Lecture 01 Basic Sensory and Motor Mechanisms and Pathways Flashcards
Describe interneurons in the reflex pathway
may terminate directly or indirectly upon motor neurons in ventral horn cells
Processes may ascend, descend or cross (commissural fibers) in the spinal cord.
describe efferent neurons in the reflex arcs
alpha or gamma motor neurons in the motor nuclei or columns
What are the receptors for free nerve endings?
interoceptors, exteroceptors and peritrichial nerve endings
What is the stimulus for interoceptors and exteroceptors ?
pain, temp and touch
What is the stimulus of peritrichial nerve endings ?
light touch
What are intrasegmental reflexes?
interneurons terminates within the same level as the afferent stimulus
What are intersegmental reflexes?
interneurons extend the influence of the incoming info among more than one spinal segment
What is a major fiber bundle associated with intersegmental reflexes?
fasiculus proprius (fasciculus spinospinalis)
What is the fasiculus proprius?
fiber bundle that is distributed around the periphery of the gray matter of spinal cord and is continuous superiorly with the reticular formation
Comprised of ascending and descending processes of interneurons
What are contralateral reflexes?
a reflex may be conveyed to the other side by way of a commissural neurons (interneuron)
this may be intra or intersegmental depending upon the type, quality and quantity of the stimulus
What do alpha motor neurons innervate?
extrafusal fibers
What do gamma motor neurons innervate?
intrafusual
Describe monosynaptic reflexes?
comprise of a two-neuron reflex arc with only one synapse
Do not have an interneuron
What reflex is tested during the clinical examinations of the knee-jerk or jaw-jerk reflexes?
myotatic reflex: rapid stretching of the muscle and contraction of the corresponding muscle
type of monosynaptic reflex
Afferent: Ia n
Efferent: alpha motor
What is a polysynaptic reflex ?
involve interneurons to elicit stereotypical response patterns to a particular type of stimulus
What type of reflex is the pain reflex?
polysynaptic reflex Stimulus: noxious stimulus (pain) Response: withdrawal Receptor: naked nerve endings Efferent: alpha motor
What are the common features in a conscious sensory pathway?
Primary neuron Secondary neuron Tertiary Neuron Primary Somesthetic Cortex Association Cortex
Describe the primary neuron features in a conscious sensory pathway
Pseudounipolar neuron
Cell body is located in a spinal ganglion
Dendrite courses peripheral nerve
Central process enters CNS and bifurcates to ascend and descend a variable # of segments
Sends off collaterals to interneurons for reflexes
primary sensory fiber eventually terminates upon a secondary neuron
Describe the common features of a secondary neuron in the conscious sensory pathway
located in the spinal cord (pain and temperature pathway) or in the medulla (proprioceptive pathway)
Collaterals from the secondary axon terminate directly or indirectly via interneurons upon motor neurons for various reflexes
What will secondary axons end in a conscious sensory pathway?
Always decussates and ascends as a lemniscus and terminates upon a tertiary neuron in the dorsal thalamus
also sends collateral fibers to the reticular formation and tectum
Describe tertiary neurons in the conscious sensory pathway
soma of the neuron is located in a specific nucleus of the dorsal thalamus
axon projects to the primary somesthetic cortex via the thalamic radiations of the internal capsule and corona radiata
What does the primary somestehtic cortex play a role in the conscious sensory pathway?
perception and discrimination of sensory stimuli
What is the conscious sensory pathway for fast pain and temp?
primary neuron: spinal ganglion
Conveyed by: dorsolateral fasciculus
Secondary Neuron: Substantia gelatinosa
Converyed by: spinal lemniscus ->Lateral spinothalamic tract
Tertiary neuron: ventral posterior lateral nuclues
Cerebral cortex: primary somesthetic cortex
What is the crude tactile pathway in the conscious sensory pathway?
Primary neuron: spinal ganglion
Conveyed by: short ascending fibers in the posterior columns
Secondary Neuron: nucleus proprius intermediate gray
Conveyed by: ventral spinothalamic tract
Tertiary neuron: Ventral posterior lateral nucleus
Cerebral Cortex: ventral posterior lateral nucleus
There are many reflex patterns, most of which are programmed by what connections?
interneurons
What is the proprioception 2-pt tactile pathway in the conscious sensory pathway?
