BIO 302 Lecture 12 Spinal Control of Motor System

Describe the flexor reflex using stepping on a tack as an example.

1. We step on a tack, which activates the pain axon, entering through the dorsal root of the spinal cord. 2. This axon synapses on an excitatory interneuron in the spinal column 3. The signal is then sent to the alpha motor neuron that projects to the flexor muscle 4. The signal is then sent for the leg to lift off the tack

Describe the crossed-extensor reflex as it relates to stepping on a tack

1. When we step on a tack, the pain is activated, which sends a signal to the synapse of an excitatory interneuron that activates the alpha motor neurons that contract/activate the flexors (as described in the flexor reflex) 2. At the same time, the signal synapses on inhibitory interneurons that inhibit the extensors, which allows us to lift the foot off the tack 3. At the same time, the signal synapses on other excitatory interneurons that cross over to the contralateral part of the spinal cord, where we see the reverse occur in the other foot 4. So basically at the same time in the other foot, the signal synapses on an excitatory interneuron that ACTIVATES the EXTENSOR and synapses on inhibitory interneurons that INHIBIT the FLEXORS (decreasing the firing rate of APs)

Describe the process of the 1A sensory axons/Myotatic reflex when weight is added to a muscle

1. add weight to the muscle, increasing the load 2. this causes increase in the length of the muscle (stretch) 3. Sensed by muscle spindles, causing 1A sensory axons to increase firing rate as muscle is stretched in the dorsal horn 4. Will then synapse on alpha motor neuron that goes to the same muscle (in ventral horn), causing increase in firing rate of alpha motor neuron 5. Causes muscles to contract/shorten = decreases stretch of muscle 6. Decreases firing of 1A sensory neuron 7. decreases firing of alpha motor neuron

Describe the reaction of the central pattern generator

1. the excitatory interneurons release Glutamate and have CA2+ activated K+ channels that will lead to rhythmic firing 2. Will then activate a flexor motor neuron + lead to rhythmic contraction of flexor neuron 3. When the flexor neuron is activated, it also wants to inhibit the extensor 4. This is achieved when excitatory interneurons fire, also activating inhibitory interneurons that act on extensor motor neurons (when this fires, the extensor doesn't, vice-versa) 5. Inhibitory interneurons inhibit themselves through feedback

What kind of axons wrap the muscle fibers?

1A sensory axons

What are the differences between 1a sensory axons and 1b sensory axons?

1a = larger, help innervate the muscle spindles to prevent stretching 1b = slightly smaller, help innervate the golgi tendon organ to prevent too much contraction/tension

What part of the spinal cord do the 1a, 1b, and alpha motor neurons travel?

1a and 1b go through the dorsal root, whereas alpha motor neurons go out the ventral root

What innervates the golgi tendon organ?

1b sensory axons

Where is the mechanism for the central pattern generator?

ALL contained in the spinal cord (NOT THE MUSCLE) and can occur without input from sensory organs but cat is able to adjust through this reflex

What do muscle spindles detect?

Changes in the length of the muscle to determine the position of body parts

What do 1A sensory axons do?

Detect changes in muscle length

What transmitter does the 1b sensory neurons/inhibitory interneurons use?

GABA (inhibitory)

Describe what happened to IW (Ian Waterman)

He lost all sensations of touch, proprioception, limb spatial position below the neck due to a virus that attacked sensory neurons able to walk if he has visual contact with his limbs at all times

Do muscle spindles contribute to contractile strength of the muscle?

Nope, but they ARE telling us the amount of stretch on the muscle

What is the golgi tendon organ sensitive/not sensitive to?

Sensitive to increase in muscle tension from ACTIVE muscle contraction, but relatively insensitive to PASSIVE stretch

Describe the knee jerk reflex

Tapping on a tendon of the quadriceps causes the muscle to stretch, which activates the muscle spindle of the muscle, causing firing of 1A axons, thereby activating the alpha motor neuron which fires to cause a contraction in the muscle it's also monosynaptic (one synapse between the 1A sensory axons and alpha motor neurons)

Differentiate the 1a, 1b, alpha motor neurons, and gamma motor neurons

The 1a and 1b axons carry information out of the muscle to the dorsal root of the spinal cord, whereas the alpha motor neuron and gamma motor neuron carries information from the ventral root to the muscle. 1a sensory axons are affected by activation of the muscle spindle and 1b sensory axons are affected by activation of the golgi tendon organ (b/w tendon and muscle). They send signals from these areas to the spinal cord. 1a sensory axons synapse directly at alpha motor neurons which activate them when the muscle is being stretched, allowing the alpha motor neurons to send a signal for the muscle to contract and gamma motor neurons to signal the muscle spindles to contract. 1b sensory axons synapse at an inhibitory interneuron which synapses at the alpha motor neuron, causing inhibition of the alpha motor neuron, causing it to stop sending signals to the muscle to contract, also inhibiting the gamma motor neurons to stop contracting the muscle spindles, therefore relaxing the muscle.

