Simple Reflexes Flashcards
what is a reflex?
Neural reflexes are STEREOTYPED INVOLUNTARY reactions of the CNS to specific sensory input
Stereotyped – predictable and reproducible (if we activated a reflex we could accurately predict the response as it will always be the same)
Involuntary – happens without conscious thought, just happens rapidly
what is used instead of voluntary/involuntary in the context of reflexes?
SOMATIC and AUTONOMIC are the contexts of a reflex and not voluntary or involuntary
Somatic – activating skeletal muscle
Autonomic – activating smooth or cardiac muscle
So i.e. the somatic nervous system through neural reflexes may exhibit an involuntary response.
what is the clinical relevance of reflexes?
give examples
If reflexes are predictable (as they’re stereotyped) it allows a clinician to test a reflex
If the response is abnormal there is a problem with a specific part of the nervous system
E.g. the pupillary reflex – constriction of pupil in response to bright light, caused by smooth muscle contraction so under control of autonomic nervous system.
E.g. deep tendon reflexes – contraction of the skeletal muscle – under control of somatic nervous system.
what are the 3 general functions of reflexes?
Protective
Postural control
Maintaining homeostasis
give examples of when reflexes are used for the purpose of protection
e.g. limb withdrawal (removing hand from hot surface)
e.g cough reflex (stops food entering the lungs –> cause damage and infection)
e.g. prevent skeletal muscle damage (example further along)
May use the somatic and autonomic NS
give an example of when reflexes are used for the purpose of postural control
e.g. when walking you do not have to think about what muscle to contract (involuntary)
so you don’t have to take up conscious thinking activity
Mainly somatic as it uses skeletal muscles
give an example of when reflexes are used for the purpose of maintaining homeostasis
e.g. blood pressure
if blood pressure falls, reflexes will bring it back to the right level.
Mainly autonomic, involved smooth and cardiac muscle
what are the neural components of a reflex arc?
Sensory receptors
Afferents (sensory neurones)
Integration centre (CNS)
Efferent neurones
Effectors
what do sensory receptors do?
Detect stimuli
Generate action potentials
what do afferent (sensory neurones) do?
they allow the AP to flow from the receptor to the CNS
what does the integration centre (CNS) do?
Once the impulse reaches the CNS they pass along interneurons.
Interneurons – where the information from a sensory neurone is passed on to a motor neurone. They can be excitatory or inhibitory.
Interneurons are found in the spinal cord, brain stem nuclei and in the enteric NS (controls the gut).
how does the integration centre integrate info?
They produce outputs depending on the inputs they receive (modulation)
The Reflex arc doesn’t work in isolation but it interacts with the rest of the nervous system (NS -Brian)
e.g. as impulse are sent to your muscles to remove your hand from a hot surface, impulses are also sent to the brain which causes you to feel the pain due to the heat. (what the arrow pointing to NS means)
you can also override reflexes e.g. you know a surface is hot but you are making a conscious decision to hold your hand there. Therefore, impulse from the brain are being sent to the integration centre causing the reflex to be overridden (what the arrow pointing away from NS means)
If there is damage to neurones that take information to or from the (NS-Brian) we will see different activity.
E.g. individuals with Cerebral Palsy have exaggerated contractions as information coming down from the brain to influence reflex arc is being disrupted. So reflex arc works without check – hyperreflexia
where in the body does the somatic and autonomic NS operate via?
most somatic reflexes operate via spinal cord whereas most autonomic reflexes operate via the brain stem nuclei
what do efferent neurones do?
Called “Motor neurones” - somatic reflexes
Called just efferent neurones - autonomic reflexes
Innervate effectors by passing on the AP from interneurons or sensory neurones.
what do effectors do?
E.g. Glands, muscle (all types)
what are muscle spindles?
Found within all skeletal muscle
They are a specialised set of muscle fibres (narrow at each end)
They are made up of nerve endings of sensory axons wrapped around muscle fibres.
If there is change in the muscle fibres, the nerve endings can detect the change.
There is a direct synapse between the sensory neuron and the motor neurone: no interneurons and no modulation potentials in between
describe mechanism 1 of simple stretch reflex mediated via muscle spindle
- Muscle is stretched and elongated (e.g. by a weight) which stretches the muscle spindle
- This generates an action potential (if the stimulus is large enough to reach the threshold value) which travels along to the axon terminal.
