Biology Notes: Chapter 23

Biology Notes, Chapter 23, page 1 Nervous System Biology Notes: Chapter 23 I. Neurons A. Supporting Cells: Neuroglial cells are accessory cells within nerve tissue. They fill spaces and surround or support various parts. B. Basic Characteristics 1. Neurons are masses of nerve cells that make up the nervous tissue of the brain and spinal cord. a. they are structural and functional units of the nervous system b. they are specialized to react to specific physical and chemical changes occurring in their surroundings 2. Neuron Cell Body: Every neuron has a cell body and tubular processes filled with cytoplasm 3. Neuron Processes: The tubular processes are: dendrites and axons a. Dendrites are short and highly branched. These with the cell membrane make up the receptive surfaces of its neurons. b. The Axons are long, slender, cylindrical processes whose ends are finely branched. c. Larger axons of the peripheral nerves are enclosed in sheaths of neuroglial cells called Schwann cells. 1) These cells are tightly bound to the axon by a sheath composed of a lipidprotein. This lipid protein is myelin. 2) Axons posses myelin sheaths are called myelinated nerve fibers and those that lack these sheaths are unmyelinated nerve fibers. 3) Myelinated fibers appear white. Unmyelinated fibers appear gray. 4) Neurilemma: portion of the Schwann cell that surrounds the myelin sheath 5) Nodes of Ranvier: gaps in the myelin sheath 4. If an injury to a cell body causes the death of the neuron there is no possibility of regeneration. However, a damaged axon may be regenerated. C. Functional Classification 1. Sensory neurons (afferent nerves): carry nerve impulses from peripheral body parts into the brain or spinal cord 2. Interneurons: transmit impulses from one part of the brain or spinal cord to another; direct incoming sensory impulses to appropriate parts for processing and interpreting. 3. Motor neurons (efferent nerves): carry nerve impulses out of the brain or spinal cord to effectors (part of the body capable of responding, such as muscles or glands.) D. Nerve impulse 1. Nerve impulse: is an electrochemical pulse that moves along the membrane of a neuron a. Electrolytes (sodium and potassium especially) move in and out of the membrane b. This movement causes depolarization followed rapidly by repolarization of the membrane 2. Nerve impulses continue along the membrane until it reaches the axon terminal

Biology Notes, Chapter 23, page 2 3. Synapse: the junction between two neurons; space between the end of an axon and the end of a dendrite a. nerve impulses must jump the synapse in order to continue to the next nerve b. Neurotransmitters: substances that directly or indirectly aid in the conduction of an impulse between neurons c. Neuromodulators are substances that alter a neuron s response to a neurotransmitter or block the release of a neurotransmitter 4. Pathway of nerve impulses a. An impulse travels from a dendrite or cell body and then moves along the axon to the presynaptic terminal at its end. b. There the impulse crosses a synapse and continues to the dendrite or cell body of another neuron. c. The process of crossing the gap at a synapse is called synaptic transmission. II. Central Nervous System A. Protection of the CNS 1. Meninges a. The dura mater is the outermost layer that forms the periosteum of the surrounding skull bones. b. The arachnoid mater is thin, netlike membrane that lacks blood vessels. It spreads over the brain and spinal cord, but generally does not dip into the grooves. c. The pia mater is very thin and contains many nerves and blood vessels that service the underlying cells of the brain and spinal cord. 1) The pia mater follows the irregular contours of the brain and spinal cord. d. Meningitis is an inflammation of the meninges. 1) It is usually caused by certain bacteria or viruses that invade the cerebrospinal fluid. 2) It is most common in infants and children. 2. Cerebrospinal Fluid: fluid that is between the arachnoid and pia maters a. The cerebrospinal fluid is secreted by tiny masses of specialized capillaries from the pia mater call choroid plexuses. b. Its primary functions are to protect and to help maintain a stable ionic concentration, and to be a pathway for blood wastes. B. The brain 1. Brain tissue a. White Matter: region of CNS that contains dense collections of myelinated fibers b. Gray Matter: region of CNS that contains mostly nerve cell bodies and unmyelinated fibers 2. Major structures a. The cerebrum is the largest part of the brain. 1) It consists of two large masses called cerebral hemispheres, which are essentially mirror images of each other. 2) The hemispheres are connected by a deep bridge of nerve fibers called the corpus callosum 3) The surface of the cerebrum is marked by ridges called gyri, which are separated by grooves.

