Chapter 12 Nervous Tissue - Syllabus

Describe the cellular properties that permit communication among neurons and effectors

1) Sensory receptor forms a graded potential that triggers a sensory axon to send an action potential into the CNS; action potential triggers release of neurotransmitter at synapse with interneuron 2) neurotransmitter stimulates an interneuron to form a graded potential in its dendrites and cell body 3) Formation of the graded potential in the interneuron stimulates a nerve action potential to travel from the axon of the interneuron to the next synapse where another neurotransmitter is released 4) The steps of graded potential turning into action potential is repeated as higher motor neurons are stimulated such as thalamus and cerebral cortex Membrane Potential: Present in Plasma membranes of excitable cells; electrical potential difference (voltage) across the membrane. Resting Membrane Potential: Voltage stored in a battery; connect positive and negative terminals of the battery to a piece of wire, electrons will flow through the wire.

List the sequence of events involved in generation of an action potential.

Action Potential: Sequence of naturally occurring events that will decrease and reverse the membrane potential and eventually restore it back to its resting state. Depolarizing Phase: Negative membrane potential reaches 0mV, then more positive. This occurs from the voltage gated Na+ channels opening in the axon terminals and axon plasma membranes, causing Na+ to rush inside the cell. Repolarizing Phase: Positive membrane potential becomes more negative due to the opening of K+ voltage gated channels in the axon plasma membrane and axon terminals that allow K+ to rush out of the cell, restoring the cell to its resting membrane potential at -70mV. After-hyperpolarizing phase: Causes the membrane potential to become even more negative due to K+ voltage gated ion channels remaining open for a short time after the Na+ voltage gated channels have already closed. Threshold: When depolarization reaches a certain level, usually -50mV in most neurons, an action potential occurs. Different neurons have different thresholds, but are usually constant. An action potential occurring is dependent upon whether the stimulus was able to depolarize the membrane potential to threshold. Subthreshold stimulus: An action potential will not occur because the stimulus was not strong enough to depolarize the membrane to threshold in the neuron. Threshold stimulus: An action potential will occur because the stimulus was just enough to depolarize the membrane to threshold. Resting State: All voltage gated Na+ and K+ channels are closed. A slight positive charge is built up on outside surface of membrane; Slight negative charge is built up on inside surface of plasma membrane; Axon plasma membrane is at resting membrane potential -70mV. Depolarizing Phase: When membrane potential of axon reaches threshold, Na+ voltage gated ion channels open and Na+ flows inside the axon to rest along inner surface of axon membrane, causing it to depolarize from -55mV to +30mV. As the depolarization occurs more Na+ voltage gated channels open, causing a positive feedback mechanism. Na+ K+ pumps expel the Na+ out of the cell quickly to maintain a low Na+ content inside the cell. Repolarizing Phase: Voltage gated K+ channels open slowly as the Na+ voltage gated channels are closing. The flow of K+ out of the cell causes the cell to repolarize dropping the membrane potential from +30mV to -70mV. After-hyperolarizing Phase: K+ voltage gated channels remain open after the membrane has reached resting membrane potential at -70mV. This causes the membrane to polarize even further from -70mV to -90mV. K+ voltage gated channels alternate between open (activated) and closed (resting). Refractory Period: A period when the excitable cell cannot generate another action potential in response to a normal threshold stimulus. Absolute Refractory Period: A very strong stimulus cannot generate an action potential. Relative Refractory Period: A period in time when a second action potential can occur, but a larger than normal stimulus must occur.

Identify the major structures and functions of the nervous system in the maintenance of homeostasis

CENTRAL NERVOUS SYSTEM Brain: Located in skull, contains 100 billion neurons. Spinal cord: Connected to brain through foramen magnum and encircled by spinal cord, contains 100 million neurons Source of thoughts, emotions, memories, glandular secretions and stimulation of muscle contraction PERIPHERAL NERVOUS SYSTEM Somatic: Sensory neurons convey information from head, body wall, limbs, vision, taste, smell to CNS; Motor neurons convey information from CNS to skeletal muscles only and is voluntary Autonomic: Sensory neurons convey information from visceral organs in stomach, and lungs to CNS; Motor neurons are sympathetic and parasympathetic, convey information from CNS to smooth muscle, cardiac muscle and glands; involuntary Enteric: "brain of the gut"; involuntary; monitor chemical changes in the GI tract and stretching of walls; sympathetic and parasympathetic neurons; contraction of GI tract smooth muscle to move food, secretions of GI tract organs, activity of GI tract endocrine cells that secrete hormones. Nerves: Bundles of 100-1000 of axons and associated connective tissues and blood vessels that lie outside CNS Cranial Nerves: 12 pairs emerge from the brain Spinal Nerves: 31 pairs emerge from spinal cord Ganglia (ganglion): Small mass of nervous tissue, consists primarily of neuron cell bodies, lies outside the CNS Enteric Plexus: Neurons that lie in walls of organs in GI tract, regulates digestive system Sensory Receptor: Structure in the nervous system that monitors changes in the internal and external environment; examples: touch receptors in skin, photoreceptors in the eye, olfactory receptors in the nose

