Muscle Contraction and Respiratory Volumes: A Detailed Look

Phases of Neuromuscular Transmission

Motor Neuron Transmission:

1. The action potential occurs in the axonal cone and travels along the axon (nodes of Ranvier). When it reaches the presynaptic membrane, voltage-gated Ca channels open.
2. Ca enters the neuron, and the presynaptic membrane attracts vesicles of acetylcholine (ACh) found in the axon's cytosol.
3. Acetylcholine is released into the synapse.
4. Chemical-gated channels open when coupled to ACh, allowing Na to enter the cell.
5. An action potential occurs in the muscle cell membrane and spreads throughout the plasmolemma.
6. In the triad, dihydropyridine receptors (DHPR) are close to the terminal cisternae. When stimulated by the action potential, they stimulate ryanodine receptors. Ryanodine receptors (calcium channels) open, releasing Ca into the sarcoplasm.
7. Once Ca is in the sarcoplasm, it acts on troponin C, coupling to it.

Stepper Theory of Muscle Contraction

1. ATP has not yet appeared. The myosin head and actin are linked.
2. ATP appears and binds to the myosin head, detaching it from actin.
3. ATP is broken down, releasing energy and P. This energy structurally changes the myosin head. ADP and P are joined at the myosin head.
4. ADP + P are released, causing the head to strike another actin-myosin.
5. Through a passive mechanism, the myosin head returns to its initial position, producing the drag of the filaments (contraction).

Muscle Relaxation

Muscle contraction relaxation occurs through two mechanisms:

1. Withdrawal of cytosolic calcium.
2. Action of acetylcholinesterase (AChE).

Additional Steps:

1. The myosin head is attached to the active point of actin. ATP is separated from the actin.
2. ADP and Pi are held together by myosin.
3. The myosin head rotates, reaching the height of actin.
4. The output of Pi starts the recovery movement. The head rotates on its hinge to drive the actin filament.
5. ADP is released, and the head resumes its original place.
6. Pumps of the SR Ca++ withdraw Ca from the sarcoplasm. The back cover tropolina sites on actin.

Respiratory Volumes

• Tidal Volume (VT): Volume of air inspired or expired at each ventilatory cycle.
• Dead Space (VD): Part of the VT that does not reach the alveoli and does not undergo gas exchange (150 ml).
• Alveolar Volume (VA): Part of the VT that reaches the alveoli (350ml).
• Inspiratory Reserve Volume (IRV): Additional volume that can be inspired above the VT (3L).
• Minute Respiratory Volume (MV): MV = F × VT
• Alveolar Ventilation Rate (AVR): AVR = F × VA
• Expiratory Reserve Volume (ERV): Volume you can breathe out through forced expiration after a normal expiration (1.1 L).
• Residual Volume (RV): Volume remaining in the lungs after forced expiration (1.2 L).
• Inspiratory Capacity (IC): IRV + VT
• Vital Capacity (VC): IRV + VT + ERV
• Total Lung Capacity (TLC): VC + RV
• Functional Residual Capacity (FRC): ERV + RV