Lecture Outline - Chapter 17

17.1. Neuron Structure (p. 306)

1. Nerve cells are called neurons.
2. Neurons are generally divided into three parts. (Fig. 17.1)
   • a. Dendrite conducts nerve impulses toward cell body.
   • b. Cell body contains nucleus and other organelles of cell.
   • c. Axon conducts nerve impulses away from cell body.
3. Nervous system consists of two main divisions.
   • a. Central Nervous System (CNS) includes brain and spinal cord along midline.
   • b. Peripheral Nervous System (PNS) includes nerves that extend beyond the midline CNS.
4. Types of Neurons
   • a. Sensory neuron (afferent neuron) conducts nerve impulse from sense organ to CNS; usually has a long dendrite and a short axon.
   • b. Motor neuron (efferent neuron) conducts a nerve impulse from CNS to a muscle fiber or gland; has short dendrites and a long axon.
   • c. Interneuron (association neuron) found within the CNS conveys messages between parts of system; has short dendrites and short or long axon.
5. Neuroglial Cells
   • a. Most long axons and dendrites are surrounded by neuroglial cells.
   • b. Schwann cells (neurolemmocytes) form many layers by spiral wrapping.
   • c. Gaps in these encircling fibers are nodes of Ranvier (neurofibril nodes).
   • d. Cell membranes contain myelin, an insulating lipid substance that gives nerve fibers their white color.
   • e. Outermost sheath directs path for nerve to regenerate if severed.

1. An oscilloscope measures electrical potential difference (voltage) between two points, one inside and the other outside of an axon.
2. Speed of conduction of this impulse can be 200 meters per second by jumping between nodes of Ranvier (saltatory conduction).
3. Resting Potential: Inside Is Negative (Fig. 17.2)
   • a. When axon is not conducting a nerve impulse, there is positive polarity outside and negative polarity inside axon membrane.
   • b. Polarity difference is due to active transport sodium/potassium pump in membrane which pumps sodium ions out potassium ions in.
   • c. Membrane is more permeable to potassium than sodium.
   • d. There are also large, negatively charged proteins inside axoplasm.
4. Action Potential: Upswing and Downswing
   • a. A rapid polarity change in membrane of a neuron results from physical stimulus, electrical shock, pH change, etc.
   • b. Stimulation of axon causes sodium gates to open membrane channels; as sodium ions rapidly move inside, oscilloscope records a depolarization in which charges inside change from a negative - 65 mV to a positive +40 mV. (Fig. 17.2b)
   • c. Potassium gates (channels) open as action potential swings down from +40 mV to at least - 65 mV, potassium ions rapidly move to outside, and inside becomes negative.
   • d. Membrane rests during refractory period; sodium/potassium pump returns sodium and potassium ions to original positions; during refractory period, neuron is unable to conduct a nerve impulse.

1. . Each axon tip or bulb comes close to dendrite or cell body of another neuron; region between them is called synapse.
2. Axon membrane at synapse is presynaptic membrane, dendrite provides postsynaptic membrane and gap is synaptic cleft.
3. Neurotransmitters are chemicals stored in synaptic vesicles of axon.
4. Nerve impulse causes presynaptic membrane to open channels to calcium ions that interact with actin filaments that pull synaptic vesicles to membrane.
5. Neurotransmitter in vesicles is released into synaptic cleft and diffuses across to postsynaptic membrane.
6. Neurotransmitter binds with postsynaptic membrane receptor in lock-and-key fashion; neurotransmitters can result in excitation or inhibition of next neuron. (Fig. 17.3)
7. If excitatory, the postsynaptic membrane potential is decreased, sodium ion channels open, and likelihood of neuron firing is increased.
8. Synapses that use gated ion channels are fast acting; other synapses are slow acting similar to hormones.
9. Neurotransmitters Are Varied
   • a. Twenty-five different types of neurotransmitters include gases CO and NO, and neuropeptides or amino acids and derivatives.
   • b. Acetylcholine (ACh) and norepinephrine (NE) can be excitatory or inhibitory depending on type of receptor at postsynaptic membrane.
   • c. They are either reactivated or inactivated; enzyme acetylcholinesterase breaks down acetylcholine.
   • d. Failure to repackage or eliminate neurotransmitters in synapse causes continuous stimulation or inhibition.