Summary of neuronal structure and function

Dendrites: Receive stimuli from ligand-gated or mechanically gated ion channels; in sensory neurons, produce generator or receptor potentials; in motor neurons and interneurons, produce excitatory and inhibitory postsynaptic potentials (EPSP and IPSP) Cell Bodies: Receives stimuli and produces EPSP and IPSP through activation of ligand-gated ion channels. Junction of Axon Hillock and Initial Segment of Axon: Trigger zone for many neurons; integrates EPSP's and IPSP's and, if sum depolarization that reaches threshold, initiates action potential (nerve impulse). Axon: Propogates nerve impulses from initial segment (or from dendrites of sensory neurons) to axon terminals in self regenerating manner; impulse amplitude does not change as it propogates along axon. Axon Terminals and Synaptic End Bulbs (for varicosities): Inflow of Ca2+ caused by depolarizing phase of nerve impulse triggers exocytosis of neurotransmitter from synaptic vesicles.

Histological characteristic of neurons

Electrical excitability: Ability to respond to a stimulus and convert it into an action potential. Stimulus: Any change in the environment that is large enough to trigger an action potential. Action Potential (nerve impulse): an electrical signal that travels along the membrane surface of a neuron; starts with movement of ions between interstitial fluid and inside of neuron through specific ion channels in the plasma membrane. PARTS OF A NEURON Cell Body (Perikaryon, Soma): Nucleus, Cytoplasm with Organelles such as mitochondria, free Ribosomes Nissl Bodies Golgi Complex, etc Nissl Bodies: Rough endoplasmic reticulum; proteins synthesized are used to replace cellular components and material for growth or repair axons in the PNS. Dendrites: Receive and input portion of neuron; receptor sites for binding chemical messengers from other cells; short, tapering, highly branched; contains nissl bodies, mitochondria and other organelles. Axon: Propogates nerve impulses towards another neuron, muscle fiber or gland cell; long, thin, cylindrical; joins cell body at the Axon Hillock. Axon Hillock: Cone shaped elevation that joins the axon to cell body. Initial Segment: Part of axon closest to Axon Hillock. Trigger Zone: Where nerve impulses start at the junction of the axon hillock and initial segment; travel along the axon to destination. Axon Collateral: Side branches of an axon that may branch off at right angles to an axon. Axon Terminal: End of an axon and its collaterals by division into many fine processes. Synapse: Site of communication between two neurons or a neuron and effector cell. Synaptic End Bulbs: Tips of axon terminals that swell into bulb-shaped structures. Synaptic Vesicles: Synaptic End Bulbs and Varicosities contain tiny membrane-enclosed sacs that store a chemical called a neurotransmitter. Neurotransmitter: Chemical released from synaptic vesicle used to inhibit or excite another neuron, muscle fiber or gland cell. Structural Diversity: Axons are shaped differently depending on where in the nervous system they are located. STRUCTURAL CLASSIFICATION Multipolar Neuron: Several dendrites and one axon; most neurons of brain and spinal cord Bipolar Neuron: One main dendrite, one axon; found in retina of eye, inner ear, olfactory area of brain Unipolar Neuron (Pseudounipolar neurons): Dendrites, one axon fused together to form a continuous process that emerges from the cell body; begin in embryo as Bipolar neurons; function as sensory receptors for touch, pressure, pain, thermal stimuli; nerve impulses triggered at junction of dendrites and axon, impulses move towards the synaptic end bulbs; cell bodies located in ganglia of spinal and cranial nerves

Comparison of Graded Potentials and Action Potentials in Neurons

GRADED POTENTIALS Origin: Arise mainly in dendrites and cell body Types of Channels: Ligand-gated or mechanically gated ion channels. Conduction: Decremental (not propogated); permit communication over short distances. Amplitude (size): Depending on strength of stimulus, varies from less than 1 mV - 50 mV. Duration: Typically longer, ranging from several milliseconds to several minutes. Polarity: May be hyperpolarizing (inhibitory to generation of action potential). Refractory Period: Not present, thus summation can occur ACTION POTENTIALS Origin: Arise at trigger zones and propogate along axon. Types of channels: Voltage-gated channels for Na+ and K+. Conduction: Propogation permitting communication over long distances. Amplitude (size): All or none: typically about 100 mV. Duration: Shorter, ranging from 0.5 to 2 msec. Polarity: Always consist of depolarizing followed by repolarizing phase and return to resting membrane potential. Refractory period: Present, thus summation cannot occur.

