A muscle twitch is the response of a muscle to a single, brief threshold stimulus or response to a single action potential.

It is too short or too weak to be useful E. g. blinking of the eye There are three phases to a muscle twitch

Latent period Period of contraction Period of relaxation

Latent period – first few msec after stimulus; EC coupling taking place

Period of contraction – cross bridges form; muscle shortens

Period of relaxation – Ca2+ reabsorbed; muscle tension goes to zero

The entire contractile response last to a single AP last for about 100msec

Figure 9.14a

Increased intracellular Ca2+ concentrations (Ca2+ released from Sarcoplasmic reticulum and T – tubules) lasts longer than action potentials

even longer lasts single muscle twitch

- as skeletal muscle has short refractory period and

action potential delivered to the sarcolema spreads along all membrane, T – tubules, and Sarcoplasmic reticulum

Figure 9.14b

Muscle consists of groups of muscle fibers bundled together and attached to bones

Connective tissue covering muscle divides muscle internally into bundles

Connective tissue extends beyond ends of muscle to form tendons Tendons attach muscle to bone

Each person has 600 skeletal muscles which range in size from delicate external eye muscles that control eye movements having few hundred fibers to the large powerful leg muscles having several thousands of fibers

Connective tissue separates muscle cells - each one must be stimulated by axons of a motoneuron

Motor unit - all muscle cells innervated by the same motoneuron – they will contract at the same time

Motor units vary in size - mostly mixture of motor units of different sizes

large motor units >100 cells (typically slow postural muscles)

small motor units about 10 cells (precise control fast acting muscles –

those moving the eye)

Twitch Brief, weak contraction Produced from single action potential Too short and too weak to be useful e.g. Blinking of the eye

Contractions of whole muscle can be of varying strength

Two primary factors which can be adjusted to accomplish gradation of whole-muscle tension Number of muscle fibers contracting within

a muscle (motor unit recruitment) Tension developed by each contracting

fiber

Adding together individual twitch contractions to increase the intensity of overall muscle contraction.

Summation occur in 2 ways:1. By increasing the number of motor units

contracting simultaneously (multiple fiber summation)

2. By increasing the frequency of contraction which can lead to tetanization. (frequency summation)

Motor unit One motor neuron and the muscle fibers it

innervates Number of muscle fibers varies among

different motor units Number of muscle fibers per motor unit and

number of motor units per muscle vary widely Muscles that produce precise, delicate

movements contain fewer fibers per motor unit Muscles performing powerful, coarsely controlled

movement have larger number of fibers per motor unit

Muscle fibers from a motor unit are spread throughout the muscle; therefore, contraction of a single motor unit causes weak contraction of the entire muscle

Increasing level of central (moto-neuronal) activation – successive activation of higher threshold motor units

When a weak signal is sent by the CNS to contract a muscle, the smaller motor units, being more excitable than the larger ones, are stimulated first. As the strength of the signal increases, more motor units are excited in addition to larger ones, with the largest motor units having as much as 50 times the contractile strength as the smaller ones. As more and larger motor units are activated, the force of muscle contraction becomes progressively stronger. Also called recruitment which brings more and more muscle fibers into play

Importance of recruitment: Asynchronous recruitment of motor units helps delay or prevent fatigue

Figure 9.16

A concept known as the size principle, allows for a gradation of muscle force during weak contraction to occur in small steps, which then become progressively larger when greater amounts of force are required.

Cause of size principle: Smaller motor units are driven by small

motor nerve fibers and the small motor units are more excitable than the larger ones so they naturally are excited first.

Figure 9.17

A single stimulus results in a single contractile response – a muscle twitch

Frequently delivered stimuli (muscle does not have time to completely relax) increases contractile force – wave summation. Motor unit stimulated by a high frequency of action potentials from its motoneuron.

Figure 9.15

More rapidly delivered stimuli result in incomplete tetanus

If stimuli are given quickly enough, complete tetanus results

Figure 9.15

Twitch summation Results from sustained elevation of

cytosolic calcium Tetanization:

Occurs if muscle fiber is stimulated so rapidly that it does not have a chance to relax between stimuli

Contraction is usually three to four times stronger than a single twitch

Tension : a force exerted on an object by a contracting muscle

Load: force exerted on the muscle by an object (usually its weight)

Both are opposing forces. In order for muscle fiber to shorten and thereby move a load muscle tension must be greater than the opposing load

• DEFINITION: When a series of maximal stimuli are delivered to

the muscle at a frequency just below tetanizing frequency

(when muscle twitch due to previous stimulus has just completed), the tension/amplitude developed during each twitch increases till a max. height is reached & a plateau is formed. This is called the

Treppe/ staircase effect.Because the tension rises in stages, like the steps in

a staircase, this phenomenon is called treppe, a German word meaning "stairs."

