Muscle Contraction, Heartbeat Regulation, and Blood Vessels
Energy Consumption in Muscle Contraction
Energy Consumption in Contraction: Requires large quantities of ATP for this process. ATP is needed for three different processes:
- For the union and disunion between actin and myosin.
- To sequester calcium.
- For the recovery of the membrane after depolarization.
The concentration of ATP in muscle is relatively low and provides enough energy to maintain the contraction for a short period. If the muscles continue to contract after that initial contribution has been exhausted, the ADP that is produced is phosphorylated again at the expense of creatine phosphate (CP) found at higher levels in the muscle cell. Additional input of ATP is required, which can come from the metabolism of glucose supplied by blood circulation and glycogen stored in muscle fiber. Blood sugar is converted to glycogen by the process of glycogenesis.
Factors Affecting Muscle Contraction
Treppe (action or step of the staircase phenomenon) is the successive increase in the intensity of the contractions of a muscle fiber due to stimuli that are repeated every few seconds. The effect may be due to increased concentration of Ca2+ ions inside the fiber, which increases activation of the myofibrils.
Summation: Every large muscle is capable of contracting with various degrees of intensity. This is the result of summation of contractions in two ways: by increasing the number of motor units that contract at a time or by increasing the frequency of contraction, which is the “summation frequency” or “tetanization.”
Tetany: As frequency increases, there comes a time when the new contraction begins before the previous one has finished. When it reaches a critical level, the contractions succeed each other so fast that they join completely, becoming a continuous contraction.
Fatigue: This is a decrease in working capacity caused by work. As energy content decreases in the cell, contractile force decreases, and the muscle becomes weaker.
Rigor: If ATP is depleted, calcium cannot be taken back into the sarcoplasmic reticulum by the calcium pump. Therefore, there can be no relaxation because the actin and myosin are united in a state of continuous contraction.
Tone: This is the tension that all muscles show in a relaxed state. It is due to the continuous transmission of low-frequency pulses from the spinal cord to the muscles.
Regulation of Heartbeat
Intrinsic Regulation:
- Heterometric Regulation: Due to variations in the degree to which the myocardium is stretched by the volume of blood entering. As the volume of blood entering increases, the myocardium undergoes a progressive stretching and contracts with greater force.
- Homeometric Regulation: High contraction causes a high heart rate (by increasing the entry of Ca2+ into the cell).
Extrinsic Regulation (Neural Control): Cardioinhibitory and cardioexcitatory centers are located in the medulla.
- Parasympathetic Influence: Ongoing cardioinhibitory nerve (vagus nerve) stimulation causes a decrease in frequency, conduction velocity, and decreased energy of contraction.
- Sympathetic Influence: Cardioaccelerator nerves (upper, middle, and lower cervical lymph nodes) cause opposite effects to the parasympathetic system.
Endocrine Control: The action of adrenaline is similar to the sympathetic nervous system. Ion environment: If you double the extracellular K+, it decreases the frequency, conduction velocity, and energy of contraction.
Types of Blood Vessels
Arteries: These have strong and elastic walls. This elasticity is important to maintain high blood pressure during diastole, i.e., the period during which the ventricles are distended.
Arterioles: In them, the middle layer is muscular in nature, so that changes of expansion and contraction are more active. Thanks to this property, the amount of blood reaching the capillaries may increase or decrease in response to the needs of tissues.
Capillaries: These are small vessels into which arteries divide and penetrate all body organs, to rejoin and form veins. They are primarily formed of endothelium.
Venules: These collect blood from capillaries and merge to form veins.
Veins: These are thin-walled vessels with little elasticity that collect blood and return it to the heart, emptying into the atria.