W6: Reflexes 1

Question 1

What is a reflex?

Answer 1

Neural reflexes are stereotyped, involuntary reactions of the CNS to specific sensory input

Question 2

What does involuntary mean

Answer 2

Without conscious thought. Leads to it being rapid

Question 3

What is stereotyped?

Answer 3

If the same action were to be repeated again the same outcome would occur. Characteristic/predictable,

Question 4

Are reflexes somatic or autonomic?

Answer 4

Somatic AND autonomic. Can be either.

Question 5

Clinical relevance of reflexes?

Answer 5

Test whether the nervous system is working. E.g. pupillary reflex, deep tendon reflexes

Question 6

General functions of reflexes

Answer 6

1. Protective
2. Postural control
3. Homeostasis (primarily autonomic)

Question 7

What are the neural components?

Answer 7

Reflex arc:

Sensory receptor - afferent - integration - efferent - effector

Question 8

What are sensory receptors?

Answer 8

Where information comes into the reflex arc.

Part of a sensory afferent neurone or in close association with them.

Question 9

What do the afferent neurones do?

Answer 9

Convey the AP from site of sensory receptor towards the CNS

Question 10

What is the integration centre?

Answer 10

Contains interneurones in the CNS (somatic via the spinal cord, autonomic via the brainstem nuclei) or the enteric NS
Role of modulation. Can be adjusted via interneurones. Can alter inputs and outputs.
Interacts with the other parts of the NS.

Question 11

What are the efferents?

Answer 11

Efferent neurones take information away from the CNS/integration centre to the effector, e,g, somatic alpha motoneurones innervate skeletal muscle.

Question 12

What are effectors?

Answer 12

Glands, muscle (all types)

Question 13

Function of simple stretch reflexes (myotatic)

Answer 13

Adjusting posture - adjusts the degree of skeletal muscle contraction
Sensory receptors = proprioceptors
E.g., Muscle spindles and Golgi tendon organs.

Question 14

What are muscle spindles?

Answer 14

Found within skeletal muscles. Narrow at ends and fatter in middle.
Specialised set of muscle fibres. The nerve endings of the sensory axon is myelinated and wrap around the muscle fibres. Organised so if there is a change in muscle fibres they will detect the change.

Question 15

How can muscle spindles influence activity of a reflex arc?

Answer 15

Pulling on a muscle will stretch it and increase muscle length. If stimulus is large enough so the receptor potential reaches threshold potential an AP is generated. Thus increased sensory activity. Releases excitatory neurotransmitter, exciting the motor neurone, releasing ACh to cause muscle contraction. This prevents damage to prevent muscle tearing.

Question 16

What is a monosynpatic reflex pattern?

Answer 16

Only one synapse between the sensory neurone and the alpha motor neurone efferent.
Neurone to neurone synapses. Muscle spindles are an example.

Question 17

What reflex are muscle spindles involved in?

Answer 17

Knee jerk reflex

Question 18

Describe the knee jerk reflex - sensory receptors

Answer 18

Sensory receptors = muscle spindle in quadricep (front muscle)
Sensory afferent goes upto the spinal cord then branches into 2.
Right hand branch directly connects to a motor neurone which innervates the muscle that the muscle spindle lives in, e.g. quadriceps
Left hand branch connects with an inter neurone, which will integrate information.

Question 19

What is hyperreflexia?

Answer 19

When the information coming down from the brain to influence reflex arcs has been disrupted. And reflexes can work without check from the CNS.

Question 20

What are proprioceptors?

Answer 20

A group of receptors whose job it is to monitor the internal environment of the body. Found in muscles and joints etc. Allows us to get information about the state our muscles are in and how our joints are functioning.

Question 21

Knee jerk reflex - what is the stimulus?

Answer 21

Rubber hammer is gently hit beneath the knee cap/patella, where the tendons which attached the quadriceps to the bone is found. This stretches the quadriceps muscle, thus the spindle stretches, activating the sensory neurone, passing information upto the spinal cord.

Question 22

Knee jerk reflex - what does the right hand branch of the sensory afferent do?

Answer 22

RH branch releases excitatory transmitter which activates the motor neurone. This activates it and thus the motor neurone sends APs back to release ACh onto the quadriceps muscle, contracting it.

Question 23

Knee jerk reflex - what else must happen apart from quadriceps contraction and why?

Answer 23

Muscles in the limbs act in pairs so when the quadriceps contract, the hamstrings must relax to allow movement of the limb in one direction.

Question 24

Knee jerk reflex - what does the left branch of the sensory afferent do?

