NervousSystem - Nervous System Functions • Regulates...

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Unformatted text preview: Nervous System Functions • Regulates internal body metabolism (internal environment) – body temp, urine volume, blood volume, gas exchange, circulation, movement • Llnk to the external environment- (interpreter) • sensory devices- sight, hearing, taste, smell, touch (pressure, pain, hot, cold) • emotional response to external stimuli (these drive desire to satisfy physical needs to preserve homeostasis) – hunger, thirst, temperature, rest, sexuality The Two Types of Neural Pathways • Sensory (afferent) nerves transmit messages from sensors to CNS – sensory receptors located the ends of peripheral nerves that detect external and internal environmental changes and relays info to brain • • • • light and sound intensity touch- changes in skin pressure temperature variation oxygen concentration in fluids of the body – impulses are brought to CNS where they are integrated to create sensations – Conscious or subconscious decisions are made and acted upon by motor functions • Motor (efferent)- nerves transmit messages from CNS to effectors – muscles that contract – glands that secrete Nervous Tissue • Central- consist of the brain and the spinal cord – incoming sensory signals from PNS travel to the CNS for processing – out going signals to from CNS to PNS • Peripheral- connects the sensory receptors,glands, and muscles in the peripheral part of the body to the CNS Peripheral NS • consist of the cranial nerves (derived from the brain) and the • spinal nerves (emerge from the spinal cord). Peripheral Nervous System Types of Neurons • Input components consist of sensory (afferent) neurons. • The output component consist of motor (efferent) neurons. • Interneurons (association) neurons carry nerve impulses from sensory to motor neurons and are located in the CNS. Peripheral Nervous System Sub divided into • Somatic nervous system – consist of sensory neurons that convey information from cutaneous and special sense receptors primarily in the head, body wall, and limbs to the CNS and – motor neurons from the CNS that conduct impulses to skeletal muscles only. • is voluntary Peripheral Nervous System • Autonomic nervous system – consist of motor neurons from the CNS that conduct impulses to smooth muscle, cardiac muscle, and glands. – Is involuntary Consist of two branches • sympathetic- controls those process that tend to expend energy (speedup heart beat). • parasympathetic- control those process that tend to conserve energy (slow the heart beat). Nervous System Histology Consist of two types of neurons • Neuroglia- serve to support and protect the neurons. – generally smaller than neurons and outnumber neurons 5-50 times. • Neurons- the main cell of the nervous system that is specialized for nerve impulse conduction. Neuroglia • Central Nervous System – – – – Astrocytes Oligodendrocytes Microglia cells Ependymal cells • Peripheral neuroglia – schwann cells (neurolemmocytes) – satellite cells Central Nervous System Neuroglia Central Nervous System Neuroglia • Astrocytes – are the most abundant and functionally diverse – Help form the blood brain barrier with cytoplasmic extensions called perivasular feet that stimulate the endothelial cells to from tight junctions – convert glucose to lactate for neuronal nourishment – Participate in the metabolism of neurotransmitters – form brain scar tissue (astrocytosis, gliosis sclerosis) Astrocytes form end feet which form a barrier to materials passing into the CNS from the meninges. These end feet fuse with the innermost layer of the meninges called the pia mater and are called subpial foot processes. Blood Brain Barrier • Regulates the passage of substances into the brain – Capillary wall – Astrocyte processes Central Nervous System Neuroglia • Oligodendrocyte – Form myelin sheath around CNS neurons Oligodendrocytes forms myelin in the CNS. • Myelin is deposited with no neurolemma formed. • Axons in the CNS display little regrowth after injury which is thought to be partly because of the lack of a neurolemma. Central Nervous System Neuroglia • Microglia – Phagocytic cells that engulf destroy microbes and cellular debris in the CNS – active in response to inflammation or injury Microglia Astrocyte & Microglia Central Nervous System Neuroglia • Ependymal Cells– Line the ventricles (spaces filled with CSF) of the brain and the central canal of the