Neuron: Difference between revisions

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==Structure==

[[File:MRuueFj.png|400px|thumb|right|Structure of the neurone. (click to expand)]]

[[File:MRuueFj.png|400px|thumb|right|Structure of the neurone (click to expand)]]

A typical neurone 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 and branching multiple times, 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 neurone to a dendrite of another. There are, however, many exceptions to these rules: neurones that lack dendrites, neurones that have no axon, synapses that connect an axon to another axon or a dendrite to another dendrite, etc.

A typical neurone 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 and branching multiple times, 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 neurone to a dendrite of another. There are, however, many exceptions to these rules. For example, neurones that lack dendrites, neurones that have no axon, and synapses that connect an axon to another axon or a dendrite to another dendrite, etc.

[[File:Axon.png|428px|thumb|right|Structure of axon membrane.]]

[[File:Axon.png|428px|thumb|right|Structure of axon membrane]]

==Propagation of Action Potentials==

All neurones 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 comprises 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~~, and~~ this is known as the resting potential, 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).

A nerve impulse is a self-propagating wave of electrical disturbance that travels along the surface of the axon membrane. This electrical disturbance comprises 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===

To begin with, 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.

To begin with, 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.

@@ Line 2: / Line 2: @@
 ==Structure==
-[[File:MRuueFj.png|400px|thumb|right|Structure of the neurone. (click to expand)]]
+[[File:MRuueFj.png|400px|thumb|right|Structure of the neurone (click to expand)]]
-A typical neurone 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 and branching multiple times, 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 neurone to a dendrite of another. There are, however, many exceptions to these rules: neurones that lack dendrites, neurones that have no axon, synapses that connect an axon to another axon or a dendrite to another dendrite, etc.
+A typical neurone 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 and branching multiple times, 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 neurone to a dendrite of another. There are, however, many exceptions to these rules. For example, neurones that lack dendrites, neurones that have no axon, and synapses that connect an axon to another axon or a dendrite to another dendrite, etc.
-[[File:Axon.png|428px|thumb|right|Structure of axon membrane.]]
+[[File:Axon.png|428px|thumb|right|Structure of axon membrane]]
 ==Propagation of Action Potentials==
 All neurones 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 comprises 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, and this is known as the resting potential, 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).
+A nerve impulse is a self-propagating wave of electrical disturbance that travels along the surface of the axon membrane. This electrical disturbance comprises 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===
-To begin with, 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.
+To begin with, 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.