HPS 490 Unit 1: Neuromuscular Physiology

Isometric contraction

Action in which the muscle develops tension, but does not shorten (no change in length) Also called a static contraction No movement occurs

Propioceptor

Receptors that provide information about the position and movement of the body Includes muscle and joint receptors as well as the receptors in the semicircular canals of the inner ear

Isotonic contraction

Contraction in which a muscle shortens against a constant load or tension, resulting in movement

Describe the structures and processes leading to voluntary movement.

Control of voluntary movement is complex and requires the cooperation of many areas of the brain as well as several subcortical areas. The first step in performing a voluntary movement occurs in subcortical and cortical motivational areas, which send signals to the association cortex, which forms a "rough draft" of the planned movement. The movement plan is then sent to both the cerebellum and the basal ganglia. These structures cooperate to convert the "rough draft" into precise temporal and spatial excitation programs. The cerebellum is important for making fast movements, while the basal ganglia are more responsible for slow or deliberate movements. From the cerebellum and basal ganglia, the precise program is sent through the thalamus to the motor cortex, which forwards the message down to spinal neurons for "spinal tuning" and finally to skeletal muscle. Feedback to the CNS from the muscle receptors and proprioceptors allows the modification of motor programs, if necessary.

Cerebellum

Coordinates and monitors complex movement Incorporates feedback from proprioceptors Has connections to motor cortex, brain stem, and spinal cord May initiate fast, ballistic movements Essential for coordination of movement, comparing and correcting movement, and motor learning Output is excitatory

Mechanical work

Product of force applied (or load moved) and the distance through which it is applied (force x distance)

Sarcoplasm

Protoplasm which contains (in addition to the contractile material) proteins, fats, glycogen, nuclei, mitochondria, and enzymes

Golgi tendon organ

Receptive to tension generated in tendon AP directed to spinal cord Synapse on inhibitory interneuron to inhibit muscle and prevent stretch

Ca++

A cation which is released by the SR and signals muscle contraction

Tropomyosin

A long protein structure in the helix of the actin filament that prevents interaction between actin and myosin Covers actin sites "Top of" myosin

Sarcoplasmic reticulum

A membranous structure that surrounds the myofibrils of muscle cells Location of the terminal cisternae or lateral sacs that store the Ca++ needed for muscle contraction Conduction or tubular "irrigation" system

Motor unit

A motor neuron and all the muscle fibers innervated by that single motor neuron Response in an "all-or-none" matter to a stimulus

Muscle biopsy

A procedure in which a piece of muscle tissue is removed and examined microscopically

Action potential

A progressive change in polarity along the length of the membrane Includes depolarization (reversal of polarity) and repolarization (re-establishment of polarity) Sometimes referred to as a depolarization wave

Troponin

A small protein on the surface of the actin that regulates the interaction between actin and myosin Moves tropomyosin out of the way Protein, associated with actin and tropomyosin, that binds Ca++ and initiates the movement of tropomyosin on actin to allow the myosin cross-bridge to touch actin and initiate contraction

Hypertrophy

An increase in cell size

Hyperplasia

An increase in the number of cells in a tissue Not conclusive (shown in Japanese quail and cats)

Dendrites

Neuron's receivers and carry impulses toward cell body

Force-velocity relationship

As force increases, velocity decreases (more cross-bridges) Greatest tension development occurs at zero shortening velocity (isometric) contractions

Show graphically the relationship between the force exerted and velocity of contraction (shortening).

As the force developed by a muscle (or load lifted) is increased, the velocity of muscle shortening (also limb movement) decreases.

Define and list the steps involved in excitation-contraction coupling.

Ca+ binds with troponin, and then troponin moves the tropomyosin molecules off the active sites on the actin filament, opening these sites for binding with the myosin head. Once it binds with the actin active site, the myosin head tilts, pulling on the actin filament so that the two slide across each other. The tilting of the myosin head is the power stroke. Energy is required before the muscle action can occur. The myosin head binds to ATP and ATPase found on the head splits ATP into ADP+P, releasing energy to fuel the contraction.

