L1-Spinal Cord, brainstem, cortex control of motor function Flashcards
groups of neurons in spinal cord:
- sensory (afferent)
- anterior motor neurons (efferents)
- interneurons
- renshaw cells
what do alpha motor neurons give rise to?
alpha fibers
describe characteristics of interneurons
- 30x as numerous as anterior motor neurons
- small and highly excitable
- capable of spontaneous activity
- responsibility for most of spinal cord integrative function
describe the characeristics of renshaw cells
- inhibitory cells in anterior horns of spinal cord
- receive collateral branches from alpha motor neurons
- transmit inhibitory signals to surrounding motor neurons: result in lateral inhibition; enhance fluidity of limb movement
- transmit inhibitory signals to same motor neuron: results in recurrent inhibition
describe 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
describe upper motor neuron:
- descends the spinal cord to level of the appropriate spinal nerve root
- synapses with lower motor neuron or interneuron
- neurotransmittor of UMN to LMN is GLUTAMATE, which binds to glu receptors
describe lower motor neurons:
- 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
describe the motor unit components:
- motor unit
- motor neuron pool
- recruitment
- small motor neurons
- larger motor neurons
describe the motor unit
- composed of a single motor neuron and the muscle fibers it innervates
- composed of extrafusal fibers
describe motor neuron pool
group of motor neurons that innervate fibers within the same muscle
recruitment
recruitment refers to the increase in tension of muscle contractions by the activation of additional motor units (size principle)
describe small motor neurons
- innervate a few muscle fibers
- lowest thresholds
- fire first
- generate smallest force
describe large motor neurons
- innervate many muscle fibers
- highest thresholds
- fire LAST
- generate largest force
differentiation upper motor neuron lesions:
- muscle bulk: preserved
- muscle tone: spastic
- spontaneous movements: none
- reflexes: increased
- babinski reflex: present
differentiation of lower motor neuron lesions:
muscle bulk: atrophy muscle tone: flaccid spontaneous movements: fasciculations reflexes: absent babinski reflex: absent
describe interneurons
- 30x as numerous as anterior motor neurons
- bipolar (shape)
- capable of spontaneous activity
- responsible for most of spinal cord integrative function
- most are inhibitor - GABA is neurotransmitter
- excitatory - glutamate is nt
neural connections in the SC: renshaw cells
- inhibitory cells use GLYCINE as nt
- receive collateral branches from alpha-MNs
- may be supplied by more than 1 alpha-MN and may synapse with multiple MNs
- transmit inhibitory signals to surrounding MNs: results in lateral inhibition // enhances fluidity of limb movement
- transmit inhibitory signals to same MN: negative feedback.
what are the two inhibitory neurons:
I-alpha and I-beta
describe I-alpha
- inhibits antagonistic muscle
- signal from muscle spindle
- avoids spasticity
describe I-beta
- autogenic inhibition reflex
- golgi tendon activated with stretch
- connects with l-beta to inhibit same muscle
- responsible for smooth contraction and protects muscle from tears
describe excitatory interneurons
- involved in spinal reflex to remove extremity from damaging stimulus
- place foot on track, sensory information must be translated into motor output
- sensory group 3 afferents send signal to excitatory interneurons that send ipsilateral and contralateral signals
list the muscle sensors
- muscle spindle (group Ia and II afferents)
- golgi tendon organ (group Ib afferents)
- Pacinian corpuscles (group II afferents)
- Free nerve endings (groups III and Iv afferents)
describe muscle spindle groups Ia and II
- arranged in parallel with extrafusal fibers
- detect both dynamic and static changes in muscle length
describe golgi tendon organ
- arranged in series with extrafusal fibers
- detect muscle tension
what do pacinian corpuscles detect?
vibration
what do free nerve endings detect?
noxios stimuli
what groups are and are not myelinated?
is: I, II, III
is not: IV
list and describe fiber types:
A FIBERS: somatic, myelinated; alpha: largest, AKA type 1; beta, aka type II; gamma; delta, smallest aka Type IV
B FIBERS: lightly myelinated, preganglionic fibers of ANS
C FIBERS: unmyelinated, found in somatic and autonomic systems, aka Type IV Fibers
sensory fiber types and description
- A-alpha or A-beta fibers (conduction rate = 30-120 m/sec)
- A-delta fibers (4-30 m/sec)
- C fibers (less than 2.5 m/s)
what are nociceptors and thermoreceptors related to?
