Carlson's Movement
MusclesSo far, I have described the nature of neural communication, the basic structure of the nervous system, and the physiology of sensation. Now it is time to consider the ultimate function of the nervous system, control of behavior. The brain is the organ that moves the muscles. It does many other things, but all of them are secondary to making our bodies move. This chapter describes the principles of muscular contraction, some reflex circuitry within the spinal cord, and the means by which the brain initiates behaviors. The rest of the book describes the physiology of particular categories of behaviors and the ways in which our behaviors can be modified by experience.
Mammals have three types of muscles: skeletal muscle, smooth muscle, and cardiac muscle.
Skeletal Muscle
Skeletal muscles are the ones that move us (our skeletons) around and thus are responsible for our behavior. Most of them are attached to bones at each end and move the bones when they contract. (Exceptions include the eye muscles, the tongue muscles, and some abdominal muscles, which are attached to bone at one end only.) Muscles are fastened to bones via tendons, strong bands of connective tissue. Several different classes of movement can be accomplished by the skeletal muscles, but I will refer principally to two of them: flexion and extension. Contraction of a flexor muscle produces flexion, the drawing in of a limb. Extension, which is the opposite movement, is produced by contraction of extensor muscles. These are the so-called antigravity muscles-the ones we use to stand up. When a four-legged animal lifts a paw, the movement is one of flexion. Putting it back down is one of extension. Some-times, people say they "flex" their muscles, which is an incorrect use of the term. Muscles contract; limbs flex. Bodybuilders show off their arm muscles by simultaneously contracting the flexor and extensor muscles of that limb.
Anatomy
The detailed structure of a skeletal muscle is shown in Figure 8. 1. Contractions of the extrafusal muscle fibers provides the muscle's motive force. These fibers are served by axons of the alpha motor neurons; thus, it is the activity of the alpha motor neurons that determines the strength of contraction of a muscle. The muscle also has sensory organs known as muscle spindles. The central region of the muscle spindles contains sensory endings that are sensitive to stretch. The spindles also contain a muscle fiber: the intrafusal muscle fiber (fusus means "spindle"). The efferent axon of the gamma. motor neuron causes the intraftisal muscle fiber to contract; however, this contraction contributes an insubstantial amount of force. As you will see, the function of this contraction is to modify the sensitivity of the fiber's afferent ending to stretch. The muscle and sheath of connective tissue that covers it are supplied with free nerve endings, which are sensitive to noxious stimuli.
Figure 8.1 Anatomy of skeletal muscle.
A single myelinated axon of an alpha motor neuron serves several extraftisal muscle fibers. In primates, the number of muscle fibers served by a single axon varies considerably, depending on the precision with which the muscle can be controlled. In muscles that move the fingers or eyes, the ratio can be less than one axon to ten muscle fibers; in muscles that move the leg, it can be one to several hundred. An alpha motor neuron, its axon, and associated extrafusal muscle fibers constitute a motor unit.
A single muscle fiber consists of a bundle of myofibrils, each of which consists of overlapping strands of two kinds of protein, actin and myosin. These strands of protein are responsible for the production of muscular contractions.
The Physical Basis of Muscular Contraction
The synapse between the terminal button of a motor neuron and the membrane of a muscle fiber is called a neuromuscular junction. The terminal buttons of the neurons synapse on motor endplates specialized regions of postsynaptic membrane located in grooves along the surface of the muscle fibers. When an axon fires, acetylcholine is liberated by the terminal buttons and produces a depolarization of the postsynaptic membrane (endplate potentiao). An endplate potential triggers an action potential in the muscle fiber to fire. This action potential induces a contraction, or twitch, of the muscle fiber.
The depolarization of a muscle fiber opens the gates of voltage-dependent calcium channels, permitting calcium ions to enter the cytoplasm. This event triggers the contraction. Calcium acts as a cofactor that permits the myofibrils to extract energy provided by the mitochondria. Contraction of a muscle fiber occurs when strands of Myosin crawl along neighboring strands of actin in a ratcheting movement, which causes the muscle fiber to shorten.