1. The peripheral nervous system (PNS) consists of cranial and spinal nerves that extend outside the CNS.
   • a. Somatic system contains sensory neurons from sense organs and motor neurons to muscles.
   • b. Autonomic system contains sensory neurons from viscera and motor neurons to control glands, heart muscle, and smooth muscles of internal organs.
2. Cranial and Spinal Nerves
   • a. Nerves are structures that contain bundles of fibers; sensory nerves contain long dendrites, motor nerves contain long axons, and mixed nerves contain both.
   • b. Cell bodies are in brain, spinal cord, or ganglia (collection of cell bodies within PNS).
   • c. Twelve pair of cranial nerves serve the head, neck, and face with a vagus nerve that branches to pharynx, larynx, and internal organs.
   • d. Thirty-one pairs of spinal nerves have paired roots in spinal column; all are mixed nerves serving regions of body. Spinal nerves have a dorsal root conducting impulses inward from sensory neurons, and a ventral root containing axons of motor neurons. (Fig. 17.5)
   • e. A neuron, but not a nerve, obeys the all-or-none law (it fires maximally or it does not fire).
3. Somatic System Serves Skin and Muscles
   • a. Refers to all those nerves that serve musculoskeletal system and exterior sense organs (receptors) of skin.
   • b. Muscle fibers are effectors that bring about a reaction to a stimulus.
4. Reflexes are Automatic
   • a. Reflexes are automatic, involuntary responses to changes occurring inside or outside body; includes blinking of eye, withdrawing from hot object, etc.
   • b. Path of a Spinal Reflex or Reflex Arc (Fig. 17.6)
     • i. Receptor generates nerve impulse due to stimulus.
     • ii. Sensory neuron carries nerve impulse from dendrite to cell body in the dorsal root ganglion to axon.
     • iii. Interneurons pass nerve impulse to the motor neuron in the gray matter of spinal cord.
     • iv. Motor neuron provides nerve impulse that leaves gray matter of spinal cord via motor neuron's axon in ventral root and stimulates effector.
     • v. Effector receives nerve impulse and reacts; glands secrete or muscles contract.
5. Autonomic System Serves Internal Organs (p. 315)
   • a. Part of PNS that controls internal organs automatically and subconsciously.
   • b. Innervates all internal organs.
   • c. Uses two motor neurons and one ganglion for each impulse.
     • i. First of two neurons has cell body within CNS and a preganglionic fiber.
     • ii. Second neuron has cell body within ganglion and a postganglionic fiber.
6. Sympathetic System: Fight or Flight
   • a. Cell body of preganglionic nerve fibers is found in thoracic-lumbar region of spinal cord and terminates in a ganglia near cord; postganglionic fiber is long and innervates the organ.
   • b. Used in emergency situations and associated with "fight or flight"; axons of the postganglionic nerves release norepinephrine (NE), which inhibits digestion, accelerates heartbeat and breathing rate.
7. Parasympathetic System: Relaxed State
   • a. Cell bodies of preganglionic nerve fibers arise from craniosacral part of spinal cord and have a long axon that terminates in ganglia near or within the organ.
   • b. The post-ganglionic fiber is short and releases acetylcholine. It promotes the relaxed state ("housekeeper system"): decreases heartbeat, promotes digestion of food, and causes pupil of eye to contract. (p. 314)

1. Central Nervous System (CNS) includes spinal cord and brain.
   • a. Spinal cord is surrounded by vertebrae; brain is enclosed inside skull.
   • b. Brain and spinal cord are wrapped in three membranes, called meninges, that are filled with cerebrospinal fluid that cushions and protects.
   • c. Cerebrospinal fluid is also found within the central canal of spinal cord and ventricles of brain.
2. Neuroglial Cells Are Versatile
   • a. Several varieties of neuroglial cells exist; comprise over half of brain volume.
   • b. Glial cells outnumber neurons nine-to-one.
   • c. Astrocytes provide nutrients to neurons, absorb neurotransmitter glutamate, and produce growth factor (may provide cure for Parkinson disease).
3. Spinal Cord Communicates
   • a. Functions as center for reflex actions.
   • b. Communicates between brain and spinal nerves.
   • c. Gray matter shaped like a butterfly or letter "H"; colored by unmyelinated cell bodies and short fibers; made of axons of sensory neurons, dendrites/cell bodies of motor axons, and interneurons.
   • d. White matter is colored white due to myelinated long fibers of interneurons; this outermost region contains interneurons grouped into dorsal tracts that send information toward brain (ascending tracts) or into ventral tracts that carry information down from brain (descending tracts).
   • e. Tracts cross over so that left side of brain controls right side of body.
4. The Brain Commands
   • a. Brain is divided into: medulla oblongata, cerebellum, pons, midbrain, hypothalamus, thalamus, and cerebrum.
   • b. Brain has four cavities or ventricles.
   • c. Brain stem consists of medulla oblongata, pons, and midbrain.
     • i. Medulla oblongata:
       - lies between spinal cord and pons anterior to cerebellum.
       - contains centers for heartbeat, respiration, vasoconstriction.
       - contains reflex centers for vomiting, coughing, swallowing, etc.

       ii. Pons means "bridge" and contains tracts bridging the cerebellum with the rest of CNS; helps regulate breathing rate and some reflexes.

