Outline and chapter notes to accompany chapter 13 THE NERVOUS SYSTEM Dec., 2003 A. THE NERVOUS SYSTEM CARRIES MESSAGES THROUGHOUT THE BODY. THE NERVOUS SYSTEM AND NEURONS: CENTRAL NERVOUS SYSTEM: Brain and spinal cord PERIPHERAL NERVOUS SYSTEM: Peripheral nerves SPECIAL SENSE ORGANS: Eyes, ears, taste buds, nasal epithelium, cutaneous senses NEURONS are nerve cells whose membranes carry nerve impulses. Sensory neurons transmit messages from sense organs toward the central nervous system. Motor neurons transmit messages from the central nervous system outward to the various muscles and glands. Association neurons make connections within the spinal cord and brain. Each neuron has a central cell body, plus two kinds of processes: dendrites, which conduct impulses toward the cell body, and axons, which conduct impulses away from the cell body . Most axons are surrounded by a fatty myelin sheath. Bundles of axons are called nerves. Nerve impulses cross from one neuron to another across a gap called a synapse. NEUROGLIA (supporting or nonneural cells) include astrocytes, oligodendrocytes, etc. NERVE IMPULSES-- HOW MESSAGES TRAVEL ALONG NEURONS Often studied in giant axons of squids Resting cell (a nerve cell which is not at the moment conducting an impulse): membrane is polarized (i.e., electrical charges are separated). Outside is more positive than inside because there are more positive ions outside the cell than inside. Sodium-potassium pump: Sodium is actively transported to the outside of cell, against a concentration gradient; a smaller amount of potassium is transported in the opposite direction, also against a concentration gradient. Subthreshhold stimuli: a very small depolarizing stimulus, or any size of hyperpolarizing stimulus, travels along the membrane and decays (lessens) with distance. Nerve impulse (action potential): a depolarizing stimulus that reaches or exceeds a threshhold value causes the membrane to quickly and dramatically reverse its polarity; this reversal is called an action potential or spike. The action potential stimulates the nearby membrane regions in such a way that the action potential propagates along the membrane surface. Recordings with sensitive voltmeters show that the nerve impulse consists of a wave of depolarization that proceeds along the membrane (Fig. 10.6). Events at the molecular level include the action of three types of membrane proteins: 1. Sodium channels, which open to allow Na+ ions to pass across cell membrane; 2. Potassium channels, which open to allow movement of potassium ions; 3. The sodium-potassium pump (described previously). Molecular events are thought to proceed as follows: 1. depolarization opens sodium channels slightly at first 2. once a threshhold is reached, sodium channels open completely; the influx of positively charged sodium ions results in an action potential in which the cell interior becomes positively charged 3. the action potential stimulates potassium channels to open; the outpouring of potassium ions makes the cell interior negative once again 4. potassium channels close and the action of the sodium- potassium pump restores the resting potential Many axons are enclosed in a myelin sheath formed by many layers of plasma membrane of a type of neuroglia cell called a Schwann cell. The myelin, a fatty material, forms an insulating layer that prevents nerve signals in one neuron from interfering with signals in other nearby neurons. NEUROTRANSMITTERS-- HOW MESSAGES TRAVEL BETWEEN NEURONS Signals cross the synapse in the form of a chemical neurotransmitter substance rather than as electrical impulses. The neurotransmitters bind to receptors on the next neuron, depolarizing or hyperpolarizing its membrane. Various types of neurotransmitters exist: Amines such as epinephrine, norepinephrine, dopamine, serotonin, histamine Amino acids such as aspartate, glutamate, glycine, & gamma- aminobutyric acid (GABA) Proteins or peptides such as somatistatin, beta-endorphin, leu- enkephalin, etc. Acetylcholine (an ester of choline) Nitric oxide (a gas) Neurotransmitters can be withdrawn from the synapse after they have done their job by either of two processes: - enzyme degradation (acetylcholine degraded by cholinesterase, or amine