Chapter 11
Nervous System II: Divisions of the Nervous System
1. Name the layers of the meninges and explain their functions.
The layers of the meninges surround the brain and spinal cord. They are, from the outermost to the innermost layers:
Dura mater—The dura mater is a tough, fibrous connective tissue layer containing many blood vessels and nerves. It functions as a protective layer, surrounding the brain and spinal cord.
Arachnoid mater—The arachnoid mater is a thin weblike membrane that lacks blood vessels and nerves. It is attached to the pia mater by thin strands.
Pia mater—The pia mater is a thin membrane containing many nerves and blood vessels that provide nourishment to the underlying brain cells and spinal cord. It is attached directly to the surface of the brain and spinal cord.
2. Describe the location of cerebrospinal fluid within the meninges.
Cerebrospinal fluid (CSF) is found between the arachnoid and pia mater of the brain and spinal cord in the space called the subarachnoid space.
3. Describe the location of the ventricles of the brain.
The lateral ventricles (first and second ventricles) extend into the cerebra hemispheres and occupy part of the frontal, temporal, and occipital lobes. The third ventricle is found in the midline of the brain, below the corpus callosum, and connects the lateral ventricles through openings in the anterior ends. The fourth ventricle is found in the brain stem in front of the cerebellum. The cerebral aqueduct connects openings in its roof that lead into the subarachnoid space of the meninges.
4. Explain how cerebrospinal fluid is produced and how it functions.
Cerebrospinal fluid (CSF) is secreted by tiny reddish cauliflower-like masses of specialized capillaries in the pia mater called choroid plexuses that project into the ventricles. CSF is important in the protection and support of the CNS by absorbing the forces of impact, maintaining a stable ion concentration, and providing a route for waste products to be removed.
5. Describe the structure of the spinal cord.
The spinal cord is a long slender column of nerve fibers that begins at the foramen magnum of the skull and extends downward to a point near the first and second lumbar vertebrae. The cord is actually a group of thirty-one segments that give rise to pairs of spinal nerves. These nerves connect all of the body to the CNS.
A thickening in the neck region, called the cervical enlargement, supplies the nerves to the arms and a similar thickening, the lumbar enlargement, supplies the nerves to the legs. Inferior to the lumbar enlargement, the spinal cord tapers into a structure (conus medullaris) that is connected to the coccyx by a thin cord of connective tissue (filum terminale).
Along the length of the cord are two grooves, the anterior median fissure and posterior median sulcus, which divide the cord into left and right halves. A cross section of the cord shows a gray matter core surrounded by white matter. The gray matter resembles a butterfly. The upper wings are called the posterior horns and the lower wings are called the anterior horns. Between these horns is a small protuberance called the lateral horn. A horizontal bar of gray matter surrounds the central canal and connects the wings on both sides. The white matter is divided on each side into three regions, the anterior, lateral, and posterior funiculi.
6. Describe a reflex arc.
A reflex arc is the simplest response to a stimulus. It begins with a receptor at the end of sensor nerve fibers. It travels to a reflex center in the CNS and an impulse is sent to an effector along a motor nerve fiber.
7. Define reflex.
A reflex is an automatic, subconscious response to stimuli inside or outside the body.
8. Describe a withdrawal reflex.
When a person touches something painful, receptors in the skin send impulses to interneurons in a reflex center in the spinal cord. The reflex center sends impulses to the flexor muscles of the affected part causing the part to be moved away. At the same time this is happening, impulses to the extensor muscles of the affected part are inhibited, so that the flexors can work more effectively. A phenomenon, called a crossed extensor reflex, occurs simultaneously with the initial reflex that causes the extensors of the opposite limb to contract.
9. Name the major ascending and descending tracts of the spinal cord, and list the functions of each.
The major ascending tracts are:
Fasiculus gracilis and fasiculus cuneatus—These tracts are found in the posterior funiculi and conduct sensory impulses from the skin, muscles, tendons, and joints. Most of the nerve fibers cross over in the medulla oblongata to their opposite sides.
Lateral and anterior spinothalamic—These tracts are located in the lateral and anterior funiculi. The lateral tracts conduct pain and temperature sensations from the body. The anterior tract conducts touch and pressure sensations from the body.
