The Muscular System

The Muscular System Chapter 11

Introduction • Muscles make things happen: • Supply force for motion • Hold animals in position • Act upon the viscera • Act in heat regulation • Produce electrical currents

Types of Muscle Tissues • The criteria for the classification of muscles is based on the reason for classifying them at the time: • According to color (Red & White) • According to position (Somatic & Visceral) • Nervous control (Voluntary & Involuntary) • Microscopic appearance (Cardiac, Skeletal, & Smooth)

Skeletal Muscle • Skeletal muscle tissue is the most common type of muscle in the body. • The cells, or muscle fibers, of skeletal muscles are very large, multinucleated cells. • Each muscle fiber is known as a syncytium, a single cell with multiple nuclei. • The actin and myosin filaments of skeletal muscle partially overlap. • Makes the muscle appear striated. • Contraction is initiated by nervous impulses and is described as neurogenic.

The partial overlap of actin and myosin filaments increases the ability of the myofilaments to interact with each other • Therefore, skeletal muscles can contract with considerable force.

When a muscle fiber is stimulated, • A redistribution of ions across the plasma membrane leads to the development of an electrical action potential, • Which travels rapidly to all parts of the muscle fiber, • And the muscle contracts. • The activity of skeletal muscles is under the voluntary control of the animal, • The complex sequence of events to activate the muscle occurs subconsciously.

Cardiac Muscle • Cardiac muscle, is composed of moderately elongated cells that frequently branch • Each cell originates from a single myoblast, so it contains a single nucleus, located near the center of the cell. • Like skeletal muscles, the actin and myosin filaments partially overlap appear striated. • The individual cells are interconnected with each other end to end, at specialized intercalated disks. • Unlike skeletal muscles the action of cardiac muscle is not voluntary and it does not fatigue.

Contraction is myogenic, originating within the muscle, and is triggered within a region called the pacemaker, or sinoatrial node. • Nerve fibers terminate on the pacemaker, and many other cells, so the inherent muscle rhythm is modulated by neurogenic control. • The design of the cardiac muscles allows them to contract with nearly as much force as skeletal muscles.

Smooth Muscles • Smooth muscle fibers are elongated, spindle shaped cells. • Because each arises from a single myoblast the cell has a single nucleus located near its center. • Actin and myosin filaments are present in the cytoplasm of the cell • They do not line up in a regular manner. • Thus, smooth muscle appears to have a homogenous texture.

Smooth muscle fibers are part of the walls of blood vessels and visceral organs, and they also attach to the hairs in mammal skin. • Their actions are involuntary, and the contractions tend to be slow and sustained. • Smooth muscles do not fatigue.

Muscle Organization and Connective Tissue • In all types of muscle tissues the individual muscle fibers are enveloped by a thin layer of connective tissue, known as endomysium, through which the blood vessels and nerves that supply the fibers travel. • Groups of skeletal muscle fibers surrounded by their endomysium form small bundles, or fasciculi, held together by a layer of connective tissue called perimysium. • Many fascicule, in turn, are surrounded by an epimysium and aggregate into units that we recognize as individual muscles.

Tendons • Skeletal muscles have distinct attachments to skeletal elements or the well defined connective tissue septa by tendons. • Tendons consist of an extension of the connective tissue within the muscle into the connective tissue that surrounds the bone. • They are cord-like bands that frequently penetrates the bone.

Skeletal muscles, or their tendons, extend across one or more joints and either; • Move skeletal elements relative to one another, or • Stabilize the skeletal elements at the joint s they don’t move. • It is convenient to describe the opposite attachment points of skeletal muscle as the origin and insertion points. • The origin is thought of as the point of attachment that remains fixed during contraction, • The insertion point is the point that moves; • However, the point that moves may change with circumstance. • By convention the origin is the proximal end of he muscle, and the insertion is the distal end.

Muscle Function • The muscle fibers within a skeletal muscle are organized into motor units • Consisting of a motor neuron and the muscle fibers it supplies. • All muscle fibers in a motor unit are activated when the neuron supplying them is activated. • Muscles differ with respect to the number of muscle fibers within motor units.

During normal muscle activity, an ever-changing rotation of active, relaxing, and quiescent motor units occurs. • As functional demands change the proportion of active motor units increases or decreases. • When a very fine regulation of contraction is needed each motor unit contains only a dozen or fewer muscle fibers. • Activating only one, or a few, motor units can cause delicate movements. • When a strong force is required each unit may contain as many as 2000 or more muscle fibers.

Muscle Contraction • Muscle contraction results from the interaction of actin and myosin myofilaments. • An action potential initiated on the membrane of a muscle fiber by a nerve impulse or by other means initiates a series of biochemical changes that lead to the formation of cross bridges between actin and myosin myofilaments. • The amount of tension or force that a muscle fiber can generate is a function of the number of actin-myosin attachments that can be made at one time.

Modes of Contraction • The tension that results from the interaction of myofilaments within a muscle is called a muscle contraction. • Depending on the circumstances the muscle may or may not shorten. • If the muscle shortens it causes bones, or other structures to which it attaches to move. • A muscle contraction that initiates a shortening of the muscle fiber is called an isotonic contraction.

Movements of the body are caused by isotonic contractions. • In isometric contractions, tension develops, but little if any shortening of the muscle takes place. • Muscles that hold an animal or hold a part in a fixed place contract isometrically.

Types of Muscle Action • Muscles perform their functions by developing tension and often shortening. • They are restored to their resting length upon relaxation by an antagonistic force that operates in a direction opposite to the direction of contraction. • Usually, muscles are arranged into antagonistic groups such that one pulls a structure in one direction, and its antagonist pulls in the opposite direction.