spinal cord – Produce cerebrospinal fluid – Assist in the circulation of the CSF Peripheral Nervous System Neuroglia • Schwann cells (neurolemmocytes) Each cells produces part of the myelin sheath around a single axon in the PNS. – Neurolemma (sheath of Schwann)- the outer nucleated cyotplasmic layer of the neurolemmocyte. – Nodes of Ranvier (neurofibral nodes)- periodic gaps in the myelin sheath. Myelinated Axons Satellite Cells • surround the neuron cell bodies in ganglia of the PNS. Little is known of their function. Neuron The basic information processing unit of the nervous system that conducts impulses from one part of the body to another. • Structure – have 3 distinct parts • cell body • Dendrites • Axon Neuron Cell Body (soma, perikaryon)contains a nucleus with a prominent nucleous surrounded by a granular cytoplasm and various organelles. •does not contain a mitotic apparatus as a result, neurons do not divide •has distinctive cytoplasmic feature called Nissl bodiesrough ER formed into large bodies seen in the light microscope as dark staining granules Lipfuscin- golden brown pigmented cytoplasmic inclusion • an end product of lysosomal digestion of worn-out organelles and other products (wear and tear granules) •collects with age and pushes the nucleus aside •abundant in all neurons and re harmless Cytoskeleton Has an extensive network of microtubles, microfilaments,and neurofilaments • Microtubles- tracks along which organelles are transported • Microfilaments-actin, located under plasma membrane, contractile, transport materials down axon • Neurofilaments- maintains shape of cell, main support for cell, neurofibrils (bundles of neurofilaments Dendrites- short, thick, highly branched cytoplasmic process that functions to receive impulses and conduct them toward the cell body. •The more dendrites a neuron has, the more information in can receive and incorporate into its decision making. Axon- a single long, thin extension that sends impulses to another neuron or tissue. • Axon hillock- where axon originates (does not contain Nissl bodies) • Vary in length from .04 inches to 3.28 feet • axon collaterals- side branches along the length of the axon • axon terminals- many tiny end filaments derived from branching of axon and collaterals • synaptic bulbs (knob)- bulb-like structures at the end of axon terminals which contain synaptic vesicles that store neurotransmitters. •cytoplasm called axoplasm •limiting membrane called axolemma Most axons are surrounded by a many layered white, lipid and protein covering produced by neuroglia called myelin sheath. It electrically insulates the axon and increases the speed of nerve impulse conduction. Axons with myelin sheaths are said to be myelinated and those without are unmyelinated. Axonal transport •fast axonal transport, 400mm/day •anterograde- motor protein: kinesin •retrograde- dynein •Slow axonal transport, 10mm/day, also called axoplasmic transport all ways anterograde Axonal transport • Fast axonal transport, 400mm/day -anterograde- motor protein: kinesin -retrograde- dynein •Slow axonal transport, 10mm/day, also called axoplasmic transport all ways anterograde •motor proteins carry materials “on their backs” while they reach out, like myosin heads of muscle, to bind repeatedly to the microtubules and crawl along them. Grouping of Neural Tissue Nerve fiber- any process projecting from the cell body (dendrite or axon). Nerve- a group of myelinated nerve fibers in the PNS. most nerves contain both sensory and motor fibers Peripheral Nerve Peripheral Nerve Grouping of Neural Tissue White matter- a group of myelinated axons from many cell bodies Tract- indicates a bundle of fibers located in the CNS. may run long distances up and down the cord or connect parts of the brain with each other. • ascending tracts- tracts that conduct sensory impulses up the cord. • descending tracts- tracts that carry motor impulses down the cord. Grouping of Neural Tissue Gray matter- neurons and there cell bodies that do not contain myelin Ganglia- groups of neuron cell bodies located in the PNS. Nuclei- groups of neuron cell bodies and dendrites in the CNS. Horns - the section of the spinal cord where gray matter is located Columns- groups of white matter tracts in the cord. Nerve Impulses Tiny electric currents that pass along neurons • result from the movement of ions in and out through the plasma membrane. Nerve Impulses Ion Channels • the membrane has a variety of ion