C fibers or A-delta fibers
describe the muscle spindle
- 3-10 mm long
- consists of 3-12 intrafusal fibers: innervated by small gamma motor neurons (group II fibers); encapsulated within a sheath to form muscle spindle; run parallel to extrafusal fibers
-central region of spindle has NO contractile fibers; functions as a sensory receptor. (sensory fibers originate from central region; stretching of central region of intrafusal fiber stimulates sensory fibers)
what happens with finer movements?
the number of MSs required increases
what are they innervated by?
small gamma motor neurons
overall, what does a muscle spindle do?
detects CHANGES in muscle length
what are the types of intrafusal fibers?
nuclear bag fibers
nuclear chain fibers
describe the nuclear bag fibers
- detect rate of change in muscle lenght
- innervated by group Ia afferents and dynamic gamma efferents
- multiple nuclei concentrated in “bag” in center of receptor area
describe nuclear chain fibers
- detect static change in muscle lenght
- innervated by group II afferents and static gamma efferents
- more numerous than nuclear bag fibers
- multiple nuclei arranged in a single row
alpha motor neuron (related to MS)
- most numerous
- innervate effector - skeletal muscle
- neurotransmitter - acetylecholine
gamma motor neuron (related to MS)
- innervates intrafusal muscle fibers
- keeps muscle spindles taut
- neurotransmitter - acetylcholine
what does the muscle spindle correct for?
increase in muscle length (stretch)
when muscle is stretched, spindle is stretched
what does stimulation result from?
- lengthening of entire muscle
- contraction of ends of intrafusal fibers
*stimulation results in stimulation of alpha-motor neurons, resulting in contraction and shortening of muscle
describe the muscle spindle gamma motor neurons
- innervate intrafusal fibers
- adjust sensitivity of muscle spindle
- coactivated with alpha-motor neurons (stimulation results from lengthening of entire muscle)
- equal to about half the number of alpha motor neurons
what types of gamma MNs are there?
A-gamma: 5 mue-meters diameter, supply small intrafusal fibers in middle of muscle spindle
Gamma-dynamic: excite nuclear bag intrafusal fibers
Gamma-static: excite nuclear chain intrafusal fibers
what brain areas control gamma fibers?
- bulboreticular region of brain stem
- cerebellum
- basal nuclei
- cerebral cortex
what are the three muscle spindle functions?
- dynamic stretch reflex
- static reflex
- prevents jerkiness of body movements (damping)
describe dynamic stretch reflext
- signals transmitted from primary nerve endings
- elicited by rapid stretch or unstretch
- opposes sudden changes to muscle length
describe static reflex
- transmitted by both primary and secondary endings
- causes degree of muscle contraction to remain relatively constant
describe the general stretch reflex
- stretching of muscle stretches group Ia afferent fibers
- group Ia afferents synapse directly on alpha-motor neurons of same muscle
- the muscle contracts and decreases tension on muscle spindle
- synergistic muscles are activated and antagonistic muscles are inhibited
describe the 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
- gogli tendon reflex is opposite of the stretch reflex
- has no motor component
what is the circuitry of the golgi tendon organ?
type Ib afferent (+) —> inhibitory interneuron (-) —> anterior motor neuron
what does after-discharge result from?
it is the result of persistent neural activity in polysynaptic circuits; it prevents muscle from relaxing for some time
what is the function of the premotor and supplementary motor cortices?
they generate a plan for movement
they 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
what does the anterior part of the premotor cortex do?
develops a “motor image” of the total muscle movement that is to be performed
describe what the supplementary motor cortex does
it programs complex motor sequences and is responsible for mental rehearsal for a movement.
what else does the premotor and supplementary motor areas do?
image in posterior motor cortex excites each successive pattern of muscle activity required to achieve the image
where does the posterior motor cortex sends signals to?
–> primary motor cortex –> basal nuclei and thalamus –> primary motor cortex
how are cells in the motor cortex organized?
vertical columns
describe the verticle columns
- each column stimulates a group of synergistic muscles or even a single muscle
- each column has 6 distinct layers
- neurons arising in the 6th layer communicate with other regions of the cerebral cortex
what layer are the pyramidal cells found in?
5th layer
where do input signals enter through?
layers 2-4
describe the generalizations of the motor paths
typical descending pathways consists of a series of two motor neurons
what are those two?
UMNs and LMNs
does not take into consideration the association neurons between UMNs and LMNs
descibe UMNs
- are entirely within the CNS
- originate in: cerebral cortex, cerebellum, and brainstem
- form descending tracts
describe LMNs
- begin in CNS: from anterior horns of SC, from brainstem cranial nerve nuclei
- made up of alpha motor neurons (A-alpha)
- make up spinal and cranial nerves
describe UMN classification
they are 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
medial UMN axial and girlde muscles: vertical organization of motor control
- medial corticospinal tract
- lateral vestibulospinal tract
- tectospinal tract
- medial reticulospinal tract
- medial vestibulospinal tract
lateral UMN limb muscles: vertical organization of motor control
- rubrospinal tract
- lateral corticospinal tract
- medullary (lateral) reticulospinal tract