon is at +40mV more than the outside of the axon. This value of +40mV is reached by the movement of sodium and potassium ions in and out of the axon. Sodium-potassium pumps transport 2K+ into the axon for every 3Na+ transported out of the axon. However, more sodium is removed from the axon compared to the potassium brought. This means the overall electronegativity is decreasing in the axon, and the axon is getting closer to reaching the potential difference of +40mV. Sodium ions then begin to diffuse back into the axon naturally, and potassium ions diffuse back out. At this stage, however, potassium gated channels are open whereas sodium gated channels are closed. This means the K+ can diffuse out faster than the Na+ can diffuse back into the axon. This increases the potential difference further between the inside and the outside of the axon.
Once an action potential has been established, it “moves” along the axon in a neuron. The action potential does not move in a physical sense of the word; the reversal of electrical charge is instead reproduced at different points along the axon in a “Mexican wave” effect. One point in an axon will become depolarised (depolarisation is a change in a cell’s membrane potential, making it more positive or less negative), and this depolarisation is a stimulus for the next region of the axon to depolarise. As the next region depolarises, the previous region returns to normal and repolarises. Eventually, the action potential will reach the end of an axon, known as a synaptic knob.
Action upon other neurons
A neuron affects other neurons by releasing a neurotransmitter that binds to chemical receptors. The effect upon the postsynaptic neuron is determined not by the presynaptic neuron or by the neurotransmitter but by the type of receptor that is activated. Receptors can be classified broadly as excitatory (causing an increase in firing rate), inhibitory (causing a decrease in firing rate), or modulatory (causing long-lasting effects not directly related to firing rate). The distinction between excitatory and inhibitory neurotransmitters is not absolute. Rather, it depends on the class of chemical receptors present on the postsynaptic neuron. In principle, a single neuron releasing a single neurotransmitter can have excitatory effects on some targets, inhibitory effects on others, and modulatory effects on others still.
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