Spinal Reflexes

Question 0

What are the two main types of responses that reflexes produce?

Answer 0

1. Protective - e.g. Withdrawal of a limb from a hot surface or a cough reflex.
2. Postural control - so that you don’t fall over when you move a limb.

Question 1

What are reflexes?

Answer 1

They are rapid, automatic, involuntary reactions if the CNS to a specific sensory unit.
They produce a rapid characteristic response to a stimulus.
They are different to voluntary, rhythmic motor patterns (e.g. walking) or learned experience.

Question 2

What are the two main types of reflexes?

Answer 2

1. Somatic reflexes - involve somatic motor neurons and skeletal muscles.
2. Autonomic reflexes - responses are controlled by autonomic neurons.

Question 3

What is a reflex arc?

Answer 3

A reflex arc is a somatic reflex. Somatic motor neurons supply skeletal muscles, the components if smelts, muscle reflexes form the reflex arc which is the most basic unit of integrated activity in the nervous system.

Question 4

What are the components of a reflex arc?

Answer 4

It involves a receptor, sensory afferent neuron, central neuron(s), motor efferent neuron and an effector.

Question 5

What are receptors?

Answer 5

They detect stimuli including light, sound, smell, pain and muscle tension. They act as signal transducers by converting the energy of a stimulus into electrical signals.

Question 6

Where are receptors located?

Answer 6

On the surface of the skin - e.g. nociceptors which detect painful stimuli or thermoreceptors which detect changes in temperature.

Within the muscle: the muscle spindles which are receptors that detect changes in muscle length or the Golgi tendon organs which detect changes in muscle tension.

Question 7

What is the function of a sensory afferent neuron?

Answer 7

It carried information from the receptors to the CNS. The main types of sensory afferent neurons are group 1a and group 1b.

Question 8

Where are central neurons/neurones?

Answer 8

Sensory afferent fibres enter the spinal cord and synapse onto either two of the central neurons:

1. Interneurons - e.g. Ia and Ib interneurones or
2. Cell body (soma) of efferent motor neurons.

Question 9

Name the two types of motor neurons in a spinal reflex and name where they are located.

Answer 9

1. Alpha motor efferent neurons - located in the extrafusal (bulk of the muscle) or
2. Gamma motor efferent neurons - located in intrafusal fibres of muscle spindles.

Question 10

Give two main examples of skeletal muscle reflexes.

Answer 10

1. Mono synaptic - e.g. stretch reflex.

2. Polysynaptic - Golgi tendon reflexes, crossed-extensor reflex

Question 11

Describe what the muscle spindle is and the nerves that innervate it.

Answer 11

The muscle spindle detects muscle length changes.
It is a specialised skeletal muscle fibre(s) contained in a fibrous capsule.
Group 1a sensory afferents wrap around the swollen middle of the capsule.

Question 12

Describe the role of the muscle spindle.

Answer 12

Any movement that increases muscle length also stretches the muscle spindle and causes it’s sensory fibres (group 1a) to fire rapidly (e.g. Placing a load on a hand stretches the muscle and the spindles).

Question 13

Why does the stretching of the muscle spindle cause a reflex action/stretch reflex?

Answer 13

To prevent the muscle from over-stretching a reflex contraction of the muscles is produced to restore the arm position.

Question 14

Describe the components of the stretch reflex and how they are linked.

Answer 14

The muscle spindle is the receptor which is connected to a group 1a sensory afferent fibre which sends signals to the CNS and the alpha motor neuron which originates from the spinal cord sends signals to the effector (the muscle) to carry out an action (contract).

Question 15

Describe the stretch reflex.

Answer 15

The muscle spindle stretches in response to a stimulus
This increases sensory activity as the group 1a sensory afferent fibre fires more rapidly.
This also causes the alpha motor neuron to fire action potentials causing the muscle to contract.

Question 16

What is the Golgi tendon organ? Where is it located? Describe it’s function.

Answer 16

It is another type of receptor in the muscle just like the muscle spindle.
It is located at the junction of the muscle and tendon.
It acts like a strain gauge and monitors tension. It is innervate by group 1b sensory afferent fibres.

Question 17

What is the role of the Golgi tendon reflex?

