Flashcards in Spinal Cord, Brainstem, Cortex Control Of Motor Function Deck (57):
What is a motor unit?
It is composed of a single motor neuron and the muscle fivers it innervates.
It is also composed of extrafusal fibers.
What is a motor neuron pool?
A group of motor neurons that innervate fibers within the same muscle.
What is recruitment?
It refers to the increase in tension of muscle contractions by the activation of additional motor units (size principle).
What are characteristics of small motor neurons?
They innervate a few muscle fibers
They have low thresholds
They fire first
They generate the smallest force
What are characteristics of large motor neurons?
They innervate many muscle fibers
They have the highest thresholds
They fire last
They generate the largest force.
What ar the functions of interneurons?
They are responsible for most of the spinal cord integrative function.
What are renshaw cells?
Inhibitory cells in the anterior horns of the spinal cord.
They receive collateral branches from alpha motor neurons.
Where do Renshaw cells transmit inhibitory signals to?
Surrounding motor neurons.
This results in lateral inhibition and enhances the fluidity of limb movement.
They also transmit inhibitory signals to the same motor neuron, which results in recurrent inhibition.
What do muscle spindle (groups Ia and II afferent) muscle sensors do?
Detect both dynamic and static changes in muscle length.
They are arranged in parallel with extrafusal fibers.
What do Golgi tendon organ (group Ib affterent) sensors do?
They detect muscle tension.
They are arranged in series with extrafusal fibers.
What do Pacinian corpuslces (group II afferents) do?
What do free nerve endings (groups III and IV afferent) do?
Detect noxious stimuli
What are the intrafusal fibers in the muscle spidle innervated by?
Small gamma motor neurons.
They are encapsulated within a sheath to form muscle spindle.
They run parallel to extrafusal fibers.
What are the features of the central region of the muscle spindle?
It has no contractile fibers and functions a sensory receptor.
In a muscle spindle, where do sensory fibers originate from?
A central region.
The stretching of central region of intrafusal fibers stimulates sensory fibers.
What does the muscle spindle do?
It detects changes in msucle length.
What do extrafusal fibers make up?
They make up muscle fibers, which are innervated by alpha motor neurons.
With finer movement, the number of muscle spindles required ___
They are innervated by small gamma motor neurons (group II afferents).
What are two types of intrafusal fibers?
Nuclear bag fibers
Nuclear chain fibers.
What do the nuclear bag fibers do?
They detect the rate of change in muscle length.
What are nuclear bag fibers innervated by?
By group Ia afferents and dynamic gamma efferents.
What do nuclear chain fibers do?
Detect static change in muscle langth.
What are nuclear chain fibers innervated by?
Group II afferents and static gamma efferents.
They are more numerous than nuclear bag fibers.
What does stimulation of sensory fibers result from?
Lengthening of the entire muscle
Contraction of ends of intrafusal fibers.
What are the two types of sensory fibers?
Ia (primary fibers)
II (secondary fibers)
What does stimulation of primary and secondary fibers result in?
Stimulation of alpha motor neurons, resulting in contraction and shortening of muscle.
What do muscle spindle gamma motor neurons innervate?
They adjust the sensitivity of muscle spindle.
What does stimulation of muscle spindle gamma motor neurons result in?
Lengthening of the entire muscle.
Gamma motor neurons are equal to about half the number of alpha motor neurons. What are the types of gamma motor neurons?
A gamma (supply small intrafusal fibers in the middle of the muscle spindle)
Gamma-dynamic (excites nuclear bag intrafusal fibers)
Gamma-static (excites nuclear chain intrafusal fibers)
What brain areas scontrol gamma fibers?
Bulboreticular region of brainstem
When you rapidly stretch or unstretch your muscles, a reflex may occur that opposes sudden changes to muscle length. What is this reflex?
Dynamic stretch reflex.
When lifting something heavy, a reflex causes a degree of muscle contraction to remain relatively constant. What is this reflex?
It prevents jerkiness of body movement (damping)
The stretching of muscle stretches what group of afferent fibers?
Group Ia afferents synapse directly on alpha motor neurons of the same muscle.
During a stretch reflex, Group Ia afferents synapse directly on alpha motor neurons of the same muscle. How does this affect tension?
The muscle contracts and decreases tension on the muscle spindle.
Synergistic muscles are activated and antagonistic muscles are inhibited.
What is the Golgi tendon organ?
An encapsulated sensory receptor through which muscle tendon fibers pass.
It is arranged in series with extrafusal fibers.
What is the Golgi organ stimulated by?
Contracting or stretching of muscle.
This detects muscle tension
The golgi tendon reflex is the ___ of the stretch reflex.
What is the circuitry of the golgi tendon organ?
Tibe Ib afferent -> inhibitory interneuron -> anterior motor neuron
Describe flexion and extension.
Flexsion occurs on the ipsilateral side
Extension occurs on the contralateral side.
What is the afterdischarge the result of?
Persistent neural activity in polysynaptic circuits.
It prevents muscle for relaxing for some time.
What are the functions of the premotor and supplementary motor areas?
They generate a plan for movement, and transfer the plan to the primary motor cortex.
Signals generated here cause more complex patterns of movement than the more discrete pattern generated by the primary motor cortex.
What does the anterior part of the premotor cortex do?
It develops a motor image of the total muscle movement that is to be performed.
What is the supplementary motor cortex responsbile for?
It programs complex motor sequences and is responsible for mental rehearsal for a movement.
The image in the posterior motor cortex excites each sucessive pattern of muscle activity required to ..
Achieve the image.
What does the posterior motor cortex send signals to?
Primary motor cortex
Basal nuclei and thalamus -> primary motor cortex
How does damage to Broca's area affect speech?
It makes word formation difficult.
How is Broca's area related to associated respiratory and voluntary eye movement fields?
Cells in the motor cortex are organized into vertical columns. What is the function of each column?
It stimulates a group of synergistic muscles or even a single muscle.
Where do input signals enter the motor cortex?
Layers 2 - 4
Where are pyramidal cells found in the motor cortex?
The fifth layer
Where do neurons in the sixth layer communicate with?
Other regions of the cerebral cortex.
A typical descending pathway consists of a series of two motor neurons. What are they?
Upper motor neurons
Lower motor neurons
Where do upper motor neurons originate in?
They form descending tracts
Where are upper motor neurons found?
Entirely within the CNS.
Where are lower motor neurons found?
They begin in the CNS.
From the anterior horns of the spinal cord
From brainstem cranial nerve nuclei
What do lower motor neurons make up?
Alpha motor neruons, as well as spinal and cranial nerves.