       iii. The midbrain also is a relay center for tracts and has reflex centers for head movements in response to visual, auditory, and tactile stimuli.
   • d. The diencephalon consists of the hypothalamus and thalamus.
     • i. Hypothalamus forms floor of third ventricle of brain; functions in homeostasis by regulating hunger, sleep, thirst, temperature, water balance, blood pressure, and controls the pituitary gland (link between nervous and endocrine systems).
     • ii. Thalamus is in roof of third ventricle; serves as central relay station for sensory impulses (except smell) traveling to appropriate regions of the cerebrum.
   • e. Cerebellum
     • i. Below posterior portion of cerebrum; separated from brainstem by fourth ventricle.
     • ii. Surface is gray matter; interior is mostly white matter.
     • iii. Functions in muscle coordination, muscle tone, and to maintain posture and balance.
   • f. Cerebrum is Paramount
     • i. Cerebrum is responsible for consciousness and is largest portion of brain.
     • ii. Outer layer is cerebral cortex; it is divided into right and left cerebral hemispheres connected by corpus callosum.
     • iii. Various regions of cerebrum specialize:
       - Frontal contains motor areas for voluntary skeletal muscles.
       - Parietal receives certain sensory input.
   • g. The Limbic System (Fig. 17.11)
     • i. Composed of basal ganglia, contains neural pathways that join frontal lobes, temporal lobes, thalamus, and hypothalamus.
     • ii. Causes person to experience pain, pleasure, rage, etc., to guide behavior to increase survival.
     • iii. Also critical in learning and long- and short-term memory.

1. Drugs that affect the nervous system may affect the limbic system or promote or decrease action of a particular neurotransmitter. (Fig. 17.12)
   • a. Stimulants either increase action of an excitatory neurotransmitter or block action of an inhibitory neurotransmitter; depressants work the opposite.
   • b. Drug abuse occurs when a person takes a drug at a dose level and under circumstances that increases potential for harmful effects.
   • c. Physical dependence (formerly termed addiction) on the drug occurs when a person develops a tolerance (must take more of the drug to get the same effect) and has withdrawal symptoms.
2. Alcohol
   • a. Alcohol may affect action of the inhibiting transmitter GABA or glutamate, an excitatory neurotransmitter.
   • b. Alcohol is primarily metabolized in liver, using up dehydrogenase enzymes needed to run glycolysis and Krebs cycle; this increases fermentation and lactic acid buildup and makes blood acidic; excess acetate is converted to fat.
   • c. Cirrhosis of liver occurs when cells engorge with fat droplets, scar tissue appears; early signs of deterioration reverse if excess drinking is given up.
   • d. Fetal alcohol syndrome is serious consequence of drinking while pregnant.
   • e. Heavy alcohol diet lacks proteins and vitamins.
3. Marijuana
   • a. Obtained from leaves, flowers, and stem of Cannabis sativa.
   • b. Active ingredient is THC (tetrahydrocannabinol) in plant resin.
   • c. Effects depend upon strength and amount consumed, expertise, and setting.
   • d. Effects include mild euphoria, distortions of space and time, inability to concentrate and speak coherently and motor incoordination.
   • e. Heavy use may produce chronic intoxication, recognized by hallucinations, anxiety, and paranoid reactions.
   • f. It does not seem to produce physical dependence but psychological dependence.
   • g. Known as the gateway drug for use of other drugs, heavy use may lead to chronic respiratory disease, lung cancer, and possible brain impairment and reproductive dysfunctions.
4. Cocaine
   • a. Alkaloid derived from the shrub Erythroxylum cocoa.
   • b. Often sold as potent extract termed "crack" (Fig. 17.13).
   • c. After a feeling of euphoria (rush), repeated use leads to a "crash."
   • d. Prevents reuptake of dopamine by the presynaptic membrane.
   • e. Highly addictive, over-dosing is a real possibility leading to cardiac and respiratory arrest.
   • f. Babies born to addicts suffer withdrawal and may have developmental problems.
5. Heroin
   • a. Derived from morphine, an alkaloid of opium.
   • b. Provides a feeling of euphoria along with relief of pain.
   • c. Alleviates pain by preventing release of substance P from sensory neurons in spinal cord and binds to receptors meant for body's own opiates (endorphins).
   • d. Individuals who inject heroin become physically dependent, develop tolerance, and suffer withdrawal symptoms.
6. Methamphetamine (Ice)
   • a. Used as an alternative to cocaine; has appearance of sheetlike crystals.
   • b. Related to amphetamine, a stimulant; easily synthesized in laboratories.
   • c. Can be smoked similar to crack, but has a stimulatory effect that lasts for hours.
7. Designer Drugs
   • a. Analogues are slightly altered molecules that maintain activity of controlled substances.
   • b. MPPP is one example of an analogue of narcotic fentanyl; very toxic even at low dosages.