myofilaments remain the same length, but that the overall length of the sarcomere shortens because the myofila­ments slide together. Calcium molecules first uncover sites on the actin where the myosin can attach. Once those sites are exposed, the myosin’s cross bridges latch onto the actin filaments like Velcro. Temporarily connected, the actin fila­ments are pulled closer together, overlapping the myosin filaments. Figure 3-38 illustrates the sliding filament mecha­nism. The overlapped filaments create a shorter sarcomere,

resulting in a shorter fibril. When many fibrils shorten, the whole muscle cell shortens. Clearly, calcium is necessary for muscle contraction, which is one of the reasons calcium should be included in a balanced diet.

A skeletal muscle is made up of bundles of muscle cells held together with connective tissue. This fibrous connective tis­sue, called fascia, carries blood vessels and nerves into the muscle and mechanically transmits the force of the contrac­tion from one end of the muscle to the other. The basic component of the muscle tissue bundle is the muscle cell, or muscle fiber. The cells are very fragile, but each one is wrapped in a connective tissue covering called endomysium (EN-doh-MAHY-see-um). Several cells and the nerve cells that supply them are bundled together to create a fascicle (FAS-sih-kul), which is wrapped with connective tissue called the perimysium. The fascicles, along with blood vessels, nerves, and muscle spindles (special sensory receptors of muscles that are discussed below in the nervous system sec­tion), are then bundled together by the epimysium, which is the connective tissue covering of the whole muscle. The epimysium blends into the tendons or aponeuroses, which attach the muscles to bones, cartilage, or other connective tissue coverings. The thickest part of a muscle that is pri­marily made of muscle cells and has a pinkish appearance is sometimes called the belly of the muscle. Tendons are very strong and thin and can easily anchor at transverse bony projections and joints to provide durability and save space. Once the small, delicate cells are wrapped in connec­tive tissue and bundled together, a very strong and resilient structure is created. Figure 3-41 illustrates the structures that form a skeletal muscle.

All muscle cells have the ability to contract, bringing the ends of the cell closer together. On a cellular scale, this movement is amplified by the many bundles of muscle cells to create movement of entire muscles. Cardiac and smooth muscles in the walls of organs squeeze contents out or push contents through organs or, in the case of blood vessels, change the diameter of the tube.

attaches to the bone. If muscles attach directly to a bone, their connective tissue covering fuses with the connective tissue that covers the bone.

All muscles have at least one of each of these two attachments. If muscle inserts into a bone, the bone will be pulled toward the origin of the contracting muscle. If a muscle inserts into a connective tissue structure, such as the lips, the lips will be pulled toward the origin of the contract­ing muscle (Fig. 3-42).

In addition to moving the skeleton and connective tis­sue structures, muscles also help move blood and lymph. Muscles become shorter and wider when they contract, which you see in a bodybuilder who strikes a pose. As the muscles contract, they constrict the blood and lymph ves­sels, squeezing the fluids out.

The neurilemma produces a chemical growth factor involved in a cellular repair mechanism for nervous tissues. Nerve cells are not regenerated on a regular basis, nor are

hope to those with nerve injuries. The brain and spinal cord contain no Schwann cells, and repair of the brain or spinal cord is nearly impossible.

The nerves are supported and connected with accessory cells called neuroglia. They come in five different forms (Fig.3-44):

• Astrocytes—support and anchor nerve cells to blood capillaries

• Schwann cells—form the myelin along a nerve cell in the PNS

The nerve has a structure similar to that of a muscle, with bundles of axons wrapped in connective tissue that are then bundled together with blood vessels in more connec­tive tissue. Each neuron is wrapped with a connective tis­sue covering called an endoneurium (EN-doh-nur-ee-um). The neurons are grouped together in fascicles and wrapped in a perineurium (PAIR-ih-nur-ee-um). Several fascicles are grouped together, along with blood vessels, to form a nerve. The entire nerve is wrapped with a fibrous connective tissue covering called the epineurium (Fig. 3-45).

to the CNS. Motor (efferent) neurons carry messages from the CNS to the muscle or organs that must react. Groups of sensory neurons form sensory nerves, and groups of motor neurons are called motor nerves. Most nerves, however, are mixed nerves, which have both sensory and motor neurons that carry information to and from the CNS.

• Merkel cells, found in the lower layer of the epider­mis, detect very light touch.

Once the impulse reaches the end of the axon, it will send a signal to the next cell. There, the action potential may contin­ue along the next cell or stimulate activity of a gland or organ.

The electrical nerve impulse on the presynaptic cell triggers the axonal terminal to release a neurotransmitter chemical into the synapse. Approximately 30 different neu­rotransmitters are made in the brain and stored in vesicles at the axonal terminals all over the body. Some of the neu­rotransmitters are epinephrine (adrenaline), norepinephrine (noradrenaline), dopamine, serotonin, and acetylcholine. Acetylcholine, discussed in the muscular system section, is the neurotransmitter released at the neuromuscular junc­tion. Once the vesicles release the neurotransmitter into the synapse, special receptor sites on the postsynaptic cell receive the neurotransmitters. The receptor site is activated and starts the action potential along the postsynaptic cell.