Compare the basic types of ion channels, and explain how they relate to action potentials and graded potentials

Ion Channel: Ions move from lower concentration to higher concentration (chemical part of gradient); positively charged cations move toward a negatively charged area; negatively charged anions move toward a positively charged area; as ions move they create an electrical current that can change the membrane potential; ions open/close by gates that move aside to open pore or seal pore shut; four channels (Leakage channel, gated channel, ligand-gated ion channel, mechanically gated ion channel, voltage-gated ion channel) Electrochemical Gradient: a concentration difference plus an electrical difference. Leakage Channel: randomly open/close; more Potassium (K+) leak channels than Sodium (Na+) leak channels; Potassium leak channels are leakier than Sodium leak channels so Potassium membrane permeability is higher than Sodium; found in nearly all cells including dendrites, cell bodies and axons. Ligand-gated Channel: opens/closes in response to ligand (chemical) stimulus; hormones, neurotransmitters, and particular ions can open/close ligand-gated channels; acetylcholine will open Na+ and Ca+ to diffuse inward and K+ to diffuse outward; are found in dendrite of some neurons such as pain receptors, and dendrites and cell bodies of interneurons and motore neurons. Mechanically Gated Channel: opens/closes in response to mechanical stimulation such as sound waves, touch, pressure, tissue stretching that changes the shape of the channel; found in auditory receptors in ears, receptors that monitor stretching of internal organs, touch and pressure receptors in skin. Voltage-gated Ion Channel: opens in response to membrane potential (voltage); take part in generation and conduction of action potentials in axons of all types of neurons.

Examples of Sensory Receptors that are Unipolar Neurons

Meissner Corpuscle: Touch receptor that consists of a mass of dendrites enclosed by a capsule of connective tissue Merkel Disc: Touch receptor consisting of free nerve endings (bare dendrites) make contact with Merkel Cells of Stratum Basale of skin Pacinian Corpuscle: Pressure receptor composed of multilayered connective tissue capsule that encloses a dendrite. Nociceptors: Pain receptor consisting of free nerve ending (bare dendrites). Thermoreceptors (detect thermal sensations); itch receptors and tickle receptors resemble nociceptors that serve as sensory receptors.

Describe the various types of neural circuits in the nervous system

Neural Circuits: functional groups of neurons that process specific types of information. CNS contains billions of neurons organized into complicated networks. Simple Series: Presynaptic neuron stimulates a single postsynaptic neuron; second neuron stimulates another, so on. Diverging Circuit: One presynaptic neuron stimulates many postsynaptic neurons or several muscle fibers and gland cells; nerve impulse from presynaptic neuron causes stimulation of increasing number of cells along the circuit. One presynaptic neuron in the brain stimulates a large amount of neurons in the spinal cord. Converging Circuit: Several presynaptic neurons stimulate one postsynaptic neuron; effective for stimulation or inhibition of postsynaptic neuron; receives input from several different sources; single motor neuron that synapses with skeletal muscle fibers at the neuromuscular junction received input from several pathways that originate in different brain regions. Reverberating Circuit: Stimulation of a presynaptic cell causes a postsynaptic cell to transmit a series of nerve impulses; incoming impulse stimulates the first neuron, which stimulates the second, second stimulates the third and so on, like a wave; branches from later neurons stimulate the earlier ones so that the nerve impulse flows through the circuit again and again lasting from a few seconds to several hours depending on number of synapses and arrangement of neurons in the circuit; inhibitory neurons may turn the circuit off after period of time; breathing, coordinated muscular activity, waking up, short-term memory Parallel After-discharge Circuit: Single presynaptic cell stimulates a group of neurons, each synapses with a common postsynaptic cell; differing number of synapses between first and last neurons causes a synaptic delay so that last neuron can exhibit multiple EPSP's (send stream of impulses out right away) and IPSP's; precise activities such as mathematical calculations.

