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- Describe a nerve impulse as an electrical signal that passes along nerve cells called neurones
A nerve impulse is an electrical signal that passes along nerve cells called neurones.
- Describe the human nervous system in terms of:
- The central nervous system consisting of brain and spinal cord
- The peripheral nervous system
- Coordination and regulation of body functions
The human nervous system consists of two main parts:
- The peripheral nervous system – this consists of receptor cells (the cells that detect changes in stimuli and send information down the sensory neurones), sensory neurones (the neurones that carry information from receptors) and the motor neurones (the neurones that carry information to the effectors).
- The central nervous system – the main components of the CNS are the brain and the spinal cord.
The role of the CNS is to coordinate messages travelling through the nervous system. When a receptor detects a stimulus, it sends an electrical impulse to the brain or spinal cord, which then sends an electrical impulse to the appropriate effectors.
Examples of receptors include taste receptors (in your taste buds), thermoreceptors (these detect temperature changes and are present in your skin and in the hypothalamus in your brain) and osmoreceptors (these detect changes in the water potential of your blood).
A stimulus is any factor in the environment (light, temperature, etc.) or inside your body (blood sugar, blood water potential, etc.) that changes.
- Distinguish between voluntary and involuntary actions
Two types of actions are controlled by the human nervous system: voluntary actions and involuntary actions.
- Identify the motor (effector), relay (connector) and sensory neurones from diagrams
The part of the neurones that contain the nucleus is called the cell body.
A long cytoplasmic branch stretches out from the cell body – these are called axons. Axons are very long – in fact, there’s actually one that starts in your brain and ends in your big toes! The electrical impulse that neurones transmit sweep along axons.
In some sensory and motor neurones, the axons are insulated by ‘Schwann cells’. These cells wrap around the axon to form myelin sheaths, as a form of insulation. The exposed space between the myelin sheaths are called nodes of Ranvier. As the area under the myelin sheath is insulated, impulses can skip this part of the axon, and instead jump from node to node, allowing them to travel along the axons much faster.
The thin cytoplasmic processes that extend from the cell bodies and carry information towards them are called dendrites. This is true for motor neurones, relay neurones and sensory neurones. Note: dendrites carry information towards the cell body, while axons carry information away.
At the end of every axon, the neurone branches out, and at the end of each branch are the synaptic knobs (they’re labelled as synaptic endings on the relay and sensory neurone diagrams, and the motor neurone diagram doesn’t even mention them. Don’t worry about that, though. We’ll call them synaptic knobs.)
When there are multiple neurones in one ‘pathway’, they don’t actually touch each other – instead, there is a space between them called the synaptic cleft. The neurone membrane before the synaptic cleft (the presynaptic membrane), the synaptic cleft, and the neurone membrane after the synaptic cleft (the postsynaptic membrane) make up the synapse.
When an electrical impulse reaches the end of the neurone (i.e. it reaches the presynaptic membrane), the neurone releases chemical transmitter substances such as acetylcholine. These substances travel along the synapse, allowing the electrical impulse to be transmitted to the postsynaptic neurone.
Also, this isn’t really mentioned in the syllabus either, but I’ve seen related questions in some past papers:
The brain and spinal cord actually have two major parts: grey matter and white matter.
Grey matter contains cell bodies, dendrites, unmyelinated axons (axons without myelin sheaths) and axon terminals. White matter is composed primarily of myelinated axons (axons with myelin sheaths) and contains some blood vessels.
- Describe a simple reflex arc in terms of receptor, sensory neurone, relay neurone, motor neurones and effector
The reflex arc:
Stimulus -> receptor cells (not always present – some sensory neurones can act as receptors themselves) -> sensory neurones -> relay neurones (this part is entirely in the brain or spinal cord) -> motor neurones -> effector -> the reaction
The interneuron in the diagram is the relay neurone.
Note: technically, in a scientific context, the spelling is neuron, not neurone, but Cambridge uses the spelling neurone, and neurone Is the traditional British spelling, so I recommend that you spell neurone with the e.
- Describe a reflex action as a means of automatically and rapidly integrating and coordinating stimuli with the responses of effectors (muscles and glands)
A reflex action is a means of automatically and rapidly integrating and coordinating stimuli with the responses of effectors (muscles and glands)
Notes submitted by Sarah
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