- At the synapse the sensory neurone will release excitatory neurotransmitter.
- This excites the motor neurone (increases motor neuron activity) - action potentials travel along the motor neuron back towards the muscle.
- When the AP gets to the NMJ the alpha motor neurone releases ACh
- This leads to the skeletal muscle contracting.
- Contraction prevents damage - prevents tearing of muscle (by the heavy weight) by resisting the stretch. This mechanism also allows us to pick up heavy objects.
describe mechanism 2 of simple stretch reflex mediated via muscle spindle – knee jerk reflex
- The tendon being hit causes stretching of the quadriceps muscle which causes the stretching and therefore activating of the muscle spindle
- This causes an action potential to be sent to spinal cord
- The sensory neurone releases excitatory neurotransmitter at both synapses.
- Excitatory neurotransmitter (ACh) at the α-motor neurone will cause action potentials to be sent to quadricep causing contraction.
- Excitatory neurotransmitter at the interneuron causes the interneuron to releases inhibitory neurotransmitter stopping/reducing action potentials being fired by the hamstring α-motor neurone causing the hamstring to relax.
- This simultaneous contraction and relaxation allow for extension at the knee.
- This causes myotatic reflex (contraction of quadriceps muscle)
explain the mechanism for a Cross Extensor Reflex (withdrawal reflex)
VERYYY LONG
E.g. Someone walking steps on something sharp
1. The sharp object will activate nociceptors once it penetrates the skin. Nociceptors are associated with sensory neurones.
2. AP travels along sensory neurone to the dorsal part of spinal cord
3. In the spinal cord the sensory neurone branches multiple times
4. One of the branches interacts with a neurone which carries information to the brain – allows for the pain experience.
5. Other branches: NB: (black = inhibitor interneurons and red = excitatory)
6. Excitatory transmitter is released via the sensory neuron. This causes an increase in the activity of the excitatory interneuron. This causes the interneuron to release excitatory neurotransmitter. This cause an increase in the activity of the inhibitory interneuron that it is connected to. This causes the inhibitory interneuron to releases more inhibitory neurotransmitter. This prevents the activation of a motor neurone. Therefore, it cannot release ACh. This decreases right quadricep muscle activity so it contracts less/not at all.
7. Excitatory transmitter released via the sensory neuron also causes an increase in the activity of the excitatory interneuron. This causes the interneuron to releases more excitatory neurotransmitter. This causes an increase in activity of the second excitatory interneuron. This causes the interneuron to releases more excitatory neurotransmitter. This activates the motor neurone. Excitatory neurotransmitter (ACh) is released by the α-motor neurone at the NMJ which will cause the right hamstring to contract.
8. This allows for simultaneous excitation of the flexor muscle and inhibition of extensor muscle. This allows flexion at the knee allowing you to move foot away from source of the pain.
9. If this occurred whilst a person is walking, the lifting of the leg would cause them to fall over due to forward momentum. So, we need to change muscle activity of the left leg – it needs to be firmly planted on the ground to allow for stabilisation. So, we need to activate motor neurones on the left side of the spinal cord to influence muscles on the left side.
10. Excitatory transmitter released via the sensory neuron causes an increase in the activity of an excitatory interneuron. This causes the interneuron to releases more excitatory neurotransmitter. This result in the activation of another two excitatory interneurons. The final excitatory interneuron releases excitatory neurotransmitter. This then activates the motor neurone. Excitatory neurotransmitter (ACh) is released by the α-motor neurone at the NMJ causing the left quadricep to contract.
11. Excitatory transmitter released via the sensory neuron will cause an increase in activity of an excitatory interneuron. This causes it to release excitatory neurotransmitter. This activates an inhibitory interneuron. This causes an increase in the activity of the inhibitor interneuron. This causes the interneuron to releases more inhibitory neurotransmitter. This prevents the activation of a motor neurone. Therefore, it cannot release ACh. This decrease left hamstring muscle activity so it relaxes.
12. This allows for extension of the knee at the left side to provide a stable base.
