Flashcards in Module E-09 Deck (45)
A hypertonic state with increased deep tendon reflexes.
Weakness, laxness and softness of muscle that offer limited
resistance to passive movement.
Rhythmic palpable oscillations between flexion and extension
Monosynaptic Reflex (i.e., Deep Tendon or Myotatic Reflex)
- Afferent and efferent limbs are directly connected (one synapse)
- Afferent and efferent limbs are connected via one or more interneurons
- multiple synapses involved
ex of Polysynaptic reflexes
- Pupillary light reflexes
- abdominal reflex
Components of Reflex Arcs
- Sensory receptor (e.g., muscle spindle, Golgi tendon organ, free nerve ending)
- Afferent axon
- Synapse on interneurons or efferent neuron (LMN)
- Neuromuscular junction
Components of Afferent LImb of Reflex
- Muscle spindles and Ia fibers
- Golgi tendon organs and Ib fibers
- Free nerve endings and Aδ fibers
Components of Efferent limb of Reflex
Alpha motor neurons, homonymous, synergist and antagonist muscles
Nature and Purpose of Deep Tendon Reflexes
- Passive stretching of muscle causes contraction.
- The reflex regulates muscle tone under modulation by upper motor neurons.
Muscle and segment being tested in Inspiration
Segment: C3,4,5 (keep the phrenic alive ;P)
Muscle and segment being tested in Shoulder Abduction
Muscle and segment being tested in Elbow Flexion
Muscle: Biceps Brachii, Brachialis
Muscle and segment being tested in Wrist extension
Muscle: Extensor Carpi , Radialis Longus and Brevis
Muscle and segment being tested in Elbow Extension
Muscle: Triceps Brachii
Muscle and segment being tested in Finger Flexion
Muscle: Flexor digitorum superficialis and profundus
Muscle and segment being tested in Finger abduction and adduction
Muscle and segment being tested in Thigh adduction
Muscle: Adductor longus and brevis
Muscle and segment being tested in Knee extension
Muscle and segment being tested in Great Toe extension
Muscle: Extensor hallucis longus
Muscle and segment being tested in Ankle Plantar flexion
Muscle: Gastrocnemius, soleus
Muscle and segment being tested in Anal Contraction
Muscle: Sphincter ani externus
Scale for Reflex testing
- 4+ - Very brisk or hyperactive with clonus
- 3+ - Brisker than average (high normal)
- 2+ - Average (normal)
- 1+ - Somewhat diminished (low normal)
- 0 - No response
Describe the pathway for the Stretch Reflex
1) The sensory signal for the stretch reflex arises in the muscle spindles.
2) The tapping a tendon with a reflex hammer stretches the muscle spindles, activating the associated Ia afferents.
3)The signals are transmitted to the spinal α motor neurons and then directly to the muscle.
What causes Muscle Tone?
- Muscles are usually under some degree of stretch, so these reflex arcs are a vital determinant of ‘muscle tone’.
- Muscular tone reflects basal partial contraction of extrafusal fibers.
Purpose of Stretch reflex
The stretch reflex constitutes a feedback loop that maintains muscle length and hence dictates posture.
Role of cortical pathways in Posture maintenance ( tone)
Descending motor pathways (e.g., corticospinal) comprising upper motor neurons can regulate muscle tone and hence posture by influencing components of the reflex arc.
Deviations in muscular length from optimal levels trigger adjustments in the activity of α-motor neurons, yielding contraction of the stretched muscle and relaxation of opposing muscles
Two systems involved in modulating the stretch reflex
1) Descending fibers (such as corticospinal tract)
2) Efferent fibers (gamma motor neuron)
Role of Gamma motor neurons (gamma loop) in posture maintence
- Smaller γ motor neurons modulate the excitability of
- When the muscle contracts, the tension of the muscle spindle is reduced, and the spindle is no longer able to measure changes of the muscle length.
- The γ motor neurons therefore activate the contractile ends of the muscle spindle fibers and maintain the tension of the middle region of the intrafusal fibers where the sensory axons are located.
- This co-activation of α and γ motor neurons (i.e., co-activation within the gamma loop) enables the spindles to function at all muscle lengths, thereby regulating both precision of movement and maintenance of posture.
Why is the inverse stretch reflex (inverse myotatic or Golgi tendon reflex) is slower than the myotatic reflex?
1) The Ib fibers that constitute the afferent limb of the inverse myotatic reflex may be of marginally smaller
diameter and hence, on average, support slightly lower conduction velocities than the Ia fibers.
2) It is polysynaptic (not monosynaptic), delaying responses.
Function of the inverse stretch reflex
Protection of the muscle and tendon. Dangerously contracted muscles relax via negative feedback.
Pathway of the inverse stretch reflex
1) Starts at golgi tendon organ
2) 1b fiber carries the signal to the spinal cord
3) synapses with inhibitory interneuron
4) Alpha motor neuron is inhibited and muscle relaxes
What is the flexion reflex?
Noxious stimulation of the skin induces a polysynaptic
reflex which produces ipsilateral excitation of flexor muscles (and inhibition of extensors) and contralateral
excitation of extensor muscles (and inhibition of
- causes withdrawal or protective flexion following a noxious stimulus
Components of the Flexion reflex
• Afferent limb: nociceptive fibers (A delta and C).
• CNS units: a number of excitatory and inhibitory interneurons over many spinal segments (Lissauer's Tract).
• Efferent limbs: alpha motor neurons, ipsilateral flexors and contralateral extensors
What is the flexion crossed extension reflex?
In the flexion reflex, when pulling away from a noxious stimulus ( activation of flexors and inhibition of extensors), some fibers cross over cause extension ( activation of extensors and inhibition of flexors) of the contralateral limb
What is the normal response seen to the Plantar Reflex ( in those over 2 years old )?
Flexion of the toes, a response that may be masked by vigorous withdrawal of the leg ( if patient is too sensitive)
What is the abnormal response seen to the Plantar Reflex ( in those over 2 years old )?
Dorsiflexion of the big toe, often accompanied by fanning of the other toes.
What is the abnormal plantar response called?
What does a Babinski's sign indicate?
a dysfunctional lateral corticospinal system. Such dysfunctions could reflect cortical or axonal disruptions, demyelination, etc.
Why do newborns show an extensor plantar response?
because their pyramidal (corticospinal) tract is immature (incompletely myelinated).
Is there a non pathlogical reason for exhibiting the extensor plantar response?
yes, during Sleep
What does Hyperreflexia indicate?
- suggest an upper motor neuron lesion.
- The damage may affect the motor cortex (e.g., precentral gyrus) or other corticospinal sites
- In spinal cord lesions, hyperreflexia may develop after a
phase of areflexia, which characterizes ‘spinal shock’
What causes Spinal shock?
Extensive bilateral spinal damage
What occurs in spinal shock?
- Transient areflexia occurs caudal to the site of the lesion.
- Disrupted reflexes will typically re-emerge after several weeks.