Muscle Receptors And Spinal Reflexes

Question 1

Where are the cell bodies of lower motor neurones located?

Answer 1

Spinal cord (lamina IX)
Cranial nerve nuclei eg facial nucleus

Question 2

What is the effect of most upper motor neurones? Inhibitory or excitatory?

Answer 2

Inhibitory 
Only one (normally inhibitory) can predominate over the lower motor neurones at one time

Question 3

What controls the lower motor neurones?

Answer 3

The upper motor neurones, normally have an inhibitory effect

Question 4

What allows limb movements to occur?

Answer 4

When the cortex gives permission for movements to occur by removing inhibition on a segment of the spinal cord - results in voluntary movement

Question 5

What allows there to be motor tone in a motionless but conscious individual?

Answer 5

They have a large input from the descending inhibitory neurones
But the lower motor neurones still have sufficient output to cause motor tone

Question 6

During deep sleep, which are the only muscles not to be paralysed by descending inhibition?

Answer 6

Breathing muscles

Extra-ocular muscles

Question 7

What does addition of extra interneurones (to produce disynaptic, trisynaptic etc. reflexes) allow?

Answer 7

Greater control to the finer movements of the body

They are also inhibited by the descending tracts

Question 8

What is a spinal reflex?

Answer 8

An involuntary, unlearned, automatic reaction to a specific stimulus that does not require the brain

Question 9

What are the five components of a reflex arc?

Answer 9

A receptor eg muscle stimulus

An afferent fibre eg muscle spindle afferent

An integration centre eg lamina IX of the spinal cord

An efferents fibre eg α-motor neurone

An effector eg muscle

Question 10

What are the two types of lower motor neurones and what do they innervate?

Answer 10

α motorneurones: extrafusal fibres (normal skeletal muscle fibres)

γ motorneurones: intrafusal fibres (found in muscle spindles) - causes contraction of the poles of the intrafusal fibre, stretching the central zone and activating the peripheral process of the afferent neurone

Question 11

Differences in structure between α and γ motorneurones?

Answer 11

α:

• myelinated
• large cell bodies
• α conduction velocity

γ

• small cell bodies
• γ conduction velocity

Question 12

Where are muscle spindles found?

Answer 12

In skeletal muscles

More numerous in muscles that control fine movements

Question 13

What does each muscle spindle consist of?

Answer 13

A connective tissue capsule in which there are 8-10 intrafusal fibres - known as a fusical

Efferents innervation provided to polar ends of intrafusal fibres innervated by gamma motorneurones

Therefore, muscle spindles detect changes in length of muscle

Question 14

Function of muscle spindles?

Answer 14

Detect muscle length

Question 15

Function of Golgi tendons?

Answer 15

Detect tension in tendons - activated by contraction or stretching of the muscle

Question 16

Why do gamma neurones need to innervate muscle spindles?

Answer 16

To keep them taut

Allows firing of alpha neurones to continue without descending inhibition allowing muscle tone when at rest

Question 17

What is the structure of the Golgi tendon organ?

Answer 17

A large unmyelinated fibre that enters a connective tissue capsule and subdivided into many unmyelinated receptor endings that intermingle and encircle collagenous fascicles

Question 18

What is motor tone produced by?

Answer 18

Tonic contraction of lower motor neurones to their muscle fibres

Question 19

What prevents muscle fibres from becoming fatigued in muscle tone?

Answer 19

Fibres contract randomly to produce sufficient tone

Question 20

What happens when tone in a muscle needs to be increased?

Answer 20

Get an orderly recruitment pattern caused by size principle

• smaller motor units are recruited first
• larger ones recruited last

Question 21

What is the reflex when providing muscle tone?

Answer 21

Feedback from muscle spindle afferents
Causes reflex contraction of the muscle that that spindle innervates
Allows for muscle tone and an ability to judge passive displacements

Question 22

Do babies have muscle tone?

Answer 22

No - if it does not develop, is a sign of brain injury

Question 23

What is the reflex seen in the knee jerk response?

Answer 23

Myotatic/stretch reflex

Question 24

What happens in the stretch reflex?

Answer 24

Afferent receptor: muscle spindle

Fast-conducting, large, myelinated axons (group 1a afferents) provide the afferent response

Muscle is stretched, action potentials are produced by the muscle spindle due to deformation

They synapse directly with α-motorneurones in the spinal cord

α-motorneurones innervate the extrafusal fibres causing contraction of the homonymous muscle

Question 25

What role does inhibition have in the stretch reflex?

Answer 25

An excitatory synapse is made with an inhibitory interneurone in the spinal cord

This makes an inhibitory synapse with the α-motorneurones innervating the antagonist muscle, inhibiting it

Question 26

What happens in the flex ion/withdrawal reflex?

Answer 26

Nociceptors stimulate by a noxious stimulus

Causes impulses to be conducted through myelinated afferent fibres (group 3) and unmyelinated afferent fibres (group 4)

They synapse directly with the α-motorneurones in the spinal cord (3 or 4 interneurones are normally involved) resulting in contraction of the ipsilateral flexor muscles

Causes withdrawal of the limb from noxious stimuli

Question 27

What is the sign that shows an impaired withdrawal reflex?

Answer 27

Babinski - upper motor neurone lesion

Question 28

Difference between cortical efferents and brainstem/bulbar efferents in upper motor neurones?

Answer 28

Cortical efferents have cell bodies in the cerebral cortex, know as pyramidal motor neurones

Brainstem/bulbar efferents have cell bodies in the sub-cortical areas, known as extra-pyramidal neurones