1. Describe the differences between Type I and Type II muscle fibers.

2. Define a motor unit. 3. Define muscle tone 4. Explain the statement that tone in skeletal muscle is

neurogenic 5. Describe the how the tension in skeletal contraction

is graded 6. Describe the acute and chronic effects of

denervation in skeletal muscle. 7. Describe the how EMG can be used to assess the

properties of skeletal muscle. 8. Explain what is meant by denervation

hypersensitivity and the mechanisms involved.

WHITE RED

Pale or white (type II), fast, glycolytic

Low oxidative capacity correlates with less- mitochondria, capillary density etc

Motor unit contains few fibers

Rapid contraction Fatigues more easily Prominent cross striation Rapidly acting muscles of

limbs eg. flexor muscles

Red or dark (type I), slow, oxidative

High oxidative capacity correlates with high-mitochondria, myoglobin, capillary density

Motor unit contains more fibers Contracts slowly Fatigue less easily Cross striations less marked Long continuous contractions

Eg. antigravity muscles back

Most muscles contain both types of fibers but proportions differ depending on the usual function of the muscle

Different people have different proportions of the three types of fibers (is genetically determined) .

All fibers in one MU have similar characteristics (type).

Changes in muscles with age ↓ max force of contraction (by 30-

40% between ages of 30 and 80) Result from atrophy of muscle fibers

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DISTRIBUTION OF THE THREE TYPES OF FIBERS

Fast-twitch Slow-twitch

Marathon 18% 82%Runners

Swimmers 26 74

Average 55 45 man

Weight 55 45Lifters

Sprinters 64 37

Jumpers 63 37

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Include muscle fibers which are homogenous in their functional and structural properties

NEURAL REGULATION OF SKELETAL MUSCLES: MOTOR UNITS (MU)

MU - a single motor neuron and the group of muscle fibers innervated by the ranches of its axon

MN pool - the group of MN that innervates fibers within the same muscle

In general, small muscles that react rapidly & precisely have fewer muscle fibers (two or three muscle fibers per motor unit) & more nerve fibers,e.g. in some of the laryngeal muscles.

Large muscles such as the soleus muscle, may have several hundred muscle fibers in a motor unit.

In large limb muscles, the small, slow units are first recruited in most movements, are resistant to fatigue, and are the most frequently used units.

The fast units, which are more easily fatigued, are generally recruited with more forceful movements.

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MOTOR UNITS: ORDER OF RECRUITMENT

MU are recruited according to the size principle: small and more excitable MU are activated by threshold stimuli; an increase in the stimulus strength activates (recruits) less excitable larger MUs (the order of activation is from smallest MUs to largest MU)

Recruitment of additional MUs increases the force of muscular contraction. The force is max when all MU in the muscle are recruited

Isometric Isotonic

Same length but ↑Tension

Examples …….

Same tension but ↓Length

Examples …….

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ISOMETRIC CONTRACTION: LENGTH-TENSION RELATIONSHIP

Overstretching of the resting muscle:

overlap between ↓actin and myosin

active tension→↓

Length-tension relationship : the active tension depends on the initial fiber length or degree of overlap of actin and myosin filaments (preload, passive tension):

Maximal tension can be generated when the muscle is approximately at its natural length in the body (B to C)

At lengths greater or less than natural length muscle develops less tension

Minimal prestretch: overlapping of actin filaments, some cross bridges can not be formed active → ↓tension

Optimal prestretch: actin filaments overlap all myosin cross bridges →Max active tension

No overlapping between actin and myosin filaments No →active tension

Tone: The resistance offered by the muscle to

passive stretch is tone or tonus. Mechanism: depends on the integrity of

Stretch reflex arc.

If the motor nerve to a muscle is cut, the muscle offers no resistance and is said to be flaccid.

A hypertonic (spastic) muscle is one in which the resistance to stretch is high because of hyperactive stretch reflexes.

Somewhere between the states of flaccidity and spasticity is the ill-defined area of normal tone.

