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Roles of synapses in the nervous system

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Lydia Grimshaw

on 31 October 2014

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Transcript of Roles of synapses in the nervous system

Roles of synapses in the nervous system
The main role of the synapses is to connect two neurones together so that a signal can be passed from one to another however they can also:
Botulism
Botulism is a rare and potentially fatal paralytic illness caused by a toxin produced by the bacteria Clostridium botulinum. The disease begins with weakness, trouble seeing, feeling tired, and trouble speaking. This may then be followed by weakness of the arms, chest muscles and legs. Botulinum inhibits the release within the nervous system of acetylcholine, the chemical that produces a bridge across synapses, where nerve cell axons and dendrites connect with each other. All forms of botulism lead to paralysis.
Comparing myleinated and non-myleinated neurones
Myelinated - 1/3 of the neurones in vertebrates are myelinated The sheath is created by a series of Schwann cells that are wrapped around the axon so it consists of several layers of membrane. There are gaps in the sheath called nodes of Ranvier where the action potential is able to jump from one to another.
Advantage of myelination
Myleinated neurons can transmit an action potential more quickly than non-myelinated neurones. They can also carry signals from the sensory receptors to the Central Nervous System and then to the effectors. They can carry signals over long distances and the increase in speed means a more rapid response to a stimulus.
Multiple sclerosis
An inflammatory disease in which the insulating covers of nerve cells in the brain and spinal cord are damaged. This damage disrupts the ability of parts of the nervous system to communicate, resulting in a wide range of signs and symptoms, including physical, mental and sometimes psychiatric problems.
Tetanus
Tetanus is a medical condition characterized by a prolonged contraction of skeletal muscle fibers.The tetanus toxin initially binds to peripheral nerve terminals. It is transported within the axon and across synaptic junctions until it reaches the central nervous system. There it becomes fixed to gangliosides at the presynaptic motor nerve endings and is taken up into the axon by endocytosis. The effect of the toxin is to block the release of neurotransmitters glycine and gamma-Aminobutyric acid across the synaptic cleft, which is required to check the nervous impulse. If nervous impulses cannot be checked by normal mechanisms, muscular spasms are produced.
Synapses
Snake Venom
Several presynaptic neurones can converge to one postsynaptic neurone so that multiple parts of the nervous system can create the same response
One signal can be transmitted to several parts of the nervous system
They ensure signals are transmitted in the right direction
Allow reflex arcs to occur
They can filter out unwanted low-level signals
Low-level signals can be amplified by summation
Acclimatisation where the vesicles run out so a response can no longer occur (habituation)
The creating of specific pathways in the nervous system are thought to be the basis of conscious thought and memory
Non - Myelinated - The rest of the neurones are non-myelinated where several neurones are surrounded by one loosely wrapped Schwann cell. This means that the action potential moves along the neurone in a wave rather than jumping.
Giant Axon
The three main characteristics of MS are the formation of lesions in the central nervous system, inflammation, and the destruction of myelin sheaths of neurons. MS involves the loss of oligodendrocytes, the cells responsible for creating and maintaining a fatty layer. This results in a thinning or complete loss of myelin and as the disease advances, the breakdown of the axons. When the myelin is lost, a neuron can no longer effectively conduct electrical signals.
Effects of neurotoxins are described/explained as interference of neuromuscular signal transmission and can be subtle alterations of neurotransmitter release to complete neuromuscular block. The activity of neurotoxins can start at the presynaptic elements, post-synaptic elements or both. Most snake venoms contain toxins that affect both.
Fasciculins: These toxins attack cholinergic neurons by destroying acetylcholinesterase ACh therefore cannot be broken down and stays in the receptor. they cause severe, generalized and long-lasting rapid muscle contractions.

Dendrotoxins: These inhibit neurotransmissions by blocking the exchange of positive and negative ions across the neuronal membrane lead to no nerve impulse, thereby paralysing the nerves.

a-neurotoxins: This is a large group of toxins, a-neurotoxins attack cholinergic neurons. They mimic the shape of the acetylcholine molecule and therefore fit into the receptors so they block the ACh flow creating feelings of numbness and paralysis.
Types
The squid giant axon is the very large - up to 1 mm in diameter (double that of a typical axon) it is the axon that controls part of the water jet propulsion system in squid. Between the tentacles of a squid is a siphon through which water can be rapidly expelled by the fast contractions of the body wall muscles of the animal. This contraction is initiated by action potentials in the giant axon. Action potentials travel faster in a larger axon and squid have evolved the giant axon to improve the speed of their escape response. The increased diameter of the squid axon decreases the internal resistance of the axon, as resistivity is inversely proportional to the cross sectional area of the object.
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