Neuron

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[1]Neurones are excitable cells that make up the nervous system.  There are elongated cells that transmit electrical signals. On average a neurone is 20-30µm in diameter, however this varies depending on the type of neurone.  There are 3 types of neurons: motor, sensory and relay neuron. Both the motor neuron and internerone are described as multipolar, meaning they have two or more processes emenating out of their cell body which gives rise to numerous dendrites. This function of these particular neurones enables signals to be integrated and passed among an entire network of neuronal cells. Whereas, the sensory neuron has only one cell body, which lies to the side (pseudo-unipolar). Within the central nervous system of one individual, there are approximately 1011 neuron cells, most neuron cells cannot regenerate, therefore damage to neurones could be permanent. A typical neuron consists of a cell body, dendrites, axon and an axon terminal [2].

A neuron transmits electrical signals by action potentials. The axon hillock membrane contains sodium potassium ATPases which pump three sodium atoms out for every two potassium molecules pumped into the cell. This helps maintain the resting potential at -70mv. Once an action potential is generated it is transmitted down the axon to the synaptic knob, where neurotransmitter is released from vesicles in the presynaptic membrane and then diffuse across the synaptic cleft to the postsynaptic membrance. It binds to receptors on the post synaptic membrane, causing Sodium voltage-gated ion channels to open and Sodium to diffuse down its concentration gradient into the neuron causing depolarisation of the membrane (an electrical signal).

A nerve cell consists of a cell body (also known as the Soma), dendrites and an axon, which passes the action potential along to the axon terminals..  The cell body contains many of the normal organelles, such as the nuclues.  Sometimes, the axon is surrounded by a myelin sheath. The myelin sheet is made by schwann cells and forms an insulating layer around the axon.  The myelin sheath has gaps in it, called nodes of ranvier which allow the transmitted signal to jump from node to node. This is known as saltatory conduction. Synapses with other cells occur at the dendrites and axon terminals, stimuli (e.g. chemical or electrical) cause the nerve to depolarise and an action potential is produced. Once at the axon terminalthe signal can be transferred to neighbouring nerve cells or effector cells by a chemical neurotransmitter e.g. acetylcholine. A high temperature, large axon diameter and the presence of a myelin sheath increase the rate at which the signal is transmitted along the axon [3].

The axon is where saltatory conduction causes the propagation of an action potential to move down the axon towards the axon terminal. The action potential is all or nothing, therfore if the threshold voltage is reached in the axon hillock then the electrical signal will always be transmitted. Temporal summation, which is how closely together in time the singals reach the postsynaptic neurone, also effect the strength at which the signal is transmitted. The axon terminal is where presynaptic modulation occurs, and where neurotransmitters are released causing the action potential to be passed to another neurone across the synapse. The dendrites are where graded potentials occur. These can either be excitatory postsynaptic potential (EPSP) or inhibitory postsynaptic potential (IPSP). A graded potential is a subthreshold voltage (below -55mV) however if a number of graded potentials sum together at the axon hillock this can lead to a suprathreshold value and and an action potential is generated.

Neurones have a resting membrane potential (RMP) of -70mv, this is set up by a potassium ion leak channel. This allows potassium to move by facilitated diffusion from inside the cell out, down its concentration gradient, creating a slightly positive charge on the outside of the membrane compared to the inside. RMP is also set up by an ATP-ase pump, sodium and potassium. This uses ATP to pump, against their concentration gradients, three sodium outside the cell and two potassium back in. This is a symport protein channel. Both of these ion movements cause the inside of the cell to be slightly negative when compared to the outside (usually around -70mV). This is the resting membrane potential for a neurone [4]

References:

  1. http://www.naturalhealthschool.com/9_2.html
  2. Developmental Biology, 8th Edition, Scott F. Gilbert, Pg 394
  3. http://hyperphysics.phy-astr.gsu.edu/hbase/biology/nervecell.html#c2
  4. Mathews G. (2003) Cellular Physiology of Nerve and Muscle, 4th Edition, Oxford: Blackwell Pub.


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