Week 9, Monday

The Motor System for Quiz 4 pdf (continued)

Review how to differentiate cross-sections

• know the functions of each structure visible at each level

Reflexes:

• Involuntary

• Stereotyped response to a sensory input

• Almost all reflexes involved a receptor, an afferent neuron, an inter neuron and an efferent neuron

  • Exception

    • stretch reflex doesn’t use an interneuron

• Several forms:

  • Stretch reflex

  • Golgi tendon refleX

  • Withdrawal reflex

What reflexes function with the motor system:

• Stretch reflex:

  • A muscle will contract in response to being rapidly stretched

  • Thought to produce postural corrections

• Golgi Tendon Organ Reflex:

  • A muscle can adjust its force of contraction in response to tension within the muscle

  • Thought to either protector against excessive tension or make fine adjustments to force contraction

• Withdrawal Reflex:

  • Muscles contract in whatever direction is necessary to move the body away from a painful stimulus

Parts contributing to the the Stretch Reflex:

• Muscle Spindle

  • Small capsule within muscle

  • Senses muscle stretch

• Intrafusal fibers

  • Muscle fibers within the muscle spindle

• Extrafusal fibers

  • Muscle fibers outside the muscle spindle

  • As they stretch, they stretch intrafusal fibers

• Alpha motor neuron

  • The lower motor neuron innervation the muscle

  • An Aa fiber

• Gamma motor neuron

  • The neuron innervation intrafusal fibers

  • An Ay fiber

How is the Stretch Reflex wired:

• Afferent limb

  • Is afferent fiber attached to muscle spindle

  • Sends central processes into spinal cord

  • Synapses directly with alpha motor neuron

• Efferent limb

  • Alpha motor neuron (LMN) Innervation muscle containing the spindle

The Stretch Reflex tells us:

• it is common to test clinically

• Testing reflexes tells you information about the sensory nerve, spinal cord segment, and motor nerve

• Biceps reflex

  • Biceps Brachii muscle

  • C5

  • Musculocutaneous peripheral nerve

• Brachioradialis Reflex

  • Brachioradialis muscle

  • C6

  • Radial peripheral nerve

• Triceps Reflex

  • Triceps Brachii muscle

  • C7

  • Radial peripheral nerve

• Knee-Jerk (patellar) reflex

  • Quadriceps femoris muscle

  • L4

  • Femoral peripheral nerve

• Ankle-Jerk (Achilles) reflex

  • Gastrocnemius and Soleus muscles

  • S1

  • Tibial peripheral nerve

Golgi Tendon Organ:

• Capsule at junction between muscle and tendon

• Fibers sit between collagen bundles of the tendon

• As the muscle contracts, tension is placed on the capsule, squeezing the nerve fibers, stimulating them

How is Golgi Tendon organ wired?

• can produce excitatory or inhibitory reflex actions depending on limb position and activity

• Auto genie inhibition

  • Inhibiting the muscle fiber it is connected to

  • Involves inhibitory inter neurons

The Withdrawal Reflex, and How is it Wired:

• removes a limb from a painful stimulus

• Widely Distributed

  • Several muscles and spinal segments

  • Many branches of sensory neurons and inter neurons

• Higher motor centers normally inhibit this pathway

  • A strong, painful stimulus is required to produce this relfex

  • Tough receptors wired to this pathway, it generally not strong enough

Can reflexes be modified?

• Reciprocal Effects

  • Reflex doesn’t just produce changes in the agonist, but also the antagonist and synergist

  • Can affect anatomist muscles and muscles of the opposite limb

  • Reciprocal inhibition

    • What a reflex activates a muscle, it simultaneously activates the appropriate synergism’s and inhibits the antagonists

• Crossed Effect

  • Reflex may produce effects in both ipsilateral and contralateral limbs

  • A withdrawal reflex in one lower limb causes the opposite effects in the contralateral limb

• Reflex sensitivity can be adjusted base on needs

  • I.e. siting, walking, standing on the limb, etc.