channels formed by membrane proteins • most channels are subject to regulation where they are either open (conducting) or closed (nonconducting). Neural Impulses • the protein gate controls the passage of ions by conformational changes in response to various signals. – ion concentration across the channel – electrical impulses – neurotransmitters and hormones Neural Impulses Membrane Potentials • in a resting membrane their is a difference in the electrical charge between the inside and outside of the neuron. – the outside is positive and the inside is negative – this occurs because of the differences in the relative concentrations of K+ and negatively charged proteins in side the cell and the Na+ outside the cell. • the resting charge of a cell at rest is -70 millivolts • A cell that has a resting membrane potential is said to be polarized Neural Impulses • Sodium/Potassium pump- maintains the K+ and Na+ concentrations (thus charge difference or resting membrane potential) across the plasma membrane (Polarized). Action Potential • Is a sequence of rapidly occurring events that decrease and reverse the membrane potential and then eventually restore it to the resting state. • Depolarization- the loss and reversal of polarization due to the rapid opening of sodium ion channels. • Repolarization- the recovery of the resting membrane potential due to the slower opening of potassium ion channels and the closing of sodium ion channels. • Depolarization and repolarization comprise a nerve impulse and takes only 1 millisecond Initiation of a Nerve Impulses • Excitability- the ability of nerve cells to respond to stimuli and convert them into nerve impulses. • Stimulus- anything in the environment capable of changing the membrane resting potential. Initiation of Action Potential • Caused by the release of neurotransmitter – Causes change in membrane potential Propagation of Action Potential Propagation of Action Potential Continuous Conduction Depolarization Hyperpolarization Action Potential Refractory Period Threshold Stimulus Subthreshold Stimulus All or None Principle Continuous Conduction Local Potentials • Response of the neuron begins at the dendrites • Na+ ligand channels are opened by stimulus causing local depolarization (local potential) This is not an action potential Characteristics that distinguish local potentials from the action potential • Local potentials are graded- meaning that they vary in magnitude (voltage) according to the strength of the stimulus – more intense or prolonged stimulus opens more ion gates than weaker stimulus, thus more Na+ enters the cell and the voltage changes more than it does a weaker stimulus Characteristics that distinguish local potentials from the action potential • Local potentials are decremental- meaning they get weaker as they spread from point of stimulation – due to the spreading out of the Na+ under the plasma membrane Characteristics that distinguish local potentials from the action potential • Local potentials are reversible- meaning if stimulation ceases, K+ diffusion out of the cell quickly returns the membrane voltage to its resting potential. Characteristics that distinguish local potentials from the action potential Local potentials can be either • excitatory- Na+ flowing in • inhibitory– produced by opening Cl- channels to Cl- to flow in making the inside more negative – opening K+ channels Summation • a neuron receives input from thousands of presynaptic neurons simultaneously of which can be a mixture of IPSP or EPSP • summation is the process of adding up postsynaptic potentials and responding to their effects Temporal Summation • when a single synapse generate EPSP at such short time intervals that each is generated before the previous decays • This allows the EPSP to add up over time to a threshold that triggers an AP Spatial Summation • when EPSP’s from several different synapses add up to threshold at the axon hillock – any one synapse may admit only a moderate amount of Na+ but several synapses acting together admit enough to reach threshold Facilitation • a process in which one neuron enhances the effect on another one – ex. one neuron acting alone is not enough to cause a AP but when combined with the effects of another neuron, then it maybe enough to cause firing. Presynaptic Inhibition • the opposite of facilitation: when one presynaptic neuron suppresses another one Action Potential • A more dramatic change produced by voltage-gated ion channels