Monosynaptic reflex

Central regions forcefully elongated Cause receptor potentials in the annulospiral endings AP propagated back to spinal cord, where they synapse directly to motor nerve Muscle contracts Muscle spindle: Contract to resist force (stretch reflex) Only known monosynaptic reflex Very rapid Example: Knee jerk response Important in controlling muscle length

Sarcolemma

Connective tissue surrounding muscle cell

Synapse

Contact points between axon of one neuron and dendrite of another neuron

Describe the movement of ions involved in depolarization and repolarization of the cell.

Depolarization: Reversal of polarity; opens Na+ channels and Na+ diffuses into cell; inside becomes more positive Repolarization: Re-establishment of polarity; return to resting membrane potential; K+ leaves the cell rapidly; Na+ channels close

Cross-bridges

Extensions of the heavy meromyosin between the myosin and actin filaments Where actin and myosin connect to allow contraction

The golgi tendon organ monitors change in muscle length. True or false? Explain briefly.

False; the GTO monitors changes in tendon tension, while muscle spindles monitor changes in muscle length. If a muscle lengthens sufficiently to apply rapid tension to the tendon however, the GTO will perceive this.

Type II fibers

Fast-twitch glycolytic (type IIb) and fast-twitch oxidative glycolytic (type IIa) Light color, fast contraction speed, strength/power (high force) Anaerobic

Axon

Neuron's transmitter Conducts impulses away from cell body

Length-tension relationship

Force of a maximal concentric contraction depends on the length of the muscle Muscle length influences tension development because excessive stretch and inadequate length decreases actin and myosin interaction Inverted-U (optimal length ~90 degrees)

Fast oxidative (type IIa) fibers

High myosin ATPase activity Fast speed of contraction Intermediate fatigue resistance High oxidative capacity Intermediate anaerobic enzyme content Many mitochondria Many capillaries High myoglobin content Red color Intermediate glycogen content Intermediate fiber diameter

Fast glycolytic (type IIb) fibers

High myosin ATPase activity Fast speed of contraction Low fatigue resistance Low oxidative capacity High anaerobic enzyme content Few mitochondria Few capillaries Low myoglobin content White color High glycogen content Large fiber diameter

Contrast hypertrophy, hyperplasia, and atrophy.

Hypertrophy is the increase in cell size, hyperplasia is the increase in cell number, and atrophy is the degradation or wasting of the cell.

Show graphically the relationship between the load and work.

If a particular load can be moved a given distance, the mechanical work completed increases in direct proportion (linearly) with increasing load.

Temporal summation

Increase in frequency at which motor units contract

Spatial summation

Increase in number of motor units contracting

Diagram a schematic representation of a motor unit and explain innervation ratio.

Innervation ratio is the number of muscles innervated per motor neuron. Muscle groups which perform fine movement have a very low innervation ratio; muscle groups which perform large gross movements have a very high innervation ratio.

Autonomic nervous system

Involuntary nervous system composed of two divisions: sympathetic (fast, excites effector, releases norepinephrine) and parasympathetic (slow, inhibits effector, releases ACH) Body uses these to regulate cellular, tissue, and organ functions Maintain internal environment

List and define the four different properties of muscles.

Irritability: Receive and propagate action potential Contractility: Contract/shorten and exert force Extensibility: Ability to lengthen Elasticity: Return to a pre-contraction length

Differentiate among isometric, isotonic, and isokinetic muscle contraction.

Isometric contractions maintain a constant joint angle through the contraction. Isotonic contractions maintain the same muscle tension throughout the contraction. Isokinetic contractions maintain the same velocity throughout the contraction.

Slow oxidative (type I) fibers

Low myosin ATPase activity Slow speed of contraction High fatigue resistance High oxidative capacity Low anaerobic enzyme content Many mitochondria Many capillaries High myoglobin content Red color Low glycogen content Small fiber diameter

Muscle spindle

Main skeletal muscle receptor Responds to stretch by contracting Continually provides info to CNS regarding static stretch, dynamic stretch, and changes in muscle length A muscle stretch receptor oriented parallel to skeletal muscle fibers The capsule portion is surrounded by afferent fibers, and intrafusal muscle fibers can alter the length of the capsule during muscle contraction and relaxation

Show graphically the relationship between the length of a muscle and maximal tension exerted.