Biology Notes, Chapter 23, page 3 i. A shallow groove is called a sulcus and a very deep groove is called a fissure. ii. A longitudinal fissure separates the right and left cerebral hemisphere. iii. A transverse fissure separates the cerebrum from the cerebellum. 4) The thin layer of gray matter that makes up the outermost portion of the cerebrum is the cerebral cortex. b. thalamus: receives all sensory impulses and channels them to appropriate regions of the cortex for interpretation c. hypothalamus: maintains homeostasis by regulating visceral activities such as heart rate, blood pressure, body temperature, hunger, sleep, etc. d. The midbrain: myelinated nerve fibers join lower parts of the brain stem and spinal cord with higher parts of the brain; several gray matter masses serve as reflex centers e. The pons appears as a rounded bulge on the underside of the brain stem. 1) It separates the midbrain from the medulla oblongata. 2) Its functions are to relay impulses from the medulla oblongata and peripheral nerves to the cerebrum and help regulate the rate and depth of breathing f. The medulla oblongata is an enlarged continuation of the spinal cord. 1) It transmits all ascending and descending impulses. 2) It also contains several vital and non-vital reflex centers including the cardiac center, vasomotor center (controls diameter of blood vessels thus controlling blood pressure), and the respiratory center. g. The cerebellum is the center for processing and interpreting impulses from the motor cortex and sensory pathways. 1) It coordinates motor activity so that smooth, well-timed movements occur. 2) It helps to coordinate posture C. The spinal cord 1. The spinal cord begins where nerve tissue leaves the cranial cavity covered in a single layered dura mater called the spinal dural sheath. 2. The central canal is continuous with the ventricles of the brain and contains cerebrospinal fluid. 3. It consists of thirty-one segments each of which gives rise to a pair of spinal nerves that branch out to various body parts. 4. Spinal Cord Enlargements a. In the neck region a thickening in the spinal cord, called the cervical enlargement, gives off nerves to the arms. b. A similar thickening in the lower back, the lumbar enlargement, gives off nerves to the legs. 5. Outer region of the spinal cord is white matter. The inner region (butterfly shape) of the spinal cord is gray matter. III. Peripheral Nervous System: the peripheral nerves that connect the CNS to other body parts A. Peripheral nerves 1. Cranial nerves: nerves that branch from the brain 2. Spinal nerves: nerves that branch from the brain 3. Afferent Neurons: sensory neurons

Biology Notes, Chapter 23, page 4 4. Efferent Neurons: motor neurons a. Somatic Nervous System: conducts impulses from the CNS to skeletal muscles b. Automatic Nervous System: visceral motor nerve fibers that regulate the smooth muscles, cardiac muscles and glands 1) Sympathetic Division: mobilizes body systems during emergency situations 2) Parasympathetic Division: conserves energy and promotes nonemergency functions B. Sensory Organs 1. Eye 2. Ear 3. Smell receptors 4. Taste receptors 5. Touch receptors Sensory Organs I. Sensory Receptions A. Groups of sensory receptors 1. chemoreceptors: stimulated by changes in the concentration of chemical substances senses of smell and taste 2. nociceptors: stimulated by damage to tissue by exposure to excessive mechanical, electrical, thermal, or chemical energy 3. thermoreceptors: sensitive to temperature change; heat receptors respond to heat energy increases above a certain level; cold receptors respond to heat energy decreases below a certain level 4. mechanoreceptors: sensitive to changes in pressure or movement of fluid; detect changes that cause the receptor to become deformed 5. photoreceptors: respond whenever they are exposed to light energy of sufficient intensity -- occur only in the eyes B. Sensations 1. A sensation is a feeling that occurs when sensory impulses are interpreted by the brain. 2. Projection of a sensation a. At the same time that a sensation is created, the cerebral cortex causes it to seem to come from the receptors being stimulated. b. This process is called projection, because the brain projects the sensation back to its apparent source. 3. Sensory adaptation a. When sensory receptors are subjected to continuous stimulation, many undergo a sensory adaptation (impulses leave at decreasing rates, until the receptors may completely fail to send signals) C. Sense of touch, pressure and temperature 1. The senses of touch and pressure employ three kinds of receptors a. Sensory nerve fibers are associated with touch and pressure b. Meissner s corpuscles are sensitive to light touch and used to judge textures.

