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B5 C) The Reflex Arc
B5 C) The Reflex Arc
In the previous section, we learnt that there are 3 different types of neurones:
The order of the electrical impulses through the different neurones is, sensory neurone first, then relay neurone and finally motor neurone.
The gaps between neurones are known as synapses. There are billions of synapses in the CNS. Each neurone may have thousands of synapses with other neurones. A diagram of a synapse is shown below.
- Sensory neurones – take electrical impulses from receptors to the central nervous system (CNS)
- Relay neurones in the CNS – take electrical impulses from a sensory neurone to a motor neurone
- Motor neurones – take electrical impulses from the CNS to the effector
The order of the electrical impulses through the different neurones is, sensory neurone first, then relay neurone and finally motor neurone.
The gaps between neurones are known as synapses. There are billions of synapses in the CNS. Each neurone may have thousands of synapses with other neurones. A diagram of a synapse is shown below.
The electrical impulse on the above diagram is travelling from left to right. The electrical impulse travels along the neurone on the left until it reaches the end of the neurone. When it reaches the end of the neurone, a chemical is released – this chemical is known as a neurotransmitter. The neurotransmitters diffuse across the gap to the membrane of the next neurone (on the above diagram, the neurotransmitters will diffuse from left to right). The neurotransmitters reaching the membrane of the next neurone results in an electrical impulse being sent along this neurone.
Reflex Actions
A reflex is a very quick involuntary response to a stimulus, which is a useful way to protect ourselves. Reflexes occur because of reflex arcs – a reflex arc is the nerve pathway that results in a fast and involuntary response. For example, a reflex will happen if we were to touch a hot object. When we touch a hot object, the receptor cells in our skin send electrical impulses along various neurones and the end result will be that we move our hand away from touching the hot object.
Reflex arcs go through the spinal cord and the unconscious part of the brain; the conscious part of the brain is not involved in the reflex arc. This is because if the conscious part of the brain was involved, the time between the receptors noticing the stimuli and the response being produced would be considerably longer, which increases the chance of the individual being injured. The conscious part of the brain is informed of what is happening because electrical impulses are also sent to the conscious part of the brain; but the conscious part of the brain is not involved in the reflex arc.
The reflex arc is shown in the diagram below for the touching of a sharp object (a pin).
A reflex is a very quick involuntary response to a stimulus, which is a useful way to protect ourselves. Reflexes occur because of reflex arcs – a reflex arc is the nerve pathway that results in a fast and involuntary response. For example, a reflex will happen if we were to touch a hot object. When we touch a hot object, the receptor cells in our skin send electrical impulses along various neurones and the end result will be that we move our hand away from touching the hot object.
Reflex arcs go through the spinal cord and the unconscious part of the brain; the conscious part of the brain is not involved in the reflex arc. This is because if the conscious part of the brain was involved, the time between the receptors noticing the stimuli and the response being produced would be considerably longer, which increases the chance of the individual being injured. The conscious part of the brain is informed of what is happening because electrical impulses are also sent to the conscious part of the brain; but the conscious part of the brain is not involved in the reflex arc.
The reflex arc is shown in the diagram below for the touching of a sharp object (a pin).
The reflex arc starts with the receptor noticing a change in the surroundings (a stimuli); the receptor cells in the skin on our finger notice that the object is sharp. The receptor then sends electrical impulses along a sensory neurone to the central nervous system (CNS), which is the spinal cord and the unconscious part of the brain. The sensory neurone enters the CNS where it reaches a relay neurone. Neurotransmitters diffuse across the synapse/ gap between the sensory and relay neurone. The neurotransmitters reaching the membrane of the relay neurone initiates an electrical impulse to be sent along this neurone. The electrical impulse travels along a relay neurone until it reaches a motor neurone (there is another synapse). An electrical impulse is then sent along a motor neurone from the CNS to the effect, which brings about a response. For our example, the effector will be the bicep muscle, which will contract. The contraction of the bicep will result in the individual pulling their hand away from the sharp object, thus protecting the individual.
Other Examples
There are many other examples of reflex actions in our bodies.
Another example of a reflex action is in our eyes to make sure that our eyes are not damaged as light levels change. When light levels increase (you go outside on a sunny day or a light is shinned into your eye), a reflex arc causes our pupils to become smaller so that less light is let into your eye, thus preventing your eye from becoming damaged.
Another example of a reflex action is the releasing of adrenaline when we become scared. We will be looking at the effects of adrenaline in a later section.
There are many other examples of reflex actions in our bodies.
Another example of a reflex action is in our eyes to make sure that our eyes are not damaged as light levels change. When light levels increase (you go outside on a sunny day or a light is shinned into your eye), a reflex arc causes our pupils to become smaller so that less light is let into your eye, thus preventing your eye from becoming damaged.
Another example of a reflex action is the releasing of adrenaline when we become scared. We will be looking at the effects of adrenaline in a later section.