Nervous control and coordination

Introduction

Many living creatures can run faster than human beings can. Some can see better in dim light. Others can hear better, and still others can smell faint odors better at a dis­tance. What, then, has enabled humans to achieve high levels of technology and com­munication while other species have not? Humans have a highly developed brain that makes it possible to learn, to remember what has been learned, and, most important, to reason. The brain, however, is just part of the complex nervous system, which controls and coordinates essential body functions. The nervous system sends special signals to and receives responses from every organ and tis­sue of the body. These signals from the ner­vous system make it possible for you to play the piano, thread a needle, throw a baseball, write in your notebook, or just sit and think. Without a nervous system, the brain is com­pletely unable to function.

The nervous system

The nervous system has two main subdivisions. One part, the central nervous system, consists of the brain and the spinal cord. The central nervous system receives stimuli from inside and outside the body and then coordinates the body’s response. The second part of the nervous system is the peripheral (pun rihf uhr uhl) nervous system. It provides the pathways to and from the central nervous system for electrochemical signals called impulses.

Three types of body structures are needed for the entire process of picking up stimuli and responding to them. They are receptors, conductors, and effectors. To understand how these different structures are coordinated, consider what happens when a doorbell rings. First, the ear acts as a receptor that picks up the sound of the ringing bell. A receptor is a cell, group of cells, or organ, that detects a stimulus. The receptor then gener­ates impulses that travel along conductors, or nerve cells. Ulti­mately, the impulses reach effectors—structures that may react to the original stimulus. In this case, muscles are the effectors. The reaction to the original stimulus is to walk to the door and open it.

The Neuron

The basic functional unit of the nervous system is the nerve cell, called a neuron. Three types of neurons interact in the nervous svstem. Neurons that receive stimuli and transmit them to the central nervous system are sensory neurons. Neurons that carry impulses away from the central nervous system to muscles or glands are motor neurons. The third type of neuron, an inter-neuron, links sensory and motor neurons.

Every neuron consists of a cell body, which contains the nucleus and cytoplasm, and threadlike extensions of cytoplasm called nerve fibers. A neuron has two kinds of nerve fibers. Dendrites are fibers that carry impulses from other neurons or receptors toward the cell body. Dendrites are generally short, branched fibers. The second kind of nerve fiber is the axon, which carries impulses away from the cell body to other neurons or to effectors. A neuron has many dendrites but only one axon. Axons are usually longer than dendrites, and any branches axons have exist only at the end of the fiber.

An axon may be wrapped in a fatty insulating layer known as a myelin sheath. The sheath is formed by special cells called Schwann cells. The sheath supports, insulates, and nourishes the axons. It also helps maintain the chemical balance of the axon. Gaps between the Schwann cells, called the nodes of Ranvier, occur about every 1 mm (0.04 in.) along the myelinated axons.

Neurons are the largest cells in the body. Some neurons may measure almost 2 m (2.2 yd.). Bundles of nerve fibers, containing hundreds or even thousands of axons, form a nerve. Within a nerve, each fiber carries a separate impulse, just as each wire inside a telephone cable can carry a separate phone call at the same time.

How a Nerve Impulse Travels

Impulses travel not only along the length of a nerve cell but also from cell to cell. Within a neuron the impulse is transmitted electrically. However, chemicals are generally involved in mov­ing the impulse from cell to cell.

The Nerve Impulse

Like all cells, neurons have a certain electrical charge on the inside and outside of their cell mem­branes. The axon of a neuron when the neuron is at its resting potential—that is, when it is not carrying an impulse.

The outside of the axon has about 10 times as many sodium (Na⁺) ions as the inside. Inside the membrane are negatively charged organic ions and about 30 times as many potassium (K⁺) ions as outside. The membrane keeps the Na* ions out­side and the negatively charged organic ions inside. The K⁺ ions move in and out of the axon freely. At resting potential, the inside of the cell membrane has a slightly negative charge, and the outside has a slightly positive charge. In this case the cell is said to be polarized.

When the nerve fiber is stimulated, its membrane suddenly becomes permeable to Na⁺ ions at the place where the stimula­tion occurs. The negative ions inside the membrane then attract the Na* ions. Some Na⁺ ions move rapidly to the inside of the cell. The presence of these positively charged ions causes that part of the interior to become more positive than the out­side. These electrical changes create an action potential, and the neuron is said to be depolarized.

The membrane remains permeable to Na⁺ ions for only half a millisecond. However, this brief electrical charge is enough to start the action potential moving down the nerve fiber. How does this movement occur? The positively charged ions inside the cell move toward the negatively charged area next to the region of stimulation. The positive ions cause this area to become depolarized and the membrane to become permeable to Na⁺. More Na⁺ ions then rush inside the membrane, causing that section of the interior to become positive. Again, positively charged ions are attracted to the adjoining negatively charged area, and thus the action poten­tial moves along the nerve fiber.

The rapid change from negative to positive charge within the membrane is an electrical wave called a nerve impulse. A nerve impulse can be described as the movement of the action . potential along a neuron.