Biology Notes, Chapter 23, page 5 c. Pacinian corpuscles are stimulated by heavy pressure and the detection of vibrations in tissues. 2. Sensing temperature a. The sensation of temperature involves thermoreceptors. b. Heat receptors are sensitive to temperatures between 77 o F and 113 o F. As 113 o F is approached, pain receptors are triggered, producing a burning sensation. c. Cold receptors are most sensitive to temperatures between 50 o F and 68 o F. If the temperature drops below 50 o F, pain receptors are stimulated, and the person feels a freezing sensation. II. The Chemical Senses: Taste and Smell A. Sense of Taste 1. Taste buds are the special organs of taste. a. They occur primarily on the surface of the tongue with tiny elevations called papillae. b. Each taste bud includes a group of modified epithelial cells called gustatory cells (taste cells) that function as chemoreceptors. c. Tiny projections called taste hairs, protrude from the outer ends of the taste cells and are believed to be the sensitive parts of the receptor cells. d. Before the taste of a particular chemical can be detected, the chemical must be dissolved in the watery fluuid surrounding the taste buds. This fluid is supplied by the salivary glands. 2. Basic Taste Sensations a. Sweet: tip and front sides of tongue b. Sour: back sides of tongue c. Salty: tip and upper portion of the tongue d. Bitter: toward the back of the tongue 3. Influence of Other Sensation on Taste a. The olfactory receptors are similar to those for taste in that they are chemoreceptors stimulated by chemicals dissolved in liquids. b. These senses function closely together and aid in food selection because food is usually smelled at the same time it is tasted. c. It is often difficult to tell what part of a food sensation is due to smell and what part is due to taste. d. If excessive mucous secretions cover the olfactory receptors when a person has a head cold, food may seem to lose its taste. 4. Effect of aging on the sense of taste a. The sense of taste is not as likely to diminish with age as the sense of smell. b. This is because taste cells are reproduced continually, so that any one of these cells functions for only about a week before it is replaced. B. The Sense of Smell 1. The olfactory organ and its function. a. The olfactory organs, which contain the olfactory receptors, include various epithelial supporting cells. They appear as yellowish brown masses surrounded by pinkish mucous membrane. b. Chemicals that stimulate olfactory receptors enter the nasal cavity as gases.

Biology Notes, Chapter 23, page 6 b. They must be dissolved in the watery fluids that surround the cilia before they can be detected. This is one of the roles of mucus. 3. The seven primary olfactory sensations. a. Camphorarceous: scent of camphor b. Musky: scent of musk c. Floral: scent of flowers d. Pepperminty: scent of oil of peppermint e. Ethereal: scent of ether f. Pungent: scent of spices g. Putrid: scent of decaying meat 4. Effect of aging on the sense of smell a. Elderly persons may require special stimulation to encourage eating because their olfactory sensitivities have diminished. b. For this reason, it is important that their foods be visibly attractive and that mealtimes be pleasant. c. Disinterest in eating may contribute to malnutrition. II. The Sense of Hearing and Balance A. Structures of the Ear 1. Auricle (Pinna): outer, funnel-like structure that helps collect sound waves 2. External auditory meatus: S-shaped tube leading inward; lined with hair and wax to keep insects and microbes out of ear 3. Ceruminous Glands: wax glands that line the meatus 4. Tympanic Membrane: the ear drum 5. Eustachian tube: connects each middle ear to the throat. a. Allows air to pass between the tympanic cavity and the outside of the body by way of the throat and mouth b. Helps maintain equal air pressure to both sides of the eardrum. c. Swallowing, yawning, or chewing can hasten the equalization of air pressure if discomfort is experienced. 6. Auditory ossicles: three bones that form a bridge connecting the eardrum to the inner in order to transmit vibrations a. The auditory ossicles are called the malleus (hammer), incus (anvil) and the stape (stirrup). b. the stape vibrates against the oval window (membrane-covering of the inner ear) 7. The Cochlea: bony structure shaped like the coiled shell; contains the organ of Corti 8. Organ of Corti: receptor organ for hearing B. Balance 1. The Semicircular Canals: bony canals that function in equilibrium; each canal is oriented to one of the three planes of space a. Static equilibrium: helps maintain the stability and posture of the head and body when these parts are motionless b. Dynamic equilibrium: helps maintain stability when the head and body are suddenly moved or rotated