Briefly outline two selected disorders (homeostatic imbalances

Multiple sclerosis (MS): Progressive destruction of myelin sheaths in the CNS; between ages 20-40, females twice as often as males; autoimmune disease; myelin sheaths degenerate into sclerosis or hardened scar tissue or plaques, especially in white matter of brain and spinal cord; destruction of myelin slows and then short circuits the propagation of nerve impulses; relapse-remitting appears in early adulthood; double vision, heaviness or weakness of muscles, abnormal sensations; beta interferon lengthens time between relapses, decreases symptoms when a relapse occurs and slows formation of new lesions, very hard to tolerate and effectiveness decreases with time. Epilepsy: Temporary attacks (seizures) of motor, sensory or psychological malfunction, almost never affects intelligence; many synchronous discharges from millions of neurons in the brain, abnormal reverberating circuits; skeletal muscles may contract involuntarily; light, noise, or smells may be sensed, when eyes, ears, and nose have not been stimulated; partial seizures occur in small area, one side of the brain, milder symptoms; generalized seizures affect larger areas, both sides of brain, loss of consciousness; causes are brain damages at birth (most common); metabolic disturbances (hypoglycemia, hypocalcemia, uremia, hypoxia); infections (encephalitis, meningitis); toxins (alcohol, tranquilizers, hallucinogens); vascular disturbances (hemorrhage, hypotension); head injuries; tumors and abscesses of the brain; fever in children; often no cause; antiepileptic drugs (phenytoin, carbamazepine, valproate sodium, implantable device that stimulates vagus nerve (CNX) great results, surgical intervention may be option in severe cases.

Describe Myelination and Grey and White Matter

Myelination: Two types of cells., Schwann Cells in PNS myelinate one axon segement between two nodes of Ranvier (gaps in a myelin sheath); begin myelinating axons during fetal development; Schwann cell wraps around the axon multiple times with glial plasma membrane closest to axon, middle layer are the Schwann cells membrane and outer layer is the Schwann cells cytoplasm and nucleus called a neurolemma. Neurolemma aids in regeneration of the axon using a regeneration tube that guides and stimulates regrowth of axon. Oligodendrocytes in CNS myelinates parts of several axons by sending out 15 broad flat processes that spiral around the CNS axons; no neurolemma because oligodendrocyte cell body and nucleus do not surround the axon; Nodes of Ranvier present but few in number; very little regrowth of an axon, possibly because of no neurolemma present and oligodendrocytes inhibit regrowth of axon. Gray Matter: Neuronal cell bodies, dendrites, unmyelinated axons, axon terminals, neuroglia; nissl bodies appear gray; blood vessels present White Matter: Myelinated axons

Describe the propagation of nerve impulses.

Nerve Impulse Propagation (or Conduction): Action potentials travel where they start at the trigger zone of the axon to the axon terminals. As they travel down the axon they maintain their strength, relying on a positive feedback mechanism. The action potential regenerates as it propagates (travels) from trigger zone to terminal along the entire length of the neuron for long distance communication. Neuronal propagation can only go in one direction, away from the cell body because the areas that have undergone an action potential is in an absolute refractory period. Continuous Conduction: Repolarization and Depolarization of each segment of the plasma membrane as in nerve impulse propagation. Occurs in unmyelinated axons and muscle fibers. Saltatory Conduction: Occurs along a myelinated axon from one Node of Ranvier to another. Electrical current flows through extracellular fluid surrounding the myelin sheath from one node to the next. An action potential propagates from one node to the next by sending an electric current (carried by ions) through extracellular fluid surrounding the myelin sheath and through the cytosol from one node to the next. Action potential at first node sends an ionic current to the next node that depolarizes the membrane to threshold causing the next node Na+ voltage gated channels to open. Each node depolarizes and then repolarizes as the action potential propagates down the axon. More energy efficient, less ATP is used by sodium-potassium pumps to maintain low intracellular concentration current seems to leap from one node to the next. Effect of Axon Diameter A fibers: Largest diameter axons; myelinated; brief absolute refractory period; action potentials 12-130 m/sec (27-290 mi/hr); sensory neurons propogate impulses for touch, pressure, position of joints, thermal and pain sensations. B fibers: Myelinated; saltatory conduction speeds up to 15 m/sec (34 m/hr); medium diameter (2-3um); somewhat longer refractory period than A fibers; sensory information from viscera to the brain and spinal cord; all autonomic motor neurons from brain and spinal cord to ANS relay stations in autonomic ganglia. C fibers: Smallest diameter (0.5-1.5 um); unmyelinated; nerve impulse propogation 0.5-2 m/sec (1-4 mi/hr); longest absolute refractory period; sensory impulses for pain, touch, pressure, cold, heat from skin, pain impulses from viscera; autonomic motor fibers that stimulate the heart, smooth muscle and glands. Motor functions in B/C fibers are constricting and dilating the pupils; increasing and decreasing the heart rate; contracting and relaxing the urinary bladder. Encoding of stimulus frequency: The nervous system can detect different sensations based on nerve impulse frequency that propagates down the axon and number of sensory neurons; light touch generates a low frequency nerve impulse with few sensory neurons stimulated; heavy pressure generates a higher nerve impulse frequency with more sensory neurons stimulated. Comparison of Electrical Signals Produced by Excitable Cells: Neurons and muscle fibers produce two types of electrical signals: graded potentials/action potentials (impulses) Graded potentials: travel over short distances because it is not propagated Action potentials: travel over long distances because it is propagated; propagation of muscle action along sarcolemma into the T-tubule system initiates muscle contraction. Typical resting membrane potential in neuron is -70mV and nerve impulse is 0.5-2 msec ; cardiac and skeletal muscle tissue is -90mV with a muscle action potential for skeletal muscle tissue 1.0-5.0 msec and cardiac muscle and smooth muscle action potential at 10-300 msec

