Chapter 13: Spinal Control of Movement Flashcards
Intro
What is the motor system?
Muscles and neurons that control muscles.
* coordinated movements are produced by spatial and temporal patterns of muscle contractions orchestrated by the brain and spinal cord.
Intro
What is the role of the spinal cord and the brain?
Spinal cord —> Contains motor programs necessary for generating coordinated movements
Brain—> Controls motor programs in spinal cord
Majority of into is descending. Starts in upper neurons then goes down
Intro
What are the two types of muscles and their purposes?
- Smooth: digestive tract, arteries, viscera
- innervated by automic nervous system fiber - Striated:
* Cardiac (heart)
* Skeletal (bulk of body muscle mass)
Intro
What are flexors and extensors?
Flexors and extensors pull on the joint in opposite directions
Intro
What are the 3 locations joints act on and fucntions?
- Axial Muscles: trunk movement (maintains posture)
- Proximal Muscles: Shoulder, elbow, pelvis, kneww movement
- Distal Muscles: Hands, feet, digits (fingers and toes) movement
Upper and lower motor neurons
What are lower motor neurons and their function?
Somatic muscles are innervated by somatic motor neurons in the ventral horn of the spinal cord
**Lower motor neurons are the “final common pathway”
* directly command muscle contraction
* When LMNs are activated, we get muscle contractions and they’re two types: alpha and gamma. **
Upper and lower motor neurons
What are upper motor neurons and their function?
Soma is in the cerebral cortex or brainstem
* **Provides input to LMNs
* Only LMNs command muscles **
Upper and lower motor neurons
How can scientists tell if their is damage to lower or upper motor neurons?
- Upper motor neurons when damaged cannot provide input which leads to a muscle constantly contracting.
- Lower motor neuros when damaged leads to a range of diseases including ALS, not producing muscle contraction at all.
Upper and lower motor neurons
Are skeletal muscles evenly distributed throughout the body?
Naur!
* Limb enlargements
* regions of the cord where innervation of the muscles of the arms and legs occur
* dorsal and ventral horns are swollen
Upper and lower motor neurons
How are lower motor neurons mapped?
- Motor neurons controlling flexors lie dorsal to extensors
- Motor neurons controlling axial muscles lie medial to those controlling distal muscles.
Alpha Motor Neurons & Motor Units
What are the two categories of LMNs in the spinal cord and their purpose?
- Alpha: innervated extrafusal muscle fibers (triggers the generation of force by muscles)
- Gamma: Innervate intrafusal muscle fibers
Alpha Motor Neurons & Motor Units
What is a motor unit?
One alpha motor neuron and all the muscles fibers it innervates
* Muscle contraction is due to the combined action of motor units
Alpha Motor Neurons & Motor Units
What is a motor neuron pool?
All alpha motor neurons that innervate a single muscle
Alpha Motor Neurons & Motor Units
What are the two types of muscle fibers and their characteristics?
- Red muscle fibers
* Large # of mitochondrias, rich in myoglobin, rich in capillary beds
* SLOW to contract
* SLOW twitch - White muscle fibers
* Sparse mitochondria, anaerobic metabolism, contract and fatigue repidly
* FAST twitch
Alpha Motor Neurons & Motor Units
What are the 3 different types of motor units and describe them
1. Slow motor units (s)
* Innervated by small alpha motor neurons
* “red” muscles that contracts slowly and generates small forces.
* Resistant to fatigue
* ex. Marathon
2. Fast fatigue - resistant motor units (FR)
* Intermediate in size - innervated by intermediate size alpha motor neurons
* Generate twice the force of slow motor units
3. Fast Fatigue motor units (FF)
* Large motor neurons - innervated by large aplha motor neurons
* Pale muscle fibers
* Brief exertions that require large forces
* Ex. spinters
All 3 exists in all muscles each motor unit has muscle fiber of one type
Alpha Motor Neurons & Motor Units
What are the two mechanism used to control the force of muscle contraction?
