Karius Reflexes BS Flashcards
What reflexes are under cortical control?
Placing and hopping reaction
What are the spinal reflexes?
Stretch (myotatic), golgi tendon reflex, and crossed extensor reflex
What reflexes are under brainstem/midbrain control?
Vestibular, righting, suckle, yawn, and eye/head movements reflexes
Describe a myotatic reflex in terms of appearance, purpose, and its characteristics
Appearance: contraction (shortening) of a stretched muscle
Purpose: protect muscle from tearing due to stretch
Characteristics: initiated by muscle spindle; monosynaptic, segmental reflex
Describe the muscle spindle
Found within skeletal muscle, embedded in a fusiform capsule, parallel to the muscle fibers; contains both afferent (sensory) and efferent (contractile) components
What are intrafusal fibers? Extrafusal fibers?
Intra: fibers within the capsule
Extra: make up bulk of the muscle and are outside the capsule
T/F: Intrafusal fibers of the muscle spindle only have a motor component.
False; intrafusal fibers have both a motor and sensory component
What are the characteristics of the sensory portion of the muscle spindle?
Not contractile, it is the portion that is sensitive to length, it is actually 2 sensors with different afferents (a nuclear bag fiber and a nuclear chain fiber)
What are the characteristics of the primary afferent (Ia fiber)?
Innervates both the nuclear bag and nuclear chain; large, myelinated Ia fiber; it is sensitive to both length of muscle and how fast the length is changing
What are the characteristics of the secondary afferent?
Smaller, myelinated group II fiber; innervates ONLY nuclear chain fiber; sensitive only to the length of the muscle
What are the characteristics of the motor portion of a myotatic reflex?
Consists of intrafusal fibers; same histology as skeletal muscle; innervated by a gamma-motor neuron; controls the length of the sensory portion
What happens to the sensory portion when we contract the intrafusal fibers?
Stretch sensory portion, rendering it more sensitive to a superimposed stretch
Describe alpha motor neurons
Large, heavily myelinated fiber that innervates skeletal musle (via NMJ, extrafusal fibers); responsible for activating muscle and activity directly leads to motion
Describe gamma motor neurons
Slightly smaller, slower than alpha, but still fast overall; innervates contractile component (intrafusal fibers) of the muscle spindle via NMJ; activity causes contraction; controls sensitivity of muscle spindle; activity does NOT directly lead to motion
T/F: In a myotatic reflex, the Ia afferent from the muscle spindle synapses directly onto the alpha motorneuron innervating the stretched muscle
True
Describe the golgi tendon reflex in terms of appearance, purpose, and characteristics
Appearance: sudden (abrupt) relaxation of a contracted muscle
Purpose: protect muscle from damage due to excessive force
Characteristics: initiated by golgi tendon organ; polysynaptic, segmental reflex
Describe golgi tendon organs
Innervate tendon; bare nerve endings with lots of branches; action potentials increase w/ tension; Ib fiber to spinal cord
Compare and contrast the myotatic reflex and the golgi tendon reflex
Myotatic: initiated by muscle spindle; passive stretch of the muscle; monosynaptic reflex; contraction of stretched muscle back to normal length
Golgi Tendon (reverse myotatic): initiated by golgi tendon; active contraction of muscle; polysynaptic reflex; abrupt relaxation of contracted muscle to prevent damage
What is spinal shock?
Occurs after an injury/transection; even though neurons producing a reflex are below the level of injury and are intact, the reflex fails to occur
What does the recovery from spinal shock result from?
Axonal sprouting below the level of the transection; expression of receptor phenotypes that are self-activating (5HTC receptor)
What does decerebrate posturing result from?
Loss of all structures rostral to the pons (caudal to red nucleus)
What is rigidity?
Resists motion in all directions; results from maintained muscle contraction; continual activation of alpha-motorneurons; brainstem is activating alpha-motorneurons but we don’t know why
What is spasticity?
Resists motion in a given direction; myotatic reflex is hyperactive; continual activation of gamma-motor neurons; contracts intrafusal muscle, lengthens nuclear bag/chain fibers; brainstem controls this
What is the brainstem facilitatory region?
It is spontaneously active; by activating gamma-motor neurons, makes muscle spindle more sensitive
What is the brainstem inhibitory region?
Region requires activation from cortical regions; inhibits gamma-motorneurons, making muscle spindle less sensitive
What happens when you lose the cortex?
Brainstem inhibitory region is not activated, leaving the facilitatory region to dominate
What is decorticate posturing?
Flexion of the upper limb joints, extension of the lower limbs, and is dependent on head position
Disinhibition of red nucleus and its control of UE flexors, disinhibiton of reticulospinal and vestibulospinal pathways, and release of postural reflexes
What does decorticate posturing result from?
Strokes in the vicinity of the internal capsule, a relatively common site for a stroke
What is decerebrate posturing?
Contraction of all anti-gravity muscles (arms in extension), loss of input from all structures rostral to the pons (caudal to red nucleus), and usually indicative of severe brain injury
