Basic Sensory And Motor Mechs And Pathways

Question 1

Describe the interneuron in a reflex arc

Answer 1

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)

Question 2

What is the fasciculus proprius?

Answer 2

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

Question 3

What is the efferent neuron in a reflex arc?

Answer 3

Two types:
Alpha motor neuron: innervates extrafusal spindles
Gamma motor neuron: innervates intrafusal spindles

Question 4

What is the effector in a reflex arc?

Answer 4

Motor end plate (MEP) of a neuromuscular junction may innervate either extrafusal fibers or intrafusal fibers

Question 5

What are the components of a basic reflex pathway?

Answer 5

Receptor
Afferent neuron
Interneuron (excitatory/inhibitory, intrasegmental, intersegmental, commissural)
Efferent neuron
Effector (motor end plate)

Question 6

Describe the myotatic reflex

Answer 6

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

Question 7

Describe pain reflex

Answer 7

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

Question 8

Describe autogenic inhibition reflex

Answer 8

Stimulus: excessive tension on tendon
Receptor: golgi tendon organ 
Afferent: Ib neuron
Interneuron: inhibitory
Efferent: alpha motor
Effector: extrafusal muscle
Response: relaxation of muscle

Question 9

Describe reciprocal inhibition

Answer 9

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

Question 10

Describe primary neuron in a conscious sensory pathway

Answer 10

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

Question 11

Describe secondary neuron in a conscious sensory pathway

Answer 11

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

Question 12

Describe tertiary neuron in conscious sensory pathway

Answer 12

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

Question 13

Describe the primary somesthetic cortex in the conscious sensory pathway

Answer 13

Plays a role in perception and discrimination of sensory stimuli

Question 14

Describe association cortex in conscious sensory pathway

Answer 14

Involved in integration, modification, and interpretation of sensory information

Question 15

Describe the pathway for fast pain/temperature

Answer 15

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

Question 16

Describe the pathway for crude tactile

Answer 16

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

Question 17

Describe pathway for proprioception, 2-pt tactile

Answer 17

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

Question 18

What are often referred to as the “final common pathway”?

Answer 18

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

Question 19

What causes lower motor neuron paralysis?

Answer 19

Destruction of motor neurons or axons of one or more cranial or spinal motor nuclei

Question 20

What is flaccid paralysis?

Answer 20

Muscle is completely “limp,” and there is no resistance to passive movement

Question 21

What is areflexia?

Answer 21

Loss of efferent component of reflex arc to a muscle results in absence of associated muscle reflex

Question 22

What is atonia?

Answer 22

Destruction of gamma motor neurons or their axons results in absence of muscle tone

Question 23

What is atrophy?

Answer 23

Denervated muscle atrophies due to loss of stimulation from motor neurons

Question 24

What are fasciculations?

Answer 24

Twitching of denervated muscle, probably due to hypersensitivity of motor end plate

Question 25

What is poliomyelitis?

Answer 25

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)

Question 26

From where do corticospinal and corticobulbar tracts arise?

Answer 26

From large pyramidally-shaped neurons (cells of Betz) located in primary motor and premotor cortices

Question 27

Describe the pyramidal system

Answer 27

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

Question 28

Describe the descent of the corticospinal tract (CST)

Answer 28

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)

Question 29

Describe lateral corticospinal tract (LCST)

Answer 29

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

Question 30

What happens if there is a unilateral lesion of the LCST?

Answer 30

Ipsilateral paralysis or paresis of distal limb musculature innervated by those spinal segments below the level of the lesion

Question 31

Describe the anterior corticospinal tract (ACST)

Answer 31

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

Question 32

What happens if there is a unilateral lesion of the the ACST?

Answer 32

There is minimal clinical effect

Question 33

What are the signs and symptoms of upper motor neuron (UPN) lesions?

Answer 33

``` 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 ```

Question 34

Describe spinal cord injury (SCI)

Answer 34

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

Question 35

Define spasticity

Answer 35

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

Question 36

Define rigidity

Answer 36

Abnormal passive resistance to movement in all directions

Question 37

What is decerebrate rigidity

Answer 37

Spasticity of extensors of both upper and lower extremities | These patients usually do not survive

Question 38

Describe decorticate rigidity

Answer 38

Spastic hemiplegia of flexors of upper extremity and extensors of lower extremity, usually due to a lesion of the internal capsule

Question 39

Compare/contrast the characteristics of the clinical syndrome of UMN and LMN lesions

Answer 39

``` 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

Question 40

Describe Amyotrophic lateral sclerosis (ALS)

Answer 40

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

Question 41

What is clinically important about posterior columns?

Answer 41

Ascending sensory tracts for proprioception, 2-pt tactile discrimination and vibratory sensations

Question 42

What is clinically important about lateral funiculus?

Answer 42

Descending UMN tracts for volitional control of limb musculature Descending UMN tracts for autonomic control of bladder and bowe

Question 43

What is clinically important about anterior funiculus?

Answer 43

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

Question 44

Describe the afferent neuron in a reflex arc

Answer 44

``` 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 ```