14. Proprioception

Question 1

What determines what muscle is moved and where?

Answer 1

The amount and area of the motor cortex activated

Question 2

What is Henneman’s size principle?

Answer 2

Smaller motor units are recruited first to minimise fatigue

Question 3

What is proprioception?

Answer 3

The sensing of the bodies position and motion

Question 4

What types of proprioceptors are located in the limbs?

Answer 4

Muscle spindles - give info about changes in muscle length
Golgi tendon organ - provides info about changes in muscle tension
Also joint angle info

Question 5

Where are muscle spindles found?

Answer 5

Throughout the body of the muscle, in parallel with extrafusal fibres (typical muscle fibres)

Question 6

What are intrafusal fibres?

Answer 6

Small, specialised muscle fibres found in the muscle spindle

Question 7

What is the structure of an intrafusal fibre?

Answer 7

Contractile proteins at the ends, with a central region devoid of contractile proteins. Central region wrapped by sensory dendrites of the muscle spindle afferent

Question 8

How are APs triggered in the spindle afferents?

Answer 8

The muscle lengthening opens ion channels on the stretched spindle. AP begins

Question 9

What type of efferent neurons innervate intrafusal fibres?

What type of efferent neurons innervate extrafusal fibres?

Answer 9

Gamma Motor neuron

Alpha motor neuron

Question 10

How are muscle spindle afferents keep responsive regardless of muscle length?

Answer 10

The gamma motor neuron stimulates contraction at the two ends of the intrafusal fibre to keep the central region taut.

Question 11

What is meant by alpha-gamma coactivation?

Answer 11

When extrafusal fibres are stimulated to contract by alpha motor neurons, the gamma motor neuron is simultaneously excited

Question 12

Where are Golgi tendon organs found?

What is their structure?

Answer 12

Located in tendons, in series with the muscle fibres.

Sensory dendrites interweave with collagen fibrils in tendon

Question 13

What activates the Golgi tendon organ afferent?

Answer 13

The contraction of the muscle resulting in collagen fibrils being pulled tight

Question 14

Why does the Golgi tendon organ not detect muscle stretch, only contraction?

Answer 14

Most of the stretch of the muscle is absorbed by the muscle itself, so the Golgi tendon organ is not stimulated

Question 15

What are the four types of mechanoreceptors?

Answer 15

Merkel
Meissen
Ruffini
Pacinian

Question 16

What are slowly adapting fibres?

Answer 16

Respond while the stimulus is present.

Merkel discs and Ruffini cylinders

Question 17

What are rapidly adapting receptors?

Answer 17

Meissner corpuscles and Pacinian corpuscles.

Respond to stimulation with a burst of firing at the beginning and end of stimulation

Question 18

What does the size of the receptive field of a receptor determine?

Answer 18

Its position in the skin. Small receptor fields are located closer to the surface of the skin, eg Merkel in epidermis, Meissner in top of dermis

Question 19

What type of stimulation do each of the mechanoreceptor types respond to?

Answer 19

Merkel - pressure
Meissner corpuscle - flutter
Ruffini cylinder - stretching
Pacinian corpuscle - vibration

Question 20

What is the structure of complex reflexes?

Answer 20

First order neuron - delivers sensations to CNS, ie brainstem
Second order neuron - delivers to higher level CNS, ie thalamus
Third order neuron - carries sensory info to appropriate area of the cerebral cortex

Question 21

Why does dizziness occur?

Answer 21

Vestibulo-ocular reflex gone wrong. Sensory cells fire after stimulus stops and cannot tell which way is up