Efferent Motor Pathways - Lower Motor And Local Circuit Neurons (10A) Flashcards
What does the input of local circuit neurons ultimately allow for lower motor neuron coordination of?
Provide coordination between muscle groups that is essential for coordinated movements, such as walking, chewing, etc.
* these activities are “preprogrammed” events that may be initiated independent of higher brain input (local circuit/lower motor neuron interactions literally responsible for a chicken running around w/ it’s head cut off)
What do local circuit neurons do?
Are the major source of synaptic input for lower motor neurons - serve to integrate info coming in from sensory systems and also descending projections from higher brain centers
What is the function of lower motor neurons?
Final common pathway for movement - all commands for movement, whether reflexive or involuntary are ultimately conveyed to the muscles by lower motor neurons
Where are cell bodies of upper motor neurons found? (2 places)
In the brainstem and cerebral cortex
What is the main different feature of an upper motor neuron compared to lower motor neurons?
Upper motor neurons are entirely contained w/in the CNS(no part leaves it), while the axons of lower motor neurons project into the periphery
What do upper motor neurons synapse with?
Usually with local circuit neurons (less frequently may directly synapse with lower motor neurons)
Upper motor pathways arising from where are responsible for initiation of voluntary movement and complex skilled movements like fine motor control?
Cerebral cortex
- projections from primary motor cortex, promotor cortex! and supplementary motor cortex are essential for planning, initiating, and directing the sequences of voluntary movement
Upper motor neurons that regulate muscle tone, and orient the eyes head and body with respect to incoming sensory info originate from where?
Brainstem
- these are critical for basic navigational movement and control of posture
How do the cerebellum and the basal ganglia alter the output of motor function?
They modulate the output of the upper motor neurons to the lower motor neurons (don’t influence lower motor neurons directly)
Where are the cell bodies of the 1st order neurons in the Corticospinal (pyramidal) Tracts located? What type do motor neurons are these?
In the precentral gyrus ; these are upper motor neurons
What structure do fibers in the corticospinal (pyramidal) tract form after leaving the primary motor cortex (precentral gyrus)?
Corona radiata; they then converge to pass through the posterior limb of the internal capsule
What structure do fibers in the corticospinal (pyramidal) tract pass through in the anterior midbrain?
Crus cerebri (basis pedunculi)
What structure do fibers of the corticospinal tract form in the anterior medulla?
Pyramids
Where do fibers of the corticospinal tract decussate? What are they called afterwards?
At the junction of the medulla and spinal cord - now known as the lateral corticospinal tract
*85-90% of the fibers decussate; the rest remain on the ipsilateral side
What are the few fibers of the corticospinal tract that don’t decussate at the pyramids called as they progress caudally?
Ventral (anterior) corticospinal tract
Where do the ventral (anterior) corticospinal tract neurons decussate? Via what structure do they cross over?
At the level they synapse on the lower motor neurons in the ventral horn of the spinal cord
* fibers that decussate cross over via the anterior white commisure
Which corticospinal tract innervates limb (distal) musculature? Do they do this ipsilaterally or bilaterally?
Lateral corticospinal tract upper motor neurons; synapse only ipsilaterally to innervate limb musculature
Which corticospinal tract neurons innervate axial (proximal) musculature lower motor neurons? Do they do so ipsilaterally or bilaterally?
Anterior (ventral) corticospinal tract upper motor neurons synapse bilaterally on lower motor neurons to axial musculature
What lamellae in spinal cord grey matter are local circuit interneurons found?
VII-VIII
*in between the intermediate zone(VII) and ventral horn(VIII)
What lamella are the lower motor neurons found in?
IX (ventral horn)
Where in the ventral horn spinal cord grey matter are neurons innervating distal structures located? Axial structures? (Somatotropin distribution)
Distal structures - lateral portions
Axial structures - medial portions
What is a motor unit? What kind of motoneuron is involved?
An alpha-motoneuron and the several muscle fibers in a single muscle it innervates
*activation of a single motor unit represents the smallest unit of force that can be generated by a single muscle fiber
What does the size of the alpha-motoneuron cell body have to do with its function?
The greater the size of the motor neuron the greater number of fibers it synapses with.
* alpha-motoneurons are the same as lower motor neurons
How is more precise control achieved for fine motor movements, like those in the fingers or eyes?
By having a single motor unit innervating fewer fibers.
But having more fibers per motor unit gives more power
Which of the three types of muscle fiber types Is dark meat composed of? Do they have a high or low ratio of muscle fibers per neuron?
Slow, fatigue resistant fibers - are red due to increased blood supply and ample myoglobin; tire slowly due to aerobic metabolism
* have a low number of muscle fibers per neuron(thus smaller motoneuron cell bodies)
Aka Type I fibers
What type of muscle fiber types are characteristic of white meat? Why are they pale? What is the ratio of muscle fiber types per neuron in these fibers?
Fast, fatiguable fibers - are paler due to less blood supply and less myoglobin; have a high ratio of fibers to neurons (so large motoneuron cell bodies, and more powerful, less control)
Aka Type II fibers
What neuron subtype send afferents from muscle spindle cells to the spinal cord to synapse on alpha-motoneurons as part of the muscle stretch (monosynaptic) reflex? Why do they also synapse on inhibitory interneurons?
