Lecture 9 - Spinal Cord and Brainstem Control of Motor Function I Flashcards
Groups of Neurons in Spinal Cord
Sensory (afferent)
Anterior motor neurons (efferents)
Interneurons
Renshaw Cells
Sensory Neurons
afferent
Anterior Motor Neurons
efferent
Alpha motor neurons give rise to A alpha (Aα) fibers
Interneurons
- 30x as numerous as anterior motor neurons
- Small and highly excitable
- Capable of spontaneous activity
- Responsible for most of spinal cord integrative function
- Bipolar (shape)
- Most are inhibitor –GABA is neurotransmitter
- Excitatory –Glutamate is neurotransmitter
Renshaw cells
- Inhibitory cells in anterior horns of spinal cord use glycine as a neurotransmitter
- Receive collateral branches from alpha motor neurons
- May be supplied by more than 1 alpha MN and may synapse on multiple MNs.
- Transmit inhibitory signals to surrounding motor neurons:
- Results in lateral inhibition
- Enhance fluidity of limb movement
•Transmit inhibitory signals to same motor neuron:
•Results in recurrent inhibition
negative feedback
Motor neurons (efferent):
- Cell body is in the motor cortex, brainstem or spinal cord
- Axon projects to SC or to effector organ/muscle
- Upper motor neuron synapse onto interneurons in SC or to lower motor neuron
- Lower motor neuron go from spinal cord to effector
- Lower motor neurons are classified as alpha, beta or gamma
Upper Motor neuron:
- Descends the spinal cord to level of the appropriate spinal nerve root
- Synapses with lower motor neuron or interneuron
- Neurotransmitter of UMN to LMN is glutamate, which binds to glutamatergic receptor
- Are entirely within the CNS.
- Originate in: Cerebral cortex, Cerebellum, Brainstem
Lower Motor neuron
- Typically found in anterior gray portion of SC or cranial nerve nuclei of brainstem
- Terminate at effector with acetylcholine as neurotransmitter
- Cranial nerves are unique LMN
- Begin in CNS.
- From anterior horns of spinal cord.
- From brainstem cranial nerve nuclei.
- Made up of alpha motor neurons (A-α).
- Make up spinal and cranial nerves.
Motor unit:
- Composed of a single motor neuron and the muscle fibers it innervates
- Composed of extrafusal fibers
Motor neuron pool:
Group of motor neurons that innervate fibers within the same muscle
Recruitment
Refers to the increase in tension of muscle contractions by the activation of additional motor units (size principle)
Small motor neurons
- Innervate a few muscle fibers
- Lowest thresholds
- Fire first
- Generate smallest force
Large motor neurons
- Innervate many muscle fibers
- Highest thresholds
- Fire last
- Generate largest force
Ia Inhibitory Interneurons
- Inhibits antagonistic muscle
- Signal from muscle spindle
- Avoids spasticity
Ibinhibitory interneurons:
- Autogenic inhibition reflex
- Golgi tendon activated with stretch
- Connects with Ib to inhibit same muscle
- Responsible for smooth contraction and protects muscle from tears
Excitatory Interneurons
- Involved in spinal reflex to remove extremity from damaging stimulus
- Place foot on tack, sensory information must be translated into motor output
- Sensory Group III afferents send signal to excitatory interneurons that send ipsilateral and contralateral signals
Muscle spindle (group Ia and II afferents):
- Arranged in parallel with extrafusal fibers
* Detect both dynamic and static changes in muscle length
Golgi tendon organ (group Ib afferents):
•Arranged in series with extrafusal fibers •Detect muscle tension
Pacinian corpuscles (group II afferents):
Detect Vibration
Free nerve endings (Groups III and IV afferents):
detect noxious stimuli
What groups are myelinated?
1, 2, 3 (I, II, III)
Group IV is NOT
A Fibers
Somatic; myelinated Alpha=Type I Beta= Type II Gamma Delta=Type IV (smallest)
B Fibers
Lightly myelinated; preganglionic fibers of ANS
C Fibers
unmyelinated; found in somatic and autonomic systems
Type IV fibers
Sensory Fibers are either:
A-alpha or A-beta fibers A-gamma fibers or C fibers (decreasing conduction rate)
extrafusal fibers make up…
the “muscle fibers,” which are innervated by alpha motor neurons.
Muscle Spindle
- 3-10 mm long
- Consists of 3-12 intrafusal fiber
- Innervated by small gamma motor neurons.
