Topic 9 Control and Coordination
Synapse:
- Synapse is a microscopic functional gap between two successive neurons.
- In this telodendrites of pre-synaptic neuron are in close proximity with dendrites of post-synaptic neuron.
- This gap is also called synaptic cleft.
- During transmission of nerve impulse, the synapse get filled with neurotransmitters like acetyl choline.
- When the telodendria are connected to muscle fibre, it is called motor end plate or neuro-muscular junction.
Properties of nerve fibres
A. Excitability/Irritability –
- The nerve fibres, on account of presence of a polarised membrane, have the ability to perceive stimulus and enter into a state of activity.
B.Conductivity –
- It is ability to transmit the excitation.
C.Stimulus –
- It is any detectable, physical, chemical, electrical change in the external or internal environment which brings about excitation in a nerve/muscle/organ/organism.
- In order to be effective, the stimulus must have a minimum intensity called threshold stimulus. Subliminal (weak) stimulus will have no effect while supraliminal (strong) stimulus will produce the same degree of impulse as the threshold stimulus.
D.Summation effect
- A single subliminal stimulus will have no effect but if many such weak stimuli are given in quick succession they may produce an impulse due to addition or summation of stimuli.
E.All or none law
- The nerve will either conduct the impulse along its entire length or will not at all conduct the impulse, as in case of subliminal or weak stimulus.
F.Refractory period
- It is the time interval (about millisecond) during which a nerve fails to respond to a second stimulus however strong it is.
G.Synaptic delay
- The impulse requires about 0.3 to 0.5 milliseconds to cross a synapse.
- This time is required for release of neurotransmitter from the axon terminal and excitation in the dendron of the next neuron.
H.Synaptic fatigue
- The transmission of nerve impulse across the synapse halts temporarily due to exhaustion of its neurotransmitter
I.Velocity:
- The rate of transmission of impulse is higher in long and thick nerves. It is higher in homeotherms than in poikilotherms.
- The velocity of transmission is higher in voluntary fibres (100 - 120 m/second in man) as opposed to autonomic or involuntary nerves (10-20 m/ second).
- synapse where the neurons communicate with one another.
- The neuron carrying an impulse to the synapse is the pre-synaptic neuron.
- The neuron receiving input at the synapse is the post synaptic neuron or generator region (gland or muscle).
- A synaptic cleft or a small intercellular space lies in between two cells.
- The process by which the impulse from the pre-synaptic neuron is conducted to the post-synaptic neuron or cell is called synaptic transmission.
- It is a one way process carried out by neurotransmission.
Types of synapses:
- Electrical synapse
- Chemical synapse.
- Electrical synapses
- In this type of synapse gap between the neighbouring neurons is very narrow.
- The synapse between such closed neurons is mechanical.
- The electrically conductive link is formed between the pre and post synaptic neurons.
- At the gap junction, the two cells are within almost 3.8 nm distance of each other.
- Transmission across the gap is faster but depends on the connection located at the gap junctions between the two neurons.
- Electrical synapses are found in those places of the body requiring fastest response as in the defence reflexes. also they are bidirectional, allowing transmission in either direction or may be unidirectional.
- These are specialized junctions through which cells of the neural system send chemical signal to the other neurons and to non-neuronal cells, such as gland and muscle.
- Synaptic gap is larger than that in electrical synapse and it is 20-40 nm.
- A chemical synapse between a motor neuron and a muscle cell is called a neuromuscular junction. There are three components of a typical chemical synapse.
- The presynaptic terminal (mostly Axonic terminal),
- Synaptic cleft (usually on the dendrite of the next neuron/ gland cell/ muscle)
- The post synaptic neuron. ( postsynaptic terminal )
- The impulse travels along the axon of the pre-synaptic neuron to the axon terminal. several synaptic knobs at their ends or terminals.
- These knobs have arrays of membranous sacs, called synaptic vesicles, that contain neurotransmitter molecules.
- When an impulse reaches a synaptic knob, voltage sensitive Ca++ channels open and calcium (Ca++) diffuses inward from the extracellular fluid.
- Once the neurotransmitters bind to the receptors of the post-synaptic cell, the action is either excitatory (turning a process on) or inhibitory (turning a process off).
- Once the impulse has been transferred across the synapse, the enzyme like cholinesterase destroys the neurotransmitter and the synapse is ready to receive a new impulse.
Transmission of nerve impulse :
- The neurons are cells with some specials features. The cells can be excited.
- The nerve impulse is a wave of bioelectrical or electrochemical disturbances passing along a neuron.
- The transmission of the nerve impulse along the long nerve fibre/axon tube is a result of electrical charges across the neuronal membrane during conduction of an excitation.
- Each neuron has a charged cellular membrane with a voltage which is different on the outer and inner side of the membrane.
Transmission of nerve impulse across a synapse
- This transmission takes place with the help of neurotransmitters.
- Once the neurotransmitters bind to the receptors of the post-synaptic cell, the action is either excitatory or inhibitory depending on the type of neurotransmitter.
- The enzyme like acetyl cholinesterase destroys the neurotransmitter after the transmission and the synapse is ready to receive a new impulse.
Transmission of nerve impulse along the axon :
- The excitable neurons transmit the impulse through changes in electrical charges across the neuronal membrane.
- The external tissue fluid has both Na+ and K+ ions.
- This process is called sodium pump or Na-K exchange pump.
- Generation of nerve impulse : Occurs through depolarization.
- Saltatory conduction takes place in medullated nerve fibres.
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