Lecture 2: Motor Reflexes Flashcards
What are the 2 Cortically mediated reflexes?
1) Placing reaction: baby automatically trying to step on something when being held and feet slightly brush up against a surface
2) Hopping reaction: being pushed and hopping to maintain balance
What are the 5 brainstem/midbrain mediated reflexes?
1) Vestibular
2) Righting reflex
3) Suckle (children)
4) Yawn
5) Eye/head movements
What are the 3 spinally mediated reflexes?
1) Stretch (myotatic) and (deep-tendon reflex)
2) Golgi tendon reflex
3) Crossed extensor
How do reflexes compare to volitional motion; what are the major differences?
- In order for relfexes to be effective, must be very precise and this precision makes reflexes look like volitional movement
- BUT reflex movements occur FASTER than even the fastest voluntary movement
- Reflexes mediated at all levels of CNS, voluntary motion originates in cortical areas
- Reflexes elicited even when unconscious, voluntary motion requires conscious awareness
What is the appearance, purpose, and characteristics (i.e., initiated by and what type of reflex) of the Myotatic (stretch) reflex?
Appearance: contraction (shortening) of a stretched muscle
Purpose: protect muscle from tearing due to over-stretch
Characteristics: initiated by muscle spindle.
- Monosynaptic, segmental reflex
α-motor neurons innervate which muscle fibers, resulting in?
Innervate extrafusal muscle fibers (the fibers resulting in contraction)
γ-motor neurons innervate which muscle fibers, which are a component of what?
- Innervate the intrafusal muscle fibers, a component of the muscle spindle
- Intrafusal fibers are distributed in parallel arrangement w/ the extrafusal fibers of skeletal muscle
What are the components of a muscle spindle?
A motor component and a sensory component
What are the characteristics of the sensory portion of the muscle spindle, sensitive to what, and what does it contain?
- Is NOT contractile
- Is the portion sensitive to length
- Is acutally TWO sensors - with different afferents
1) Nuclear bag fiber
2) Nuclear chain fiber
What fiber is the primary afferent of the muscle spindle, innervates what, and is sensitive to?
- The Ia fiber (LARGE and myelinated)
- Innervates both the nuclear bag and nuclear chain
- Sensitive to both: length of muscle AND how fast length is changing
What fiber is the secondary afferent of the muscle spindle, innervates what, and is sensitive to?
- The smaller, myelinated Group II fiber
- Innervates only the nuclear chain fiber
- Sensitive only to the length of the muscle
What innervates the motor portion (intrafusal contractile elements) of a muscle spindle and what is the function of this portion?
- γ-motoneuron
- By contracting the intrafusal portion, we stretch the sensory portion and it becomes even more sensitive to a superimposed stretch
- Controls the sensitivity
How does the activity of the γ-motoneuron correlate with sensitivity of the muscle spindle?
- More active γ-motoneuron = more sensitive muscle spindle
- Less active γ-motoneuron = less sensitive muscle spindle
What does the afferent portion of the muscle spindle do within the spinal cord and this leads to what?
- Forms an excitatory synapse directly on the homonymous muscle’s alpha motor neuron in the ventral horn of the SC
- This is a monosynaptic connection
- Also forms an excitatory synapse on GABAergic inhibitory interneurons that inhibit the antagonist muscle (forms the basis of reciprocal innervation)
How does the end result of the activity of alpha vs. gamma motor neurons differ?
Alpha = directly leads to motion
Gamma = causes contraction, but does NOT directly lead to motion
What is reciprocal inhibition and how is it involved in the myotatic (stretch) reflex; why is it important?
- The Ia afferent enters the SC, synapses and releases EAA’s on the alpha-motor neuron, BUT..
- The Ia afferent also gives off a branch which synapses on an interneuron which will inhibit the alpha-motor neuron of the antagonist muscle
- Allows the antagonist muscle to relax and lengthen
The muscle spindle can be stretched by what 2 mechanisms?
1) Stretch of the whole muscle
2) By stimulation of the gamma motor neurons that produce contraction of the ends of the muscle spindle
What are the 5 sequences of events involved in the stretch reflex (i.e., the spindle, afferents, and excitation)?
- Muscle stretch, either by manipulation or by tendon tap
- Muscle spindle stretch (due to the in-parallel arrangement)
- Increase discharge of spindle Ia afferents
- Excitation of α-motoneurons
- Muscle contraction increases to oppose the stretch
What is the Golgi Tendon reflex also referred to?
Inverse myotatic w/ autogenic inhibition
What is the appearance of the Golgi Tendon reflex, the purpose, and what initiates it/type of reflex?
- Sudden (abrupt) relaxation of a contracted muscle
- Protect muscle from damage due to excessive force
- Initiated by golgi tendon organ
- Polysynaptic, segmental reflex
The Golgi Tendon organs innervate what and what type of fiber do they utilize?
- Innervate tendon
- Bare nerve ending with lots of branches; Ib fiber to spinal cord
What happnes to the AP’s in golgi tendon organs as tension increases?
AP’s increase w/ tension
What is the order of events (5 of them) for the Golgi tendon reflex?
1) Muscle contracts putting massive strain on tendon
2) Increased tension increases AP’s in Ib afferent fiber
3) Ib afferent synapses on inhibitory interneuron in SC
4) Inhibition of the α-motorneuron of the same muscle
5) Muscle abruptly relaxes and tension on tendon is relieved
What type of synaptic reflex is a Myotatic (stretch) vs. Golgi tendon (reverse myotatic)?
Myotatic (stretch) is a monosynaptic reflex; contraction of a stretched muscle back to normal length
Golgi tendon is a polysynaptic reflex; abrupt relaxation of contracted muscle (protective)
What is Spinal shock; how long does it last; what helps with recovery?
- An acute spinal cord injury w/ loss of reflexes and sensation below the level of the transection
- All reflexes abolished even if circuit is intact
- Gradual recovery in humans: week-years; partly due to loss of excitatory bias from CNS
- Some of the recovery seems to be due to sprouting of axons below the transection to form synapses on neighboring cells
What is Spasticity vs. Rigidity; which motor neurons contribute to each?
Spasticity: is an increased resistance to the passive movement of a joint in a given direction; from continual activation of gamma-motor neurons
Rigidity: increased resistance in all directions; from continual activation of alpha-motor neurons
Decerebrate posturing is a result of; what is seen with the pathology?
- Loss of all structures/input rostral to the pons; extensive UMNL
- Contraction of all anti-gravity muscle (arms in extension, head is arched back)
- Rigidity due to an increase in α-motoneuron excitability
- Spasticity due to an increase in γ-motorneuron firing
What is seen with the upper limbs, lower limbs, and head position with decorticate posturing?
- Flexion of the upper limb joints
- Extension of the lower limbs
- Dependent on head position!
The brainstem inhibitory regions requires activation from what regions and what is its actions on gamma-motor neurons; how is the region related to spasticity/decerebrate posturing?
- Requires activation from cortical regions and inhbits gamma-motor neurons
- Makes muscle spindle less sensitive
- An injury producing decerebrate posturing abolishes activity of cortex so activation of gamma-motor neurons is unopposed
Why are clinicians extremely likely to see patients with unilateral decorticate posturing?
- Is a result of strokes in the vicinity of the internal capsule
- A relatively common sign for a stroke
How does head position affect the positon of someone with Decorticate posturing?
- If head neutral, affected leg extended and affected arm somewhat flexed
- If you rotate the head either direction - increases extensor tonus of arm on side head is rotated towards but flexion of the opposite arms
