NERVES AND NERVOUS SYSTEM
NEURON ->nerve cells; many types and sizes but same architecture.
-> can't survive on their own, need GLIAL CELLS for support and supply
Typical nerve cells
Nerve Impulses
- Signals are sent along neuron by electrical current of depolarization.
RESTING POTENTIAL
A sodium potassium pump in the neuron actively transports NA+ ions out of the cell. K+ ions are pumped in at the same time but K+ can diffuse back out of the cell. Na+ can not diffuse back in:. there is a [] difference of positive ions inside and outside the neuron membrane. The neuron is polarized and the potential difference across the membrane is called the resting potential.
+ + + + + + + + + + + + + + + + + + + Na+ and K+
cell membrane ------
------K+ diffuse out
cytoplasm
------K+ (-70 mV)
cell membrane ------
+ + + + + + + + + + + + + + + + + Na+ and K+
Stages of nervous impulse ( ACTION POTENTIAL) 524-526
1) initiation of the impulse
- some change (stimulus) in the environment surrounding a sensory neuron changes the shape of proteins in the neuron membrane causing NA+ to enter the cell through passive Na+ channels.
- results in a flood of Na+ inward called a depolarization and end up with a positive charge at the spot inside the neuron. - The Na+ channels close quickly after depolarization and Na+ is actively pumped back out. K+ gates open and K+ flows outwards and the membrane becomes polarized again. The whole process takes 5 milliseconds. A brief refractory period occurs for 1 to 10 ms while the membrane repolarizes. During this time, no stimulus will be able to cause another action potential.
2) transmission of the Action Potential.
- neighbouring areas of membrane depolarize beside the initial area, causing a chain reaction down along the axon of the neuron. This is caused by a small localized electrical current which induces local Na+ ions channel to open resulting depolarization.
3) The impulse requires a certain amount of depolarization to occur before it will be transferred down the axon. This minimum amount is called the threshold value and the consequent shift in the electrical charges is called Action Potential. This action potential is an all-or-none event.
4) The action potential will travel down the axon until it reaches the dendrites and the synapses (where nerves associate with other cells at dendrites). At the synapse, there is no contact (10-20 nanometers) and the space between is called the synaptic cleft.
Figure 13 page 426: Action Potential
SALTATORY CONDUCTION
- the axon is wrapped in a myelin sheath of Schwann cells with Nodes of Ranvier located in between.
- when the action potentials travel down the axon, it jumps (saltatory) from one node to the next very quickly up to 120m per second. Speeds up transmission.
CHEMICAL SYNAPSES
- When action potentials reach output zone (terminal boutons) they do not go any further.
- they usually do induce the neuron to release one or more neurotransmitter that diffuse across chemical synapses and stimulate adjacent cells.
- synapses are narrow clefts between the output zone of one neuron and the input zone of an adjacent neuron, muscle cell, or gland cell. (see figure 6, page 527)
- neurotransmitter chemicals are stored in synaptic vesicles in the cytoplasm of the presynaptic cell. When an action potential reaches the end of the axon, it causes calcium ion gates to open and calcium flow in. This induces the synaptic vesicles to fuse to the presynaptic membrane which releases the neurotransmitter into the synaptic cleft.
- The neurotransmitter molecules diffuse across the cleft and will bind to specific receptor proteins on the postsynaptic membrane. When they bind on it changes the shape of the receptors which opens up a channel and allows ions to enter the postsynaptic cell.
- There are hundreds of different types of neurotransmitter chemicals and they often will affect the postsynaptic cells differently: either have an excitatory effect or an inhibitory effect Example: The neural transmitter acetylcholine (Ach) will have an inhibitory or excitatory effect depending on the type of postsynaptic receptor cells.
- Synaptic integration is when more than one neurotransmitter is released into a synaptic cleft by different adjacent nerve cells. There is often a competition for control of the postsynaptic membrane. Excitatory and inhibitory signals allow for a fine control and may give priority to certain impulses.
- What prevents the neurotransmitter molecules left in the cleft from stimulating the postsynaptic cell even after the action potential has ended?
Neurotransmitter are removed from the cleft by different methods.
1) Most are degraded by enzymes in the cleft such as acetycholinestrase.
2) Some diffuse out of the cleft.
3) Membrane transport molecules also actively pump them back into the presynaptic membrane.
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