5.5.10 Reflex Actions Flashcards
What are reflex actions?
Reflex actions are involuntary responses to certain stimuli
They are very fast and usually have a protective purpose or survival value
Many different reflex actions are carried out by the human body, including:
Yawning
Saliva production
Swallowing
Pulling a part of the body away from a source of pain (sometimes known as the ‘withdrawal reflex’)
Blinking
Constricting the muscles of the iris in response to bright light (the pupil reflex)
Some of these actions, such as blinking, can also be carried out on purpose
In these cases, therefore, they are not reflex actions
Some of these actions, such as the pupil reflex, are always automatic (involuntary)
What are the sequences of the components in a reflex action?
All reflex actions consist of the following sequence:
Stimulus → Receptor → Coordinator → Effector → Response
Reflex actions in response to a change first require detection
Detection involves a stimulus being detected by a receptor cell
There are different types of receptors
Some receptor cells produce electrical activity in nerve cells in response to stimuli
Other receptor cells secrete substances in response to stimuli
The nerve impulses sent by receptor cells travel to a coordinator in the central nervous system
In most reflexes the coordinator is the spinal cord
Some reflexes may involve unconscious parts of the brain, e.g. the medulla in the brainstem
From the coordinator the impulse is conducted to the specific effector that will produce the appropriate response
The response is the action that is carried out and it is always the same for a given stimulus
What is the knee-jerk reflex?
The knee-jerk reflex is often used by doctors to assess whether the nervous system of a patient is working properly or not
Doctors use a specialised hammer to hit a ligament between the knee cap and the tibia, and the leg of the patient will involuntarily straighten in a small kicking motion
The knee-jerk reflex consists of the following sequence of components:
Stimulus - stretching of the quadriceps muscle caused by pressure on the ligament; this pressure is created by the hammer
Receptor - stretch receptors in the quadriceps muscle
Coordinator - the spinal cord
Effector - the quadriceps muscle
Response - contraction of the quadriceps muscle, causing the leg to straighten
In the nervous pathway of the knee-jerk reflex:
The stretch receptors send impulses down a sensory neurone which connects directly, via a synapse, with a motor neurone in the spinal cord
Note that although relay neurones are normally present in the nerve pathways of reflex actions, there is no relay neurone in the knee-jerk reflex
The motor neurone then carries the impulses to the effector (the quadriceps muscle), which contracts
The knee-jerk pathway shows why reflex actions are so fast and automatic
Nerve impulses are delayed by synapses
If these impulses are transmitted via the brain, as occurs in voluntary actions, they have to travel across many synapses
In this reflex action the signal only has to cross a single synapse, allowing for a very rapid response
Connections from the spinal cord to the brain will still allow information about the stimulus (the pressure caused by the hammer) to be sent to the brain but by the time it receives and processes this information, the response will have already occurred
This is why the response happens automatically - the brain has no chance to make a decision
What is the blinking reflex?
Blinking is a reflex response caused by something travelling towards the eye at high speed, something contacting the cornea, or by drying of the cornea
The nervous pathway of this reflex action does, in fact, go via the brain but not via any decision-making areas and the number of synapses is still minimal
In the nervous pathway of the blinking reflex:
Irritation or drying of the cornea sends impulses down the trigeminal sensory nerve to the medulla of the brain, where it connects with other neurones to transmit the signal to the effector muscles
Relay neurones are involved in the transmission of impulses to the effectors in the lower eyelid
The effectors for the blinking reflex include the superior levator palpebrae muscle, which lowers the upper eyelid, and the orbicularis oculi muscle, which pulls the eyelids inwards and helps to close them
