Chapter 13 Spinal Motor

Question 1

When your biceps contracts, the triceps typically relaxes. Why?

Answer 1

triceps are antagonistic extensor muscles and relaxing the antagonist allows movements to be faster and more efficient because the muscles are not working against one another

Question 2

what is the motor system?

Answer 2

The motor system consists of all our muscles and the neurons that control them.

Question 3

what are the parts of motor control?

Answer 3

1. the spinal cord’s commands and control of coordinated muscle contraction
2. the brain’s commands and control of all the motor programs in the spinal cord.

Question 4

what are the two types of muscles in the body?

Answer 4

smooth and striated

Question 5

what is smooth muscle

Answer 5

smooth muscle lines the digestive tract, arteries, and related structures and is innervated by nerve fibers which plays a role in peristalsis (the movement of material through intestines) and the control of blood pressure and blood flow

Question 6

what are the two types of striated muscles?

Answer 6

cardiac and skeletal

Question 7

what is a cardiac muscle

Answer 7

cardiac muscle contracts rhythmically even in the absence of any innervation. Innervation of the heart from ANS functions to accelerate and slow down the heart rate.

Question 8

what is skeletal muscle

Answer 8

skeletal muscle is responsible for the bulk of the muscle mass of the body and functions to move bones around joints, to move eyes within the head, to inhale and exhale, to control facial expression, and to produce speech. skeletal muscle is enclosed with a connective tissue sheath which forms tendons at the end of the muscle

Question 9

what are synergists and antagonists muscles

Answer 9

synergists are flexor muscles and antagonists are extensor muscles

Question 10

what are axial muscles

Answer 10

muscles that are responsible for movements of the trunk. axial muscles are important for maintaining posture

Question 11

what are proximal (girdle) muscles

Answer 11

muscles that are responsible for the movements of the shoulder, elbow, pelvis, and knee. proximal muscles are important for locomotion

Question 12

what are distal muscles

Answer 12

muscles that are responsible for the movements of hands, feet, and digits (fingers and toes). distal muscles are important for the manipulation of objects

Question 13

what are lower motor neurons

Answer 13

the ventral horn of the spinal cord contains motor neurons that directly innervate skeletal muscle fibers. also known as the final common pathway. however the upper motor neurons supply input to the spinal cord

Question 14

The segmental organization of lower motor neurons

Answer 14

-The cervical enlargement of the spinal cord contains the motor neurons that innervate the arm muscles.
-The lumbar enlargement contains the neurons that innervate the muscles of the leg.
-Axial muscles are found at all levels
-Distal and proximal muscles are found in cervical and lumbar-sacral segments of the spinal cord

Question 15

what are the two types of lower motor neurons

Answer 15

alpha and gamma

Question 16

what are alpha neurons

Answer 16

alpha motor neurons directly triggers the generation of force by muscles

Question 17

what is a motor unit

Answer 17

A motor unit is an alpha motor neuron and all the muscle fibers it inner- vates

Question 18

what is a motor neuron pool

Answer 18

A motor neuron pool is all the alpha motor neurons that innervate one muscle.

Question 19

how does an alpha motor neuron communicates with a muscle fiber

Answer 19

releases the neurotransmitter acetylcholine (ACh) at the neuromuscular junction, the specialized synapse between a nerve and a skeletal muscle. ACh released in response to a presynaptic action potential causes an excitatory postsynaptic potential in the muscle fiber also called end plate potential

Question 20

what are the three major sources of input to an alpha motor neuron

Answer 20

1. dorsal root ganglion cells with axons that innervate a specialized sensory apparatus embedded with a muscle that tells us about the muscle length is called muscle fiber
2. upper motor neurons in the motor cortex and brain stem tell about the initiation and voluntary movement
3. interneurons in the spinal cord. this input may be excitatory or inhibitory and is part of the circuitry that generates the spinal motor programs

Question 21

what are slow motor unit

Answer 21

slow motor unit contains slowly fatiguing red fibers
-high frequency and small diameter

Question 22

what are fast motor units

Answer 22

Fatigue-resistant (FR) fibers (moderate) Fast fatigable (FF) (fastest) white fibers
-low frequency and large diameter

Question 23

The muscle fiber structure

Answer 23

Muscle fibers are enclosed by an excitable cell membrane called the sarcolemma. Within the muscle fiber are a number of cylindrical structures called myofibrils, which contract in response to an action potential sweeping down the sarcolemma. Myofibrils are surrounded by the sarcoplasmic reticulum (SR), an extensive intracellular sac that stores Ca2. Action potentials sweeping along the sarcolemma gain access to the sarcoplasmic reticulum deep inside the fiber by way of a network of tun- nels called T tubules (T for transverse). These are like inside-out axons; the interior of each T tubule is continuous with the extracellular fluid.
Where the T tubule comes in close apposition to the SR, there is a specialized coupling of the proteins in the two membranes. A voltage- sensitive cluster of four calcium channels, called a tetrad, in the T tubule membrane is linked to a calcium release channel in the SR. Some Ca2 flows through the tetrad channels, and even more Ca2 flows through the calcium-release channel, and the resulting increase in free Ca2 within the cytosol causes the myofibril to contract.