Maximal strength is greatest during eccentric contraction, followed in descending order by isometric and concentric contraction.

Joint receptors

Mechanoreceptors sensitive to metabolites from metabolism May elicit increases in cardiovascular parameters

Isokinetic contraction

Muscle contracts moving the limb at a single velocity, that is the velocity of shortening remains constant Mainly used in rehab

Describe the general structure of a muscle spindle and discuss its physiology function.

Muscle spindles consist of extrafusal fibers, intrafusal fibers, and annulospiral endings. It allows neural information to be relayed back to the CNS of muscle stretch, muscle length, and the rate of change in muscle length.

Motor neurons

Neuron located in spinal cord that innervates muscle fibers Composed of cell body (soma), dendrites, and axon

If the sarcoplasmic reticulum was removed from a muscle fiber, would that fiber contract in response to an impulse reaching the myoneural junction? Why or why not?

No, because there would be no Ca++ released with which to move the tropomyosin from the actin active sites.

Describe and diagram an EMG for the different types of muscle contraction at the same load for a fatigued and non-fatigued muscle.

Non-fatigued: More electrical activity for concentric contractions Fatigued: Electrical activity increases in eccentric contractions and begins to look similar to electrical activity produced by concentric contractions

Synaptic vesicles

Packets of transmitter substance (ACH) with the axon terminal

Transverse tubules

Pass through cell, contact with SR forms triads, provide contact between inside of cell and fluid bathing it Connect with SR to carry signal An extension of the muscle membrane that conducts the action potential into the muscle to depolarize the terminal cisternae, which contain the Ca++ needed for muscle contraction

Response

Physiological changes that occur going from one exercise state to another Short-term change to physical state Returns to baseline Examples: HR, BP, RR, Q, etc.

Adaptation

Physiological changes that occur with regular (chronic) participation in exercise Long-term Doesn't return to baseline Referred to as "training effects" Examples: RHR, BMD, BF%, etc. Specific for activity

Show graphically the relationship between the load and power output.

Power increases with load only to a certain point and then it decreases (inverted-U).

List the important components of the muscle cell and diagram a schematic representation of the sarcomere.

Satellite cell Skeletal muscle fiber Sarcomere A-band I-band H-zone M-line Z-line Myofibril Myofilaments: Actin and myosin

List three functional characteristics that differentiate slow motor units from fast motor units.

Slow motor units do not fatigue easily, fast motor units utilize a glycolytic metabolism as opposed to an oxidative metabolism, and fast motor units generate more power.

Basal ganglia

Slow, deliberate movements Output is inhibitory "Brake hypothesis" Examples: Parkinson's and Huntington's

Type I fibers

Slow-twitch oxidative Dark color, slow contraction speed, endurance (low force) Aerobic

Compare and contrast slow- and fast-twitch skeletal muscle with respect to biochemical and contractile characteristics.

Slow-twitch: Type I, oxidative, dark in color, slow contraction speed, endurance performance, many mitochondria, high myoglobin, small fiber, low anaerobic enzyme. Fast-twitch: Type II, glycolytic, lighter in color, fast contraction speed, poor endurance, few mitochondria, low myoglobin, large fiber, high anaerobic enzyme.

Myofibrils

Small, string-like structure within muscle fiber containing contractile proteins A series of sarcomeres where the repeating pattern of the contractile proteins gives the striated appearance to skeletal muscle

Describe the resting membrane potential of the cell.

The magnitude of the negative charge of the muscle cell, which is a measure of possible action potential.

Describe and draw a schematic of the withdrawal and crossed-extensor reflex.

Stimulation causes you to push away on the contralateral side Related to withdrawal reflex (pull away)

Contrast temporal and spacial summation at a motor neuron.

Temporal summation is the increase in frequency at which muscle units contract, whereas spatial summation is the increase in the number of muscle units contracting. Both increase force output and improve the efficiency of the nervous system.

Discuss the function of the Golgi tendon organs in monitoring tension.

