Basic Sensory And Motor Mechs And Pathways Flashcards
Describe the interneuron in a reflex arc
Terminates directly or indirectly (via another interneuron) upon a ventral horn cell
The course and termination of an interneuron determines the pattern of the reflex arc (intrasegmental, intersegmenta, contralateral)
What is the fasciculus proprius?
Major fiber bundle associated with intersegmental reflexes
It is distributed around periphery of gray matter of spinal cord and is continuous superiorly with reticular formation
Comprised of ascending and descending processes of interneurons
What is the efferent neuron in a reflex arc?
Two types:
Alpha motor neuron: innervates extrafusal spindles
Gamma motor neuron: innervates intrafusal spindles
What is the effector in a reflex arc?
Motor end plate (MEP) of a neuromuscular junction may innervate either extrafusal fibers or intrafusal fibers
What are the components of a basic reflex pathway?
Receptor Afferent neuron Interneuron (excitatory/inhibitory, intrasegmental, intersegmental, commissural) Efferent neuron Effector (motor end plate)
Describe the myotatic reflex
Comprised of a two neuron reflex arc with only one synapse (no interneuron) Stimulus: rapid stretching of muscle Receptor: neuromuscular spindle Afferent: Ia neuron Interneuron: none Efferent: alpha motor Effector: extrafusal muscle Response: contraction of muscle Example: patellar, jaw-jerk
Describe pain reflex
Polysynaptic Stimulus: noxious stimulus (pain) Receptor: naked nerve endings Afferent Interneuron: multiple Efferent: alpha motor Effector: extrafusal muscle Response: withdrawal from stimulus Example: thumb-nail stimulus
Describe autogenic inhibition reflex
Stimulus: excessive tension on tendon Receptor: golgi tendon organ Afferent: Ib neuron Interneuron: inhibitory Efferent: alpha motor Effector: extrafusal muscle Response: relaxation of muscle
Describe reciprocal inhibition
Stimulus: contraction of agonist Receptor: neuromuscular spindle Afferent: Ia neuron Interneurons: to agonist (+) and antagonist (-) Efferent: alpha motor Effector: extrafusal muscle Response: contraction of agonist and relative relaxation of antagonist Example: biceps/triceps, eye movements
Describe primary neuron in a conscious sensory pathway
Pseudounipolar neuron whose cell boy (soma) is located in a spinal ganglion Peripheral process (dendrite) courses in peripheral nerve. Its ending is associated with a receptor Central process enters CNS and bifurcates to ascend and descend a variable number of segments Along its course, it sends off collaterals to interneurons for reflexes Primary sensory fiber eventually terminates upon a secondary neuron
Describe secondary neuron in a conscious sensory pathway
Located in spinal cord (pain and temperature pathway) or in medulla (proprioreceptive pathway)
Collaterals from secondary axons terminate directly or indirectly via interneurons upon motor neurons for various reflexes
For conscious sensory:
Always decussates and ascends as a lemniscus
Terminates upon a tertiary neuron in dorsal thalamus
Sends collateral fibers to reticular formation (RF) and tectum
Describe tertiary neuron in conscious sensory pathway
Soma is located in a specific nucleus of dorsal thalamus
Axon projects to primary somesthetic cortex via thalamic radiations (thalamocortical fibers) of internal capsule and corona radiata
Describe the primary somesthetic cortex in the conscious sensory pathway
Plays a role in perception and discrimination of sensory stimuli
Describe association cortex in conscious sensory pathway
Involved in integration, modification, and interpretation of sensory information
Describe the pathway for fast pain/temperature
Primary neuron: spinal ganglion
Conveyed by dorsolateral fasciculus
Secondary neuron: substantia gelatinosa
Conveyed by spinal lemniscus or lateral spinothalamic tract
Tertiary neuron: ventral posterior lateral nucleus
Cerebral cortex: primary somesthetic cortex
Describe the pathway for crude tactile
Primary neuron: spinal ganglion
Conveyed by short ascending fibers in posterior columns
Secondary neuron: nucleus proprius or intermediate gray
Conveyed by ventral spinothalamic tract
Tertiary neuron: ventral posterior lateral nucleus
Cerebral cortex: primary somesthetic cortex
Describe pathway for proprioception, 2-pt tactile
Primary neuron: spinal ganglion
Conveyed by long ascending fibers in posterior columns, fasciculus gracillis, or fasciculus cuneatus
Secondary neuron: nucleus gracillis (lower limb) or nucleus cuneatus (upper limb)
Conveyed by medial lemniscus
Tertiary neuron: ventral posterior lateral nucleus
Cerebral cortex: primary somesthetic cortex
What are often referred to as the “final common pathway”?
Lower motor neurons (LMN)
Without them, there is no way to respond to physical enivonment
LMN are the final effectors of the motor systems
Their processes form the motor nerves that innervate skeletal muscles
What causes lower motor neuron paralysis?
Destruction of motor neurons or axons of one or more cranial or spinal motor nuclei
What is flaccid paralysis?
Muscle is completely “limp,” and there is no resistance to passive movement
What is areflexia?
Loss of efferent component of reflex arc to a muscle results in absence of associated muscle reflex
What is atonia?
Destruction of gamma motor neurons or their axons results in absence of muscle tone
What is atrophy?
Denervated muscle atrophies due to loss of stimulation from motor neurons
What are fasciculations?
Twitching of denervated muscle, probably due to hypersensitivity of motor end plate
What is poliomyelitis?
Acute anterior poliomyelitis selectively involves motor neurons of the anterior (ventral) horns and the cranial nerve motor nuclei
Initially, there is often severe inflammation, vasodilation, edema, and macrophage activity.