- Central region of spindle has no contractile fibers; functions as a sensory receptor.
- Detects changes in muscle length.
With finer movements, the number of muscle spindles required ____.
increases
two types of intrafusal fibers
Nuclear bag fibers and nuclear chain fibers
Nuclear bag fibers
Nuclei concentrated in “bag” in center of receptor area
•Detect rate of change in muscle length
•Innervated by group Ia afferents and dynamic γ efferents
•Multiple nuclei located in a central “bag-like” configuration.
Nuclear chain fibers
Nuclei aligned in chain throughout receptor area
•Detect static change in muscle length
•Innervated by group II afferents and static γ efferents
•More numerous than nuclear bag fibers
•Multiple nuclei arranged in a single row.
Alpha motor neuron
- Most numerous
- Innervate effector –skeletal muscle
- Neurotransmitter -acetylcholine
Gamma motor neuron
- Innervates intrafusal muscle fibers
- Keeps muscle spindles taut
- Neurotransmitter -acetylcholine
Muscle spindle corrects for increase in _______.
muscle length (stretch)
Sensory fibers from central region of intrafusal fiber:
Stimulation results from:
•Lengthening of entire muscle
•Contraction of ends of intrafusal fibers
Types of sensory fibers: •Ia(primary fibers): •Forms annulospiral ending •II (secondary fibers) •smaller
Stimulation results in stimulation of α-motor neurons, resulting in contraction and shortening of muscle.
When muscle is stretched, ____ is stretched.
spindle
Brain areas controlling gamma fibers
- Bulboreticular region of brain stem
- Cerebellum
- Basal nuclei
- Cerebral cortex
Dynamic stretch reflex
•Signals transmitted from primary nerve endings. •Elicited by rapid stretch or unstretch •Opposes sudden changes to muscle length
Static reflex
- Transmitted by both primary and secondary endings
* Causes degree of muscle contraction to remain relatively constant
Muscle Spindle Function
Dynamic stretch reflex
static reflex
Prevents jerkiness of body movements (damping)
In a normal muscle, what does the graph of contraction look like?
sigmoidal and reaching asymptote at peak force of contraction
In a muscle whose spindles were denervated, what does the graph of contraction look like?
oscillating
Stretch Reflex
- Stretching of muscle stretches group Ia afferent fibers.
- Group Ia afferents synapse directly on α-motor neurons of same muscle.
- The muscle contracts and decreases tension on muscle spindle.
- Synergistic muscles are activated and antagonistic muscles are inhibited
Golgi Tendon Organ
•ENCAPSULATED sensory receptor through which muscle tendon fibers pass.
•Arranged in series with extrafusal fibers
•10-15 muscle fibers attached to each Golgi organ.
•Golgi organ is stimulated by contracting or stretching of muscle.
•Detects muscle tension.
•Golgi tendon reflex is OPPOSITE OF STRETCH REFLEX.
NO MOTOR COMPONENT
Sensory neuronal endings with tendon/muscle interface
Functions of premotor and supplementary motor areas
- Premotor and supplementary motor cortices generate a plan for movement.
- Transfer plan to primary motor cortex
- Signals generated here cause more complex patterns of movement than the more discrete pattern generated by the primary motor cortex.
- Anterior part of the premotor cortex develops a “motor image” of the total muscle movement that is to be performed.
- Supplementary motor cortex programs complex motor sequences and is responsible for mental rehearsal for a movement.
- Image in posterior motor cortex excites each successive pattern of muscle activity required to achieve the image.
- Posterior motor cortex sends signals to: •→ Primary motor cortex •→ Basal nuclei and thalamus → Primary motor cortex
Cells in motor cortex are organized into vertical columns:
•Each column stimulates a group of synergistic muscles or even a single muscle. •Each column has 6 distinct layers: •Pyramidal cells are in the 5th layer. •Input signals enter layers 2-4. •Neurons arising in 6thlayer communicate with other regions of the cerebral cortex.
Typical descending pathway consists of a series of two motor neurons:
- Upper motor neurons (UMNs)
- Lower motor neurons (LMNs)
- Does not take into consideration the association neurons between UMNs and LMNs
UMN Classification
Classified according to where they synapse in the ventral horn
Medial activation system
Innervate postural and girdle muscles
Lateral activation system
Associated with distally located muscles used for fine movements
Nonspecific activating system
Facilitate local reflex arcs