Describe the steps in repair and regeneration of nervous tissue

Neurogenesis in the CNS: Formation of new neurons from undifferentiated stem cells; epidermal growth factor can help with renewal, tissue repair and regrowth in nonneuronal cells and has been proven to proliferate into neurons and astrocytes; new neurons form in the hippocampus that is crucial in learning. Plasticity: Capability to change based on experience; chemical and electrical signals causes growth of new dendrites, synthesis of new proteins, changes in synaptic contacts with other neurons. Regeneration: Capability for neuron to replicate or repair itself is limited; Repair possible in PNS if cell body in tact and schwann cells that myelinate are not damaged. Repair in CNS is limited because Oligodendrocytes myelinate the axons which prevents regeneration and astrocytes proliferate creating scar tissue. Damage and Repair in the PNS: Axons and dendrites associated with neurolemma can repair if cell body in tact, schwann cells are functional, rapid scar tissue does not occur; axon damages changes occur in cell body and portion of axon distal to injury and possibly in portion of axon proximal to site of injury Injury in normal peripheral neuron 1) 24-48 hours after injury, Nissl bodies break up into fine granular masses (Chromatolysis) 2) 3-5 days part of axon distal to injury becomes swollen, breaks up into fragments and myelin sheath deteriorates (Wallerian Schwann Degeneration), neurolemma remains in tact 3) Following chromatolysis recovery of cell body is occurring 4) Synthesis of RNA and proteins accelerates which helps with regeneration of axon 5) Schwann cells multiply by mitosis, grow toward each other and form regeneration tube across injured area; tube guides growth of new axon from proximal area to distal area; growth cannot occur if gap is too big or collagen fibers present in the gap 6) Axons invade tube formed by schwann cells from proximal area and grow at rate of 1.5mm per day, find way into distal regeneration tubes, grow towards distally located receptors and effectors; some function and sensory functions restored; schwann cells form new myelin sheath.

Describe the major functions of Neuroglia

Neuroglia or Glia: half of volume of CNS; glue that holds nervous tissue together; smaller than neurons; 5-25 times more than neurons; multiply to fill spaces in injury or disease; do not generate action potentials NEUROGLIA OF CNS Astrocytes: Star shaped cells; two kinds Protoplasmic Astrocytes (long branching processes found in gray matter) and Fibrous Astrocytes (long unbranched processes found in white matter); branches make contact with blood capillaries, neurons, pia mater (thin membrane in brain and spinal cord) Functions: 1) Blood brain barrier created from the astrocyte processes surrounding the blood capillaries, isolating neurons in the CNS, restricting movement of substances between blood and interstitial fluid. 2) Neuron support due to containing strong microfilaments 3) Secrete chemicals during embryonic growth that regulates growth, migration and interconnection between neurons and brain 4) Helps maintain proper chemical composition for nerve impulse generation; regulate appropriate ion concentration; take up excess neurotransmitters; serve as a pathway for nutrients and other substances between blood capillaries and neurons. 5) Influence the formation of neural synapses that helps in learning and memory Oligodendrocytes: Resemble astrocytes but smaller and contain fewer processes. Forms and maintains myelin sheath around CNS axons. Myelin Sheath: Multilayered lipid and protein covering surrounding and insulating some axons, increasing the conduction speed of nerve impulses. Microglia: Small cells, slender processes with numerous spine like projections; function as phagocytes; remove cellular debris formed during normal nervous system function and phagocytize microbes and damaged tissue. Ependymal Cells: Cuboidal to columnar cells that have microvilli and cilia; line ventricles of brain and central canal of spinal cord; protects and nourishes the brain and spinal cord; produce, possible monitor and assist in circulation of cerebrospinal fluid. NEUROGLIA OF PNS: completely surround axons and cells bodies Schwann Cells: Encircle PNS axons; form myelin sheath around a single axon at a time; can enclose 20 or more unmyelinated axons; participate in axon regeneration Satellite Cells: Flat cells, surround PNS neuronal cell bodies; provide structural support, regulates exchanges of materials between neuronal cell bodies and interstitial fluid.

