Overview Of Muscle Tissues

OVERVIEW OF MUSCLE AND NERVOUS TISSUES

The three types of muscle tissue are skeletal, cardiac, and smooth.

First, skeletal and smooth muscle cells (but not cardiac muscle cells) are elongated and, for this reason, are called muscle fibers.

Second, muscle contraction depends on two kinds of myofilaments, which are the muscle equivalents of the actin- or myosin-containing microfilaments.

These two proteins play a role in motility and shape changes in virtually every cell in the body, but this property reaches its highest development in the contractile muscle fibers.

Third, whenever you see the prefixes myo or mys or sarco (flesh), the reference is to muscle

SKELETAL MUSCLE

Each skeletal muscle is a discrete organ, made up of several kinds of tissues.

Although skeletal muscle fibers predominate, blood vessels, nerve fibers, and substantial amounts of connective tissue are also present.

A skeletal muscle’s shape and its attachments in the body can be examined easily without the help of a microscope.

Overview Of Muscle Tissues

In general, each muscle is served by one nerve, an artery, and by one or more veins, all of which enter or exit near the central part of the muscle and branch profusely through its connective tissue sheaths.

Unlike cells of cardiac and smooth muscle tissues, which can contract in the absence of nerve stimulation, each skeletal muscle fiber is supplied with a nerve ending that controls its activity.

Overview Of Muscle Tissues

Contracting muscle fibers use huge amounts of energy, a situation that requires more or less continuous delivery of oxygen and nutrients via the arteries.

Muscle cells also give off large amounts of metabolic wastes that must be removed through veins if contraction is to remain efficient.

Muscle capillaries, the smallest of the body’s blood vessels, are long and winding and have numerous cross-links, features that accommodate changes in muscle length.

CONNECTIVE TISSUE SHEATHS

In an intact muscle, the individual muscle fibers are wrapped and held together by several different connective tissue sheaths.

Together these connective tissue sheaths support each cell and reinforce the muscle as a whole, preventing the bulging muscles from bursting during exceptionally strong contractions.

Overview Of Muscle Tissues

• Endomysium. Each individual muscle fiber is surrounded by a fine sheath of connective tissue consisting of areolar and reticular fibers.
• Perimysium and fascicles. Within each skeletal muscle, the endomysium-wrapped muscle fibers are grouped into fascicles that resemble bundles of sticks.

Surrounding each fascicle is a layer of fibrous connective tissue called perimysium.

Epimysium. An overcoat of dense irregular connective tissue surrounds the whole muscle.

This coat is the epimysium, a name that means outside the muscle.

Sometimes the epimysium blends with the deep fascia that lies between neighboring muscles or the superficial fascia deep to the skin

ATTACHMENTS

• Most skeletal muscles span joints and are attached to bones (or other structures) in at least two places.
• When a muscle contracts, the movable bone, the muscle’s insertion, moves toward the immovable or less movable bone, the muscle’s origin.

In the muscles of the limbs, the origin typically lies proximal to the insertion

Muscle attachments, whether origin or insertion, may be direct or indirect.

In direct, or fleshy, attachments, the epimysium of the muscle is fused to the periosteum of a bone or perichondrium of a cartilage.

In indirect attachments, the muscle’s connective tissue wrappings extend beyond the muscle either as a ropelike tendon or as a sheetlike aponeurosis.

The tendon or aponeurosis anchors the muscle to the connective tissue covering of a skeletal element (bone or cartilage) or to the fascia of other muscles

SMOOTH MUSCLE

• Smooth muscle fibers are spindle-shaped cells, each with one centrally located nucleus.
• Typically, they have a diameter of 5–10 µm and are 30–200 µm long.

Skeletal muscle fibers are some 10 times wider and thousands of times longer.

Smooth muscle lacks the coarse connective tissue sheaths seen in skeletal muscle.

However, a small amount of fine connective tissue, secreted by the smooth muscles themselves and containing blood vessels and nerves, is found between smooth muscle fibers.

Most smooth muscle is organized into sheets of closely apposed fibers.

These sheets occur in the walls of all but the smallest blood vessels and in the walls of hollow organs of the respiratory, digestive, urinary, and reproductive tracts

CARDIAC MUSCLE

like skeletal muscle, is striated, and it contracts by the sliding filament mechanism.

However, in contrast to the long, cylindrical, multinucleate skeletal muscle fibers, cardiac cells are short, fat, branched, and interconnected.

Each fiber contains one or at most two large, pale, centrally located nuclei.

The intercellular spaces are filled with a loose connective tissue matrix (the endomysium) containing numerous capillaries.

This delicate matrix is connected to the fibrous skeleton, which acts both as a tendon and as an insertion, giving the cardiac cells something to pull or exert their force against

Overview Of Muscle Tissues

Large mitochondria account for 25–35% of the volume of cardiac cells and give cardiac cells a high resistance to fatigue.

Most of the remaining volume is occupied by myofibrils composed of fairly typical sarcomeres.

The sarcomeres have Z discs, A bands, and I bands that reflect the arrangement of the thick (myosin) and thin (actin) filaments composing them.

NERVOUS TISSUE

The nervous system consists mostly of nervous tissue, which is highly cellular.

Less than 20% of the CNS is extracellular space, which means that the cells are densely packed and tightly intertwined.

Although it is very complex, nervous tissue is made up of just two principal types of cells:

• Supporting cells, smaller cells that surround and wrap the more delicate neurons.
• Neurons, the excitable nerve cells that transmit electrical signals.

NEUROGLIA

• Neurons associate closely with much smaller cells called neuroglia or simply glial cells.

There are six types of neuroglia—four in the CNS and two in the PNS.

Each type has a unique function, but in general, these cells provide a supportive scaffolding for neurons.

Some produce chemicals that guide young neurons to the proper connections, and promote neuron health and growth.

Others wrap around and insulate neuronal processes to speed up action potential conduction.

Neuroglia in the CNS include astrocytes, microglia, ependymal cells, and oligodendrocytes.

The two kinds of PNS neuroglia—satellite cells and Schwann cells

NEURONS

The neurons, also called nerve cells, are the structural units of the nervous system.

They are highly specialized cells that conduct messages in the form of nerve impulses from one part of the body to another.

Besides their ability to conduct nerve impulses, neurons have some other special characteristics

1. They have extreme longevity. Given good nutrition, neurons can function optimally for a lifetime (over 100 years).

• They are amitotic.
• They have an exceptionally high metabolic rate and require continuous and abundant supplies of oxygen and glucose.

Neurons cannot survive for more than a few minutes without oxygen