Posterior and anterior spinocerebellar—Both tracts are located near the surface of the lateral funiculi. The anterior tracts cross over in the spinal cord, while the posterior tracts do not. Both tracts conduct impulses from the legs and trunk to the cerebellum and aid in muscle coordination.
The major descending tracts are:
Lateral and anterior corticospinal—These tracts are found in the lateral and anterior funiculi. Most of the fibers in the lateral tracts cross over in the spinal cord, while the anterior tracts’ fibers do not. Both of these tracts conduct impulses through the spinal nerves to various skeletal muscles to control voluntary movements. These tracts are also called pyramidal tracts because they pass through pyramid-shaped regions in the medulla oblongata.
Lateral, anterior, and medial reticulospinal—The lateral tracts are found in the lateral funiculi and the anterior and medial tracts are found in the anterior funiculi. Some of the nerve fibers in the lateral tracts are the only ones that cross over. None of the other tracts do. These tracts conduct impulses that control muscle tone and sweat gland activity.
Rubrospinal—These fibers are found in the lateral funiculi and cross over in the brain. These fibers conduct impulses to skeletal muscles to aid in muscle coordination and control posture.
10. Explain the consequences of nerve fibers crossing over.
Crossing over causes the impulses from one side of the body to be received and controlled by the opposite side of the brain.
11. Describe how the brain develops.
During embryonic development, the brain begins as a neural tube that gives rise to the CNS. At one end there are three major cavities or vesicles: the forebrain (prosencephalon), midbrain (mesencephalon), and hindbrain (rhombencephalon). The forebrain divides into the anterior (telencephalon) and posterior (diencephalon) portions. The hindbrain partially divides into the metencephalon and myelencephalon. These five cavities in the mature brain become the ventricles and the tubes that connect them. The tissue of the telencephalon becomes the cerebrum and basal ganglia while the diencephalon remains unchanged. The midbrain continues to mature and is still called the midbrain in the adult structure. The hindbrain matures into the cerebella, pons, and medulla oblongata. The brain stem is comprised of the midbrain, pons, and medulla oblongata and connects the brain to the spinal cord.
12. Describe the structure of the cerebrum.
The cerebrum consists of two cerebral hemispheres separated by a layer of dura mater called the falx cerebri and connected deeply by a nerve fiber bundle called the corpus callosum. The hemispheres are marked by many convolutions separated by shallow grooves called sulci (sing. sulcus) and deep grooves called fissures. These grooves form distinct patterns. For instance, the longitudinal fissure separates left and right hemispheres, and the transverse fissure separates the cerebrum from the cerebellum.
Various sulci divide each hemisphere into lobes names after the skull bones they underlie. They are:
Frontal lobe—The frontal lobe forms the anterior portion of each cerebral hemisphere, and lies in front of the central sulcus (fissure of Rolando) and above the lateral sulcus (fissure of Sylvius).
Parietal lobe—The parietal lobe lies behind the central sulcus and frontal lobe.
Temporal lobe—The temporal lobe lies below the frontal and parietal lobes, separated by the lateral sulcus.
Occipital lobe—The occipital lobe is the posterior portion of each hemisphere separated from the cerebellum by the tentorium cerebelli. There is no clear boundary between the temporal, parietal, and occipital lobes.
Insula—The insula (island of Reil) is found deep in the lateral sulcus and is separated from the frontal, parietal, and temporal lobes by a circular sulcus.
13. Define cerebral cortex.
The cerebral cortex is the outermost layer of the cerebrum and is a layer of gray matter that contains 75 percent of all neuron bodies in the nervous system.
14. Describe the location and function of the primary motor areas of the cortex.
The primary motor areas of the cerebral cortex lie in the frontal lobes along the anterior wall of the central gyrus. Large pyramidal cells are responsible for nerve impulses sent through the corticospinal tracts to voluntary muscles. Impulses from the upper parts of the motor areas control muscles in the legs and thighs; the middle portion control muscles in the shoulders and arms; and the lower portions control the muscles of the head, face, and tongue.