Describe the classes and functions of neurotransmitters

Neurotransmitters: 100 substances known or suspected; bind to receptors to quickly open and close ion channels; slowly via second messenger systems to start a chemical reaction in a cell; either excitation or inhibition of postsynaptic neurons; can be released as hormones in blood via endocrine cells; neurosecretory cells in brain release hormones; classified into small neurotransmitters and neuropeptides. SMALL-MOLECULE NEUROTRANSMITTERS Acetylcholine: Released by PNS neurons and by some CNS neurons; binding to ionotropic receptors to open cation channels at the neuromuscular junction is excitatory; inhibitory when binding to metabotropic receptors coupled to G+ proteins that open K+ channels; acetylcholinesterase inactivates acetylcholine by breaking it down into acetate and choline fragments. Amino Acids Excitatory: Neurotransmitters in CNS; glutamate (used by most excitatory neurons in CNS and used by half the excitatory neurons in the brain) and asparate are excitatory. Binding of glutamate to ionotropic receptors opens cation channels causing an inflow of NA+ which causes a EPSP, reuptake of glutamate back into synaptic end bulbs and neighboring neuroglia inactivates it. Inhibitory: Gamma aminobutyric acid (GABA) and glycine. Gaba is used by 1/3 of brain synapses; only in CNS; binds to ionotropic receptors to open CL- channels. Glycine binding to ionotropic receptors opens CL- channels; 1/2 of inhibitory synapses in spinal cord us Glycine, rest use GABA. Biogenic Amines Certain amino acids are modified and decarboxylated (carboxyl group removed) to produce biogenic amines that bind to metabotropic receptors; may cause excitation or inhibition. Norepinephrine: Arousal from deep sleep, dreaming, regulating mood; used as a hormone; released by adrenal gland into blood. Epinephrine: serve as a hormone, released by adrenal gland into blood. Dopamine: In brain neurons; active during emotional responses, addictive behaviors, pleasurable experiences; regulate skeletal muscle tone; Parkinsons - degeneration of neurons that release dopamine causing muscular rigidity; one form of schizophrenia is caused by build up of dopamine in the brain. Catecholamines: Norepinephrine, Epinephrine and Dopamine; all include amino group (-NH2), catechol ring composed of 6 carbons and 2 adjacent hydroxyl (-OH)V groups; synthasized from amino acid tyrosine; reuptake into synaptic end bulbs causes inactivation, then recycled back into synaptic vesicles or destroyed by enzymes (catechol-o-methyltransferase or COMT and monooamine oxidase or MAO) Seratonin or 5-Hydroxytryptamine (5-HT): Concentrated in neurons in part of brain called raphe neucleus; involved in sensory perception, temperature regulation, control of mood, apetite and induction of sleep. ATP and other Purines Purine ring is characteristic structure of Adenosine portion of ATP; excitatory neurotransmitter in CNS and PNS, along with triphosphate, diphosphate, monophosphate; sympathetic neurons release ATP and norepinephrine in synaptic vesicles at the same time in PNS; parasympathetic neurons release ATP and acetylcholine in same vesicles. Nitric Oxide (NO) Simple gas; secreted by neurotransmitters in the brain, spinal cord, adrenal glands, nerves to penis, widespread throughout body; single nitrogen atom; nitric oxide synthase (NOS) catalyzes formation of NO from amino acid arginine; NO formed on demand and acts immediately, brief action; lipid-soluble and diffuses through cells that produce it and activates and enzyme for a second messenger cyclic GMP; causes vasodilation in blood vessels lowering blood pressure, helps with erectile dysfunction; released by some macrophages to kill microbes and invaders Carbon Monoxide Protect against excess neuronal activity, dilation of blood vessels, contribute to memory, olfaction, vision, thermoregulation, insulin release, anti-inflamamtory activity; produced on demand; excitatory neurotransmitter in the brain in response to neuromuscular and neuroglandular function. Neuropeptides 3-40 amino acids linked by peptide bonds; large amount and all over CNS and PNS; bind to metabotropic receptors both excitatory and inhibitory depending on metabotropic receptors at synapse; formed in neuron cell body, packaged into vesicles, transported to axon terminals; both neurotransmitters and regulate physiological responses throughout body Enkephalins: two molecules dynorphins and endorphins, each with five amino acids; potent analgesic 200 times stronger than morphine; improved memory and learning; feelings of pleasure or euphoria; control of body temperature; regulation of hormones that affect onset of puberty, sexual drive, reproduction; mental ilness such as depression and schizophrenia. Substance P: enhances perception of pain; released by neurons that transmit pain related input from PNS to CNS; enkephalin and endorphins hinders release of substance P; might be useful in nerve degeneration because it counteracts certain nerve-damaging chemicals.

