L5 LMNs

Question 1

Lower motor neurons

Answer 1

Known as the final common path

1. skeletal muscle contraction is initiated by them in the SC and brainstem
2. located within the ventral horn of SC gray matter and motor nuclei in the brainstem
3. Send axons to muscles via peripheral nerves or cranial nerves
4. Influenced by location and timing of activation
5. Help with reflexes, rhythmic, and stereotyped behaviors
6. Receive input from UMNs
7. Have Alpha and Gamma types

Question 2

Sensorimotor reflexes

Answer 2

LMNs mediate them
automatic responses to stimuli

Question 3

Rhythmic behaviors

Answer 3

actions that are repeated in a regular pattern, like walking or breathing

Question 4

Stereotyped behaviors

Answer 4

actions that follow a fixed and predictable sequence, like coordinated movements involved in chewing food

Question 5

Interactive systems that produce movement

Answer 5

1. UMN
2. LMN
3. Cerebellum
4. Basal Ganglia

Question 6

Upper Motor Neurons

Answer 6

1. Cell bodies are in the brainstem and cerebral cortex
2. Axons synapse with local neurons or with LMNs
3. Essential for voluntary movements and sequencing of skilled movements
4. Help with planning, initiating, directing of voluntary movements
5. Other purposes include speech (broca’s area), eyes/body/head in respect to sensory stimuli, expression

Question 7

Cerebellum

Answer 7

1. Detects the difference or motor error between an intended movement and what was performed
2. Mediates reductions in errors
3. Functions via efferent pathways to UMNs

Question 8

Basal Ganglia

Answer 8

prevents UMNs from initiating unwanted movements and prepares motor cicuits for initiation of movements

Question 9

Motor Neuron Pools

Answer 9

groups or clusters of motor neurons within the spinal cord that work together to control the contraction of a specific muscle or group of related muscles

groups of LMNs

remember that medial motor pools innervate proximal muscles

Question 10

Alpha motor neurons

Answer 10

large and innervate striated muscles fibers, that generate the forces needed for posture and movement

Question 11

Gamma motor neurons

Answer 11

innervate specialized muscle fibers, like spindles
sensory receptors

perform refining movements

Question 12

Motor units

Answer 12

the distribution of multiple fibers to one motor neuron helps to ensure even spread of contractile forces, damage to a motor neuron won’t hugely impact a muscle

Question 13

Slow motor units

Answer 13

contract slowly and generate small forces
rich myoglobin content, plentiful mitochondria make them resistant to fatigue

helps with sustained contraction like posture. have lower thresholds for activation

Question 14

Fast Fatigable

Answer 14

larger alpha motor neurons innervating larger muscle fibers with fewer mitochondria

important for brief exertions requiring large forces

threshold reached only during rapid movements requiring a lot of force like jumping

Question 15

Fast fatigue resistance

Answer 15

intermediate size
generate about twice force of a slow motor unit and are more resistant to fatigue

Question 16

How can muscle force be regulated?

Answer 16

increasing/decreasing the number of motor units active

increasing/decreasing the firing rate

Question 17

Size principle

Answer 17

1. progressive increases in muscle tension are produced by progressively increasing the activity of axons that provide input to the LMN pool
2. Smallest units are activated, and then as the input increases, FFR and then FF will be recruited

Question 18

Unfused tetanus

Answer 18

individual twitches are still apparent

lower neural stimulation, allows muscle to partially relax

Question 19

Fused tetanus

Answer 19

individual twitches are not longer apparent

muscle fibers are stimulated so rapidly taht there is not chance to fully relax between contractions

Not allowing for muscle relaxation

Necessary for actions like maintaining grip or posture

Question 20

Temporal summation

Answer 20

repeated firing of APs in quick sucession, allowing muscle to partially relax, and then contract again

with each subsequent impulse, the force increases

Question 21

Intrafusal fibers

Answer 21

make up muscle spindles

two classes –> nuclear bag and nuclear chain

each spindle contains 2-3 bag and 4-6 chain

large diameter sensory axons (Group 1a) are coiled around middle region of each intrafusal fiber