The GTO monitors tendon tension, the action potentials of which are directed to the spinal cord. Synapse on an inhibitory interneuron, which inhibits A-motor neuron.

Inhibition

The act of restricting or hindering a process

Describe how the force/tension developed within a skeletal muscle can be regulated (increased or decreased).

The amount of force generated during muscular contraction is dependent on types and number of motor units recruited, the initial muscle length, and the nature of the motor units' neural stimulation. The addition of muscle twitches is termed summation. When the frequency of neural stimulation to a motor unit is increased, individual contractions are fused together in a sustained contraction called tetanus. The peak force generated by muscle decreases as the speed of movement increases. However, in general, the amount of power generated by a muscle group increases as a function of movement velocity.

Z-line

The boundary at each end of a sarcomere

Perimysium

The connective tissue surrounding the fascicles of secondary bundles

Describe the action potential of the cell.

The depolarization wave that travels across a muscle cell which elicits contraction. Na+ is released into the muscle and causes the charge to become positive. Na+ channels close and K- is released, causing the charge to become negative again.

Describe the all-or-none principle with regards to the motor unit.

The force and speed of contraction is controlled at the spinal cord level by the number and frequency of motor units recruited.

Endomysium

The inner layer of connective tissue surrounding a muscle fiber Forms the fasciculus (primary bundle)

Myoneural junction

The junction between a nerve fiber and the muscle it supplies

Describe the events involved in muscle contraction beginning with the nerve impulse at the myoneural junction and ending with relaxation of the muscle.

The motor nerve releases ACH, which opens ion gates in the muscle cell membrane and allows Na+ to enter the muscle cell. Action potential travels along the sarcolemma, then through the tubule system. Ca++ is released from the SR. Ca++ binds to troponin, causing tropomyosin to roll away which exposes the active site on actin. Myosin cross-bridges attach to active sites on actin, pivot, and pull on thin filaments. ATP replaces the ADP+P, allowing myosin and actin to detach and reposition. When Ca++ is actively pumped back into the SR, tropomyosin re-covers actin sites.

Motor endplate

The nerve and muscle fiber areas that permit transfer of a nerve impulse from the nerve to the muscle Also referred to as the neuromuscular junction Transfers nerve impulse to muscle with no physical contact

Epimysium

The outer layer of connective tissue surrounding tertiary bundles Usually referred to as fascia

Sarcomere

The repeating contractile unit in a myofibril bounded by Z-lines Basic functional unit

Describe and give an example of the stretch reflex.

The stretch reflex is important in controlling muscle length. Results in immediate contraction of related muscle groups upon presentation of rapid stretching. Best example is the striking of the patellar tendon with a rubber mallet, resulting in contraction of the quadriceps.

Atrophy

The wasting away or degeneration of cells

Myosin

Thick filaments encompassing the center of the sarcomere Makes up 50-55% protein in muscle Two parts: Light meromyosin (tail) and heavy meromyosin (head)

Actin

Thin filaments attaching to the ends (Z-lines) of the sarcomere 20-25% protein in muscle Extend from Z-line to H-zone

Describe the importance of muscle fiber type and performance of various physical activities.

Type I is important for participation in activities that require the athlete to be light and perform for long durations such as cycling, distance running, and swimming. Type II is important for participation in activities that require the athlete to generate both force and power for short durations, such as weightlifting and swinging a baseball bat.

Somatic nervous system

Voluntary nervous system Nerves that innervate skeletal muscle Voluntary control

Contrast muscle fiber response to weight training and endurance training.

Weight training: Hypertrophy, new myosin and actin filaments develop, possible conversion from type IIa to type IIb fibers, increased glycogen storage capacity, increased motor recruitment. Endurance training: Increase in oxidative efficiency, possible conversion from type IIb to type IIa fibers, increased glycogen storage capacity, increase in number of mitochondria.

Describe the role of the sarcoplasmic reticulum in muscle contraction and relaxation.

When the SR is stimulated, it releases Ca++ which leads to muscle contraction. When the SR ends the supply of Ca++, it leads to muscle relaxation.