• CAUSE: The rise is thought to result from a gradual increase in the concentration of calcium ions in the sarcoplasm, in part because the ion pumps in the sarcoplasmic reticulum are unable to recapture them in the time between stimulations.

Figure 9.18

• Length of fiber at onset of contraction is a very important factor influencing extent to which tension can be developed in a muscle

Muscles operate with greatest active force when close to resting length 2.5μm. When stretched or shortened beyond this, the maximum active force generated decreases

 This decrease is minimal for small deviations, but the force drops off rapidly as the length deviates further from the resting state

If muscle is passively stretched the thin filaments are pulled out from b/w the thick filaments so decreasing the no. of actin sites available for cross bridge binding so due to un matching of the two sites , many of them are unused. So when less cross bridge activity so less tension

Less tension will develop by the sarcomere because the actin filament become overlapped limiting the opportunity for the cross bridges to interact with actin

- The ends of the filament become forced against the z-discs so no further shortening can take place.

The extremes in muscle length that prevent development of tension occur only under experimental conditions when the muscle is removed and stimulated at various lengths.

In the body the muscles are so positioned that their relaxed length is approximately their optimal length. So they can achieve near maximal tetanic contraction.

Because attachment to the skeleton imposes limitations, a muscle cannot be stretched or shortened more than 30% of its resting optimal length

Tension is produced internally within sarcomeres(tension generating unit)

And sarcomeres are not attached to bones Tension must be transmitted to bone by

means of connective tissue and tendons before bone can be moved (series-elastic component which serve as a stretchy spring b/w the sarcomeres and the bone and that is to be moved against an external load.

Shortening the sarcomere stretches the series elastic components. Muscle tension is transmitted to the bone by tightening of these series elastic components. This force applied to the bone moves the bone against the load

There are two primary types of contraction, depending on whether the muscle changes length during contraction. They are

Isotonic Muscle tension remains constant as muscle

changes length Isometric

Muscle is prevented from shortening Tension develops at constant muscle length

• Isotonic contraction: occurs when muscle contracts with shortening of length but against a constant load, thus, the tension on the muscle remains constant (iso= same, tonic= tension)

OR A contraction that creates force & moves a

load.

Isotonic contractions are used for body movements and for moving external objects. E.g. picking up a book, a box.

Concentric = shortening muscles

Eccentric = lengthening muscles

Isometric contraction: occurs when muscle contracts without shortening in length.

(iso= same, metric= measure or length)OR

A contraction that creates force without movement.

Isometric contractions can be seen in 2 cases:1. If the object you are trying to lift is too heavy.2. If the tension developed in the muscle is

deliberately less than needed to move the load. E.g. standing for long time or holding up a glass of water while taking sips.

• shortening

• isometric

• lengthening

(Isotonic: shortening against fixed load, speed dependent on M·ATPase activity and load)

Three Potential Actions During Muscle Contraction:

Most likely to causemuscle injury

Biceps muscle shortensduring contraction

Biceps muscle lengthensduring contraction

The same internal events occur in both isotonic and isometric contractions:

Muscle excitation starts the sliding filament cycling; the cross bridges start cycling; and filament sliding shortens the sarcomeres, which exert force on the bone at the site of the

muscle’s insertion.During a given time, a muscle may shift between isotonic & isometric contractions. E.g. when you lift a book up it is isotonic contraction and when you keep holding the book up while reading it is isometric contraction.

NOTE: Since Work=Distance X Load,

Isotonic contractions do work where as Isometric do not.

ISOMETRIC CONTRACTIONS ISOTONIC CONTRACTIONS

1. Length same2. Tension changes3. W= FX D since no

distance covered so no work performed

4. e.g. walking,

1. Length changes2. Tension is same3. Work is performed

4. e.g. standing, pushing against the wall

When the muscle contract against the load it perform the work and the energy required to perform the work is derived from the chemical reaction in the muscle cells during contraction

Most of the energy is required for:1. Walk along theory 2. Ca pump3. Na K pump

Muscle contraction depends on the energy supplied by the ATP

Since ATP is the only source of energy that directly be used for contractile activity to continue, so ATP must be constantly supplied

Only limited stores of ATP are immediately available in muscle tissue which produces muscle contraction for 1-2 seconds but 3 pathways supply additional ATP as needed during muscle contraction

Transfer of high-energy phosphate from creatine phosphate to ADP First energy storehouse tapped at onset of

contractile activity Oxidative phosphorylation (citric acid

cycle and electron transport system Takes place within muscle mitochondria if

sufficient O2 is present Glycolysis

Supports anaerobic or high-intensity exercise

Figure 9.20

Creatine + ATPCreatine phosphate + ADP

Glycolytic reaction can occur even in the absence of oxygen so muscle contraction can be sustained up to a minute when when oxygen delivery from blood is not available

Rate of formation of ATP by glycolysis is 2.5 times faster as compared to oxidative phosphorylation

