Reflexes

Question 1

define motor unit

Answer 1

motor neuron and all fibers it innervates

Question 2

define motor neuron pool

Answer 2

all axons that innervate a muscle

Question 3

what are lower motor neurons, and what do they include?

Answer 3

motor neurons of the brainstem & spinal cord that innervate skeletal muscle:
- ventral horn cells & processes
- motor nuclei of CNs & processes

Question 4

name the 2 types of ventral horn cells

Answer 4

alpha motor neurons
gamma motor neurons

Question 5

what do alpha (a) motor neurons innervate?

Answer 5

extrafusal skeletal muscle fibers

Question 6

what do gamma (y) motor neurons innervate?

Answer 6

intrafusal fibers (specialized skeletal muscle fibers enclosed in a fibrous capsule called a muscle spindle)

Question 7

symptoms of LMN lesion

Answer 7

• paresis/paralysis/plegia
• atrophy
• fasciculations
• hypotonia and hyporeflexia
• fibrillations

Question 8

what are fasciculations?

Answer 8

visible muscle twitching resulting from involuntary synchronous contraction of all muscle fibers in a motor unit

Question 9

fasciculations are characteristic of…

Answer 9

slowly progressive diseases of the motor unit

Question 10

what are fibrillations?

Answer 10

spontaneous activity of one muscle fiber detected only with electromyography

Question 11

fibrillations are a sign of…

Answer 11

progressed muscle denervation or myopathy

Question 12

what is a muscle spindle?

Answer 12

• specialized skeletal muscle fibers in fibrous capsule situated in parallel with the extrafusal muscle fibers
• composed of intrafusal m fibers, sensory (afferent) axons, and motor axons (efferents which regulate sensitivity)

Question 13

what is the motor innervation of muscle spindles?

Answer 13

gamma (y) motor neurons

Question 14

describe alpha and gamma co-activation

Answer 14

• stretch-> 1a proprioceptors in muscle spindle send info in spinal cord about stretch
• contraction-> intrafusal fibers become slack
• y-motor neurons cause them to tighten back up so sensitivity is not lost

Question 15

why is alpha and gamma co-activation important?

Answer 15

ensures extrafusal and intrafusal fibers are in sync

Question 16

what is the sensory innervation of muscle spindles?

Answer 16

Ia (annulospiral endings) fibers and II (flower spray) fibers that detect muscle stretch

Question 17

name the 3 types of intrafusal fibers

Answer 17

• nuclear chain fibers
• nuclear bag fibers: dynamic & static

Question 18

what are nuclear chain fibers?

Answer 18

• thin intrafusal fibers, with nuclei arranged in a single file
• afferents include group Ia and II

Question 19

what are nuclear bag fibers?

Answer 19

thicker intrafusal fibers, with nuclei grouped in the central region

Question 20

what are dynamic nuclear bag fibers?

Answer 20

• sensitive to rate of change (rapidly adapting)
• afferents include group Ia only

Question 21

what are static nuclear bag fibers?

Answer 21

• slowly adapting stretch receptors
• afferents include both group Ia and II

Question 22

what is the stretch (deep tendon) reflex?

Answer 22

monosynaptic (no interneuron) connection between central axon of Ia fiber and the a-motor neuron, causing muscle contraction in response to stretch

Question 23

in the stretch reflex, the ____ muscle is excited via the ____ neuron

Answer 23

agonist
alpha motor

Question 24

what is the sensory receptor of the stretch reflex?

Answer 24

muscle spindle

Question 25

what is the Ia inhibitory reflex?

Answer 25

central Ia axon --< Ia inhibitory interneuron --< alpha motor neuron innervating antagonist muscle

Question 26

when testing the biceps deep tendon reflex, the biceps muscle is excited by the ____, while the triceps are inhibited by the ____

Answer 26

stretch reflex: alpha motor neuron Ia inhibitory reflex: Ia inhibitory interneuron

Question 27

what is the function of recurrent (renshaw cell) inhibition?

Answer 27

- to avoid overexcitation of agonist motor neuron pool - negative feedback system allows for control of excitability of all muscles around a joint

Question 28

explain recurrent (renshaw cell) inhibition

Answer 28

- agonist a-motor neuron (or descending tracts) --< renshaw inhibitory cell - renshaw cell withdraws inhibition of Ia interneuron so that antagonist is excited while agonist is inhibited

Question 29

what is the overall effect of renshaw cell reflex?

Answer 29

- directly inhibits agonist - withdraws inhibition of antagonist (inhibits Ia inhibitory interneuron)

Question 30

what is a golgi tendon organ?

Answer 30

encapsulated sensory receptors located in the junction between muscle & tendon, in series with the extrafusal muscle fibers

Question 31

what is the sensory innervation of golgi tendon organs?

Answer 31

Ib proprioceptors sensitive to muscle tension

Question 32

explain the Ib reflex

Answer 32

in response to tension in the muscle: central axons of Ib fibers from golgi tendon organ, cutaneous receptors, & joint receptors --< Ib inhibitory interneuron inhibits a-motor neuron pool of agonist

Question 33

what is the overall effect of the Ib reflex?

Answer 33

inhibition of the agonist muscle

Question 34

what is the pain withdrawal reflex called?

Answer 34

flexion & crossed extension reflex

Question 35

what is the flexion & crossed extension reflex?

Answer 35

polysynaptic pathways where central axons of A-delta nociceptors facilitate flexion (withdrawal) of the hurt lower limb, while facilitating extension of the contralateral lower limb for support

Question 36

explain the flexion & crossed extension reflex

Answer 36

A-delta central axons synapse on: - inhibitory interneurons inhibit a-motor neuron pool of ipsilateral extensors - excitatory interneurons that facilitate a-motor neuron pool of ipsilateral flexors - inhibitory interneurons that inhibit a-motor neuron pool of contralateral flexors - excitatory interneurons that facilitate a-motor neuron pool of contralateral extensors

Question 37

what information do fasciculus gracilis & cuneatus carry?

Answer 37

ipsilateral: proprioception vibration 2-point discrimination pressure touch

Question 38

nucleus gracilis and cuneatus give rise to ____ that decussate to form the ____ which ends in the ____

Answer 38

internal arcuate fibers medial lemniscus ventral posterior lateral thalamic nucleus

Question 39

from the thalamus, fibers of the DCML then run through the ____ and end in the ____

Answer 39

internal capsule somatosensory cortex (3,1,2)

Question 40

examination of DCML lesion

Answer 40

- vibration tuning fork on dermatomes - position sense mainly of upper extremities - romberg's test

Question 41

what tract is responsible for proprioception from the lower extremity?

Answer 41

spinocerebellar tract

Question 42

what tracts make up the anterolateral system, and what is their function?

Answer 42

spinothalamic spinoreticular spinomesencephalic pain & temperature perception

Question 43

pain & temperature fibers enter the tract of ____ and end in the ____

Answer 43

Lissauer dorsal horn

Question 44

when pain and temperature fiber cross at the ____, they form the ____ and end in the ____

Answer 44

ventral white commissure spinothalamic tract ventral posterior lateral thalamic nucleus

Question 45

spinothalamic fibers that end in the thalamus then travel through the ____ to end in the ____

Answer 45

internal capsule somatosensory cortex (3,1,2)

Question 46

examination of spinothalamic tract lesion

Answer 46

pinwheel (prick) over dermatomes