Subsequently, these neurons die, and there is significant astrocytic gliosis
The onset of this viral disease usually lasts between 2-4 days with symptoms which are characteristic of any acute viral meningitis such as pyrexia (fever), headache, vomiting, neck stiffness, and pain in back and limbs
Symptoms may subside, and the patient may completely recover (nonparalytic poliomyelitis), or result in varying degrees of paresis or paralysis (paralytic poliomyelitis)
From where do corticospinal and corticobulbar tracts arise?
From large pyramidally-shaped neurons (cells of Betz) located in primary motor and premotor cortices
Describe the pyramidal system
Primary motor cortex (precentral gyrus)
Corticospinal tract: Posterior limb of internal capsule Cerebral peduncle (mid 3/5) Pyramid Pyramidal decussation
Lateral corticospinal tract:
Fractionation of movement
Anterior corticospinal tract
Describe the descent of the corticospinal tract (CST)
Descends through corona radiata, internal capsule, cerebral peduncles, pons, and upper medulla
In the lower medulla, 85-90% of corticospinal fibers decussate at pyramidal decussation and form the lateral corticospinal tract (LCST)
The remaining uncrossed fibers continue as the anterior corticospinal tract (ACST)
Describe lateral corticospinal tract (LCST)
In the spinal cord, the LCST descends in the lateral funiculus
Most of the fibers terminate in neuronal pools (intermediate gray) at all levels of the spinal cord, where they exert collective influence upon intrinsic spinal reflex circuits
Only a small number of fibers synapse directly upon lower motor neurons (anterior horn).
Most fibers distribute to cervical (55%) and lumbosacral (25%) enlargements
What happens if there is a unilateral lesion of the LCST?
Ipsilateral paralysis or paresis of distal limb musculature innervated by those spinal segments below the level of the lesion
Describe the anterior corticospinal tract (ACST)
Some of the corticospinal fibers do not cross at the level of decussation, and therefore, descend uncrossed in anterior funiculus of cervical and upper thoracic spinal cord
What happens if there is a unilateral lesion of the the ACST?
There is minimal clinical effect
What are the signs and symptoms of upper motor neuron (UPN) lesions?
Varying degrees of spastic paresis of axial and proximal limb musculature, and some degree of spastic paralysis of distal limb musculature, especially upper extremity Hypertonia and hyperreflexia Babinski sign Clonus Rigidity Disuse atrophy
Describe spinal cord injury (SCI)
Initially, patient is in spinal shock with areflexia, atonia, and flaccid paralysis.
After a few weeks/months, return of basic spinal reflexes indicates patient’s recovery from spinal shock. The reflexes are due to a reactivation of intrinsic circuits of the spinal cord distal to the lesion
The patient displays some degree of spastic paresis of axial and proximal limb musculature in addition to some degree of spastic paralysis of distal limb musculature, especially upper extremity.
After 1-2 years, affected muscle groups will exhibit spasms of extensors, flexors, or remain flaccid
Define spasticity
Abnormal, passive resistance to movement in one direction
May be due to increased sensitivity of neuromuscular spindles to stretch of gravity muscles, increased gamma efferent activity, loss of descending inhibition, and/or denervation hypersensitivity of LMN pools
Define rigidity
Abnormal passive resistance to movement in all directions
What is decerebrate rigidity
Spasticity of extensors of both upper and lower extremities
These patients usually do not survive
Describe decorticate rigidity
Spastic hemiplegia of flexors of upper extremity and extensors of lower extremity, usually due to a lesion of the internal capsule
Compare/contrast the characteristics of the clinical syndrome of UMN and LMN lesions
UMN: Paralyzes movements in hemiplegic, quadriplegic, or paraplegic distribution, not individual muscles Atrophy of disuse only (late or slight) Hyperactive deep tendon reflexes Clonus Clasp-knife spasticity Absent abdominal-cremasteric reflexes Babinski reflex
LMN:
Paralyzes individual muscles or sets of muscles in root or peripheral nerve distributions
Atrophy of denervation (early and severe)
Fasiculation and fibrillations
Hypoactive or absent deep tendon reflexes
Hypotonia
Describe Amyotrophic lateral sclerosis (ALS)
May be due to defect in glutamate metabolism
Death is due to bulbar paralysis, i.e., vital respiratory centers, within average of 4 years of onset.
Most common form of ALS involves a combination of:
LMN (anterior horn cells, hypoglossal nucleus, nucleus ambiguus, facial motor nucleus)
UMN (chronic, progressive degeneration of corticospinal tracts)
ALS leads to LMN paresis and atrophy of intrinsic muscles of hands followed later by arms and shoulder musculature.
Patients develop dysarthria, dysphagia, and paresis of tongue
Involvement of corticospinal tract leads to spastic paralyssi, hyperreflexia, and Babinski sign.
NO SENSORY DEFICITS
What is clinically important about posterior columns?
Ascending sensory tracts for proprioception, 2-pt tactile discrimination and vibratory sensations
What is clinically important about lateral funiculus?
Descending UMN tracts for volitional control of limb musculature
Descending UMN tracts for autonomic control of bladder and bowe
What is clinically important about anterior funiculus?
Ascending sensory tracts for pain and temperature, and crude (light) tactile sensations
Decussating fibers in direct pain and temperature pathway
Lower motor neurons in anterior/ventral horn of spinal cord
Describe the afferent neuron in a reflex arc
Pseudounipolar neuron whose cell body (soma) is located in a spinal ganglion Peripheral process (dendrite) courses in a spinal nerve Central process (axon) enters spinal cord in dorsal root and bifurcates into ascending and descending processes
