Motor neurones

Motor neurones

A mammalian motor neurone can generate and conduct impulses. In doing so, it covneys information rapidly over considerable distances. A single nerve impulses may be transmitted from the spinal cord to the feet in a few milliseconds.
The structure of a motor neurone. A typical motor neurone has a cell body containing a nuleus and other organelles. In the cytoplasm of the cell body are Nissl granules- these contain parallel rows of the endoplasmic reticulum and assoicated ribosomes. Dendrites, highly branched cytoplasmic extensions about 1mm long, recieve incoming information from other cells and conduct nerve impuleses towards the body. A fine process called the axon extends from the cell body to the target cells. The conical region where an axon joins the cell body is called an axon hillock. the axon hillock integrates the incoming information from the dendrites and initiates a nerve impulse. the axon transmits outgoing nerve impulses from the cell body to the target cell. Axons and dendrons are often refered to as nerve fibres.
At the target cell, the axon divides into a number of nerve endings. the tip of each nerve ending has a swelling called the axon terminal. a narrow gap called the synaptic cleft seperates the membrane of an axon terminal from the membrance of a target cell. the junction between one neurone and another cell is called a synapse.
The axons of many mammalian motor neurones are enclosed along most of their length by a thick insulating material called the myelin sheath. the myelin sheath is produced by special supporting cells called Schwann cells. The sheath is essentially a series of cell membrances, each produced by a single schwann cell and wrapped many times around the axon. Gaps between the membranes of one schwann cell and the next are called nodes of ranvier. they play a key role in the fast transmisson of nerve impulses
Fast transmission enables mammals to respond almost instantaneously to stimuli. nerve impulses can be directly along the nerve fibres to specific points in the body so that responses can be very localised.
Invertebrate neurones are not myelinated. The speed of conduction of their nerve impulses depends, amoung other things, on the diameter of the neurone
Structures and functions
1. Dendirtes;
  1. Branched and numerous allowing many synaptic connections with other neurones
  2. Recieve incoming information
2. Cell body
  1. Has a large volume to accomodate a nucleus and organelles
  2. Densely packed with mitrochondria to provide the ATP needed to generate a nerve impule
  3. Nissl granules manufacture proteins
3. Axon
  1. Carries outgoing nerve impulses to target cells
  2. Long and thin to conduct impulses over a long distance rapidly
4. Schwann cell
  1. produce a myelin sheath that acts as an electrical insulator for the axon
5. Nodes of Ranvier
  1. Gaps in the myelin sheath that allow a nerve impulse to jump from one to the next, speeding up the condition of a nerve impulse
6. Axon terminals
  1. highly branched to increase the contact area betwen an motor neurone and its target cell
  2. Bulbous end contain neurotransmitter which is released when a nerve impulse reaches it

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