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Neuro Exam 3 > Basic Sensory And Motor Mechs And Pathways > Flashcards

Basic Sensory And Motor Mechs And Pathways Flashcards

1
Q

Describe the interneuron in a reflex arc

A

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)

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2
Q

What is the fasciculus proprius?

A

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

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3
Q

What is the efferent neuron in a reflex arc?

A

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

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4
Q

What is the effector in a reflex arc?

A

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

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5
Q

What are the components of a basic reflex pathway?

A 
Receptor
Afferent neuron
Interneuron (excitatory/inhibitory, intrasegmental, intersegmental, commissural)
Efferent neuron
Effector (motor end plate)
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6
Q

Describe the myotatic reflex

A 
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
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7
Q

Describe pain reflex

A 
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
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8
Q

Describe autogenic inhibition reflex

A 
Stimulus: excessive tension on tendon
Receptor: golgi tendon organ 
Afferent: Ib neuron
Interneuron: inhibitory
Efferent: alpha motor
Effector: extrafusal muscle
Response: relaxation of muscle
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9
Q

Describe reciprocal inhibition

A 
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
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10
Q

Describe primary neuron in a conscious sensory pathway

A 
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
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11
Q

Describe secondary neuron in a conscious sensory pathway

A

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

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12
Q

Describe tertiary neuron in conscious sensory pathway

A

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

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13
Q

Describe the primary somesthetic cortex in the conscious sensory pathway

A

Plays a role in perception and discrimination of sensory stimuli

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14
Q

Describe association cortex in conscious sensory pathway

A

Involved in integration, modification, and interpretation of sensory information

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15
Q

Describe the pathway for fast pain/temperature

A

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

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16
Q

Describe the pathway for crude tactile

A

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

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17
Q

Describe pathway for proprioception, 2-pt tactile

A

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

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18
Q

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

A

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

19
Q

What causes lower motor neuron paralysis?

A

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

20
Q

What is flaccid paralysis?

A

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

21
Q

What is areflexia?

A

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

22
Q

What is atonia?

A

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

23
Q

What is atrophy?

A

Denervated muscle atrophies due to loss of stimulation from motor neurons

24
Q

What are fasciculations?

A

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

25
Q

What is poliomyelitis?

A

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)

26
Q

From where do corticospinal and corticobulbar tracts arise?

A

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

27
Q

Describe the pyramidal system

A

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

28
Q

Describe the descent of the corticospinal tract (CST)

A

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)

29
Q

Describe lateral corticospinal tract (LCST)

A

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

30
Q

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

A

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

31
Q

Describe the anterior corticospinal tract (ACST)

A

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

32
Q

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

A

There is minimal clinical effect

33
Q

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

A 
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
34
Q

Describe spinal cord injury (SCI)

A

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

35
Q

Define spasticity

A

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

36
Q

Define rigidity

A

Abnormal passive resistance to movement in all directions

37
Q

What is decerebrate rigidity

A

Spasticity of extensors of both upper and lower extremities

These patients usually do not survive

38
Q

Describe decorticate rigidity

A

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

39
Q

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

A 
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

40
Q

Describe Amyotrophic lateral sclerosis (ALS)

A

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

41
Q

What is clinically important about posterior columns?

A

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

42
Q

What is clinically important about lateral funiculus?

A

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

43
Q

What is clinically important about anterior funiculus?

A

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

44
Q

Describe the afferent neuron in a reflex arc

A 
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

Neuro Exam 3 (27 decks)

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