L8 Proprioception

Question 1

Learning outcomes for the Proprioceptive System

Answer 1

• Name, describe the components of, compare functions of the 2 different sensory receptors and primary afferent axon types responsible for proprioception (MS and GTO)
• Demonstrate an understanding of basic principals of myotactic and reverse myotactic reflex arc.
• Name, describe anatomy of peripheral neural components of proprioception: from the proprioceptive sensory receptors to the dorsal horn of the spinal cord.
• List, demonstrate understanding of projection of the 2 spinocerebellar pathways responsible for subconscious proprioception of:
  
  • trunk and lower body
  • upper body

Question 2

2. The Proprioceptive System measures muscle length and tension. Describe the sensory receptors that monitor these aspects of proprioception.

Answer 2

(red) Muscle spindles (MS) - stretch receptors

• amongst extrafusal muscle (m) fibres
• connective tissue (CT) capsule containing specialised m fibres and various (sensory and motor) myelinated axons
• prevent damage from excessive stretch

(blue) Golgi tendon organs (GTO) - tension sensors

• in tendons amongst fascicles of collagen/CT
• made up of large myelinated axons which split/intermingle with and encircle the collagenous fascicles
• prevent damage from excessive contraction

Question 3

3. Describe the Muscle Spindle receptor components and function.

Answer 3

A capsule(grey) contains 8-10 specialisedintrafusalmuscle fibres (orange) parallel and attached toextrafusal skeletal muscle fibres (green). Because the ends of intrafusals are attached to extrafusals, the MS will stretch with the m whenever this occurs.

Intrafusal fibres - 2 types:

1. Nuclear chain fibres (pink) - narrow, single row of central nuclei
2. Nuclear bag fibres (magenta) - wider, central cluster of nuclei at central bag-like dilation

Afferents - 2 types:

1. Annulospiral endings (blue) - innervate central part of both chain and bag fibres - wrap around like spiral.
   
   • Primary afferents (Ia axons) activated by brief stretch/vibration or sustained stretch of m.
   • tonically active, ensuring optimal muscle length
2. Flow-spray endings - innervate ends of both chain and bag fibres
   
   • Secondary afferents (IIa axons) activated only when sustained stretch of m.

• Both these nerve endings send APs to the SC when the m stretches.
• Primary afferents (1a, annulospiral) synapse with alpha motor neurons (amn’s) which provide motor input to extrafusal m fibres, causing contraction - myotactic reflex

Remember - Muscle Spindles detect and respond both active/passive changes in muscle stretch (length)

Re. gamma mn’s - see card 4.

Question 4

4. What is the function of the gamma motor neurons (gmn’s) in the innervation of the intrafusal muscle fibres (mf’s)?

Answer 4

Gmn’s originate in SC (lateral horn?) and regulated sensitivity to stretch by providing motor input to poles of intrafusal mf’s (MS) to keep them (and Ia axons) ‘tight’ when extrafusal fibres are contracted.

When alpha motor neurons (amn’s) provide motor input to extrafusals, m’s contract/shorten. Amn’s may be excited by type Ia axons (response to stretch) to trigger muscle contraction which slackens the MS.

• Thus, gmn’s provide input to tighten/contract the MS and keep it responsive.

(see attached)

Question 5

5. What happens in the myotactic reflex? Mention the components of the muscle spindle involved.

Answer 5

• Patellar tendon tap pulls on quadriceps (extensor) muscle: stretches extra- and intrafusal muscle fibres
• 1a axons wrapping around intrafusals are activated - send action potentials to spinal cord
• 1a axons directly excite alpha motor neurons in spinal cord - triggers extrafusal muscle fire contraction in stretched quadriceps
• contraction of quadriceps (and relaxation of flexor hamstring; inhibition of extrafusal fibres due to inhibitory interneuron involvement) results in forward kick of leg.
  
  • signifies proprioceptive connections in SC are intact and functioning

Question 6

6. Describe the components of the Golgi Tendon Organ (GTO) and their functions.

Answer 6

Located at junction between muscle and tendon, consists of connective tissue capsule ensheathing mesh of collagen fibres intermingled with 1b primary afferents.

Muscle contraction provides tension to this collagen bundle which squashes intermingling 1b axons, which send a signal to the spinal cord.

GTO’s detect and respond to active changes in muscle tension to protect muscles from excessive contraction.

Question 7

4. How are Golgi Tendon Organs involved in the Inverse Myotactic Reflex?

Answer 7

Muscle contractions shortens muscle belly:

• increases tension along associated muscle tendons and onto the
• *GTOs**
• 1b axons in collagen bundles are compressed, stimulating them to send action potentials to the SC.

In SC 1b axons synapse, excite inhibitory interneurons - release inhibitory neurotransmitter in synapse with amn’s:

• causes dampening/cessation of extrafusal contractions linked to the tensed tendon.

Firing of 1b axons increases with muscle tension = increased inhibition of the amn, to a point where it will cease contraction to ‘drop’ a heavy load that might cause damage to the muscle.

Firing of 1b axons also decreases with decreasing muscle tension, in turn decreasing amn inhibition and allowing more muscle contraction. This helps regulate muscle tension in an optimal range, particularly for fine motor acts eg manipulation of fragile objects with fingers; allows for steady but not ‘crushing’ grip

Question 8

8. Which of the following is correct regarding primary afferents, dorsal root ganglion, roots and the spinal cord?
9. Information from golgi tendon organs and muscle spindles is conveyed to the spinal cord via myelinated type 1a and 1b axons respectively within the mixed spinal nerve.
10. Pseudounipolar axons of proprioceptive primary afferents project towards the spinal cord in the ventral root.
11. Cell bodies of pseudounipolar proprioceptive primary efferents are clustered in the dorsal root ganglion located just outside the vertebral column in the intervertebral foramen.
12. Myelinated axons project into SC and synapse in Laminae (I-VII) of the dorsal horn.

Answer 8

Correct:

4. Myelinated axons project into SC and synapse in Laminae (I-VII) of the dorsal horn. They may also project to higher levels of the neuraxis without synapsing.

Incorrect:

1. MS = 1a, GTA = 1b (To remember that Golgi Tendon Organs are Ib axons, remember that GTO is 3 words, and 3 rhymes with ‘b’.)
2. dorsal root = afferents (ventral root = motor neurons, projecting out of the SC)
3. proprioceptive primary neurons are sensory afferents (not efferents)

Question 9

9. Describe the tracts of conscious proprioception from the spinal cord to the brain.

Answer 9

Information conveyed from type 1a and 1b axons of primary afferent proprioceptors in the periphery to the spinal cord.

1. Primary afferents ascend SC in dorsal column to synapse with second order neurons in the dorsal column nuclei in medulla.
2. Secondary neurons cross to contralateral side of medulla in the medial lemniscus and project to synapse with third order neurons in thalamus.
3. Third order neurons travel to the somatosensory cortex of the cerebrum contralateral to the proprioceptive input.