Karius Reflexes BS

Question 1

What reflexes are under cortical control?

Answer 1

Placing and hopping reaction

Question 2

What are the spinal reflexes?

Answer 2

Stretch (myotatic), golgi tendon reflex, and crossed extensor reflex

Question 3

What reflexes are under brainstem/midbrain control?

Answer 3

Vestibular, righting, suckle, yawn, and eye/head movements reflexes

Question 4

Describe a myotatic reflex in terms of appearance, purpose, and its characteristics

Answer 4

Appearance: contraction (shortening) of a stretched muscle

Purpose: protect muscle from tearing due to stretch

Characteristics: initiated by muscle spindle; monosynaptic, segmental reflex

Question 5

Describe the muscle spindle

Answer 5

Found within skeletal muscle, embedded in a fusiform capsule, parallel to the muscle fibers; contains both afferent (sensory) and efferent (contractile) components

Question 6

What are intrafusal fibers? Extrafusal fibers?

Answer 6

Intra: fibers within the capsule

Extra: make up bulk of the muscle and are outside the capsule

Question 7

T/F: Intrafusal fibers of the muscle spindle only have a motor component.

Answer 7

False; intrafusal fibers have both a motor and sensory component

Question 8

What are the characteristics of the sensory portion of the muscle spindle?

Answer 8

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)

Question 9

What are the characteristics of the primary afferent (Ia fiber)?

Answer 9

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

Question 10

What are the characteristics of the secondary afferent?

Answer 10

Smaller, myelinated group II fiber; innervates ONLY nuclear chain fiber; sensitive only to the length of the muscle

Question 11

What are the characteristics of the motor portion of a myotatic reflex?

Answer 11

Consists of intrafusal fibers; same histology as skeletal muscle; innervated by a gamma-motor neuron; controls the length of the sensory portion

Question 12

What happens to the sensory portion when we contract the intrafusal fibers?

Answer 12

Stretch sensory portion, rendering it more sensitive to a superimposed stretch

Question 13

Describe alpha motor neurons

Answer 13

Large, heavily myelinated fiber that innervates skeletal musle (via NMJ, extrafusal fibers); responsible for activating muscle and activity directly leads to motion

Question 14

Describe gamma motor neurons

Answer 14

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

Question 15

T/F: In a myotatic reflex, the Ia afferent from the muscle spindle synapses directly onto the alpha motorneuron innervating the stretched muscle

Answer 15

True

Question 16

Describe the golgi tendon reflex in terms of appearance, purpose, and characteristics

Answer 16

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

Question 17

Describe golgi tendon organs

Answer 17

Innervate tendon; bare nerve endings with lots of branches; action potentials increase w/ tension; Ib fiber to spinal cord

Question 18

Compare and contrast the myotatic reflex and the golgi tendon reflex

Answer 18

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

Question 19

What is spinal shock?

Answer 19

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

Question 20

What does the recovery from spinal shock result from?

Answer 20

Axonal sprouting below the level of the transection; expression of receptor phenotypes that are self-activating (5HTC receptor)

Question 21

What does decerebrate posturing result from?

Answer 21

Loss of all structures rostral to the pons (caudal to red nucleus)

Question 22

What is rigidity?

Answer 22

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

Question 23

What is spasticity?

Answer 23

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

Question 24

What is the brainstem facilitatory region?

Answer 24

It is spontaneously active; by activating gamma-motor neurons, makes muscle spindle more sensitive

Question 25

What is the brainstem inhibitory region?

Answer 25

Region requires activation from cortical regions; inhibits gamma-motorneurons, making muscle spindle less sensitive

Question 26

What happens when you lose the cortex?

Answer 26

Brainstem inhibitory region is not activated, leaving the facilitatory region to dominate

Question 27

What is decorticate posturing?

Answer 27

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

Question 28

What does decorticate posturing result from?

Answer 28

Strokes in the vicinity of the internal capsule, a relatively common site for a stroke

Question 29

What is decerebrate posturing?

Answer 29

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