17.5.12

1. A change in resting membrane potential corresponds to a change in

(a) Charge flowing across the membrane

(b) Charge stored on membrane capacitor

2. Potential difference exists

(a) Across the two ends of axon

(b) In the intracellular space of neurons

(c) Across the neuron membrane

(d) In the extracellular space of neurons

The bright regions in the pictures above indicate unidirectional

(a) Flow of ions

(b) Propagation of action potential

Voluntary Muscles:Voluntary muscles are under the control of central nervous system

Skeletal muscles Involuntary Muscles:

Not under the control of brain

Cardiac muscles

Smooth muscles

The magnitude of the tension developed by a muscle fiber when it contracts (active tension) depends on the

1. Initial resting length of the muscle (muscle fiber)

2. Frequency of the stimulation

No stimulation No cross bridge formation because cross bridge binding

site is physically covered by troponin-tropomyyosin complex

No tension produced

An action potential at neuron terminal stimulates release of acetylcholine which diffuses across the cleft and triggers an action potential in the muscle fiber

The action potential moves across the surface membrane and into muscle fibers interior through T tubule. An action potential in the T tubule triggers release of Ca2+ from the sarcoplasmic reticulum into cytosol

Released Ca2+ binds to troponin on thin filaments

Ca2+ binding to troponin causes tropomyosin to change shape, physically moving it away from its blocking position which uncovers the binding sites on actin for the myosin cross bridges

Binding trggers the cross bridge to bend pulling the thin filament over thee thick filament. This power stroke is powered by energy provided by ATP

At resting length (2.0-2.2µm), a skeletal muscle fiber develops maximum tension if stimulated because this is the length where optimal overlap of thick filament cross bridges and thin filament cross bridge binding site occurs

If the muscle length is increased or decreased from the plateau region tension developed on stimulating is lesser

A single action potential in a muscle fiber produces a brief, weak contraction called a “twitch”

Greater tension can be produced by repeated stimulation of the muscle fiber before it relaxes.

Two twitches from two action potentials add together or sum to produce greater tension. This twitch summation is similar to temporal summation of EPSPs at the postsynaptic neuron

Twitch summation is possible only because the duration of the action potential (1-2msec) is much shorter than the duration of the resulting twitch (100ms)

If the muscle fiber is stimulated so rapidly that it does not have a chance to relax at all between stimuli, a smooth sustained contraction of maximal strength known as “tetanus” occurs. A tetanic contraction is usually four times stronger than a single twitch

As soon as Ca2+ is released in response to an action potential, the sarcoplasmic reticulum starts pumping Ca2+

back into lateral sacs As less cytosolic Ca2+ concentration subsequently declines,

less Ca2+ is present to bind with troponin, so some of the troponin-tropomyosin complexes slip back into their blocking position

Consequently, not all cross bridge binding sites remain available to participate in the cycling process during a single twitch induced by a single action potential

On repeated stimulation by successive action potentials, the cytosolic concentration of Ca2+ remains high

The prolonged availability of Ca2+ in the cytosol permits more of the cross bridges to continue participating in the cycling process for a longer time

With an increase in the frequency of action potentials, duration of elevated cytosolic Ca2+ concentration increases and contractile activity likewise increases until a maximum tetanic contraction is achieved

With the tetanus, the maximum number of cross bridge binding sites remain uncovered so that cross bridge cycling and consequently tension development is at its peak

When a motor neuron enters a muscle, it branches with each axon terminal supplying a single muscle fiber

One motor neuron supplies a number of muscle fibers but each muscle fiber is supplied by only one motor neuron

When a motor neuron is activated, all the muscle fibers it supplies are stimulated to contract simultaneously

This team of concurrently activated components : one motor neuron plus all the muscle fibers it innervates –is called a motor unit

Total tension developed by a whole muscle depends on

Number of muscle fibers in a motor unit that are activated simultaneously

Number of motor units in the whole muscle that are activated simultaneously

Because the muscle fibers are connected in parallel and exert forces on the same tendon, the force produced in all the fibers will be summed at the tendon. This is the spatial summation of tension

Total tension developed by a muscle is due to both spatial and temporal summation of tension

The minimum tension is produced by a single twitch in the smallest motor unit (with least number of muscle fibers)

The maximum tension is produced by tetanic contractions in all motor units of a muscle