Describe the process of how the alpha motor neurons and gamma motor neurons are activated and affect 1A sensory neurons

When the muscle is stretched, 1A sensory axons send APs/signal to the alpha motor neurons that will contract the extrafusal muscle fibers. However, this causes the poles of the intrafusal muscle spindles to lose tension. To fix that, gamma motor neurons are activated which cause the poles to contract, fixing the tension of the intrafusal muscle spindles so everything is back to normal. By fixing this, the gamma motor neurons allow the 1A sensory axons to fire more rapidly again, ready to respond to another stretch

Describe how muscle contraction activates the Golgi tendon organ, causing the slowing of muscle contraction

When there is increased alpha motor neuron activity due to the stretching of a muscle, these neurons release ACh which causes the muscle to contract as a response. This contraction activates the Golgi tendon organ, which wants to relax the muscle. So, it sends a signal to the 1b sensory axons to inhibit the alpha motor neuron (happens at synapse between inhibitory interneuron and alpha motor neuron) and slows the muscle contraction.

What do alpha motor neurons and gamma motor neurons do (simply) to 1A sensory output?

alpha motor neurons DECREASE 1A sensory output (since they contract the outside of the muscle) gamma motor neurons INCREASE 1a sensory output (since they contract the poles of the inside of the muscle, restoring it to normal)

Where is the golgi tendon organ located?

at the junction between the muscle and the tendon

What does activation of the gamma motor neurons cause?

contraction of the poles of the spindle

What type of spinal interneurons are there?

excitatory and inhibitory

What does the reverse myotatic reflex help with when related to real life?

helps muscle maintain constant force (picking up a brick vs. paper cup, we need feedback to understand how much force we're using)

Describe reciprocal inhibition of muscle groups

if a muscle is stretched, the 1a sensory axon activates the alpha/gamma motor neurons to contract the extrafusal/intrafusal muscle fibers/spindles. But we need to cause the antagonizing muscle to be inhibited, which happens through inhibitory interneurons. So basically, when we contract a flexor we need to relax an extensor, and vice-versa

What is a muscle spindle?

includes sensory receptors in muscles (stretch receptors) that are parallel to muscle fibers

What do gamma motor neurons do?

innervate intrafusal muscle fibers at the two ends of the spindle

What are the three major sources of input to alpha motor neurons?

input from spinal interneurons, sensory input from muscle spindles, input from upper motor neurons in the brain

Crossed-extensor reflex

involves both excitatory + inhibitory interneurons, for example as we lift our foot off the tack (contracting flexors, relaxing extensors), we want the opposite foot to do the opposite in order to stabilize it

What happens to the 1A sensory axons' AP frequency when the muscle is stretched?

it INCREASES AP frequency (preventing overexertion!)

Describe UofL research on spinal cord injuries

man had some movement, some feelings in his legs they implanted electrodes on the surface of the spinal cord Electrodes were stimulated, causing voluntary leg, foot, toe movement, full weight-bearing standing, some stepping like patterns but ONLY during electrode stimulation

Central Pattern Generator

mechanism that sets up rhythmic patterns like walking

What is the role of the golgi tendon organ?

monitors muscle tension

Describe the flexor reflex

multiple neurons involve excitatory interneurons, although it is slower than myotatic reflex

Contrast the myotatic reflex and the reverse myotatic reflex

myotatic = monosynaptic, reverse myotatic = polysynaptic myotatic = prevents the muscle from stretching too much by contracting it, reverse myotatic = prevents the muscle from contracting too much (too much tension) by relaxing it

Where do 1b sensory axons synapse?

on inhibitory interneurons which synapse to the alpha motor neurons that innervate the same muscle at the same place as 1a sensory axons they then travel to the golgi tendon organ in the muscle next to the tendon

What do muscle spindles regulate?

regulate contraction of the muscles via the stretch reflex to resist muscle stretch, thus preventing overexertion

What is the Myotatic reflex?

stretch reflex (Myo = muscle, tatic = stretch)

What happens as striated (skeletal) muscle are contracting due to alpha motor neurons releasing ACh?

the gamma motor neurons fire at the same time to maintain the sensitivity of the muscle spindles, causing the poles of the spindles to contract

What is an example of this Myotatic reflex?

the knee jerk reflex

Compare and contrast in more detail the muscle spindle and the golgi tendon organ

the muscle spindle is sensitive to stretch, golgi tendon = sensitive to tension (contraction) muscle spindle = myotatic reflex, golgi tendon = reverse myotatic reflex muscle spindle = maintain constant length, golgi tendon = maintain constant force muscle spindle = monosynaptic (1a sensory), golgi tendon = polysynaptic (1b + inhibitory interneurons)

What is the main difference between the muscle spindle and golgi tendon organ?

the muscle spindle monitors stretch of the muscle golgi tendon organ monitors tension of the muscle

Describe the features of 1A sensory axons

they are the largest axons + are myelinated (conduct APs very quickly due to lower internal resistance, faster conduction velocity)

How are alpha motor neurons and gamma motor neurons related to 1A output?

they have OPPOSITE effects on 1A output

Where do 1A sensory axons project to?

to lower motor neurons in the spinal cord with alpha motor neurons that innervate the same muscle

What is the role of gamma motor neurons?

to maintain sensitivity of the muscle spindles during contraction