Spinal Cord, Brainstem, Cortex Control of Motor Function (Lec 7) Flashcards
What are the three groups of neurons on the spinal cord?
sensory (afferent)
anterior motor neurons
interneurons
What type of fibers to alpha motor neurons give rise to?
alpha fibers
True or False?
anterior motor neurons are 30x as numerous as interneurons
False; interneurons are 30x as numerous as anterior motor neurons
What are some characteristics of interneurons?
small and highly excitable, capable of spontaneous activity, responsible for most of spinal cord integrative function
What type of cells are Renshaw cells and what do they do?
Inhibitory cells; transmit inhibitory signals to surrounding motor neurons - results in lateral inhibition
What do Renshaw cells receive collateral branches form?
alpha motor neurons
How long is a muscle spindle?
3-10 mm long
How many intrafusal fibers does a muscle spindle consist of?
3-12 intrafusal fibers
What are intrafusal fibers of a muscle spindle innervated by?
innervated by small gamma motor neurons
Does the central region of the muscle spindle have contractile fibers?
no
What does the central region of the muscle spindle function as?
sensory receptor; sensory fibers originate from the central region
What stimulates sensory fibers in the muscle spindle?
stretching of the central region of intrafusal fibers
What does the muscle spindle detect?
changes in muscle length - causes ends of intrafusal fibers to contract
What are the two types of sensory fibers in the muscle spindle?
Ia (primary fibers)
II (secondary fibers)
The following characteristics describe which type of sensory fiber in the muscle spindle:
forms annulospiral ending
17 micrometers in diameter
Transmits at 70-120 m/sec
Ia (primary fibers)
How long are II (secondary) sensory fibers in the muscle spindle?
8 micrometers
In regards to muscle spindle function, what is the dynamic stretch reflex?
elicited by rapid stretch or unstretch, opposes sudden changes to muscle length
In regards to muscle spindle function, what is the static reflex?
causes degree of muscle contraction to remain relatively constant
What do the two muscle spindle reflexes work together to prevent?
jerkiness of body movements (damping)
Signals for the dynamic stretch reflex are transmitted by what?
primary nerve endings
What transmits the signal for the static reflex?
both primary and secondary endings
Which class of intrafusal muscle fibers have nuclei concentrated in “bag” in center of receptor area?
nuclear bag fibers
Which class of intrafusal muscle fibers have nuclei aligned in chain throughout receptor area?
nuclear chain fibers
The amount of gamma motor neurons is equal to about ___ the number of alpha motor neurons
half
In regards to gamma motor neurons, which type has a 5 micrometer diameter and supplies small intrafusal fibers in the middle of the muscle spindle?
Alpha Gamma
In regards to gamma motor neurons, which type excites nuclear bag intrafusal fibers?
Gamma-dynamic
In regards to gamma motor neurons, which type excites nuclear chain intrafusal fibers?
Gamma-static
What are the brain areas that control gamma fibers?
Bulboreticular region of brain stem, cerebellum, basal nuclei, and cerebral cortex
The golgi tendon organ is an encapsulated sensory receptor through which ___ ___ fibers pass
muscle tendon
How many muscle fibers are attached to each golgi tendon organ?
10-15
What stimulates the golgi tendon organ?
contracting or stretching of muscle
What does the golgi tendon organ detect?
muscle tension
What is the circuitry pathway of the golgi tendon organ?
type Ib afferent (16 micrometer diameter) -> inhibitory interneuron -> anterior motor neuron
What does a myogram of the flexor reflex show?
rapid onset of reflex, an interval of fatigue, and after discharge after the input stimulus is over
What does a myogram of the extensor reflex show?
slow onset but prolonged afterdischarge
In regards to the motor cortex, signals generated here cause more complex patterns of movement than the more discrete pattern generated by the primary motor cortex
premotor area
Anterior part of the premotor area of the motor cortex develops a ___ ___ of the total muscle movement that is to be performed
motor image
True or False?
The motor image in the posterior motor cortex excites each successive pattern of muscle activity required to achieve the image
true
Where does the posterior motor cortex send signals to?
a. ->primary motor cortex
b. -> basal nuclei and thalamus -> primary motor cortex
How are cells in the motor cortex organized?
into vertical columns
Each column of the motor cortex stimulates what?
a group of synergistic muscles or even a single muscle
How many layers does each column in the motor cortex have?
6
Pyramidal cells are in which layer of the column of motor cortex cells?
5th
Which layer of the column of motor cortex cells do input signals enter?
2-4
Neurons arising in the 6th layer of the column of motor cortex cells communicate with what?
other regions of the cerebral cortex
A generalization of the motor paths would be that typical descending pathways consist of a series of ___ motor neurons
two: upper motor neurons, lower motor neurons
Where are upper motor neurons found and where do they originate?
entirely within the CNS; originate in cerebral cortex, cerebellum, and brainstem
Where are lower motor neurons found and where do they originate?