Primary neuron: spinal ganglion
Conveyed by: long ascending fibers in the posterior columns
Secondary neuron: Nucleus gracilis, nucleus cuneatus
Conveyed by: medial lemniscus
Tertiary neuron: ventral posterior lateral nucleus
Cerebral cortex: primary somesthetic cortex
Describe lower motor neurons
final common pathway final effectors of the motor system Cell bodies in craniospinal motor nuclei Innervate skeletal m Alpha and gamma
Describe upper motor neurons
premotor
descending processes form the rest of the motor system
What type of fibers innervate branciomeric mm?
SVE
What type of fibers innervate myotomic mm?
GSE
SVE from nucleus ambiguus go to what structures?
laryngeal and pharyngeal mm
What are the clinical manifestations of LMN paralysis ?
Flaccid paralysis: muscles limp
Areflexia: Loss of efferent component of reflex arc
Atonia: Destruction of gamma motor neurons = absence of muscle tone
Atrophy: Denervated m due to loss of motor neurons
Fasciculations: twitching of denervated m; hypersensitivity of motor end plate
What is poliomyelitis ?
Viral disease
Motor neurons of anterior horns and cranial nerve motor nuclei die due to inflammation, edema, and macrophage activity
Astrocytic gliosis
2-4 days of symptoms: fever, headache, vomit, neck stiffness, back and limb pain (similar to any acute viral meningitis)
Symptoms may subside and pt completely recover or have varying degrees of paresis or paralysis
What is the pathway of the corticospinal tract?
Primary motor cortex ->descends through the corona radiata, posterior limb of internal capsule, cerebral peduncles, pons, and upper medulla
90% decussate at pyramidal decussations and form LCST
Remaining uncrossed fibers ->ACST
What does unilateral lesions of the LCST result in?
ipsilateral paralysis or paresis of the distal limb musculature innervated by those spinal segments below the level of the lesion
Very rare to have pure isolated lesions of the corticospinal tract
What is the lateral corticospinal tract primarily involved in?
the control of the distal limb musculature, especially precise, individualized movements of the digits
Piano
What is UMN paralysis commonly due to?
interruption of the motor cortex, corticospinal and/or corticobulbar tracts
What is an UMN paralysis classically referred to? what are the signs and symptoms ?
as the spastic paralysis of the antigravity mm Symptoms: varying degrees of spastic paresis of axial and proximal limb mm, some degree of spastic paralysis of the distal limb mm esp the upper extremity hypertonia hyperrreflexia babinski sign clonus rigidity disuse atrophy
Where does the LCST terminate?
it descends in the lateral funiculus and most fibers terminate in neuronal ‘pools’ (intermediate gray) at all levels of the spinal cord
What are the three phases of physical events following UMN lesions due to spinal cord injury ?
- pt in spinal shock with areflexia, atonia, and flaccid paralysis
- weeks-mnts: return of basic sppinal reflexes indicates pts recovery from spinal shock
(reflexes due to reactivation of intrinsic circuits of the spinal cord distal to lesion)pts display some degree of spastic paresis of axial and prox limb and some spastic paralysis of distal limb mm - 1-2 yrs: affected m groups will exhibit spasms of extensors, or flexors or remain flaccid
What is spasticity?
abnormal, passive resistance to movement in one direction due to increased sensitivity of neuromuscular spindles to the stretch of gravity mm, increased gamma efferent activity and/or denervation hypersensitivity of LMN pools
What is rigidity ?
abnormal passive resistance to movement in ALL directions
Paralyzes movements in hemiplegic, quadriplegic or paraplegic distribution, not individual mm are due to UMN or LMN lesions?
UMN
Paralyzes individual mm or sets of mm in root or peripheral nerve distributions are due to UMN or LMN lesions?
LMN
Atrophy of disuse only is due to UMN or LMN lesions?
UMN (late or slight)
Fasiculations and fibrillation symptoms due to UMN or LMN lesions?
LMN
Hyperactive deep tendon reflexes due to UMN or LMN lesions?
UMN
Hypoactive or absent deep tendon reflexes due to UMN or LMN lesions?
LMN
What type of tracts are found in posterior columns?
Ascending sensory tracts for proprioception, 2-pt tactile discrimination and vibratory sensations
What type of tracts are found in the lateral funiculus?
descending UMN tracts for volitional control of limb musculature
Descending UMN tracts for autonomic control of bladder and bowel
What tracts are in the anterior funiculus?
ascending sensory tracts for pain and temp, crude (light) tactile sensations
Decussating fibers in the direct pain and temp pathway
LMN in the anterior horn of spinal cord
Lateral spinothalamic tract
