Muscle PhysiologyMuscle Physiology

Chapter 7Chapter 7

Skeletal Muscle Tissue

Striated

Elongated Fibers

Voluntary Nervous Control

Located Near Bones

Cardiac Muscle Tissue

Forms the wall of the heart

Striated

Intercalated discs to transmit electrical impulses

Involuntary nervous control.

Smooth Muscle Tissue

Located in visceral organs

Nonstriated

Nuclei are elongated and located in the center

Involuntary

Anatomy of Skeletal Muscle

• Fascia - sheet or broad band of fibrous CT underneath skin or around muscles

• Epimysium - covers entire muscle• Perimysium - Covers fasiculi• Endomysium - Covers individual

muscle fibers• Tendons & Aponeuroses - Muscles to

bones.

The Motor Unit

• A motor neuron & the muscle fibers it stimulates make up a motor unit.

• A single motor neuron may innervate as few as 10 or as many as 2000 muscle fibers. Average = 150

The Motor Unit

The Neuromuscular Junction

• Synaptic Cleft - the small gap separating the two cells.

• Motor End Plate - the Post synaptic cell (muscle fiber!!)

• Achetylcholine (Ach) - the NT contained in the synaptic vesicles released from the axon terminal.

The NMJ

Microscopic Anatomy of Muscle

• Fibers are covered by a sarcolemma. The fibers contain sarcoplasm, nuclei, sarcoplasmic reticulum, and transverse tubules.

• Each fiber contains myofibrils consisting of thin & thick filaments.

• Sarcomeres - Compartmentalization of filaments.

Myofibrils

• Myosin - Thick Filaments

• Actin - Thin filaments

• Troponin & Tropomyosin - Regulatory proteins.

Contraction of Muscle

• Sliding Filament Mechanism– Myosin cross bridges pull on actin, causing them

to slide inward toward the H zone.– Z discs come toward each other and the

sarcomere shortens, but the thick and thin filaments do not change length.

– The sliding of filaments and shortening of sarcomeres causes shortening of the muscle.

Sliding Filament cont’d

• Nerve impulse arrives• Synaptic vesicles fuse w/membrane• Ach is released into synaptic cleft• Ach diffuses across & binds to

receptors.• AP is generated in the sarcolemma that

then travels into the T-tubules & causes stored Ca ions to be released.

Sliding Filament Cont’d

• The muscle AP releases calcium ions that combine w/Troponin, causing it to pull on the tropomyosin to change its orientation.

• Myosin-binding sites on actin are exposed.

Energy for Muscle Contraction

• ATP is the immediate source of energy for muscle contraction.

• The enzyme ATPase splits ATP into ADP + P (inorganic phosphate) and the released energy activates myosin cross-bridges.

• Activated cross bridges attach to actin & the power stroke occurs - sliding of actin filaments over myosin filaments.

Energy of Contraction

• Once the power stroke is complete, ATP again combines with the ATP binding sites on the myosin cross-bridges; as ATP binds, the myosin head detaches from actin & the cycle may be reinitiated repeatedly.

Relaxation of Muscles

• Acetylcholinesterase (ACHe) and calcium ions is moved from the sarcoplasm back into the sarcoplasmic reticulum via active transport & a calcium binding protein called calsequestrin.

Relaxation of Muscles

• Relaxation Period - Muscle fibers returning to their original length.

• Refractory Period - The time when a muscle has temporarily lost excitability.– Skeletal Muscle: Short RP– Cardiac Muscle:Long RP

Muscle Twitch

• Latent Period• Contraction Phase• Relaxation Phase

Force of Contraction

• Summation: Increasing the force of contraction of the muscle fibers within the muscle.

• Recruitment: Involves increasing the number of muscle fibers contracting.

Force of Contraction

• Summation: The force of contraction of individual fibers is increased by rapidly stimulating them. If the frequency of stimulation is high, there is not enough time for relaxation, so one contraction summates, or is added onto, a previous contraction. As a result, the overall force of contraction increases.

Force of Contraction

• Tetanus: A sustained contraction that occurs when the frequency of stimulation is so rapid that there is no relaxation. The increased force of contraction produced in summation and tetanus occurs because of a buildup of Calcium ions.

Force of Contraction

• Recruitment: The strength of contraction is increased by increasing the number of motor units stimulated. As the number of motor units increases, more muscle fibers are stimulated to contract, and the force of contraction increases.