A single impulse of a motor neuron produces a single twitch of a muscle fiber. The physical effects of the twitch last considerably longer than will the action potential, because of the elasticity of the muscle and the time required to rid the cell of calcium. (Like sodium, calcium is actively extruded by a pump situated in the membrane.) A single motor unit in a leg muscle of a cat can raise a 100-g weight (about four times the weight of a mouse), which attests to the remarkable strength of the contractile mechanism.
As you know from your own experience, muscular contraction is not an all-or-nothing phenomenon, as are the twitches of the constituent muscle fibers. Obviously, the strength of a muscular contraction is determined by the rate of firing of the motor units. If, at a given moment, many units are firing, the contraction will be forceful. If few are firing, the contraction will be weak.
Sensory Feedback from Muscles
As we saw, the muscle spindles contain sensory endings that are sensitive to stretch.The intrafusal muscle fibers are arranged in parallel with the extrafusal muscle fibers. Therefore, they are stretched when the muscle lengthens and relaxed when it shortens.Thus, even though these afferent neurons are stretch receptors, they actually function as muscle length detectors. This distinction is important. Another kind of stretch receptor is located within the tendons in the Golgi tendon organ. These receptors detect the total amount of force exerted by the muscle, through its tendons, on the bones to which the muscle is attached. The stretch receptors of the Golgi tendon organ measure the degree of force by the rate of firing. They respond not to a muscle's length, but to how hard it is pulling. You will see the function of these detectors in a later section.
Smooth Muscle
Our bodies contain two types of smooth muscle, both of which are controlled by the autonomic nervous system. Multiunit smooth muscles are found in large arteries, around hair follicles (where they produce piloerection, or fluffing of fur), and in the eye (controlling lens adjustment and pupillary dilation). This type of smooth muscle is normally inactive, but it will contract in response to neural stimulation or to certain hormones. In contrast, single-unit smooth muscles normally contract in a rhythmical fashion. The efferent nerve supply (and various hormones) can modulate the rhythmical rate, increasing or decreasing it, but the contractions themselves occur independently. Single-unit smooth muscles are found chiefly in the gastrointestinal system and uterus.
Cardiac Muscle
As its name implies, cardiac muscle is found in the heart.This type of muscle looks somewhat like skeletal muscle, but acts like single-unit smooth muscle. The heart beats regularly, even if the nerve that connects it to the brain is severed. Neural activity and certain hormones (especially epinephrine and norepinephrine from the adrenal medulla) serve to modulate the heart rate. A group of cells in the pacemaker of the heart is rhythmically active and initiate the contractions of cardiac muscle that constitute the heartbeat.
INTERIM SUMMARY
Our bodies possess skeletal muscle, smooth muscle, and cardiac muscle. Skeletal muscles contain extrafusal muscle fibers, which provide the force of contraction. The alpha motor neurons form synapses with the extrafusal muscle fibers and control their contraction. Skeletal muscles also contain intrafusal muscle fibers, which detect changes in muscle length. The length of the intrafusal muscle fiber, and hence its sensitivity to increases in muscle length, is controlled by the gamma motor neuron. Besides containing the intrafusal muscle fibers, the muscles contain stretch receptors in the Golgi tendon organs, located at the ends of the muscles.
The force of muscular contraction is provided by long protein molecules called actin and myosin, arranged in overlapping parallel arrays.When an action potential, initiated by the synapse at the motor endplate, causes Ca 2+ to enter the muscle fiber, the myofibrils extract energy provided by the mitochondria and cause a twitch of the muscle fiber, producing a ratchetlike "rowing" movement of the myosin cross bridges.
Smooth muscle is controlled by the autonomic nervous system through direct neural connections and indirectly through the endocrine system. Multiunit smooth muscles contract only in response to neural or hormonal stimulation. In contrast, single unit smooth muscles normally contract rhythmically, but their rate is controlled by the autonomic nervous system. Cardiac muscle also contracts spontaneously, and its rate of contraction, too, is influenced by the autonomic nervous system
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