Describe the factors that contribute to generation of a resting membrane potential.

Resting Membrane Potential: exists due to a small buildup of negatively charged ions inside the cytosol along inside of membrane and equal buildup of positively charged ions on outside surface of membrane; form of potential energy (measured in millivolts or volts) when there is a separation of positive and negatively charged ions; larger the membrane potential = greater difference in charge across the membrane. Three Factors that causes Resting Membrane Potential 1) Unequal Distribution of Ions in ECF and Cytosol: ECF rich in Na+ and Cl- ions; cytosol main cation is K+ and two dominant anions are Phosphates attached to molecules such as ATP and amino acids in proteins; more K+ leak channels diffuses K+ down concentration gradient out of cell than Na+ leak channels that diffuse down concentration gradient into the cell so inside of cell membrane becomes increasingly negative, and outside of membrane becomes increasingly positive. 2) Inability of Most Anions to Leave the Cell: Most anions cannot leave the cell because they are attached to nondiffusible molecules such as ATP and large proteins. 3) Electrogenic Nature of the Na+/K+ ATPases: Membrane permeability to Na+ is very low because there are few Na+ leak channels; the slow diffusion of Na+ into a cell is quickly counteracted by the Na+/K+ ATPase pumps that pump Na+ out of the cell as fast as it comes in, maintaining the resting membrane potential; Na+/K+ ATPase pumps are electrogenic because they expel 3 Na+ out of the cell for every two K+ imported, contributing to the negativity of the cell but only contribute -3mV of the total -70mV resting membrane potential in a typical neuron. Polarized: a cell that exhibits a membrane potential Graded Potential: Small deviation from membrane potential that makes the membrane more polarized (inside more negative) or less polarized (inside more positive); stimulus causes mechanically gated or ligand-gated channels to open or close in an excitable cell's membrane; graded means that it varies in size and amplitude, depending on strength of stimulus. Hyperpolarizing Graded Potential: More polarized or more inside more negative Depolarizing Graded Potential: Less polarized or inside less negative Decremental Conduction: Mode of travel which graded potentials die out as they spread along the membrane away from the stimulus source. Summation: Process by which graded potentials add together. If two graded potentials occur close to each other than the the result will be a larger depolarizing graded potential.

Functions of the Nervous System

Sensory: Detect internal stimuli such as stretching of blood vessels, or outside stimuli such as feeling a raindrop landing on the arm, these signals are sent back to the spinal cord and brain through spinal and cranial nerves Integrative Function: Nervous system analyzes and processes the sensory information and makes decisions for appropriate responses Motor Function: Effectors (muscles and glands) are activated in the glands and muscles after the responses have been integrated by the nervous system. Stimulation of effectors causes muscles to contract and glands to secrete.

Distinguish between spatial and temporal summation; and between excitatory and inhibitory neurotransmitters.

Spatial Summation: Summation of postsynaptic potentials in response to stimuli that occur at different places on the postsynaptic cell at the same time. Can result from a buildup of a neurotransmitter released at the same time by several presynaptic end bulbs. Temporal Summation: Summation of a postsynaptic potential that occurs from stimuli of the postsynaptic cell at the same location but at different times. Neurotransmitter is released from one presynaptic end bulb in rapid succession. Excitatory Post Synaptic Potential (EPSP) lasts about 15msec, the second neurotransmitter release must occur shortly after for Temporal Summation to occur. EPSP: Total excitatory effects are greater than the total inhibitory effects but is not great enough to reach threshold than it is EPSP that does not reach threshold. A second stimulus can more easily generate a nerve impulse because neuron is partially depolarized. Nerve Impulses: If total excitatory effects are greater than the inhibitory effects and the membrane potential reaches threshold, then one more action potentials will be generated as long as EPSP is at or above threshold level. IPSP: If total inhibitory effects are greater than the excitatory effects that the membrane potential will hyperpolarize causing inhibition of the postsynaptic neuron and an inability to produce a nerve impulse.

Table 12.4 Neuropeptides

Substance Description Substance P: Found in sensory neurons, spinal cord pathways, parts of brain associated with pain; enhances perception of pain. Enkephalins: Inhibit pain impulses by suppressing release of substance p; may have role in memory and learning, control of body temperature, sexual activity and mental illness. Endorphins: Inhibit pain by blocking release of substance Pl may have role in memory and learning, sexual activity, control of body temperature, mental illness. Dynorphins: May be related to controlling pain and registering emotions. Hypothalamic releasing and inhibiting hormones: Produced by hypothalamus; regulate release of hormones by anterior pituitary gland. Angiotensin II: Simulates thirst; may regulate blood pressure in brain. As a hormone, causes vasoconstriction and promotes release of aldosterone, which increases rate of salt and water re-absorption by kidneys. Cholecystokinin (CCK): Found in brain and small intestine; may regulate feeding as a "stop eating" signal. As a hormone, regulates pancreatic enzyme secretion during digestion, and contraction of smooth muscle in gastrointestinal tract.