Large amount of nutrient fuel is used giving less amount of energy so glycolysis rapidly depletes the storage pool of glycogen

Lactic acid production may cause pain and stiffness in the muscle

So both factors play a role in the onset of muscle fatigue

More than 95% of all energy used by muscles for sustained long term contraction is derived from this source

The food stuffs consumed during this process are :

Carbohydrates Fats ProteinsFor long term maximal contraction (period

of hours) greatest energy production from fats

% of energy input that is converted to work instead of heat

It is about 25% with 75% becoming heat which is required in maintaining body temperature

But in isometric contraction the muscle efficiency is 0 (zero)

Every muscle of the body is composed of a mixture of fast and slow fibers and other fibers gradated b/w these two extremes

Rapidly acting muscles are composed of fast fibers mainly with small number of other varieties

Slowly acting muscles but with prolonged contraction are composed of mainly slow fibers (soleus muscle)

Classified based on differences in ATP hydrolysis and synthesis

Three major types Slow-oxidative (type I) fibers Fast-oxidative (type IIa) fibers Fast-glycolytic (type IIx) fibers

Table 9.2

Occurs when exercising muscle can no longer respond to stimulation with same degree of contractile activity

Defense mechanism that protects muscle from reaching point at which it can no longer produce ATP

Underlying causes of muscle fatigue are unclear

Occurs when CNS no longer adequately activates motor neurons supplying working muscles

Mechanisms involved in central fatigue are poorly understood

However stimulation of the nerve fiber at rates greater than 100 times per second for several minutes often diminishes the no. of acetylcholine vesicles so much that impulses fail to pass into the muscle fiber and finally muscle will not contract at all. This is called fatigue of NMJ

Cause: inability of the synthesis of acetylcholine to keep pace with its release and hydrolysis.

In intact organism it is unlikely to be the site of fatigue.

Lack of oxygen causes ATP deficit Lactic acid builds up from anaerobic

glycolysis A local increase in ADP and inorganic

phosphate from ATP breakdown which will interfere with cross bridge cycling

Accumulation of ECF K+ when Na K pump cannot transport K back into muscle so decrease in membrane potential so decrease excitability

Depletion of glycogen energy reserves

All muscles of the body are continually being remodeled to match the functions that are required of them

Their diameter are altered Their lengths are altered Their strengths are altered Their vascular supplies are altered

Definition: When the total mass of a muscle increases, this is called Muscle Hypertrophy. The resulting muscle enlargement comes from an increase in diameter of

the muscle fibers. It is in response to a regular & intensive use of that particular muscle. e.g. body

building or excessive use of steroids

Physiologic Basis: ↑in the number of actin & myosin filaments causing

increase in thickness of individual muscle fibers---called fiber hypertrophy

Rate of synthesis of actin & myosin far greater

Definition: When the total mass of a muscle

decreases, it is called Muscle Atrophy. If a muscle is not used, its actin and myosin

content decreases, its filaments become smaller and the muscle decreases in mass

and becomes weaker.

Disuse atrophy: When the muscle is prevented from doing work even though the nerve supply is intact. e.g. in bed-ridden patients, in a limb in a plaster of Paris cast. This type is thus called Disuse Atrophy.

1. Denervation atrophy: Atrophy also seen nerve supply to the muscle is lost. This can be due to an accident or when motor neurons supplying a muscle are destroyed .e.g. Poliomyelitis.

• Muscle fiber becomes thin & low in proteins, glycogen and ATP.

• Most of the fibers are destroyed and replaced by fibrous tissue which leads to contractures leading to disfiguring

Under rare conditions of extreme muscle force generation, the actual number of muscle fibers increase, in addition to the fiber hypertrophy ----This increase in fiber number is called Muscle Hyperplasia.

Mechanism: Linear splitting of previously enlarged fibers

Stimulate muscle growth and hypertrophy

That’s the reason athletes have dangerous practice of taking steroid hormones to increase their athletic performance

It is a fatal muscle-wasting disease that primarily strikes boys and leads to their death before the age of 20.

There is progressive degeneration of contractile proteins of the muscle and their replacement with fibrous tissue.

It is a genetic X-linked disease.

Mutation in the Dystrophin gene located on X-chromosome↓

Skeletal muscle lacks protein dystrophin (a large protein that provides structural stability to the muscle cell’s plasma

membrane)↓

Its absence leads to constant leakage of Ca into the muscle cell↓

Ca activates proteases that start damaging the muscle↓

Leads to increasing muscle weakness & fibrosis↓

Symptoms start at 2-3 years, patient wheel-bound at 10-12 years Usually die at about 25-30 years of age (usually Males)

↓Death is usually due to respiratory failure or heart failure as the

respiratory or heart muscles become too weak.↓

Milder disease is Becker’s muscular dystrophy