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Chapter 13: Spinal control of movement
Terms in this set (62)
What does the motor system consist of?
All our muscles and the neurons that control them
What are motor programs?
Contained in the spinal cord; they are accessed, executed and modified by descending commands from the brain
How can motor control be divided?
The spinal cord's command and control of coordinated muscle contraction
The brain's command and control of the motor programs in the spinal cord
Brain --> spinal cord --> muscle contraction
What are the 3 major components of the peripheral somatic motor system?
Joints, skeletal muscles, spinal motor neurons
What are muscle cells made of?
What is each muscle fiber innervated by?
One motor neuron
What are the 2 types of muscle in the body?
Smooth: gut (lines the digestive tract, arteries, and related structures and is innervated by nerve fibers from the ANS)
What does smooth muscle play a role in? (2)
Peritalsis (the movement of material through the intestines) and the control of BP/blood flow
What are the 2 types of striated muscle?
Cardiac: heart muscle; contracts rhythmically even in the absence of any innervation; innervation of the heart from the ANS functions to accelerate or slow down the heart rate
Skeletal/somatic: bulk of muscle mass in body; moves bones around joints, moves eyes within the head, controls respiration/facial expression and produces speech
What is each skeletal muscle enclosed by?
A connective tissue sheath that, at the ends of the muscle, forms the tendons
What are muscle fibers?
Cells of skeletal muscle - each is innervated by a single axon branch from the CNS
What makes up the somatic motor system?
Skeletal muscle and the parts of the NS that control them
Describe the somatic motor system
Under voluntary control; generates behaviour
What 3 muscles are flexors of the elbow joint?
Branchialis, biceps branchii, coracobranchialis
What are synergists?
Muscles that work together
What 2 muscles are exensors of the elbow joint?
Triceps brachii, anconeus
What are antagonists?
Muscles that pull on the joint in opposite directions (extensors/flexors)
Muscles can only pull on joints - they cannot push
What are 3 muscle types and what do they do?
Axial muscles: move the trunk; maintain posture
Proximal (girdle) muscles: move the shoulder, elbow, pelvis knee; locomotion
Distal muscles: move the hands, feet and digits (fingers/toes); object manipulation
What are lower motor neurons?
Somatic motor neurons in the ventral horn of the spinal cord that innervate the somatic musculature
What are upper motor neurons?
In the brain; supply input to the spinal cord
What did Sherrington call the lower motor neurons and why?
The final common pathway for the control of behaviour because they are the ones that directly command muscle contraction
What are the 2 types of lower motor neuron?
Alpha and gamma
What do the axons of lower motor neurons bundle together to form?
Ventral roots, each of which joints with a dorsal root to form a spinal nerve that exits the spinal cord (their cell bodies are in the ventral horn)
What do alpha motor neurons do?
Directly trigger the generation of force by muscles
What is a motor unit?
An alpha motor neuron and all the muscle fibers it innervates
What is a motor neuron pool?
All the alpha motor neurons that innervate one muscle
There are 2 ways that the NS controls muscle contraction by alpha motor neurons. Describe them
1) Varying the firing rate of motor neurons
- An alpha motor neuron communicates with a muscle fiber by releasing ACh at the neuromuscular junction in response to an AP, which causes an EPSP in the post; a single post AP causes a twitch (rapid sequence of contraction/relaxation) in the muscle fiber; high-frequency pre activity twitch summation in the post, which causes a sustained contraction as the number/frequency of incoming AP's increases = smooth contraction
- Ripples = slow-input rate; smooth = fast input rate
2) Recruitment of additional motor units
- Henneman's size principle: during contraction, smaller motor units are recruited before larger ones
- This is because small motor units have small alpha motor neurons = better for finer/acute motor control
- Larger ones have more strength = less control
3 sources of input for alpha motor neurons
1) Muscle spindles: monitor muscle contraction strength by providing feedback about muscle length
2) Spinal interneurons: largest
3) Upper motor neurons: in the brain for initiation/control of voluntary movement
What are the 2 types of muscle fibers?