The muscles are generally hypotonic when the rate of γ- efferent discharge is low and hypertonic when it is high.

Summation of individual contractions occurs:

(1) by increasing the number of motor units contracting simultaneously, which is called multiple fiber (Spatial) summation,

(2) by increasing the frequency of contraction, is called frequency summation (Temporal) and can lead to tetanization.

With rapidly repeated stimulation, activation of the contractile mechanism occurs repeatedly before any relaxation occurrs, and the individual responses fuse into one continuous contraction, a response called a tetanus (tetanic contraction).

It is a complete tetanus when there is no relaxation between contractions

an incomplete tetanus when there are periods of incomplete relaxation in between.

Muscle fatigue: a decline in the ability of the muscle to sustain the strength of contraction

Causes: - rapid build-up of lactic acid - decrease in oxygen supply - decrease in energy supply

(glucose, glycogen, fatty-acids)

- Decreased neurotransmitter at the synapse

- psychological causes

Muscle Hypertrophy, Muscle Atrophy ,Hyperplasia

• When the total mass of a muscle increases, this is called muscle hypertrophy.

• results from an increase in the number of actin and myosin filaments in each muscle fiber, causing enlargement of the individual muscle fibers; this is called simply fiber hypertrophy.

• with increasing size of myofibrils, the enzyme systems that provide energy also increase

• When total mass of a muscle decreases, the process is called muscleAtrophy

• Actual number of muscle fibers increase , in addition to the fiberhypertrophy process. This increase in fiber number is called fiber

hyperplasia.

Fibrillations are Fine, irregular contractions of individual fibers. Classic picture of a lower motor neuron lesion. Usually, the contractions are not visible.

Fasciculations, are jerky, visible contractions of groups of muscle fibers that occur as a result of pathologic discharge of spinal motor neurons.

Healthy skeletal muscle contracts only in response to stimulation of its motor nerve supply. Destruction of this nerve supply causes---------- muscle atrophy & abnormal excitability of the muscle and increases its sensitivity to circulating acetylcholine (denervation hypersensitivity.

Spontaneous fine, irregular contractions of individual fibers (fibrillations) appear on EMG (the contractions are not visible grossly). If the motor nerve regenerates, the fibrillations disappear.

Acute effects:Loss of tone Flaccid paralysisLoss of reflex It also leads to abnormal excitability of the muscle and increases its sensitivity to circulating acetylcholine called as denervation hypersensitivitySpontaneous, fine, irregular contractions of individual fibers called as fibrillations appear. Chronic effect. muscle atrophy (malfunction)-

An electromyogram (EMG) is a test that is used to record the electrical activity of muscles.

When muscles are active, they produce action potentials. This current is usually proportional to the level of the muscle activity.

EMG is used to diagnose two general categories of disease: neuropathies and myopatheies.

Myopathic disorders EMG will show decreased amplitude & duration of waves.

Electromyograph-process of recording motor unit elctrical activity

Electromyogram-the record obtained with electromyograph

Patterns of EMGMuscle tissue at rest is normally electrically silent.When muscle is voluntarily contracted gradually, few action potentials begin to appearWhen the force of muscle contraction is maximum, groups of AP’s of varying rate & amplitude will appear- pattern is called as interference pattern (due to recruitment of more motor unit potentials)

EMG and nerve conduction studies are useful in localizing peripheral nervous system deficit found on clinical examination

It detects any signs of blocking or slowing down of responses to nerve stimulation.

The test provides information about the muscle itself and shows how well it receives stimulation from the nerve.

• An EMG is used to evaluate muscle weakness,twitching or paralysis, causes of numbness, tingling , pain.

• To differentiate between true weakness and reduced use because of pain or lack of motivation.

• To differentiate whether a muscle disorder begins in the muscle itself or is caused by a nerve disorder.

• To determine the degree of axonal loss with the presence of abnormal spontaneous activity and the degree of reduced MU recruitment.