There are three kinds of muscle tissue: skeletal, cardiac, and smooth.

• These three kinds of muscle tissue compose about 50 percent of a human’s body weight.

• Skeletal muscle tissue is striated and subject to voluntary control.

• The skeletal muscles make up the muscular system.• The skeletal muscles are innervated by the somatic

nervous system.

• Skeletal muscles require stimulation from the nervous system in order to contract

• Motor neurons are the cells that cause muscle fibers to contract

cell body

dendrites

axonSynaptic terminals

(synaptic end bulbs)telodendriaaxon hillock

motor neuron

End bulbs contain vesicles filled with Acetylcholine (Ach)

A muscle fiber is a skeletal muscle cell. It is large, elongated, and cylinder-shaped.

• A muscle fiber contains contractile elements called myofibrils. A myofibril contains thick filaments called myosin and thin filaments called actin.

• Actin and myosin are arranged in units called sarcomeres. A sarcomere is found between two Z lines. The sarcomere is the functional unit of the muscle. Its regions are:

• A band - myosin (thick) filaments stacked along with parts of the actin (thin) filaments

• H zone - middle of the A band where actin does not reach• M line - extends vertically down the center of the A band• I band - has part of actin that do not project into A band

Skeletal muscle

fiber (cell)

Muscle Fascicle

Surrounded by perimysium

Surrounded by endomysium

endomysium

perimysium

Skeletal muscle

Surrounded by epimysium

epimysiumtendon

sarcolemmatransverse (T) tubules sarcoplasmic

reticulumterminal cisternae

myofibril

thin myofilament

thick myofilament

triad

mitochondria

nuclei

myoglobin

Z line Z lineA band

H zone

I band Zone of overlap M line

Zone of overlap

Thin myofilaments Thick

myofilaments

Muscle fiber

myofibril

Thin filaments Thick filaments

Thin myofilamentMyosin molecule ofthick myofilament

sarcomereZ-line

The thick filaments of myosin have cross bridges. The cross bridges can attach to actin binding sites. The cross bridges also have myosin ATPase activity.

• Actin is the main, thin structural protein in the sarcomere. Each actin molecule has a binding site that can attach with a myosin cross bridge.

• Actin and mysoin are contractile proteins.

Tropomyosin and troponin are thin proteins. They are regulatory proteins.• Tropomyosin covers the actin binding sites, preventing

their union with myosin cross bridges.• Troponin has three binding sites: one binds to

tropomyosin, one to actin, and one to Ca ions. • When calcium combines with troponin, tropomyosin slips away

from its blocking position between actin and myosin.• With this change actin and myosin can interact and muscle

contraction can occur.

Thick myofilament

(has ATP & actin binding

site)

M-line

Play IP sliding filament theory p.5-14 for overview of thin & thick filaments

Thin Myofilament

(myosin binding site)

Z-line (Z-disc)

telodendria

Synaptic terminal

(end bulb)

Synaptic vessicles

containing ACh

Motor end plateof sarcolemma

Synaptic cleftNeuromuscular

junction

Synapticcleft

Arrival of an action potential at the synaptic terminal

Sarcolemma ofmotor end plate

Arriving action potential

Vesicles

AChAChE moleculesAChreceptorsite

Action potential

Synaptic terminal

Axon

Sarcolemma

Musclefiber

• An action potential (AP), an electrical impulse, travels down the axon of the motor neuron to the end bulbs (synaptic terminals)

Synapticcleft

Vesicles in the synaptic terminal fuse with the neuronal membrane and dump their contents into the synaptic cleft.

Release of acetylcholine

Arrival of an action potential at the synaptic terminal

Sarcolemma ofmotor end plate

Arriving action potential

Vesicles

AChAChE moleculesAChreceptorsite

Action potential

Synaptic terminal

Axon

Sarcolemma

Musclefiber

•The AP causes the synaptic vesicles to fuse with the end bulb membrane, resulting in the release of Acetylcholine (ACh) into the synaptic cleft

Synapticcleft

Vesicles in the synaptic terminal fuse with the neuronal membrane and dump their contents into the synaptic cleft.

The binding of ACh to the receptors increases the membrane permeability to sodium ions. Sodium ions then rush into the cell.

ACh binding at the motor and plateRelease of acetylcholine

Arrival of an action potential at the synaptic terminal

Sarcolemma ofmotor end plate

Arriving action potential

Vesicles

AChAChE moleculesAChreceptorsite

Action potential

Synaptic terminal

Axon

Sarcolemma

Musclefiber

Na+

Na+

Na+

•ACh diffuses across the synaptic cleft & binds to ACh receptors on the motor end plate

•The binding of ACh to its receptors causes a new AP to be generated along the muscle cell membrane

•Immediately after it binds to its receptors, ACh will be broken down by Acetylcholinesterase (AChE) – an enzyme present in the synaptic cleft

Sliding Filament Theory• Myosin heads attach to actin molecules (at binding (active) site)

• Myosin “pulls” on actin, causing thin myofilaments to slide across thick myofilaments, towards the center of the sarcomere

• Sarcomere shortens, I bands get smaller, H zone gets smaller, & zone of overlap increases

• As sarcomeres shorten, myofibril shortens. As myofibrils shorten, so does muscle fiber

• Once a muscle fiber begins to contract, it will contract maximally.

• If there are no longer APs generated on the motor neuron, no more ACh will be released

• AChE will remove ACh from the motor end plate, and AP transmission on the muscle fiber will end

• Ca+2 gates in the SR will close & Ca+2 will be actively transported back into the SR

• With Ca+2 removed from the sarcoplasm (& from troponin), tropomyosin will re-cover the active sites of actin

• No more cross-bridge interactions can form

• Thin myofilaments slide back to their resting state

The two primary types of contraction are isotonic and isometric.

• By isometric contraction the muscle tension developed is less than its opposing load. The muscle cannot shorten and lift the object with that load.

• By isotonic contraction the muscle tension developed is greater than its opposing load. The muscle usually shortens and lifts an object. The muscle maintains a constant tension throughout the period of shortening.

ATP is generated three ways for the muscle contraction.

• Creatine phosphate plus ADP is converted enzymatically to creatine plus ATP. This is the first source of ATP for the first minute or less of exercise.

• Oxidative phosphorylation generates large amounts of ATP in the mitochondria if oxygen is available for the muscle cell. This supports aerobic exercise.

• Glycolysis makes a small amount of ATP in the absence of oxygen. A net of 2ATPs is formed per glucose molecule. This process uses large quantities of stored glycogen and produces lactic acid. The accumulation of this acid produces muscle soreness.Glycolysis supports anaerobic exercise. One glucose molecule is converted into two molecules of pyruvic acid.

Smooth muscle can develop tension when it is stretched significantly.

• Its contraction is slow and energy-efficient.• Single-unit smooth muscle can exist at a many

lengths without a change in tension. It is well-suited for forming the walls of distensible, hollow organs.

Smooth muscle composes the internal, contractile organs except the heart. The heart is composed of cardiac muscle. • Smooth muscle cells are small and

unstriated.• These cells have actin and

myosin. Their arrangement is not organized compared to skeletal muscle cells. Therefore, smooth muscle cells are not striated.

• Smooth muscle cells contract when calcium ions enter the cells from the ECF. Calcium is also released from intracellular stores.

• This release activates a series of biochemical reactions leading to myosin cross bridge movement.

Cardiac muscle has properties of skeletal and smooth muscle.

• It is found in the walls of the heart.• It is highly organized and striated. These are

similarities to skeletal muscle tissue.• It can generate action potentials which spread

throughout the walls of the heart. This is similar to single-unit smooth muscle.