The Reflex Arc (A-level Biology)
The Reflex Arc
Responses Types
Nervous System
- The job of the nervous system is to coordinate responses to certain stimuli using a complex network of neurones. The responses it produces are fast, short acting and localised.
- There are three types of neurone:
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- Sensory neurones carry impulses from the receptors into the CNS (central nervous system).
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- Motor neurones carry impulses from the CNS to effector organs (muscles or glands).
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- Relay neurones receive impulses from sensory neurones and transmit to motor neurones.
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Hormonal System
- The hormonal system can also be used in response to stimuli, and produces a slower, longer acting and more widespread response. Hormones are chemicals that travel through the bloodstream to activate target cells.
Simple Reflex Arcs
Simple reflex arcs are protective in function, and follow the same basic outline:
- A receptor detects a stimulus. A receptor is specific to the type of stimulus.
- The coordinator formulates a response (the spinal cord or the CNS).
- Effectors produce a response. Effectors include glands and muscle cells which respond to the instruction.
Example: Withdrawal Reflex to Heat
You need to know the action of a simple reflex arc involving sensory, relay and motor neurones. A good example is a withdrawal reflex in response to heat.
- A heat stimulus is detected by thermoreceptors in the skin.
- Sensory neurones transmit the impulse to relay neurones (in the spinal cord).
- Relay neurones transmit the impulse to motor neurones.
- Motor neurones transmit the impulse to the effector, in this case a muscle.
- The muscle contracts to remove the body from harm, the response.
Example: Blinking Reflex
- A potentially eye-damaging stimulus is detected by the body.
- Sensory nerve endings in cornea are responsive to touch, and nerve impulses from sensory neurones are sent to relay neurones.
- Relay neurones transmit the impulse to motor neurones.
- Motor neurones transmit the impulse to the effector, in this case the orbicularis oculi muscles.
- Muscles cause eyelids to close and protect the eye from damage.
Example: Knee-Jerk Reflex
- Quadriceps muscle contain stretch receptors which can detect stretch.
- Nerve impulses from sensory neurones are sent directly to motor neurones in the spina cord.
- Motor neurones transmit the impulse to the effector, in this case the quadriceps muscle.
- Muscle contracts, causing lower leg to move forward quickly leading to the ‘knee-jerk reflex’.
The Reflex Arc is the neural pathway responsible for the involuntary, rapid, and automatic response of the body to a stimulus. It is an important topic in A-Level Biology because it helps to understand the basic mechanisms of the nervous system.
The Reflex Arc works through a series of steps that involve a sensory neuron, an interneuron, and a motor neuron. When a stimulus is detected by the sensory neuron, an impulse is sent to the spinal cord, where it is processed by the interneuron. The interneuron then sends an impulse to the motor neuron, which in turn causes a muscle contraction or other appropriate response.
The Reflex Arc is important because it provides a quick and automatic response to potentially harmful stimuli, allowing the body to quickly protect itself. This reflex action is also important in maintaining posture, balance, and coordination.
The different components of the Reflex Arc include the sensory receptor, the sensory neuron, the interneuron, the motor neuron, and the effector (the muscle or gland that carries out the response).
The sensory receptor is responsible for detecting the stimulus and converting it into an electrical signal that can be transmitted to the nervous system.
The sensory neuron is responsible for transmitting the electrical signal from the sensory receptor to the spinal cord.
The interneuron is responsible for processing the signal from the sensory neuron and transmitting it to the motor neuron.
The motor neuron is responsible for transmitting the electrical signal from the interneuron to the effector, which carries out the response.
The speed of the Reflex Arc is important because it allows the body to quickly respond to potentially harmful stimuli, such as withdrawing from a hot object. The speed of the reflex arc can also be used to diagnose certain medical conditions, such as nerve damage.
The Reflex Arc can be modified or inhibited by the brain through the use of higher-order neural pathways. This can allow for more complex responses to stimuli and can also allow for voluntary control of reflex actions. Some reflexes, such as the gag reflex, can also be inhibited by certain drugs or medical procedures.
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