1A (sensory from muscle spindle)
Activation of interneurons inhibit the lower motoneurons of the antagonistic muscle set
What is the name for the connection made when a sensory afferent nerve (1A) provides excitatory information to an alpha-motoneuron?
Monosynaptic connection
What kind of amino acids are common excitatory NTs for motoneurons?
Acidic a.a.
In addition to input from sensory neurons in the dorsal root ganglion in the same spinal cord segment, what other two sources of input do alpha-motoneurons have? Do these synapse directly on the motoneurons?
- DRG neurons from spinal segments above and below
- Input from higher motor centers (upper motor neurons)
* do not synapse directly, input is received via interneurons; both excitatory and inhibitory interneurons exist
What two afferent neuron subtypes do muscle spindles give rise to?
Types IA and II
What type of motoneurons innervate muscle spindles?
Gamma-motoneurons (are the efferent fibers to muscle spindle intramural fibers)
What type of sensory organs are located at semitendinous junctions where muscle fibers terminate to form a tendon? What subtype of afferent nerves do these organs give rise to?
Golgi tendon organs; give rise to type IIb afferent nerves
What are annulospiral endings?
Specialized sensory nerve endings that encircle intrafusal nerve endings within muscle spindles - they are responsible for the APs fired when the muscle spindle is stretched (such as in response to reflex testing)
What role do gamma-motoneurons have in maintaining function of the muscle spindles?
They fire in concert with alpha-motoneurons to keep muscle spindle intrafusal fibers contracting at the same time as the extrafusal fibers (ones making up the normal muscle) - this keeps the mechanical stretch receptors (annulospiral endings) from becoming relaxed and non-functional when the rest of the muscle contracts
What is the main purpose of the muscle spindles?
Monitor and regulate muscle tone throughout the full range of muscle extension and flexion to make sure it is a smooth process and relatively constant muscle tone is maintained throughout the movement
What information do type II afferent nerves from the muscle spindle encode?
Encode muscle fiver length information - increasing length increases the frequency of AP firing; decreasing the length decreases the frequency of AP firing
*remember, type IA and II are the afferents from muscle spindle fibers
What information about muscle fibers is encoded by type IA afferent nerves from muscle spindles?
Both length and velocity of change in length
- frequency of APs increases dramatically during a rapid increase in muscle fiber length; frequency decreases dramatically when length rapidly shortens
- when length is constant, the basal frequency of AP firing reflects the length of the fiber (this info Los conveyed by the type II afferents
What does the combined action of muscle fiber afferents (II and IA) and their monosynaptic connections allow for functionally?
Allows the motor unit to monitor its own length and thus control movement at the level of the spinal cord without input from higher CNS structures; this allows you to hold objects w/o looking or to touch your nose with closed eyes, for example
What is the main purpose of Golgi tendon organs? What type of afferents do they use again?
They monitor muscle force use the info to protect against excessive and potentially harmful forces = are basically an emergency shut-off switch in the event of muscles contracting too hard
- use IB afferent nerves
What kind of neurons do type IB afferents from GTO’s synapse on?(2 targets) Are GTOs connected in series or parallel with their respective muscle/tendon?
- Inhibitory interneurons = inhibit the alpha-motoneurons that innervate the same muscle as the Golgi tendon organ
- Excitatory interneurons = contraction of the antagonist muscle groups to those of the GTO
- connected in series with muscles/tendons (in contrast to intrafusal muscle spindle fibers, which are connected in parallel w/ their muscles)
T or F? Muscle spindle and GTO reflex mechanisms do not receive input from higher cortical and brainstem centers.
False; higher cortical and brainstem centers have substantial effects on these reflexes (via input to local circuit interneurons)
What is the flexor reflex? What two components does it have?
Reflex withdrawal due to uncomfortable stimulus on skin (like touching hot stove)
Has components of agonist muscle activation (via excitatory interneurons) and antagonist muscle inhibition
Gamma-motoneuron activity reflects ______ gain or bias on muscle tone. What does higher gain mean?
Positive; higher the gain, the greater the muscle tone and force of contraction
What two inputs alter gamma-motoneuron gain/bias?
- Local reflex circuitry
2. Upper motor neuron input
What two things alter Golgi tendon organ activity?
- Local sensory circuitry
2. Upper motor neuron input
Do upper or lower motor neuron lesion cause weakness?
Yes, both do
Does an upper or a lower motor neuron cause atrophy?
Only lower motor neuron lesions cause atrophy
In what kind of motor lesion (upper or lower) are fasciculations seen?
Lower motor lesions only
What happens to reflexes in upper motor neuron lesions? Lower?
Upper motor neuron lesion - increased reflexes
Lower motor neuron lesion - decreased reflexes
What happens to muscle tone in upper motor neuron lesions? Lower motor neuron lesions?
Upper - increased muscle tone
Lower - decreased muscle tone
What is the basic function of the cerebellum?
Detects difference (error) between intended movement and the movement actually performed, both real-time and long term *long term is motor learning process - improvement over time