15. Describe the location and function of Broca’s area.
Broca’s area is found just anterior to the primary motor cortex usually in the left hemisphere. It is responsible for complex muscular coordination of the mouth, tongue, and larynx, which make speech possible.
16. Describe the location and function of the sensory areas of the cortex.
The sensory areas for temperature, touch, pressure, and pain in the skin are found in the anterior portion of the parietal lobes along the central sulcus. Vision sensory areas are found in the posterior portion of the occipital lobes. The sensory areas for hearing are found in the dorsal posterior portion of the temporal lobes. The sensory areas for taste are found near the base of the central sulci along the lateral sulci and the sense of smell arises from deep in the cerebrum.
17. Explain the function of the association areas of the lobes of the cerebrum.
The association areas are found in the anterior frontal lobes, and in the lateral areas of the parietal, temporal, and occipital lobes. These function to analyze and interpret sensory experiences involving memory, reasoning, verbalizing, judgement, and emotions. The association areas of the frontal lobes deal with concentration, planning, problem solving, and judging the consequences of behavior. The areas of the parietal lobes deal with the understanding speech and word choice for thought expression. The areas of the temporal lobes deal with complex sensory interpretation, such as reading, music, and memories of visual scenes. The areas of the occipital lobes deal with visual pattern analysis and combining these images with other sensory experiences.
18. Define hemisphere dominance.
Although both hemispheres participate in basic functions, in most people, one hemisphere is dominant over the other. For instance, in over 90 percent of the population, the left hemisphere controls language activities such as reading, speech, and writing as well as complex intellectual functions requiring verbal, analytical, and computational skills. The nondominant hemisphere seems to be more in control of the nonverbal activities such as spacial orientation, interpreting musical patterns, visual experiences, and emotional and intuitive thought.
19. Explain the function of the corpus callosum.
The nerve fibers of the corpus callosum allow the dominant hemisphere to receive sensory information sent to the nondominant hemisphere for use in decision making by the general interpretive areas. It also allows the dominant hemisphere to control the motor cortex of the nondominant one.
20. Distinguish between short-term and long-term memory.
Short-term memories are thought to be electrical in nature such that the neurons are connected in a circuit so that the last in the series stimulates the first. As long as the stimulation continues, the thought is remembered. When it ceases, so does the memory, unless it enters long-term memory.
Long-term memories appear to change the structure or function of certain neurons that enhance synaptic transmission. The synaptic patterns must meet two requirements of long-term memory. First, there must be enough synapses to encode an almost infinite number of memories. Second, the pattern of synapses can remain unchanged for years.
21. Describe the location and function of the basal nuclei.
The basal ganglia (basal nuclei) are masses of gray matter found deep in the cerebral hemispheres. They are the caudate nucleus, the putamen, and the globus pallidus and are responsible for producing most of the inhibitory neurotransmitter dopamine. Impulses from the basal ganglia inhibit motor functions, controlling certain muscular activities.
22. Name the parts of the diencephalon, and describe the general functions of each.
Diencephalon—The diencephalon contains many parts:
Thalamus—The thalamus serves as a central relay for sensory impulses ascending from other parts of the body. It receives all impulses except for smell, and routes them to the appropriate areas of the cortex. It also interprets general feelings such as pain, touch, and temperature. The thalamus also transmits sensory information by synchronizing action potentials. In this way, it serves as a messenger and an editor.
Hypothalamus—The hypothalamus is interconnected to the cortex and all areas of the brain stem so that it can send and receive impulses to and from these areas. It plays a key role in maintaining homeostasis by regulating visceral activities and serving as a link between the nervous and endocrine system.
Optic tracts and optic chiasma—These are formed as the optic nerve fibers cross over.
Infundibulum—The infundibulum attaches the pituitary gland to the brain stem.
Other parts include the: posterior pituitary gland, mammillary bodies, and the pineal gland.
23. Define the limbic system, and explain its functions.
The limbic system controls emotional experience and expression. It produces feelings of fear, anger, pleasure, and sorrow. It apparently recognizes upsets in a person’s physical or psychological condition that could be life threatening. By relating pleasant or unpleasant feelings about experiences, it guides behaviors that may increase the chance of survival. It also interprets sensory impulses from the olfactory receptors.