Sets of terms may define many antagonistic actions. • The movement of a distal limb segment toward a more proximal one is called flexion. • Extension is in the opposite direction and straitens the limb. • Adduction describes the movement of a part toward some point of reference, • Abduction is away from the point of reference. • Rotation is the movement of the bone around some fixed point.

Fiber Orientation • Muscles also differ in the length and arrangement of the muscle fibers within them: • Strap-Shaped muscles contain long, parallel fibers and have relatively broad attachments. • Fusiform muscles are similar, except that their muscle fibers lead to narrow tendons at the end of the muscle, so the force of contraction is on a smaller area. • Pennate muscles contain short, diagonally arranged fibers that insert into tendons on one side, unipennate, of the muscle or on both sides of a tendon, bipennate.

These variables in muscle architecture affect • The degree of muscle contraction, • The velocity of contraction, • The force of contraction, and • The power a muscle can develop. • Muscle power is equal to the amount of force a muscle can generate multiplied by the velocity of the contraction. • The speed degree of contraction depends on muscle length. • The strength, force, of contraction depends on the number of myofilaments in a fiber and the number of fibers within a muscle.

Embryonic Origins • A regional grouping of muscles or a functional grouping is useful in dissection and muscle studies, • Such groupings often include muscles of difference evolutionary, or phylogenetic development. • During the course of evolution muscles sometimes change their position or points of attachment. • For this reason, the group to which a muscle belongs is best seen by looking at its embryonic development and nerve supply.

Following the division of the body into a somatic and visceral half, we can differentiate muscles into somatic and visceral groups. • Most somatic muscles lie in the “outer” tube of the body and are develop from myotomes that derive from embryonic somites. • They form most of the skeletal muscles of the body. • Visceral muscles develop in the inner tube of the body and form the inner, splanchnic, layer of the body. • They contribute to the walls of the visceral organs and the heart.

Somatic Muscle Groups • Somatic muscles can be subclassified into axial muscles located along the longitudinal axis of the body or appendicular muscles that develops and migrates into the limb buds. • Axial muscles can be further subclassified according to the group of body segments from which they arise. • Extrinsic ocular muscles form the muscles of the eye. • Branchiomeric muscles form the mandibular and branchial muscles. • The first part of the trunk musculature are the epibranchial and hypobranchial muscles. • The remaining myomeres form the muscles of the trunk.

Appendicular muscles are defined as those muscles that begin their differentiation within the limb buds. • Appendicular muscles always insert on the girdles or bones of the paired appendages. • They can usually be sorted into dorsal and ventral groups based on their position relative to the skeleton and girdles.

Axial Muscles • Vertebrates have many individual muscles, and they vary greatly as the methods of support, locomotion, feeding, gas exchange, and other activities of vertebrates change during their adaptation to their many habitats and modes of life.

Branchiomeric Muscles • The branchiomeric muscles lie in the lateral wall of the pharynx • The branchiomeric and hypobranchial muscles work together in breathing movements, capturing food, and swallowing. • In some cases they are assisted by the epibranchial muscles

Extrinsic Ocular Muscles • The most rostral axial muscles belong to the extrinsic ocular group. • These small, strap-shaped muscles arise from the wall of the orbit and insert on the surface of the eyeball. • They rotate the eyeball as needed and the ability to rotate the eye is common to all vertebrates with well-developed eyes. • Nearly all vertebrates have the same 6 muscles: • The ventral oblique, ventral rectus, medial rectus, dorsal rectus, dorsal oblique, and dorsal rectus.

Trunk and Tail Muscles • Fishes: The embryonic myotomes of the trunk and tail develop into a series of folded muscle segments, the myomeres, of adult fishes. • The sequential contraction of myomeres, acting with the vert column, causes a series of lateral undulations by which the fish swims. • Individual myomeres are separated by myosepta, a horizontal skeletogenous septum divides the myomeres into dorsal epaxial and ventral hypaxial halves.

Tetrapods: Major changes occur in the trunk and tail muscles because their role in locomotion decreases in the transition to the terrestrial environment. • Although trunk muscles become less important in locomotion, they play an important role in mediating flexion and extension of the spine and in supporting the body against gravity. • The epaxial muscles are especially important in supporting the body and moving the vertebral column and head.

The hypaxial muscles of vertebrates can be divided into three groups: • A subventral group that act on the vertebral axis and assist the epaxial muscles in supporting the body. • A ventral group that includes the abdominal muscles that support the abdomen and assist in lateral and ventral trunk flexion. • A lateral group that lies on the flank and forms 3-4 layers within the abdomen.

Evolution of the Appendicular Muscles • The paired appendages of most fishes do not deliver major propulsive thrust. • The fin may provide lift, but they are primarily used in maintaining stability, breaking, and maneuvering. • Often a single dorsal, extensor, muscles is located dorsally on the fin and pulls it dorsally; • While a ventral flexor muscle pulls the fin ventrally. • The structure and movements of the paired appendages of terrestrial vertebrates are far more complex because the limbs support the body and provide propulsive thrust for locomotion.

Appendicular muscles constitute the bulk of the muscles found in terrestrial vertebrates. • Despite their complexity they can be divided into dorsal and ventral groups. • Collectively the dorsal muscles are homologous to the fish extensor, • The dorsal muscles of both tetrapod pectoral and pelvic girdles are responsible for abducting and extending the limbs as a whole and these actions occur during the swing phase of the step. • And, Those of the ventral group to the fish’s flexor muscles.

The pectoral and pelvic limbs as a whole are advanced or protracted during the swing phase. • A force is developed that could retract, or draw the limb back during the stance phase; • Because the feet remain firmly on the ground, this force advances the trunk relative to the feet. • Both the dorsal and ventral muscles participate in these actions, depending on whether the limbs lie anterior or posterior to the shoulder and hip joints.

The Muscular System