Answer 24

Action potentials travelling down the LH branch of sensory afferent will release excitatory neurotransmitter, exciting the inter neurone. The interneurone will release inhibitory neurotransmitter, which stops the motor neurones from firing, so relaxes the hamstrings, allowing the leg to kick out.

Question 25

What are Golgi tendon organs?

Answer 25

Within tendons are sensory neuron endings. Tendons are made of collagen fibres which do not contract but can be stretched out.

Question 26

What happens to the tendon when it is stretched?

Answer 26

When a muscle contracts it will pull a tendon tight, straightening the collagen fibres and causing a mild straightening of the sensory nerve ending. The sensory afferent runs away from the tendon and upto the spinal cord.

Question 27

Do Golgi tendon sensory afferents interact with interneurones?

Answer 27

Sensory neurones that innervate Golgi tendon organs always interact with and interneurone, which will interact with the motor efferent.

Question 28

Are Golgi tendon organ interneurones excitatory or inhibitory!

Answer 28

Neurotransmitter released from these interneurones is always inhibitory.

Question 29

What is the integration centre in a Golgi tendon organ reflex?

Answer 29

The spinal cord

Question 30

What does polysynaptic mean?

Answer 30

There is more than one neuron to neuron synapse. Usually have at least one interneurone. E.g., in the Golgi tendon reflex.

Question 31

What can the stimulus be for a Golgi tendon organ?

Answer 31

E.g., a muscle picking up a heavy weight, activating muscle spindles and contracting the muscle. The contracting muscle pulls on the tendon, straightening the collagen fibres and applying mild compression to the sensory nerve endings, activating the Golgi tendon organ.

Question 32

What do sensory afferents coming from the Golgi tendon organ do?

Answer 32

APs within the sensory afferent will run up the axon of the sensory afferent and enter the spinal cord, invading the synaptic terminal, depolarising it and allowing the excitatory neurotransmitter to be released. This will cause the interneurone to fire more action potentials, releasing inhibitory neurotransmitter, so preventing activity in the motor neurone. So the muscle will contract less due to no APs in the motor neurone.

Question 33

What kind of myotatic reflex is the Golgi tendon organ and why?

Answer 33

This reflex arc is a reverse (or inverse) myotatic reflex because the sensory activation results in inhibition of the motor part.

Question 34

What is the functional important of the reverse myotatic reflex?

Answer 34

Prevents damage due to overwork of a muscle, e.g., weightlifting when the weight becomes too damaging to the muscle and it stops contracting Fine control of muscle tension, e.g., small movements needed to write

Question 35

What is the withdrawal reflex?

Answer 35

Activated in response to pain. Involves nociceptive sensory neurones. E.g., crossed extensors reflex.

Question 36

What is the stimulus for the crossed extensors reflex?

Answer 36

E.g., stepping on a sharp object. This activates nociceptors once it penetrates the skin

Question 37

What needs to happen in the crossed extensors reflex?

Answer 37

Skeletal muscle needs to be activated to withdraw the foot away from the pain.

Question 38

Describe the sensory component of the crossed extensors reflex.

Answer 38

Sensory neurones take their course towards the dorsal part of the spinal cord. The sensory neurone branches several times once it enters the spinal cord. One branch interacts with a neurone that carries information upto the brain, bringing about conscious awareness of the pain. The other branches interact with many interneurons.

Question 39

Describe the innervation of the left most two branches of the sensory neurone?

Answer 39

Left hand most branch: first interacts with an excitatory interneurone, resulting in excitation of the next inhibitory interneurone, which will result in inhibition of the motor neurone, so the muscle it innervates relaxes, so the extensor muscle is inhibited. Second left branch interacts with excitatory interneurones, which interacts with an excitatory interneurone, hence exciting the motor efferent, so exciting the flexor muscle.

Question 40

What does the contraction of the flexor muscle do in the crossed extensor reflex?

Answer 40

Allows us to contract the muscles in the leg such that the leg can be picked up and removed from the source of pain.

Question 41

What would happen if only the flexor muscle is contracted and nothing else?

Answer 41

The momentum/walking activity of the person would mean that the person would fall over. So the whole posture of the person must be adjusted, by changing the muscle activity in the left leg, which must be firmly planted into the ground to support the weight.

Question 42

Describe the right 2 most branches of the sensory afferent of the crossed extensor reflex

Answer 42

Information coming in from the right side of the spinal cord (left side of the diagram) must be crossed over to the other side of the spinal cord. 3rd branch: connects to a series of 3 excitatory interneurons. So motor neurones are excited which will pass down to the quadriceps. 4th branch: a pathway of interneurons in which there is an inhibitory transmitter to the motor neurone, so the hamstrings will relax.