Explain the events of signal transmission at synapse.

Synapse: region where communication occurs between two neurons or between a neuron and an effector cell (muscular or glandular cell). Axondendritic (from axon to dendrite) or Axosomatic (from axon to cell body) or Axoaxonic (from axon to axon). Presynaptic Neuron: a nerve cell that carries a nerve impulse toward a synapse; A cell that sends a signal. Postsynaptic Cell: receives a signal Postynaptic Neuron: neve cell that carries an nerve impulse away from the synapse or effector cell that responds to the impulse at the synapse. ELECTRICAL SYNAPSE: conduct directly between plasma membranes of adjacent neurons through structurs called Gap junctions. 1) Faster communication: action potentials conduct through gap junctions which is faster than chemical synapses (separated by a synaptic cleft); action potential passes directly from presynaptic cell to post synaptic cell. 2) Synchronization: Coordinate a group of muscle fibers or neurons and produce action potentials in unison; helps move food through GI tract or produce a heartbeat. Gap Junctions: contains a 100 or so tubular connexons, act like tunnels to connect the cytosol of two cells directly. Nerve impulse spreads from cell to cell through connexons; common in visceral smooth muscle, cardiac muscle, developing embryo, brain. CHEMICAL SYNAPSE: plasma membranes of presynaptic neurons and postynaptic neurons are close but don't touch. Synaptic Cleft: Neurons seperated by a space of 20-50 nm; filled with interstitial fluid; use of neurotransmitter that is released in the presynaptic neuron that diffuses through the fluid in the synaptic cleft, binds to receptors in plasma membrane of postsynaptic neuron. Postsynaptic Potential: type of graded potential; produced by a postsynaptic neuron after receiving a chemical signal. TRANSMISSION OF CHEMICAL SIGNALS AT CHEMICAL SYNAPSE: 1) Nerve impulse arrives at a synaptic end bulb (or varicosity) of presynaptic axon 2) Depolarizing phase of nerve impulse opens voltage gated Ca+ channels (present in the membrane of synaptic end bulbs); calcium concentrated in extracellular fluid, Ca+ flows inward through open channels 3) Increase of concentration of Ca+ inside the presynaptic neuron triggers exocytosis of the synaptic vesicles. Merging of the synaptic vesicle membrane and plasma membrane of the presynaptic neuron causes the release of the neurotransmitters from the synaptic vesicles into the synaptic cleft. Each synaptic vesicle holds several thousands of neurotransmitters. 4) Neurotransmitter molecules travel across the synaptic cleft to fuse with the neurotransmitter receptors in the postsynaptic neuron's plasma membrane. 5) Binding of the neurotransmitter molecules to receptors on ligand-gated channels, opens the channels and allows particular ions to flow across the membrane. 6) Flow of ions across the membrane through the open voltage gated channels causes either a depolarizing (Na+ channels open and Na+ flows into the cell) or hyperpolarizing (Cl- or K+ channels open and Cl- flows into the cell and K+ flows out of the cell) post synaptic potential. 7) Action potential in the axon postsynaptic neuron is triggered when the depolarizing postynaptic potential reaches threshold. Postsynaptic Potential: Change in membrane voltage from the flow of ions through the open voltage gated channels. Transmission of Signals at a Chemical Synapse: One-way information transfer Presynaptic Neuron -> Postynaptic Neuron Presynaptic Neuron -> Effector (Muscle Fiber or Gland Cell) Excitatory Postsynaptic Potential (EPSP): A neurotransmitter that causes the membrane potential to depolarize, brings the membrane closer to threshold; Single EPSP may not bring membrane to threshold but will bring it closer that the next may. Inhibitory Postsynaptic Potential (IPSP): Neurotransmitter that causes the membrane potential to hyperpolarize making an action potential more difficult because the membrane potential is inside more negative. Removal of a Neurotransmitter: Removal is essential for normal synaptic function. 1) Diffusion: Neurotransmitter molecules diffuse away from the synaptic cleft, once away from its receptors it can no longer have an effect. 2) Enzymatic Degradation: Enzymes break down neurotransmitters. Acetylcholinesterase breaks down Acetylcholine. 3) Uptake by cells: Neurotransmitter transporters take up neurotransmitters back into the cells that released them to be used again in synaptic vesicles, or transported into neighboring neuroglia.