Muscle Contraction and Relaxation

Neuron Structure and Function

• Dendrites: Receive messages from other cells.
• Cell Body: The cell's life-support center.
• Axon: Passes messages away from the cell body to other neurons, muscles, or glands.
• Myelin Sheath: Covers the axon of some neurons and helps speed neural impulses.
• Neural Impulse (Action Potential): Electrical signal traveling down the axon.
• Terminal Branches of Axon: Form junctions with other cells.

Synapse

• Neurotransmitters: Chemical messengers that transmit signals across a synapse.
• Axon Terminal: The end of an axon where neurotransmitters are released.
• Synaptic Vesicles: Sacs within the axon terminal that contain neurotransmitters.
• Pre-synaptic Membrane: The membrane of the axon terminal that releases neurotransmitters.
• Synaptic Cleft: The space between the pre-synaptic and post-synaptic membranes.
• Post-synaptic Membrane: The membrane of the receiving cell that contains receptors.
• Receptors: Proteins on the post-synaptic membrane that bind to neurotransmitters.

Skeletal Muscle Contraction and Relaxation

Steps in Initiating Muscle Contraction

1. Motor Terminal and Plate: The motor neuron's axon terminal approaches the muscle fiber.
2. ACh Released, Binding to Receptors: Acetylcholine (ACh) is released from the motor neuron and binds to receptors on the sarcolemma.
3. Action Potential Reaches T-tubule: The action potential travels along the sarcolemma and into the T-tubules.
4. Sarcoplasmic Reticulum Releases $Ca^{2+}$: The sarcoplasmic reticulum releases calcium ions into the sarcoplasm.
5. Active Site Exposure, Cross-Bridge Formation: Calcium ions bind to troponin, exposing active sites on actin, allowing myosin heads to bind and form cross-bridges.
6. Contraction Begins

Steps in Muscle Relaxation

1. ACh Broken Down by AChE: Acetylcholinesterase (AChE) breaks down acetylcholine, removing the stimulus for contraction.
2. Sarcoplasmic Reticulum Recaptures $Ca^{2+}$: The sarcoplasmic reticulum actively transports calcium ions back into its lumen.
3. Active Sites Covered, No Cross-Bridge Interaction: Tropomyosin covers the active sites on actin, preventing further cross-bridge formation.
4. Contraction Ends
5. Relaxation Occurs: The muscle passively returns to its resting length.

The Contraction Cycle

• The contraction cycle begins with the arrival of calcium ions within the zone of overlap.

Active-Site Exposure

• Calcium ions bind to troponin, weakening the bond between actin and the troponin-tropomyosin complex.
• The troponin molecule changes position, rolling the tropomyosin molecule away from the active sites on actin, allowing interaction with energized myosin heads.

Cross-Bridge Formation

• Once the active sites are exposed, the energized myosin heads bind to them, forming cross-bridges.

Myosin Head Pivoting

• After cross-bridge formation, the energy stored in the resting state is released as the myosin head pivots toward the M line (power stroke).
• During this action, bound ADP and phosphate group are released.

Cross-Bridge Detachment

• When another ATP molecule binds to the myosin head, the link between the myosin head and the active site on the actin molecule is broken.
• The active site is now exposed and able to form another cross-bridge.

Myosin Reactivation

• Myosin reactivation occurs when the free myosin head splits ATP into ADP and P (inorganic phosphate).
• The energy released is used to recock the myosin head.

Fiber Shortening

• As sarcomeres shorten, the muscle pulls together, producing tension.
• Muscle shortening can occur at both ends of the muscle or at only one end, depending on how the muscle is attached.

Muscle Relaxation

• $Ca^{2+}$ concentrations fall.
• $Ca^{2+}$ detaches from troponin.
• Active sites are recovered by tropomyosin.

Rigor Mortis

• A fixed muscular contraction after death.
• Caused when ion pumps cease to function due to lack of ATP, leading to a buildup of calcium in the sarcoplasm.

Summary of Muscle Contraction and Relaxation

• Skeletal muscle fibers shorten as thin filaments slide between thick filaments.
• Free $Ca^{2+}$ in the sarcoplasm triggers contraction.
• The sarcoplasmic reticulum releases $Ca^{2+}$ when a motor neuron stimulates the muscle fiber.
• Contraction is an active process.
• Relaxation and return to resting length are passive.

Tension Production and Contraction Types

Tension Production by Muscle Fibers

• A muscle fiber is either contracted or relaxed as a whole.
• Dependent on:
  • The number of pivoting cross-bridges.
  • The fiber’s resting length at the time of stimulation.
  • The frequency of stimulation.

Length-Tension Relationships

• The number of pivoting cross-bridges depends on the amount of overlap between thick and thin fibers.
• Optimum overlap produces the greatest amount of tension.
• Too much or too little overlap reduces efficiency.
• Normal resting sarcomere length is 75% to 130% of optimal length.

Frequency of Stimulation

• A single neural stimulation produces:
  • A single contraction or twitch, lasting about 7–100 msec.
• Sustained muscular contractions require many repeated stimuli.

Twitches

• Latent period:
  • The action potential moves through the sarcolemma, causing $Ca^{2+}$ release.
• Contraction phase:
  • Calcium ions bind and tension builds to peak.
• Relaxation phase:
  • $Ca^{2+}$ levels fall, active sites are covered, and tension falls to resting levels.

Tension Production by Skeletal Muscles

• Depends on:
  • Internal tension produced by muscle fibers.
  • External tension exerted by muscle fibers on elastic extracellular fibers.
  • Total number of muscle fibers stimulated.

Motor Units and Tension Production

• Motor units in a skeletal muscle:
  • Contain hundreds of muscle fibers.
  • Contract at the same time.
  • Controlled by a single motor neuron.

Recruitment (Multiple Motor Unit Summation)

• In a whole muscle or group of muscles, smooth motion and increasing tension are produced by slowly increasing the size or number of motor units stimulated.
• Maximum tension is achieved when all motor units reach tetanus but can only be sustained for a very short time.