Lecture 10 Spinal Cord ,Brainstem , Cortex control of Motor Function Flashcards
Anterior motor neurons
Alpha motor neurons give rise to A alpha(Aa)fibers
Interneurons
- 30X as numerous as anterior motor neurons
- Small and excitable
- Capable of spontaneous activity
- Responsible for most of spinal cord integrative function
Motor unit
- Composed of a single motor nueron and muscle fibers it innervates
- composed of extrafusal fibers
Motor Neuron pool
-Group of motor neurosn taht innervate fibers within the same muscle
Recruitment
- Increase in tension of muscle contractions by the activation of additional motor units(size principle)
- -size principle- states that under load, motor units are recruited from smallest to largest. In practice, this means that slow-twitch, low-force, fatigue-resistant muscle fibers are activated before fast-twitch, high-force, less fatigue-resistant muscle fibers
Small vs Large motor neurons
- small is first to fire , lowest threshold , innervates few muscle fibers , and generate smallest force
- large is last fire , highest threshold, innervates many fibers , generates largest force
Renshaw cells
- Inhibitory cells in anterior horns of spinal cord
- receive collateral branches from alpha motor neurons
- transmit inhibiotry signals to surrounding motor neurosn
- transmit inhibitory signals to same motor neuron–>recurrent inhibition
What type of muscle sensor is muscle spindle?
- Group Ia and II afferents
- arranged in parallel with extrafusal fibers
- detect both dynamic and static changes in muscle length
What type of muscle sensor is Golgi tendon organ
- Group 1b afferents
- arranged in series with extrafusal fibers
- detect muscle tension
What type of muscle sensor is Pacinian corpuscles
- Group II afferents
- detect vibration
What type of muscle sensor are free nerve endings?
- Groups III and IV afferents
- Detect noxious stimuli
Which nerve fiber type is not myelinated?
Group IV
-1,2,3 are myelinated
Details about Muscle spindle
- 3 to 10mm long
- 3 to 12 intrafusal fibers(make up the muscle fibers , which are innervated by alpha motor neurons)
- -innervated by small gamma motor neurons(group II afferents)
- -run parallel to extrafusal fibers
- Central region of spindle has no contractile fibers; functions as a sensory receptor
- -stretching of central region of intrafusal fiber stimuatle sensory fibers
- detect changes in muscle length
- with finer movements ,the nubmer of muscle spindles requried increases
Types of intrafusal fibers in muscle spindle?
Nuclear bag fibers
Nuclear chain fibers
Nuclear bag fibers
- detect rate of chagne in muscle length
- innervated by group 1a afferents and dynamic gamma efferents
- Multiple nuclei located in a central “bag like”configuration
Nuclear chain fibers
- Detect static change in muscle length
- innervated by group II afferents and static gamma efferents
- more numerous than nuclear bag fibers
- multiple nuclei arranged in a single row
Sensory fibers from central region of intrafusal fiber?(muscle spindle)
- Stimulation results from:
- -lengthening of entire muscle
- -contraction of ends of intrafusal fibers
- Type of sensory fibers
- -Ia(primary fibers
- -II(secondary fibers )
- –smaller then Ia and stimulation results in stimulation of a-motor neurons,resulting in contraction and shortening of muscle
Muscle Spindle y motor neurons
- Innervate intrafusal fibers
- Adjust sensitivity of muscle spindle
- coacitvated with a-motor neurons
- -stimulation results from lengthening of entire muscle
Gamma Motor Neurons
Equal to about half the number of alpha motor neurons
Types:
-Ay:supply small intrafusal fibers in middle of muscle spindle
-Gamma-dynamic:excite nuclear bag intrafusal fibers
-Gamma-static: excite nuclear chain intrafusal fibers
