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GCSE Biology Nervous System GRADE 9 Notes
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GCSE Biology Nervous System GRADE 9 Notes as part of the Homeostasis topic
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24.10.2020The Nervous System
The nervous system is divided into two parts:
o Central Nervous System: Brain and Spinal cord
o Peripheral Nervous System: all other nerves of the body
Nerve cells (neurones) carry electrical impulses
o Very fast transmission, unlike hormones
Structure of Neurons
We had a look at the structure of nerves – there is a dendrite, cell body, axon, and the nerve ending. Like a
normal cell, we have cytoplasm, a cell membrane and a nucleus in the cell body.
When an impulse is generated, it travels down the neuron, from the dendrite to the nerve ending down the
axon.
At the nerve ending, the neuron communicates with muscles, glands, or other nerves via synapses.
One nerve contains multiple neurons, wrapped in connective tissue.
The axon is made of the myelin sheath, which itself is made up of Schwann cells, with nodes of Ranvier in
between. The myelin sheath insulates the neuron.
o Intervals in the sheath are nodes of Ranvier – parts of the axon not covered in myelin. These gaps
allow the rapid transmission of electrical impulses – the impulse doesn’t need to travel the whole
length of the axon – it can ‘jump’ between the nodes of Ranvier (where there is no myelin) as the
impulses cannot pass through the myelin. speeds up the length of impulse transmission
Pathway of Response
Receptors detect a stimulus and produce an electric impulse as a result.
This conveys information to sensory neurons (PNS) which carries impulses to the CNS.
The relay neuron is the communicator neuron between the two, carrying information between sensory and
motor neurons.
The CNS processes the impulses and sends information to the motor neurons (PNS), which sends impulses to
effectors.
These effectors (muscles, glands, etc) produce a response.
Synapses
Synapse: a junction between two individual neurons.
Neurones communicate with each other across these junctions, using neurotransmitters
An electrical impulse arrives at the pre-synaptic neuron. This arrival causes calcium ions to move into the
neuron, through calcium channels.
The calcium ions cause vesicles in the neuron to move towards the pre-synaptic membrane. The vesicles
contain neurotransmitters.
Once at the membrane, the vesicles fuse with it, releasing the neurotransmitters (NTs) inside them into the
synaptic cleft.
The NTs diffuse across the cleft to the post-synaptic membrane – here, they attach to receptors on the post-
synaptic neuron.
This triggers sodium channels to open in the post-synaptic neuron, causing sodium ions to enter the neuron.
This allows the electrical impulse to be initiated in the next neuron.
We can also call nerve impulses action potentials.
The junction from a neuron to a muscle fibre is called a neuromuscular junction (NMJ). This works in a very
similar way to a normal synapse – we just need to remember to call it a muscle cell, and not a post-synaptic
neuron!
Reflex Arcs
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