neurotransmitters by the enzyme MAO = monoamine oxidase) - reuptake (neurotransmitters other than acetylcholine are usually reabsorbed by the cell that secreted them) DOPAMINE PATHWAYS IN THE BRAIN-- PARKINSONISM AND HUNTINGTON'S DISEASE PARKINSONISM: Symptoms: muscle tremors, "pill-rolling" movements, difficulty intiating voluntary movements, shuffling gait with body leaning forward. Cause: degeneration of neurons in the substantia nigra in the brain, brain, resulting in insufficient dopamine, especially to the basal ganglia. Treatment: giving precursors to dopamine, such as L-dopa (may produce schizophrenia-like side effects) HUNTINGTON'S DISEASE: Symptoms: onset typically between age 40 and 50, uncontrollable spasms or twiches (choreic movements), loss of motor functions and mental functions, death a few years after onset. Cause: destruction of cells in the basal ganglia that inhibit dopamine production, thus resulting in an overproduction of dopamine. Treatment: none at present. Drugs which inhibit dopamine will reduce symptoms. Feedback inhibition is involved: Normally, the neurotransmitter GABA decreases dopamine secretion. If GABA secretion is impaired (as in Huntington's disease), dopamine is no longer inhibited and is instead secreted to excess. B. MESSAGES ARE ROUTED TO AND FROM THE BRAIN: MESSAGE INPUT-- SENSE ORGANS THE EYE: Cornea lets in light Lens focuses light on the retina Retina is sensitive to light and produces nerve impulses Ciliary body controls lens shape (focusing) and light levels Protective layers include choroid and sclera THE EAR Outer ear includes ear flap (pinna), auditory tube, and eardrum. Middle ear houses three bones (hammer, anvil, stapes). Inner ear includes: Cochlea, a coiled tube associated with hearing Semicircular canals, important in maintaining balance TASTE AND SMELL Taste buds on tongue and palate detect five types of stimuli: sweet, bitter, sour, salty, and "umami" (glutamate) Nasal epithelium responds to airborne molecules by sending nerve impulses into the brain. CUTANEOUS SENSES Light touch, deep pressure, warmth, cold, pain MESSAGE PROCESSING IN THE BRAIN: FOREBRAIN (PROSENCEPHALON) TELENCEPHALON: Olfactory bulbs, olfactory tract, olfactory lobe (all concerned with smell); cerebral hemispheres (concerned with thought and conscious activity), hippocampus (concerned with certain types of learning), basal ganglia (important integration centers whose malfunctions are described later), corpus callosum (connecting the two cerebral hemispheres), lateral ventricles I and II DIENCEPHALON: Pineal body (important in circadian rhythms), anterior and posterior commissures (connecting right and left sides of the brain), thalamus (center of certain emotions), hypothalamus (controlling temperature and appetite), infunbidulum, optic chiasma, ventricle III MIDBRAIN (MESENCEPHALON): Superior and inferior colliculi (centers for visual and auditory perception); ventral tegmental area (including reticular formation and positive reward centers); cerebral aqueduct HINDBRAIN (RHOMBENCEPHALON): METENCEPHALON: Cerebellum (important in balance and muscle coordination), pons (connects right and left portions of cerebellum) MYELENCEPHALON: Medulla oblongata (responsible for breathing and other reflexes); ventricle IV Corresponding parts of different species are homologous. All the ventricles are filled with cerebrospinal fluid; they communicate with blood vessels by diffusion across the thin tela choroidea (one in the diencephalon, another in the myelenceph- alon); the tela choroidea is part of the blood-brain barrier. Methods of studying the brain include comparative anatomy, positron emission tomography (PET scan), and ablation, among other methods. MESSAGE OUTPUT-- MUSCLE CONTRACTION Some nerve cells have synapses with muscle cells instead of with other neurons. The muscle cells have polarized membranes the way neurons do. Voluntary signals are sent along motor neurons to skeletal muscle tissues. Acetylcholine crosses the synapse and binds to receptors on the muscle cell membrane, triggering calcium channels to open on vesicle membranes inside the muscle cell. Calcium rushes into the cytoplasm and binds to troponin, a protein which is normally coupled with myosin. The release of