in the plasma membrane – occurs where there is a high enough density of channels • soma 50-75 channels per square micrometer • trigger zone- 350-500 gates per square micrometer – if excitatory local potential spreads to the trigger zone and is strong enough when it arrives (reaching threshold), it can open Na+ gates and generate an AP • Threshold- minimum level of depolarization to cause an AP inhibitory AP in muscle Synapses Axodendritic Axosomatic Axoaxonic •Nodes of Ranviergaps between myelinated segments •Internodes- segments of myelin between nodes •initial segmentsection of nerve between axon hillock and first glial cell •trigger zone- segment where nerve signal is initiated (initial segment and axon hillock PNS Unmyelinated Fibers Neural Conduction in Mylinated Fibers: Saltatory Conduction Speed of Nerve Impulses The speed of a nerve impulse is determined by • temperature- speed is faster at warmer temperatures • fiber diameter- speed is faster in larger diameter fibers due to larger surface area for conduction • presence or absence of myelin- speed is faster in myelinated fibers (saltatory conduction) – large fibers are myelinated – smaller fibers are unmyelinated A regeneration tube or guidance channel is necessary for nerve regrowth and is formed by the neurilemma and endoneurium Neurotransmitter Criteria • The chemical must be produced within a neuron. • The chemical must be found within a neuron. • When a neuron is stimulated (depolarized), a neuron must release the chemical. • When a chemical is released, it must act on a post-synaptic receptor and cause a biological effect (alter the physiology of that cell). • After a chemical is released, it must be inactivated. Inactivation can be through a reuptake mechanism or by an enzyme that stops the action of the chemical. • If the chemical is applied on the post-synaptic membrane, it should have the same effect as when it is released by a neuron. Types of Neurotransmitters Three major categories • Acetylcholine • Amino Acids – – – – GABA Glycine Aspartic acid Glutamic acid • Biogenic amines (monoamines) – Catecholamines • epinephrine • norepinephrine • dopamine – Serotonin – Hisamine • Neuropeptides -Some neurotransmitters are excitatory and some are inhibitory -for some the effect depends on what kind of receptor the postsynaptic cell has -some are ligand gated ion channels and some act through the second messenger systems Synapse Excitatory Cholinergic synapse • employs acetylcholine as its neurotransmitter (can be excitatory or inhibitory) – opens Na+ channels Inhibitory GABA-ergic synapse • opens Cl- channels causing a hyperpolarization Adrenergic excitatory synapse • employs norepinephrine • acts through second messenger systems such as cyclic AMP • the receptor is not an ion gate but an integral protein associated with G protein 1. the binding of NE activates the G protein 2. which activates adenylate cyclase 3. which converts ATP to cAMP 4. cAMP can have multiple effects such as stimulating the synthesis of new enzymes Inactivation of Neurotransmitters The action of neurotransmitters can be stopped by four different mechanisms 1. Diffusion: the neurotransmitter drifts away, out of the synaptic cleft where it can no longer act on a receptor. Inactivation of Neurotransmitters • 2. Enzymatic degradation (deactivation): a specific enzyme changes the structure of the neurotransmitter so it is not recognized by the receptor. For example, acetylcholinesterase is the enzyme that breaks acetylcholine into choline and acetate. Inactivation of Neurotransmitters • 3. Glial cells: astrocytes remove neurotransmitters from the synaptic cleft. Inactivation of Neurotransmitters • 4. Reuptake: the whole neurotransmitter molecule is taken back into the axon terminal that released it by endocytosis and then is broken down by an enzyme called monoamine oxidase (MNO – some antidepressant drugs work by inhibiting MAO • This is a common way the action of norepinephrine, dopamine and serotonin is stopped...these neurotransmitters are removed from the synaptic cleft so they cannot bind to receptors. Summary of the major biochemical effects that drugs may have at the • • • • • • • • • • • • synapse: Precursor compounds Synthesis blockade Transmitter depletion Prevention of release Receptor inhibition Mimicking Inactivation blockade Reuptake blockade False transmitters (+) False transmitters (-) Conduction blockade ...
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