Question 24

what is myasthenia gravis

Answer 24

the ACh released is far less effective, and neuromuscular transmission often fails. The name is derived from the Greek for “severe muscle weakness.” The disorder is characterized by weakness and fatigability of voluntary muscles, typically including the muscles of facial expression, and it can be fatal if respiration is compromised.

Question 25

The sliding-filament model of muscle contraction

Answer 25

Myofibrils shorten when the thin filaments slide toward one another on the thick filaments.

Question 26

The molecular basis of muscle contraction

Answer 26

The binding of Ca2 to troponin shifts tropomyosin and allows the myosin heads to bind to the actin filament. Then the myosin heads pivot, causing the filaments to slide with respect to one another. An ATP binds to each myosin head and it disengages from actin. The cycle continues as long as Ca2 and ATP are present.

Question 27

What are the two filaments

Answer 27

thick filaments myosin and thin filament actin

Question 28

what is rigor mortis

Answer 28

Starving the muscle cells of ATP prevents the detachment of the myosin heads and leaves the myosin attachment sites on the actin filaments exposed for binding. The end result is the formation of permanent attachments between the thick and thin filaments.

Question 29

Proprioception from Muscle Spindles

Answer 29

A muscle spindle, also called a stretch receptor, consists of several types of specialized skeletal muscle fibers contained in a fibrous capsule. The spindles and their associated Ia axons, specialized for the detection of changes in muscle length (stretch), are examples of proprioceptors. These receptors are a component of the somatic sensory system that is specialized for “body sense,” or proprioception, which informs us about how our body is positioned and moving in space.

Question 30

What is stretch reflex?

Answer 30

As the muscle is stretched, the discharge rate goes up; as the muscle is shortened and goes slack, the discharge rate goes down. The Ia axon and the alpha motor neurons on which it synapses constitute the monosynaptic stretch reflex arc “monosynaptic” because only one synapse separates the primary sensory input from the motor neuron output.
What is the value of these findings? The circuits of the stretch reflex play an important role during normal movement by sensing the configuration of the body and limbs and regulating the way they respond to mechanical distur- bances.

Question 31

what are gamma motor neurons

Answer 31

Activation of alpha motor neu-
rons causes the extrafusal muscle fibers to shorten. If the muscle spindle were to become slack, it would go “off the air” and no longer report the length of the muscle. Activation of gamma motor
neurons causes the poles of the spindle to contract, keeping it active and “on the air.”

Question 32

Proprioception from Golgi Tendon Organs

Answer 32

Golgi tendon organ, which acts like a very sensitive strain gauge; that is, it monitors muscle tension, or the force of contraction. When the muscle contracts, the ten- sion on the collagen fibrils increases. As the fibrils straighten and squeeze the Ib axons, their mechanosensitive ion channels are activated and ac- tion potentials can be triggered.
It is important to note that while spindles are situated in parallel with the muscle fibers, Golgi tendon organs are situated in series. he Ib axons enter the spinal cord, branch repeatedly, and synapse on special interneurons called Ib inhibitory interneurons in the ventral horn.

Question 33

muscle spindle vs golgi tendon

Answer 33

a) Muscle spindles are arranged parallel to the extrafusal fibers; Golgi tendon organs lie in series, between the muscle fibers and their points of attachment. (b) Golgi tendon organs respond to increased tension on the muscle and transmit this information to the spinal cord via group Ib sensory axons. Because the activated muscle does not change length, the Ia axons remain silent in this example. The Ib axon of the Golgi tendon organ excites an inhibitory interneuron, which inhibits the alpha motor neurons of the same muscle.

Question 34

when NMDA receptors are acti- vated by glutamate

Answer 34

1. The membrane depolarizes.
2. Na and Ca2 flow into the cell through the NMDA receptors.
3. Ca2 activates potassium channels.
4. K flows out of the cell.
5. The membrane hyperpolarizes.
6. Ca2 stops flowing into the cell.
7. The potassium channels close.
8. The membrane depolarizes, and the cycle repeats.