Make up spinal and cranial nerves; begin in CNS
What type of neurons make up lower motor neurons?
alpha motor neurons
What are some characteristics of upper motor neurons?
originate in the motor cortices; 75-85% decussate in pyramids and form the lateral corticospinal tracts; remainder decussate near synapse with lower motor neurons and form anterior corticospinal tracts; most synapse with association neurons in spinal cord central gray
The corticospinal tract and corticobulbar tract make up what system?
pyramidal
What makes up the lateral corticospinal tract? What does it supply?
corticospinal fibers that have crossed in the medulla; supplies all levels of the spinal cord
What makes up the anterior coritcospinal tract? What does it supply?
uncrossed corticospinal fibers that cross near level of synapse with LMNs; supply neck and upper limbs
What is the origin of the corticospinal tract (aka pyramidal tract)?
primary motor cortex, premotor cortex, somatosensory cortex
What is the pathway of the corticospinal tract?
site of origin -> internal capsule -> medullary pyramids in lower medulla -> lateral columns of spinal cord (lateral corticospinal tract)
What happens to the fibers in the corticospinal tract that do not cross?
continue down ipsilaterally in ventral corticospinal tract
In regards to the corticospinal tract, the following characteristics describe what type of cell?: located in motor cortex large cells large fibers transmit 70 m/sec make up about 3% of fibers in tract send collaterals back to cortex
Giant pyramidal cells
In regards to the corticospinal tract, what do other fibers from the cortex do?
pass into caudate nucleus and putamen, pass to red nucleus, pass to reticular substance and vistibular nuclei, large numbers of fibers pass to pontine nuclei
What are the functions of the corticospinal tract?
adds speed and agility to conscious movements - especially movements of hand; provides a high degree of motor control (ex: movement of individual fingers)
What do corticospinal tract lesions cause?
reduced muscle tone, clumsiness, weakness, not complete paralysis
What does the corticobulbar tract innervate?
the head
Most fibers in the corticobulbar tract terminate where?
reticular formation near cranial nerve nuclei
Association neurons of the corticobulbar tract leave reticular formation and synapse where?
in cranial nerve nuclei with lower motor neurons
In regards to red nucleus, fibers from primary motor cortex (corticorubral pathway) and branches from corticospinal tract synapse where?
in magnocellular portion of red nucleus
In regards to red nucleus, large neurons from magnocellular region of red nucleus give rise to the rubrospinal tract which decussates where?
in lower brain stem
The magnocellular region of the red nucleus has a ___ representation of all the muscles of the body
somatotopic
What does the extrapyramidal system include?
pathways that contribute to motor control but that are not part of the corticospinal system: rubrospinal tracts, vestibulospinal tracts, and reticulospinal tracts
Where does the Rubrospinal tract begin, decussate, and descend?
begins in red nucleus
decussates in midbrain
descends in lateral funiculus
The function of the rubrospinal tract is closely related to what?
cerebellar function
What do lesions on the rubrospinal tract cause?
impairment of distal arm and hand movement; intention tremors
Where does the vestibulospinal tract originate, descend, and synapse?
orginates in vestibular nuclei - receives input from
vestibular nerve
descends in anterior funiculus
synapses with LMNs to extensor muscles - primarily involved in maintenance of upright posture
Where does the reticulospinal tract originate and descend?
originates in various regions of reticular formation
descends in anterior portion of lateral fundiculus
note: thought to mediate larger movements of trunk and limbs that do not require balance or fine movements of upper limbs
What do Pontine reticular nuclei do and make up?
excite antigravity (axial) muscles; make up the pontine reticulospinal tract
What do Medullary reticular nuclei do and make up?
relax antigravity muscles;
make up the medullary reticulospinal tract
What are the vestibular apparatus components?
Utricle, Saccule, Semicircular canals
Where is the macula of the utricle located and what does it do?
located on horizontal plane; plays role in determining orientation of head when head is upright
Where is the macula of the saccule located and what does it do?
located in a vertical plane; signals head orientation when person is lying down
Each macula is covered by a gelatinous layer. Describe this layer.
contains large number of embedded small calcium carbonate crystals (statoconia); contains thousands of hair cells which project cilia into the gelatinous layer; the wight of the statoconia bends cilia in the direction of gravitational pull
What are some characteristics of hair cells found in macula?
50-70 small cilia; 1 large cilium; tips of sterocilia are connected together and to kinocilium; bending of cilia opens hundreds of cation channels causing receptor membrane depolarization; bending of cells in opposite direction causes hyper polarization; oriented such that bending the head in different directions causes different groups of hair cells to depolarize
In regards to semicircular canals, what occurs when head is bent forward 30 degrees?
lateral ducts are horizontal; anterior ducts are in vertical planes projecting forward and 45 degrees outward; posterior ducts are in vertical planes projecting backward and 45 degrees
The following characteristics describe what?:
enlargement at one end of each duct; filled with endolymph
ampulla
What is the small crest within each ampulla called?
crista ampullaris
What is the loose mass of gelatinous tissue on top of the crista called?
cupula
Describe what happens in regards to semicircular canals when the head begins to rotate in any direction
inertia of the fluid in one or more of the semicircular canals remains stationary while semicircular canal rotates with the head; hundreds of hair cells within each cupola detect bending and send signals via the vestibular nerve