As soon as an impulse passes a section of nerve fiber, the membrane once again becomes permeable only to K⁺ ions. The neuron then returns to its resting potential in preparation for the next impulse. The process of returning to resting potential involves an active transport system known as the sodium-potas­sium pump. The sodium-potassium pump carries Na⁺ ions to the outside and K⁺ ions to the inside of the membrane.

In myelinated axons, the myelin sheath acts as an insulator against electrical impulses. Because of this insulation, the ex­change of ions across the membrane takes place only at the nodes of Ranvier, where the sheath is interrupted. This periodic, rather than continuous, exchange results in a leaping of the impulse from node to node. As a result, impulses travel along myelinated axons 50 times faster than they do along unmyelinated axons, sometimes as fast as 100 meters per second (224 miles per hour).

The Synapse

Impulses travel from neuron to neuron, but adjoining neurons generally do not touch one another. There­fore, an impulse must cross from the axon of one neuron to the dendrites of another. This junction is called a synapse. An im­pulse does not “jump” across the space, however. In fact, the original impulse ends when it reaches the end of an axon. At that point, however, the impulse causes the release of chemicals that generate new impulses in the next neuron.

Many axon branches terminate in tiny bulblike structures called synoptic buttons, which contain numerous synoptic vesi­cles. A synaptic vesicle is a tiny sac that holds chemical sub­stances called neurotransmitters that stimulate nearby den­drites to start new impulses. A neurotransmitter released into the space, called the synaptic cleft, diffuses rapidly to nearby den­drites. There it disturbs the resting potential of the dendrites and so generates new impulses.

An impulse eventually reaches an effector cell, such as a muscle fiber. In this situation, a neurotransmitter is released from motor neurons through motor endplates, which are located at the ends of axons near muscle fibers. The neurotransmitter then causes the muscle to contract.

Starting a Nerve Impulse

To “fire” a neuron—that is, to get a nerve impulse going in the first place—a stimulus must have a certain level of strength called a threshold. If the energy level of a stimulus falls below the threshold, the neuron will not fire. However, a stimulus with an energy level greater than the threshold does not cause a faster or stronger impulse. The neu­ron either fires or it doesn’t, a phenomenon known as the all-or-none response. The intensity of a sensation depends on the num­ber of neurons stimulated. After an impulse, the neuron must rest for about one-hundredth of a second. A stimulus, no matter how strong, cannot fire the neuron during this time.

The Central Nervous System

Impulses travel through the central nervous system, which proc­esses incoming sensory impulses and sends out responding im­pulses. The brain and spinal cord, which make up the central nervous system, each control specific tasks.

The Brain

The brain is the control center for the human body. Its 100 billion nerve cells not only coordinate and regulate body activi­ties but also enable humans to think. The human brain weighs only about 1.4 kg (3 lb.), but it is the most complex structure on Earth. The surface is gray matter, which consists of about 6 million cell bodies and their dendrites packed into each cubic centimeter (0.06 cu. In.). Under the gray matter is white matter, formed from myelinated axons.

The brain is composed of three major structures: the cere­brum, the cerebellum, and the brain stem. Each area seems to control separate functions. However, it is not the independence but rather the interdepend­ence of its parts that makes the brain so effective.

The Cerebrum

The cerebrum makes up about seven-eighths of the total brain weight. Its two sides, called cerebral hemi­spheres, are joined by a bridge of 200 million nerve fibers. This bridgelike structure between the cerebral hemispheres is known as the corpus callosum. Deep grooves mark off four areas on each hemisphere. The four areas are the frontal, parietal, temporal, and occipital lobes.

The gray matter of the cerebrum is called the cerebral cor­tex. Its main function is to receive sensory impulses from the body and coordinate motor responses to them. Its many ridges and valleys, called convolutions, greatly increase the surface area of the brain. Each area on the section called the motor cortex controls the movement of muscles in a specific part of the body. Each area of the sensory cortex receives impulses from specific part of the body. The area devoted to each body part is proportional to its sensitivity or motor capability, not to its size. For example, a large area is devoted to the hand, a sensitive area.

Each hemisphere controls the actions and sensations of the opposite side of the body. For example, the left side controls movement of the right hand; the right side controls movement of the left hand. Scientists have discovered that in most people each side also has exclusive control over certain functions.

Several important structures lie within the cerebrum. On each side of the brain is the thalamus, a small organ that acts as a relay center for impulses. The thalamus processes incoming sensory impulses before sending them to appropriate parts of the cortex. It also sorts out and combines impulses from the cortex and other areas of the brain. Below the thalamus is the hypothalamus. Research has indicated that this structure controls body temperature, thirst, hunger, salt and water balance, and emotional behavior in general. Near the cor­pus callosum is a network of neurons called the limbic system. The limbic system is thought to translate a person’s drives and emotions into actions.

The Cerebellum

The cerebellum is located beneath the oc­cipital lobe. The white matter that composes most of the cere­bellum is covered by a thin layer of gray matter. The cerebellum coordinates voluntary muscle movements and maintains muscle vigor and body balance. Damage to the cerebellum may result in jerky, awkward movements, although the ability to make the movements is not affected.