**1. Varying the firing rate of AMNs **
* A single AP in an AMN causes a muscle twitch
* Sustained contraction requires contrinuous stimulation since APs are being contrinuously fired.
* # of and frequency of APd increases
2. Motor neurons are recruited from smallest to largest: Size principle **
**Small motor units have small AMNs, large motor units have large AMNs
* Small motor units have a lower threshold for activation
increas. # of active motor units changes the amt. of force produced.
Alpha Motor Neurons & Motor Units
What are the contractile properties of motor units?
- Single AP triggers contraction strength of differing force & time course in each of the different motor units.
- When the different muscle units experience repeated APs over a longer time period, they show different rates of fatigue.
Alpha Motor Neurons & Motor Units
What did the crossed-innervation experiment tell us?
Muscle phentotype switch can occur by changing the activity in the input motor neuron.
Alpha Motor Neurons & Motor Units
What are the three major sources of input for Alpha motor neurons?
- DRG neurons with axns innervate the muscle spindle
- Upper motor neurons in the motor cortex and brainstem
- Interneurons in the spinal cord - largest input.
Muscle Spindles and the Stretch Reflex
How is the activity of the motor neuron regulated?
- Muscle Spindles (stretch receptors)
- Group 1a sensory axons wrap around the muscle fibers
- contain mechanosensitive ion channels –sensitive to stretch.
Muscle Spindles and the Stretch Reflex
What are the characteristics of muscle spindles?
- Situated in parallel with extrafusal muscle fibers
- 1a sensory afferents wrap around intrafusal muscle fibers and inform CNS about muscle length.
Muscle Spindles and the Stretch Reflex
What is the stretch reflex?
Tapping on the knee lengthens quad - this tests if muscle spindles are reporting
* When a muscle is pulled it tends to contract
- Requires sensory feedback from the muscle
- Stretching a muscle spindle leads to increased activity in 1a afferents
- increase in. activity of alpha motor neurons that innervate same muscle to contract it.
Gamma Motor Neurons
What are gamma motor neurons?
- LMNs that innervate intrafusal inside muscle spindle
- Intrafusal fibers- skeletal muscle within the fibrous capsule
Muscle Spindles and the Stretch Reflex
What would happen without GMNs?
Spindle no longer sensitive to stetch
Golgi Tendon Organs
What are golgi tendon organs?
- Strain gauges
- innervated by 1b sensory axons
- protects muscle from being overloaded
- decreases activation of the muscle when large forces are generated
Golgi Tendon Organs
What is the main difference between muscle spindles and golgi tendon organs?
Muscle spindles: 1a axons encode info abt muscle length
Golgi tendon organs: 1b axons encode info about muscle tension
Golgi Tendon Organs
What is passive stretch?
- Both afferents discharge in response
- golgi tendon organ discharge is less than that of the spindle
Golgi Tendon Organs
What is active contraction?
- The spindle is unloaded»_space; falls silent
- Rate of golgi tendon organ firing increasess
Spinal interneurons, flexor withdrawal reflex, and spinal walking
What are spinal interneurons?
- inhibirtory interneurons plays an essential role in execution of simple reflexes
- Involved in reciprocal inhibition – contraction of one muscle get accompanied by relaxation of the antagonist muscle.
Spinal interneurons, flexor withdrawal reflex, and spinal walking
What do excitatory neurons do?
Excitatory interneurons mediate the flexor withdrawal reflex
- Used to withdrawal a limb from an aversive stimulus
- Slower than the stretch reflex
- Process begins with pain signal from delta A nociceptive axons
Spinal interneurons, flexor withdrawal reflex, and spinal walking
What is the crossed-extensor reflex?
- Used to compensate for extra load imposed by limb withdrawal on the opposite side
- Reciprocal inhibition - activation of flexors on one side is accompanied by inhibition of flexors in opposite side
