Muscle Spindles and Sensory Feedback

Sensory feedback in different sports?

American football
- can rapidly avoid tackle

Wrestling
- can't seen opponent, info from muscle stretch (proprioception)

What is the complex system of the sensory feedback?

Vestibular system gives info about gravity and positioning

GTO give info about force output

Muscle spindles give info about muscle stretch

Cutaneous receptors give info about skin touch, stretch, indentation, vibration

These all give info to brain and help us form responses

What is patient GL?

Deafferentation (destruction of the afferent connections of nerve cells) caused by demyelination of large diameter peripheral nerves responsible for touch and proprioception
- most common is acute inflammatory demyelinating polyneuropathy

- complete loss of touch, vibration, pressure and kinesthetic senses & no tendon reflexes in the four limbs
- pain and temperature sensations were present,
- the motor fibers have not been affected

When they are in the light you can feel what you can touch and reach properly but when in dark reaching everywhere can't tell what you are touching (weak sensations)

What kind of movements do Patient GL make without vision?

Inaccurate, jerky movements with large errors

Intrafusal vs Extrafusal Fibres?

Intrafusal Fibres:
- non-force producing muscle fibres that lengthen & stretch with changes in muscle length
- inside the membrane, deep inside muscle fibres
- only contract at the ends & not in middle because the sensory axons are there
- if contracted in middle the axons are compressed, no AP, & they go limp

Extrafusal Fibres:
- fibres that cause contractions
- are parallel with intrafusal fibres
- outside the membrane

Afferent vs Efferent Axons?

Afferent Axons:
- innervate sensory endings of a muscle spindle & send feedback to the spinal cord
- carry info from sensory receptors to CNS
- 1a, 2b

Efferent Axons:
- of gamma motor neurons that innervate polar ends (contractile) of intrafusal fibres & send excitatory inputs from spinal cords
- carry motor info away from CNS to muscles
- only on ends

Muscle Spindle Anatomy?

Gamma Motor Neurons:
- stimulate intrafusal muscle fibres

Sensory Endings:
- sense length of intrafusal muscle fibres

Central/Non-contractile Region:
- of the muscle spindle

Capsule:
- connective tissue surrounding intrafusal fibres & sensory receptors that compose the spindle

Muscle spindles in different actions?

Stretched:
- causes spindle receptors to increase their firing rates

Relaxed:
- normal and distributed spike discharge of sensory afferent

Contracted:
- muscle shortening causes spindle receptors to reduce their firing rates
- actively contracting without stretching

Mechanically Gated Ion Channels & muscle stretch?

These channels are connected by cytoskeletal strands that pull on them to open channels up so sodium can flow in
- the patch clamp records Na+ current when stretch is applied to membrane


Muscle stretch deforms and open stretch-sensitive ion channels exciting the spindle afferent and causing it to fire action potentials
- sodium enters

Longer and larger stimuli produce larger responses
- greater change in membrane potential, more AP, and release of neurotransmitters

What happens in stretch reflex (knee jerk reflex)?

1) Tendon tap rapidly lengthens the quadriceps muscle.
2. Stretch of quadriceps muscle lengthens intrafusal fibers, causing spindle to fire action potentials.
3. Spindle afferents enter the spinal cord via the dorsal horn.
4. Sensory neuron synapses on and excites alpha motor neuron (AMN).
5. Excitation of AMN causes reflex contraction of quadriceps
6. Sensory neuron also synapses on inhibitory interneuron, which inhibits AMN of antagonist muscle (reciprocal inhibition).
7. Hamstring (antagonist) is inhibited.
8. Information is sent to the brain

this is quickk ^ 20 - 25 ms!

Does contraction of quads lengthen hamstrings and cause stretch reflex?

No due to reciprocal inhibition
- relaxation of muscles on one side of a joint to accommodate contraction on the other side
- inhibition on alpha for hamstrings

Stretch activates spindle afferent
- large enough will cause depolarization

Motor neurons in homonymous muscle begin to depolarize (EPSP)

Inhibitory neuorns is always excitable, EPSP, but sent inhibitory signals

Motor neurons in antagonist are inhibited, IPSP

Latency of neural responses depends on number of synapses involved

How do we know reciprocal inhibition is mediated by a single inhibitory interneuron?

Time difference between the onset of activity in the stretched muscle and inhibition of the shortened muscle

What are synergists?

Act around a moveable joint to produce motion similar to or in concert with agonist muscles

- stretch of agonist muscle produces reflex contraction of agonist (homonymous;
biceps brachii) and synergist muscles (e.g.,
brachialis), and inhibition of its antagonist (e.g., triceps).

What's the purpose of gamma motor neurons?

With GMN's:
- when they are stimulated at same time as alpha MN, the intrafusal fibres contract along with extradusal (skeletal) muscle fibres
- this allows the muscle spindles to keep responding during contractions

Without GMN's:
- if muscle were to contract to a shorter length, the intrafusal fibres would become floppy & the spindle would stop providing feedback about where your limbs are

How do gamma motor neurons help maintain spindle sensitivity to measure unexpected changes in muscle length during voluntary movements?

GMNs stimulate intrafusal muscle fibers. This maintains the length of intrafusal fibers and spindle sensitivity during voluntary muscle contractions/movements.