The Brain Stem

The brain stem contains all the nerves that connect the spinal cord with the cerebrum. The principal divi­sions of the brain stem are the medulla oblongata, pons, and midbrain. The medulla oblongata is the enlarged portion of the spinal cord that enters the lower skull. It controls breathing, swallowing, digestive processes, and action of the heart and blood vessels. In the medulla, many nerve fibers criss­cross. As a result, each hemisphere receives impulses from and sends impulses to the opposite side of the body. The pons con­nects the two hemispheres of the cerebellum and links the cere­bellum with the cerebrum. The midbrain lies above the pons. It controls responses to sight, such as movements of the eyes and size of the pupils.

A complex network of nerve fibers called the reticular for­mation runs through the brain stem and thalamus. This structure plays an essential role in consciousness, awareness, and sleep. The reticular system activates the rest of the brain when a stimu­lus is received. However, it first filters every stimulus. For example, people can sleep through loud noises such as traffic sounds but be awakened instantly by the ring of a telephone. Researchers do not know exactly how the reticular formation functions during sleep, but they know that a lack of sleep can seriously affect a person’s well-being. A person deprived of sleep becomes quick-tempered, lacks concentration and energy, and is easily distracted. Too little sleep can eventually affect sight and hearing.

Protection of the Brain

The brain is protected in three ways. First, the skull helps prevent serious injury from blows to the head. Second, the brain is cushioned inside the skull by cerebrospinal fluid. Cerebrospinal fluid is tissue fluid that cir­culates constantly around the brain and spinal cord. Third, three layers of tissue known collectively as the meninges protect the surface of the brain. The innermost layer, called the pia maier) follows all brain convolutions. Its rich blood supply carries nutrients and oxygen to brain cells and carries waste products away. The middle layer, the arachnoid, is a delicate weblike structure. Fluid between the pia mater and arachnoid serves as the pathway for exchange of nutrients and waste products. The outenjiost layer is a tough fibrous mem­brane called the dura mater.

The Spinal Cord

The spinal cord is a column of nerve tissue extending from the brain through the spinal column. In adults it is about 43 cm (17 in.) long and as thick as a pencil. The spinal cord links the brain with nerves to all parts of the body and controls involuntary movements known as reflexes.

The center of the cord is filled with gray matter with a cross section shaped somewhat like the letter H. Cell bodies of motor neurons and intemeurons are in the gray matter. The cell bodies of sensory neurons form small masses called ganglia outside the spinal cord. White matter around the gray matter consists of myelinated axons. Vertebrae, mening and cerebrospinal fluid protect the spinal cord.

The Peripheral Nervous System

The peripheral nervous system carries impulses to and from the central nervous system. Twelve pairs of cranial nerves and 31 pairs of spinal.nerves make up the peripheral nervous sys­tem. The cranial nerves connect the brain primarily with sense organs, the heart, and other internal organs. The spinal nerves carry impulses between the spinal cord and skeletal muscles.

So far, the nervous system has been presented like a map. Another way of thinking of the nervous system is to focus on what its parts do and not on where they are located. When de­scribed in this way, the subdivisions are called the somatic nerv­ous system and the autonomic nervous system. These systems involve both the peripheral and the central nervous systems.

The Somatic Nervous System

The somatic nervous system transmits impulses to and from skeletal muscles, which are usually under conscious control. For this reason the somatic nervous system is sometimes called the voluntary nervous system. Each pair of spinal nerves has motor and sensory fibers. Each pair carries impulses to and from skeletal muscles in a specific part of the body.

Not all skeletal muscle movements are voluntary. Move­ments called reflexes are not under conscious control. A reflex pathway, called a reflex arc, involves two or three neurons. The simplest arc consists of one sensory and one motor neuron. A three-neuron reflex arc includes an interneuron. The brain is not involved in either arc.

Responses such as dodging a moving object are condi­tioned: they are learned from experience. Reflexes, such as blinking, are unconditioned. The advantage of reflexes is speed. Reflexes may take one one-hundredth of a second—much faster than any pathway involving the brain. However, the brain may play a role in other responses to the stimulus. Typically, you quickly withdraw your hand after touching a hot object. Three neurons carry the impulses in the reflex arc, causing the jerk of your hand. Other impulses travel to the brain. At the conscious level, then, you may look at the hot object and gasp, “Ouch.”

The Autonomic Nervous System

The autonomic nervous system controls automatic, or involun­tary, functions involving glands, internal organs, and other smooth muscle tissue. The autonomic nervous system is divided into two systems. The sympathetic nervous system enables the body to handle stress through what is called the “fight or flight” response, in which a person either fights or runs away. Fighting and running both require extra energy. Therefore, the sympa­thetic nervous system causes bodily changes that channel extra glucose and oxygen to skeletal muscles, thus supplying the extra strength needed in an emergency. When stress no longer exists, the parasympathetic nervous system is responsible for returning body functions to normal and for maintaining them at that level. For example, the sympathetic nervous system speeds the heart to supply cells more quickly; the parasympathetic nervous sys­tem slows the heart.

Date: 2014-12-22; view: 943