Group 2a axons form nuclear chain

Question 22

Group 1a afferents

Answer 22

trans it rapid changes in muscle length and rate of stretch
fire more when muscle is stretched, providing info about speed and direction of movement

Question 23

Group 2 afferents

Answer 23

respond to static changes in muscle length, contribute to proprioception

help with static fiber stretch, little “dynamic” sensitivity

Question 24

Reciprocal innervation

Answer 24

allows for rapid contraction of the stretched muscle and relaxation of the antagonist muscle

innervation of the sensory neuron to alpha motor neuron and GABA local circuit neurons

also responsible for muscle tone because the muscles are always under some kind of stretch

Question 25

Muscle gain

Answer 25

also referred to as the level of gamma motor neuron activity

Question 26

Gain of myotatic reflex

Answer 26

the amount of muscle force generated in response to a given stretch of the intrafusal fibers

how sensitive and responsive the reflex is to changes in muscle length

Higher gain = small changes in length result in strong reflex response

Low gain = larger changes are needed to trigger a reflex

Question 27

Changes in gain

Answer 27

Voluntary stretching = gain is reduced to facilitate lengthening

Standing on moving bus = gain is modulated by UMN to compensate for variable changes

level of gamma motor neuron activity can be modulated independently of alpha motor neuron to allow for fine adjustments in movements

Question 28

Challenging, precise, or unpredictable conditions…

Answer 28

tend to lead to an increase in baseline level of gamma motor neurons

Question 29

Golgi tendon organs

Answer 29

sensory receptor important in reflexive regulation of motor unit activity

encapsulated afferent nerve endings, located at the junction of a muscle and tendon

innervated by Ib sensory axons

in series w/extrafusal muscle fibers (not parallel)

Question 30

What happens when a golgi tendon organ is stretched?

Answer 30

1. Stretch causes the sensory nerve endings entertwined with GTO to become squeezed
2. Ion channels in sensory nerve endings cause AP, which travel along 1b axon
3. Signals reach the spinal cord, and synapse with inhibitory neurons (GABA interneurons).
4. GABA is synapsed with alpha motor neurons, which puts a break on the muscle activity

Question 31

GTO vs spindle

Answer 31

GTO = force
Spindle = length

Question 32

Flexor withdrawal reflex

Answer 32

1. Stimulation of nociceptive fibers
2. Withdrawal of limb by excitation of flexor muscles, inhibition of extensor muscles (ipsilateral limb)
3. Contralateral limb has the opposite stimulation to provide support

Question 33

Central pattern generators

Answer 33

neural circuits found in the spinal cord or brain that are responsible for generating rhythmic and repetitive movements

allows us to perform movements without thinking

Question 34

Areflexia

Answer 34

interruption of efferent limb of sensorimotor reflex arcs

Question 35

Hypotonia

Answer 35

in part dependent on reflex arc
reduced muscle tone or tension

Question 36

Fibrillations or fasciculations

Answer 36

spontaneous twitches characteristic of denervated muscle muscle fibers/motor units

Question 37

Lower Motor Neuron Syndrome

Answer 37

Experience paralysis of affected muscles, areflexia, hypotonia, fibrillations, and atrophy

Question 38

Type 1b Afferent Neurons

Answer 38

signal changes in muscle tension and force, helping avoid excessive force

Question 39

Nuclear Bags

Answer 39

found within muscle spindle
nuclei are located within the central region, giving it the name of nuclear bag
stretching the muscle stretches these, causing a firing of the sensory neurons located within the bag
Contain type 1a

Question 40

Nuclear Chain

Answer 40

found within muscle spindle
arranged in linear chain like fashion
sense changes in muscle length and transmit sensory information
contain Type 2 neurons