Red (dark): large number of mitochondria and enzymes specialized for oxidative energy metabolism; slow to contract but can sustain contraction for a long time without fatigue (called slow-twitch fibers); found in the antigravity muscles (legs, lungs)
White (pale): few mitochondria and rely on anaerobic metabolism; contract rapidly/powerfully, but also fatigue rapidly (fast-twitch fibers); arm muscles/escape reflex
Each motor unit contains muscle fibers of only a single type. What do fast motor units contain? Slow motor units?
Rapidly fatiguing white fibers (fast-twitch/large-diameter axons); slowly fatiguing red fibers (slow-twitch/small-diameter axons)
Describe the crossed-innervation experiment
Forcing slow alpha motor neurons to innervate a fast muscle causes the muscle to switch to assume slow properties = properties of a muscle are determined by the type of innervation it gets (innervations from a slow motor neuron = slow-twitch fiber)
Describe the AP activity of fast/slow motor neurons
Fast motor neurons: high-frequency bursts (30-60 impulses/sec)
Slow motor neurons: low-frequency activity (10-20 impulses/sec)
What are muscle spindles?
6-8 fibers located within muscle mass itself
-signal length and rate of change of length
- more found in muscles for fine movement
Stretch receptors deep inside skeletal muscle; run PARALLEL with the muscle fibers responsible for contraction; consists of several types of specialized skeletal muscle fibers contained in a fibrous capsule; the middle part of this capsule is where group Ia sensory axons wrap around the muscle fibers of the spindle
(muscle spindles contain intrafusal muscle fibers that are innervated by gamma motor neurons)
What are Ia sensory axons sensitive to? What do they do?
Muscle length (stretch) = proprioceptors (inform us how our body is positioned/moving in space)
Enter the spinal cord via dorsal roots and form excitatory synapses upon interneurons/alpha motor neurons of the ventral horns
Group I axons are the thickest myelinated axons in the body,meaning that they conduct APs very rapidly; Ia axons are the largest and fastest
What is the myotatic reflex?
Stretch reflex: when a muscle is pulled on, it pulls back (contracts) due to sensory input to the spinal cord from Ia axons in the muscle spindle
When does the reflex disappear?
When dorsal roots are cut = the reflex involves sensory input and alpha motor neurons must receive a continual synaptic input from the muscles
What happens when the muscle is stretched by the addition of weight?
Weight placed on muscle = muscle strats to lengthen and muscle spindles are stretched = Ia axons are depolarized and their AP discharge rate goes up = alpha motor neurons depolarized, which causes them to increase their AP frequency = muscle cells activated = muscle contraction = shortening of the muscle
What is an example of the myotatic reflex?
The knee-jerk reflex
What are extrafusal muscle fibers?
Lie outside the muscle spindle and form the bulk of the muscle; innervated by alpha motor neurons
When upper motor neurons command muscle contraction, the alpha motor neurons respond and the extrafusal fibers contract, causing the muscle to shorten
What would happen if muscle spindles went slack? Why does this not happen?
The Ia axons would become silent and the spindle would go off the air, no longer providing information about muscle length; this does not happen because gamma motor neurons are also activated, which innervate the intrafusal muscle fiber at the two ends of the muscle spindle; activation of these fibers causes a contraction of the muscle spindle, keeping the Ia axons active
Activation of alpha/gamma motor neurons has opposite effects on Ia output: alpha activation alone decreases Ia activity and gamma activation alone increases Ia activity
What is another sensor in skeletal muscle that is a source of proprioceptive inputs from the muscles?
The Golgi tendon organ
What does the Golgi tendon organ do?
Monitors muscle tension (force of contraction)
Describe the Golgi tendon organ
Arranged in series with muscle fibers (not side by side) - this is what distinguishes the type of information the 2 sensors provide the spinal cord; innervated by Ib sensory axons (smaller than Ia axons); synapse onto interneurons (some make inhibitory connections with the alpha motor neurons and some make excitatory connections)
What is the reverse myotatic reflex?
