Biology by Karl Irvin Baguio (the ebook reader TXT) 📕

a fatty layer of material called the myelin sheath. Bundles of axons bound together are referred to as a nerve.

 

 

 

There are three types of neurons in animals: sensory neurons, interneurons, and motor neurons. Sensory neurons receive stimuli from the external environment; interneurons (or association neurons) connect sensory and motor neurons and carry stimuli in the brain and spinal cord; motor neurons transmit impulses from the brain and spinal cord to the muscle or gland that will respond to the stimulus. The neurons are supported, protected, and nourished by cells of the nervous system known as glial cells. Together with extracellular tissue, the glial cells make up the neuroglia.

 

Nerve impulse

 

The nerve impulse is an electrochemical event that occurs within the neuron. In an inactive neuron, the cytoplasm is negatively charged with respect to the outside of the cell. This difference in electrical charge is maintained by the active transport of sodium ions out of the cytoplasm. A cell in this state is said to have a resting potential, and it is polarized.

 

A nerve impulse is generated when the difference in electrical charge disappears. This occurs when a stimulus contacts the tip of a dendrite and increases the permeability of the cell membrane to sodium ions. The ions rush back into the cytoplasm, and the difference in electrical charges disappears. This creates a pulse of electrochemical activity called the nerve impulse. A neuron displaying a nerve impulse is said to have an action potential. The cell is depolarized.

 

More specifically, the influx of sodium ions into the neuron cytoplasm activates the adjacent portion of the cell membrane to admit sodium ions also. Successively, the adjacent areas of the neuron lose their differences of electrical charge, and a wave of depolarization is generated in the neuron. This wave of depolarization is the nerve impulse. After the wave of depolarization has passed, the neuron reestablishes the difference in charges by pumping potassium ions out of the cytoplasm and then pumping sodium ions in.

 

Synapse

 

The nerve impulse passes down the dendrite, through the cell body, and down the axon. At the end of the axon, the impulse encounters a fluid-filled space separating the end of the axon from the dendrite of the next neuron or from a muscle cell. This space is the synapse. A synapse located at the junction of a neuron and muscle fiber is a neuromuscular junction.

 

As the impulse reaches the end of the axon, it induces changes in the cell membrane and the release of chemical substances called neurotransmitters (for example, acetylcholine). Molecules of neurotransmitters accumulate in the synapse and increase the membrane permeability of the next dendrite. This causes an influx of sodium ions, and a new nerve impulse is generated. After the nerve impulse has swept down the next dendrite, the neurotransmitters in the synapse are destroyed.

 

Reflex arc

 

The reflex arc, the simplest unit of nervous activity, involves the detection of a stimulus in the environment by sensory nerve endings, followed by impulses that travel via the sensory neurons to the spinal cord. Here the impulses synapse with interneurons, and the interneurons generate impulses to respond to the stimulus. The impulses travel along the motor neurons to muscles or glands that respond appropriately.

 

In some cases, a reflex arc involves an interpretation. For this activity, interneurons transmit impulses up the spinal cord to the conscious area of the brain, where an analysis occurs.

 

Human Central Nervous System

 

The human nervous system can be conveniently subdivided into two divisions: the central nervous system (the brain and spinal cord) and the peripheral nervous system (the nerves extending to and from the central nervous system).

 

Spinal cord

 

The spinal cord of the central nervous system is a white cord of tissue passing through the bony tunnel made by the vertebrae. The spinal cord extends from the base of the brain to the bottom of the backbone. Three membranes called meninges surround the spinal cord and protect it. The outer tissue of the spinal cord is white (white matter), while the inner tissue is gray (gray matter).

 

Thirty-one pairs of projections called nerve roots extend out along each side of the spinal cord. The nerve roots are sites of axons belonging to sensory and motor neurons. A central canal in the spinal cord carries cerebrospinal fluid, which provides for the nutrition and gaseous needs of the cord tissue. The neurons of the spinal cord serve as a coordinating center for the reflex arc and a connecting system between the peripheral nervous system and the brain.

 

Brain

 

The brain is the organizing and processing center of the central nervous system. It is the site of consciousness, sensation, memory, and intelligence. The brain receives impulses from the spinal cord and from 12 pairs of cranial nerves coming from and extending to the senses and to other organs. In addition, the brain initiates activities without environmental stimuli.

 

Two major hemispheres, the left and the right hemispheres, make up the tissue of the brain. The outer portion of the brain consists of gray matter, while the inner portion is white matter. Three major portions of the brain are recognized: the hindbrain, the midbrain, and the forebrain.

 

The hindbrain consists of the medulla, cerebellum, and pons. The medulla is the swelling at the tip of the brain that serves as the passageway for nerves extending to and from the brain. The cerebellum lies adjacent to the medulla and serves as a coordinating center for motor activity; that is, it coordinates muscle contractions. The pons is the swelling between the medulla and the midbrain. The pons acts as a bridge between various portions of the brain.