Biology Notes, Chapter 23, page 7 2. Motion Sickness: common equilibrium disorder appearing to be due to sensory input mismatch. The brain receives conflicting information leading to nausea and vomiting. C. Transmission of Sound to the Inner Ear 1. The malleus is attached to the tympanic membrane so that when the membrane vibrates, the malleus vibrates in unison with it 2. The malleus causes the incus to vibrate and the incus passes the movement onto the stapes. 3. The stapes is held by ligaments to the oval window. 4. Vibration of the stapes causes motion of a fluid within the inner ear. 5. These vibrations of the fluid are responsible for stimulating the hearing receptors. D. Problems with Hearing 1. Deafness a. Conduction Deafness occurs when something interferes with the conduction of sound vibrations to the fluids of the inner ear. Causes include 1) impacted earwax blocking the auditory canal 2) ruptured eardrum b. Otosclerosis: age-related problem when overgrowth of bony tissue fuses the stapes to the oval window or fuses the ossicles to one another c. Sensorineural Deafness results from damage to neural structures at any point from the cochlea to the auditory nerve 2. Tinnitus: ringing in the ears; usually a symptom of another problem such as a. cochlear nerve degeneration b. inflammation of the middle or inner ears c. a side effect of form of medication such as aspirin III. The Sense of sight A. Accessory Structures of the Eye 1. Eyebrows help shade and protect the eyes 2. Eyelids 3. Lacrimal apparatus: The lacrimal apparatus consists of the lacrimal gland which secretes tears, and a series of ducts which carry the tears into the nasal cavity. These tears lubricate the eye. 4. Extinsic muscles: There are six extrinsic muscles that function to move the eye in various directions. B. Structure of the Eyeball 1. cornea: window of the eye; helps focus entering light rays a. few cells, no blood vessels b. numerous pain receptors so very sensitive 2. sclera: white portion of the eye; provides protection and serves as an attachment for the extrinsic muscles 3. choroid coat: loosely attached to sclera; numerous blood vessels; absorbs excess light and helps keep the inside of the eye dark 4. ciliary body: Thickest part of middle tunic; forms an internal ring around the front of the eye

Biology Notes, Chapter 23, page 8 5. iris: seen as the colored portion of the eye; controls the opening and closing of the pupil 6. Lens: focuses light/image onto the retina a. Lies directly behind the iris b. Held in position by suspensory ligaments; tension in the suspensory ligaments focuses the lens 7. retina: contains the visual receptor cells 8. optic disk: an area just medial to the fovea centralis 9. Fovea centralis: the depression in the center of the macula lutea. a. Only cones are found in the fovea centralis. b. Produces the sharpest vision. 10. Macula lutea is a yellowish spot in the central region of the retina. a. The concentration of cones decreases in area further away from the macula lutea, while the concentration of rods increases in these areas. 11. optic nerve: pierces the sclera in the back of the eye C. Photoreceptors of the eye are actually modified neurons. 1. Rods: Rods produce colorless vision; allow people to see general outlines of objects a. There are about 100 million rods in the retina. b. Rods are extremely sensitive to light enabling persons to see in relatively dim light. 2. Cones: detect colors; three basic types red, yellow, blue receptors a. There are about 3 million cones in the retina. b. Cones allow people to see sharp images. D. Fluids of the eye 1. The vitreous humor is a transparent, jellylike fluid between the lens and the retina; This fluid supports the internal parts of the eye and helps maintain its shape. 2. The aqueous humor is a watery fluid that fills the space between the cornea and the lens. It helps nourish these parts and aids in maintaining the shape of the front of the eye. E. Process of sight 1. The size of the pupils change constantly in response to reflexes that are triggered by such factors as light intensity, gases, and variations in emotional state. 2. When light hits the curved surface of the lens it is refracted (bent). a. If the tension on the suspensory ligaments is relaxed, the lens surface becomes more convex. This change occurs in the lens when the eye is focused to view a close object. b. This adjustment is called accommodation 3. Because the light rays intersect the image projected on the retina is upside down and reversed from left to right F. Problems with sight 1. Glaucoma: compression of the retina and optic nerve due to pressure caused by blocked drainage of the aqueous humor 2. Cataract: clouding of the lens; usually caused by aging and the thickening of the lens 3. Myopia: nearsighted; can see near objects; usually results from the eyeball being too long

Biology Notes, Chapter 23, page 9 4. Hyperopia: farsighted, can see far objects; usually results from an eyeball that is too short or a lazy lens 5. Astigmatism: misshapen lens results in unequal curvatures in different parts of the lens 6. Color blindness: result of a congenital lack of one or more of the cone types, more common in males than in females. 7. Night blindness: condition in which rod function is seriously hampered, impairing one s ability to drive safely at night G. Stereoscopic vision is vision that involves perceiving the distance and depth as we as the height and width of objects. 1. Such vision is due largely to the fact that the pupils of the eyes are 6-7 cm apart. 2. Objects that are relatively close produce slightly different retinal images.