Regulates muscle tension within an optimal range (or protects the muscle from being overloaded in extreme circumstances); Ib axons synapse on interneurons in the ventral horn, some of which form inhibitory connections with alpha motor neurons innervating the same muscle = the basis of this reflex
As muscle tension increases, the inhibition of the alpha motor neuron = slows down muscle contraction and as muscle tension decreases, inhibition falls and muscle contraction increases
Important for proper execution of fine motor acts, such as the manipulation of fragile objects with the hands
Explain the clasp-knife reflex
Increased muscle tension = inhibition of the alpha motor neuron, which decreases muscle contraction before muscle damage can occur
Lengthening of muscle = firing rate of Ia goes up and lengthens = flexion of elbow joint
Output of Ia axon makes contact with alpha-motor neuron, as well as an inhibitory interneuron
Interneuron inhibits the alpha-motor neuron
Shortening is unopposed becaus of inhibitory function
The actions of Ib inputs from Golgi tendon organs on alpha motor neurons are mediated by intervening spinal interneurons = most alpha motor neuron input comes from interneurons of the spinal cord
What is reciprocal inhibition
The contraction of one set of muscles is accompanied by the relaxation of the antagonist muscles
What is an example of reciprocal inhibition?
Lengthening of the flexors of the elbow involves contraction of the flexors via the myotatic reflex, but also requires relaxation of the antagonist muscles (extensors) through reciprocal inhibition
This occurs because Ia axons synapse on inhibitory interneurons that inhibit alpha neurons supplying the antagonist muscles
What is an example of a reflex mediated by excitatory interneurons?
Flexor reflex: used to withdraw a limb from a painful stimulus
Pain axons entering the spinal cord activate excitatory interneurons, which excite alpha motor neurons that control the flexor muscles of the limb. Inhibitory interneurons are also recruited to inhibit the alpha motor neurons that control the extensors
An additional recruitment is the crossed-extensor reflex
What is the crossed-extensor reflex?
An example of reciprocal inhibition where activation of flexors on one side of the spinal cord is accompanied by inhibition of flexors on the opposite site;
Activation of extensor muscles and inhibition of flexors on the opposite side used to yank foot up in response to a pain stimulus
Pain signal feeds on an inhibitory interneuron that feeds to extensors = inactive and the flexor reflex is unopposed to extensor serving the same leg
How do neural circuits generate rhythmic patterns of activity such as locomotion?
Using AMPA/NMDA post receptors for glutamate
Describe AMPA/NMDA receptors
AMPA bind glutamate ASAP = depolarization = glutamate + depolarization displaces Mg2+ block of NMDA receptors = can open for Na+/Ca2+
What is a third type of receptor that spinal interneurons possess?
Ca2+-activated K+ channels co-localized with NMDA; once NMDA receptors are activated, the K+ channel opens and there is an outflow of K+ = hyperpolarization
Describe the possible pattern-generating circuit for walking
Walking is initiated when a steady input excites 2 interneurons that connect to the motor neurons controlling the flexors and extensors, respectively
Interneurons generate outputs; their activity alternates because they inhibit each other via another set of inhibitory interneurons
A burst of activity in one interneuron inhibits the other and vice versa
Then, using the circuitry of the crossed-extensor reflex, the movements of the opposite limb can be coordinated for that flexion on one side is accompanied by extension on the other
larger motor neurons innervate muscle fiber faster fatiguable contractile properties (White meat, escape reflex, flight)
smaller motor neurons innervate less muscle fiber with slower, less fatiguable contractile properties (Red meat, posture (can maintain for longer period of time))
it minimizes the development of fatigue by using the most fatigue-resistant muscle fibers most often (holding more fatguable fibers in reserve until needed to achieve higher forces); and
it permits equally fine control of force at all levels of force output (e.g, using smaller motor units when only small, refined amounts of force are required).
smaller MNs produce a larger excitatory post-synaptic potential (EPSP), which reaches threshold sooner, resulting in an action potential. more area- more resistance
small mn/slow twitch
intermediate/fast- fatigue resistant
big mn/fast twitch fatiguable
what is the mytoatic reflex
when muscle is pulled on, tends to pull back (contract)
- stretch reflex
- sensory feedback from muscle shown by cutting dorsal roots (caused it to be eliminated, and muscle tone loss)
Ia discharge goes up when it is stretched, and down when it is slack
reflex chain hypothesis
spinal cord produces rhythmic movement through reflex chain
-sensory feedback from one burst of muscle activity serves as stimulus for next
half center hypotheis
proposes that rhythmic motor activity is generated by reciprocal inhibition between 2 pools of interneurons located on each side of the spinal cord—an extensor half center activating extensor motoneurons and a flexor half center exciting flexor motoneurons
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