 

The midbrain lies between the hindbrain and the forebrain. It consists of a collection of crossing nerve tracts and is the site of the reticular formation, a group of fibers that arouse the forebrain when something unusual happens.

 

The forebrain consists of the cerebrum, the thalamus, the hypothalamus, and the limbic system. The cerebrum contains creases and furrows called convolutions that permit the cerebral hemisphere to accommodate more than 10 billion cells. Each hemisphere of the cerebrum has four lobes, and activities such as speech, vision, movement, hearing, and smell occur in these lobes. Higher mental activities such as learning, memory, logic, creativity, and emotion also occur in the cerebrum.

 

The thalamus serves as an integration point for sensory impulses, while the hypothalamus synthesizes hormones for storage in the pituitary gland. The hypothalamus also appears to be a control center for such visceral functions as hunger, thirst, body temperature, and blood pressure. The limbic system is a collection of structures that ring the edge of the brain and apparently function as centers of emotion.

 

Human Senses

 

The sense organs connect the nervous system to the external environment. They are the sources of stimuli that cause a response in the nervous system, and they are the sources of all information to the human body.

 

Eye

 

In the human eye, the nerve cells are located in a single layer called the retina, located along the back wall of the eye. Light rays enter the eye through a curved, transparent structure called the cornea, and then pass through the pupil, an opening in the eyeball. The iris regulates the size of the pupil. Next, the lens focuses the light on the retina, which contains two types of light-sensitive cells, rod cells and cone cells, which detect light. Cone cells, which detect color, are concentrated in the central portion of the retina, while rod cells, which permit vision in dim light, are concentrated at the edge of the retina. A light-sensitive pigment called rhodopsin functions in the detection of light.

 

From the eye, a series of impulses is generated for transmission to the brain. The optic nerve carries these impulses. The region of keenest vision, the fovea, is located at the center of the retina. When vision is poor, light is not focusing on the fovea, and corrective lenses are prescribed.

 

Ear

 

The ear is the organ of hearing in humans. The outer ear funnels vibrations to the eardrum, or tympanic membrane, which transmits the vibrations to three inner ear bones: the malleus (hammer), the incus (anvil), and the stapes (stirrup). These bones transmit the vibrations to the inner ear where the receptor of hearing, the cochlea, is located.

 

The cochlea is a snail-like series of coiled tubes within the skull. As the ear bones vibrate, they push and pull a membrane at one end of the cochlea, causing fluid within the tubules to vibrate. The vibrations are detected by sensitive hair cells, and nerve impulses are generated. The auditory nerve carries the impulses to the brain for interpretation.

 

Taste and smell

 

Specialized receptor cells called chemoreceptors transmit taste and smell. Chemoreceptors of the human tongue distinguish four different tastes: sweet, sour, salty, and bitter. In the human nose, chemoreceptors detect a variety of scents, including minty, floral, musky, putrid, and pungent.

In both taste and smell, chemoreceptors are stimulated by molecules and ions that reach the tongue and nose. Liquid materials affect the chemoreceptors in the taste buds of the tongue, while gaseous molecules affect the chemoreceptors in the upper reaches of the nose. The olfactory nerve carries nerve impulses from the nose to the brain for interpretation.

 

Other senses

 

The other senses of the body include receptors for touch, pain, temperature, and balance. Touch and pain receptors, called Pacinian corpuscles, are located in the skin, muscles, and tendons. The sense of balance is centered in the semicircular canals of the inner ear. Visceral senses include stretch receptors in the muscles, as well as carbon dioxide receptors in the arteries.

Chapter 29: Reproduction

  Human Reproduction

 

Reproduction is an essential process for the survival of a species. The functions of the reproductive systems are to produce reproductive cells, the gametes, and to prepare the gametes for fertilization. In addition, the male reproductive system delivers the gametes to the female reproductive tract. The female reproductive organs nourish the fertilized egg cell and provide an environment for its development into an embryo, a fetus, and a baby.

 

Human reproduction takes place by the coordination of the male and female reproductive systems. In humans, both males and females have evolved specialized organs and tissues that produce haploid cells, the sperm and the egg. These cells fuse to form a zygote that eventually develops into a growing fetus. A hormonal network is secreted that controls both the male and female reproductive systems and assists in the growth and development of the fetus and the birthing process.

 

Female Reproductive System

 

The organs of female reproduction include the ovaries, two oval organs lying within the pelvic cavity, and adjacent to them, two fallopian tubes. Also known as oviducts, the fallopian tubes are the passageways that egg cells enter after release from the ovaries. The fallopian tubes lead to the uterus (womb), a muscular organ in the pelvic cavity. The inner lining of the uterus, called the endometrium, thickens with blood and tissue in anticipation of a fertilized egg cell. If fertilization fails to occur, the endometrium degenerates and is shed in the process of menstruation.

 

The opening at the lower end of the uterus is a constricted area called the cervix. The tube leading from the cervix to the exterior is a muscular organ called the vagina. During periods of sexual arousal, the vagina receives the penis and the semen. The sperm cells in the semen pass through the cervix