Understanding Muscles: Types, Properties, and Function
Muscles: The Engine of Movement
Muscles are the active part of the locomotor system, soft organs responsible for bodily movement. They are susceptible to stimuli and have the ability to contract and relax.
Muscle Properties
- Excitability: The ability to react to stimuli received from the motor nerve.
- Conductivity: The power of a fiber to transmit electrical impulses via depolarization of the cell membrane.
- Elasticity: The ability of the fiber to recover its original shape.
- Contractility: The ability to undergo changes in internal tension, resulting in changes in length and width.
Types of Muscle Tissue
- Smooth Muscle Tissue: Found in internal organs (digestive tract, etc.) and controlled by the autonomic nervous system. Contraction is involuntary.
- Cardiac Muscle Tissue: Forms the heart and is stimulated by the myocardium. It contracts autonomously due to a structure called the sinus node.
- Striated Skeletal Muscle Tissue: Forms the locomotor apparatus, enabling voluntary movement. Stimulated by the central nervous system, it is composed of striated fibers connected by tendons, aponeuroses, blood vessels, lymphatics, nerve endings, and connective tissue.
Skeletal Muscle Structure
- Muscle Fiber: Cylindrical and arranged in parallel, separated by the endomysium.
- Fascicle: A bundle of fibers surrounded by the perimysium.
- Muscle Belly: A union of several fascicles, surrounded by the epimysium (fascia).
- Tendons: The epimysium narrows at the ends to form the tendons, which connect the muscle to the bone by joining the periosteal envelope.
Microscopic Structure of Striated Muscle
- Sarcolemma: A thin, elastic membrane surrounding the muscle fiber cell, allowing the passage and exchange of substances. It has indentations to receive nerve endings.
- Sarcoplasm: The aqueous protoplasm of the cell, containing:
- Pigments: Such as myoglobin (oxygen reserve).
- Lipid Droplets: Energy reserves floating in the sarcoplasm.
- Glycogen Droplets: Stored glucose, a medium-duration fuel for muscle exertion.
- Mitochondria: Elongated organelles that synthesize cellular energy (ATP), providing energy to other cell parts and continuously renewing themselves.
- Sarcoplasmic Reticulum: A network of canals, tubes, and vesicles parallel to the myofibrils, storing calcium for muscle contraction (L-system).
- Transverse Tubular System (T-system): A transverse network of tubules connected to the myofibrils and sarcoplasmic reticulum, forming triads.
- Myofibrils: Smaller fibers within the sarcoplasm, containing even smaller myofilaments (actin and myosin).
- Sarcomere: The functional unit of the myofibril.
- Triad: An interconnected network of tubular channels parallel to the myofibrils, with a vesicle (L-system) on each side storing calcium. The T-system, perpendicular to these, forms a triad with two T-tubule vesicles. Each sarcomere contains two triads. The T-tubule system functions as a network for transporting substances from the outer membrane to the cell.
Muscle Fiber Types
- Slow-Twitch Fibers: Low myosin ATPase levels, slow contraction speed, numerous mitochondria, fatigue-resistant, suitable for prolonged work and aerobic activities.
- Fast-Twitch Fibers: High electrochemical transmission capacity, high myosin ATPase activity (for ATP breakdown during contraction), quick calcium release and uptake, generating energy quickly for powerful and fast activities.
Muscle Function
- Contraction: First, the muscle must be excited by the nervous system. The motor end plate transmits the signal to the muscle. The neurotransmitter acetylcholine is released from neuron axons, permeabilizing the membrane, and sodium and potassium depolarize the membrane via the T-tubules.
- Relaxation: When stimulation stops, calcium levels decrease, leading to relaxation (which is never complete).
- Stretching: The ability to stretch depends on the connective tissue. The stronger the muscle, the less it can stretch.