Answer 17

It prevents damage due to overwork.
It protects the arm from heavy loads by causing the arm to relax and drop the load.
It slows down muscle contraction.

Question 18

Describe the process of the Golgi tendon reflex?

Answer 18

1. When the muscle contracts, the tension of the muscle and tendon increases and the Golgi tendon organ detects this.
2. The Golgi tendon organ is activated and the group 1b sensory afferent fibres increase their firing of action potentials.
3. The inhibitory interneurons are stimulated (via synapses between the sensory afferent neuron) and they fire action potentials to the motor neuron via a synapse.
4. Motor neuron activity decreases so the motor neurons fire fewer action potentials and muscle contraction slows down.

Question 19

Describe the process of the Golgi tendon reflex?

Answer 19

1. The Golgi tendon organ monitors an increase in muscle and tendon tension.
2. The group 1b sensory afferent fibre fires action potentials.
3. The motor neuron is inhibited.
4. The muscle relaxes.
5. The load is dropped.

Question 21

Describe the process of reciprocal inhibition?

Answer 21

Knee jerk reflex/patellar tendon reflex.
Contraction of the quadriceps muscle:
1. Stimulus - tap to the tendon
2. Receptor - muscle spindle stretches
3. Sensory afferent (group 1a) fires action potentials
4. Alpha motor neuron (connected to sensory afferent via synapse) fires action potential.
5. Muscle contracts.

Relaxation of hamstring muscle:

1. The sensory afferent (group 1a) fibre this time fires action potentials to the INHIBITORY INTERNEURON in the spinal cord which inhibits the somatic neuron.
   2: hamstring stays relaxed allowing the leg to extend.

Question 22

Which reflexes are slower? Mono synaptic or polysynaptic reflexes?

Answer 22

Polysynaptic reflexes are slower.

Question 23

What are the components involved in complex stretch reflexes (polysynaptic reflexes)?

Answer 23

Interneurones (inhibitory and excitatory)
Renshaw cells
The receptor and effector, unlike monosynaptic reflexes, can be in different parts if the body.
For example, the flexion (withdrawal) reflexes are polysynaptic reflexes causing the arm to move away from a painful stimulus like a pinprick or hit stove.

Question 24

Describe the process of a flexion/withdrawal reflex.

Answer 24

1. Painful stimulus is received by nociceptors in the skin (e.g. Due to a pin prick or hot stove).
2. Sensory fibres carry information from the nociceptors to interneurones in the spinal cord.
   Some of the interneurones excite alpha motor neurons leading to the contraction of flexor muscles.
   Other interneurones activate inhibitory interneurones that cause the relaxation of antagonist muscle groups (extensors) so that the limb is flexed, withdrawing it from the stimulus.

Question 25

What is the crossed-extensor reflex?

Answer 25

The quick withdrawal of the right foot for a painful stimulus is matched by an extension of the left leg to support the sudden shift in weight.
This is produced by the action if the crossed-extensor reflex - a postural reflex that helps maintain balance when one foot is lifted off the ground.

Question 26

What does the crossed-extensor reflex involve?

Answer 26

Whilst the withdrawal reflex acts on the flexor and reciprocal inhibition of the extensors on the right leg, the crossed-extensor reflex ensures the INHIBITION of flexors and ACTIVATION of extensors on the left leg.

This involves sensory input to multiple excitatory and inhibitory interneurones.

Question 27

What type of neurons control the most complicated spinal reflex pathways?

Answer 27

They are controlled by networks of neurones in the CNS called central pattern generators (CPGs).

Once they are activated, they create spontaneous reputation movement.

In humans, rhythmic movements controlled by CPGs include locomotion (ability to move from one place to another) and the unconscious rhythmicity of quiet breathing.

Question 28

Which higher centre of the brain control movement?

Answer 28

The motor cortex controls planning and coordinating of complex movements.

The brain stem controls posture, hand and eye movements.

The basal ganglia controls motor planning.

The cerebellum controls the adjustment of fine movements.

Question 29

What is reciprocal inhibition?

Answer 29

In addition. To the excitation of the effector muscle during the stretch reflex. The antagonist muscle is inhibited at the same time.
This is achieved by reciprocal inhibition caused by the activation of the Ia inhibitory interneuron.