How can motor system damage be classified?

• Broadly speaking, there are 3 types of damage to motor systems:

  • Damage to motor neurons produces weakness and/or paralysis together with altered reflexes, muscle tone and atrophy

  • Damage to higher motor centers produces abnormal movements without weakness or paralysis

    • I.e. Involuntary, poorly coordinated, etc.

LMN and UMN injury results:

• LMN injuries are seen at the relevant myotome

• UMN injuries are seen at all levels inferior to the site of the injury

• UMN damage more notable with LCST damage

• LMN injury =

  • Decreases strength

  • Decreased muscle tone

  • Decreased stretch reflexes

  • Severe atrophy

  • Fasciculations and Fibrillations can occur

    • Involuntary muscle twitches

    • Fasciculation is visible, fibrillation is not

• UMN injury =

  • Decreased strength

  • Increased muscle tone

  • Increased stretch reflexes

  • Mild Atrophy

  • Clonus, pathological reflexes (i.e. Babinki’s sign) may occur

    • Clasp-knife effect

      • Limb resists stretch but then collapses

    • Clonus

      • Sudden stretch resulting in rhythmic contractions for duration of stretch

    • Hemiparetic Stance/gait:

      • Likely due to hypertonia

What results from Spinal Shock:

• occurs during first few weeks after CNS injury

• LMN symptoms during this time

• UMN symptoms set in after weeks

• Autonomic system also becomes hyperreflexive

  • May lead to life threatening hyperactivity

• Effects thought to be due to:

  • loss of descending inputs, followed by formation of new synaptic connections

  • degeneration of descending fibers, etc.

What is the result of isolated damage to one tract:

• recovery can occur following isolated damage to Corticospinal tract

  • Spinal shock occurs initially, but following this strong recovery occurs

  • Primary permanent damage lies in the inability to use the fingers individually

  • Damage to tract before pyramid is more disabling than damage to tract after pyramid (collaterals)

• Damage to tract and reticular formation (reticule spinal tract) produces more severe issues

  • Medial reticular tract

    • Disability of axial muscles

  • Lateral reticular tract

    • Disability of independent use of arms

• Damage to the peripheral nerves

  • Ipsilateral LMN symptoms

  • Only affects myotome(s) of the nerve(s) affected

• Damage to the LCST

  • Ipsilateral UMN symptoms

  • All levels below the site of the injury are afffected

• Damage to the ACST

  • No notable symptoms as long as LCST is intact

• Damage to the pyramidal decussation

  • Bilateral UMN symptoms

  • All levels below sire of injury affected

• Damage above the pyramidal decussation

  • Contralateral UMN symptoms

  • All levels below injury affected

Damage to the Corticospinal tract (Summary):

• Peripheral nerve damage

  • Ipsilateral LMN symptoms

• Unilateral LCST damage

  • Ipsilateral UMN symptoms

• Spinal cord transaction or pyramidal decussation damage

  • Bilateral UMN symptoms

• Ipsilateral brainstem or cerebral cortex damage

  • Contralateral UMN symptoms

Myotomes help locate site of damage in spinal cord

• Test myotomes and reflexes to locate damage to peripheral nerves, spinal cord

Cranial nerves help locate the site of the damage in the brainstem

• CN III

  • Diplopia

  • Eye down and out

  • Mydriasis (dilated pupil)

  • Ptosis (droopy eyelid)

• CN IV

  • Diplopia

  • Head tilted

  • Difficulty looking down

• CN V3

  • Weakness of muscles of mastication

• CN VI

  • Diplopia

  • Eye deviates medially

  • Difficulty abducting eye

• CN VII

  • Weakness of muscles of facial expression

• CN IX

  • Difficulty swallowing

• CN X

  • Difficulty swallowing

  • Hoarse voice

• CN XI

  • Difficulty shrugging shoulders/ turning head

• CN XII

  • Tongue deviates to affected side

  • Difficulty speaking and swallowing