myosin from troponin frees the myosin now to bind with actin. Repeated binding between actin and myosin causes filaments of these two proteins to slide past one another, causing contraction. C. THE BRAIN STORES AND REHEARSES MESSAGES. LEARNING-- STORING BRAIN ACTIVITY Physiological adaptation is a change in response to the environment. Learning consists of lasting changes in behavior or knowledge resulting from experience. PROCEDURAL LEARNING Procedural learning (how to do a task or perform an activity) can be demonstrated by nonhuman animals (insects, etc.) and by infants. It does not require activity of the cerebral cortex. Simple types of procedural learning include: Habituation: learning not to pay attention to certain stimuli Sensitization: paying more attention to stimuli when awareness is heightened Classical conditioning: learning to associate one stimulus with another that has often followed the first in past experience. DECLARATIVE LEARNING AND MEMORY (of particular persons, places, or things) It requires activity of the cerebral cortex. MEMORY FORMATION AND CONSOLIDATION: To become a memory, information must be acquired, stored, and retrieved. Storage into long-term memory requires the action of the hippocampus in the forebrain. Memory consolidation is a process which restructures memories based on other more recent experiences. Consolidated memories last much longer and can be recalled more easily. ABSTRACTION AND GENERALIZATION: Learning can be generalized. Generalization is an aspect of information processing during memory consolidation in which general concepts develop from specific experiences. As a result, similar stimuli evoke similar responses. Oddity problems can be used to show that nonhuman primates can sometimes form abstract concepts. ALZHEIMER'S DISEASE-- A LACK OF ACETYLCHOLINE Symptoms: memory loss and mental deterioration resulting in death. Cause: defective neurotransmission of acetylcholine (exact defect not known). Treatment: none. Drugs that inhibit cholinesterase will temorarily improve memory. BIOLOGICAL RHYTHMS-- TIME-OF-DAY MESSAGES CIRCADIAN RHYTHMS are biological rhythms of approximately 24 hour duration. Most of the body's physiological and psychological functions are affected. We now have evidence that these rhythms are endogenously (internally) controlled by the suprachiasmatic nucleus in the diencephalon. Circadian rhythms maintained by this nucleus are entrained to follow the 24-hour day by the cyclic activity of the pineal body, which produces melatonin during darkness but not in the daytime (i.e., in response to exogenous or external clues). DREAMS-- PRACTICE IN SENDING MESSAGES Sleep is studied by electrical recordings (electroencephalography). Characterized by four stages (1 lightest, 2 deeper, 3 still deeper, 4 deepest) Each has characteristic electrical activity patterns. Difficulty of arousal increases from stage 1 to 4. Also a stage characterized by Rapid Eye Movements (REMs) REM sleep appears to correspond to dreaming episodes. People deprived of REM sleep suffer hallucinations and mental impairment. Typical night's sleep includes 3-5 cycles; each starts with stage 1 and ending with a REM episode; awakening often follows REM episode. Successive cycles are shorter in length and may not progress to the deepest stages (stages 3 & 4) of sleep. MENTAL ILLNESSES AND NEUROTRANSMITTERS IN THE BRAIN DEPRESSION Symptoms: feelings of despair & helplessness; attempts at suicide in many cases. Presumed cause: reduced amount of a neurotransmitter, possibly serotonin Treatment: MAO inhibitors (which block destruction of amine neurotransmitters in the synapse); also reuptake inhibitors (which block the reuptake of neurotransmitters from the synapse). SCHIZOPHRENIA (incl. frequent hallucinations): possibly due to an excess of dopamine. EPILEPSY (marked by brain seizures) may be due to a loss of feedback inhibition in the cerebral cortex or hippocampus. ---------------------------------- Dec., 2003 PERMISSION IS HEREBY GRANTED to instructors who have adopted the book BIOLOGY TODAY for classroom use to download, modify, and use these notes as needed to aid them in in their teaching. Students of such instructors may likewise use and modify these notes as study aids.