Nervous System Basics PDF

A&P2 Review Sheet Kimberly Denise Vigee’, N.D. THE NERVOUS SYSTEM BASICS There are 2 control centers in the body that regulate homeostasis: Nervous and Endocrine 1. Nervous System - PRIMARY one; targets specific areas; very fast response; secretes neurotransmitters 2. Endocrine System - produces...

A&P2 Review Sheet Kimberly Denise Vigee’, N.D. THE NERVOUS SYSTEM BASICS There are 2 control centers in the body that regulate homeostasis: Nervous and Endocrine 1. Nervous System - PRIMARY one; targets specific areas; very fast response; secretes neurotransmitters 2. Endocrine System - produces a general effect; slow response; secretes hormones 3 Major Functions of the Nervous System: 1. Sensation – the body has millions of sensory receptors to detect change (a stimulus) that occur both inside and outside the body a. Examples of External Stimuli: ~ Temperature The body will do ~ Light everything possible to ~ Sound maintain homeostasis! b. Examples of Internal Stimuli: ~ Internal environment - hormone regulation ~ Pressure - blood pressure ~ PH - blood acidity ~ CO2 ~ Electrolytes - Na+, K+, Ca+2 * Note: Both external and internal stimuli are called sensory input. 2. Integration – when sensory input is converted into electrical signals called nerve impulses that are transmitted to the brain. * After information is sent to the brain, it interprets whether the information will be a thought, sensation or memory. 3. Action – Based on the sensory input and integration, the nervous system responds by sending signals to muscles or glands. * Glands or muscles: referred to as effectors because they cause an action or motor output. Organization of the Nervous System - There are 2 principal divisions of the Nervous System: CNS and PNS 1. Central Nervous System - Controls center for the “entire” Nervous System. * CNS consists of only 2 things: Brain and Spinal cord 1 A&P2 Review Sheet Kimberly Denise Vigee’, N.D. * Brain and spinal cord will be discussed further in the respectable chapters. 2. Peripheral Nervous System * PNS consists of 2 organs: Nerves and Ganglia 1. Nerves coming off of the brain and spinal cord a. 12 pairs of Cranial nerves - emerge from the brain and transmit information to an organ, gland or muscle b. 31 pairs of Spinal nerves – emerge from the spinal cord and transmit information to an organ, gland or muscle Note: Nerves are bundles of nerve fibers – just like muscles are bundles of muscle fibers Note: The cranial nerves will be discussed in more detail in the brain and cranial nerve chapter. 2. Ganglia: - defined as a group of nerve cell bodies that look like knots * The PNS is divided into 2 parts: Somatic and Autonomic Nervous Systems 1. Somatic nervous system 2. Autonomic nervous system Somatic Nervous Systems -The somatic nervous system is also known as the voluntary nervous system. -This system controls movements of skeletal (voluntary) muscles. - Neurotransmitter for the somatic nervous system - acetylcholine Autonomic Nervous System -The involuntary nervous system (autonomic nervous system) maintains homeostasis. - ANS is the portion of the nervous system that regulates the activities of 3 things: * Smooth Muscle * Cardiac Muscle * Glands (certain ones) - What 3 parts of the brain relate the ANS? * Cerebral Cortex 2 A&P2 Review Sheet Kimberly Denise Vigee’, N.D. * Hypothalamus * Medulla Oblongata -The ANS is divided into: 1) Sympathetic 2) Parasympathetic systems. The Sympathetic Nervous System participates in a Fight or Flight response: - Examples of Fight or Flight response: 1) Increased heart rate 2) Increased force of contraction of heart 3) Dilation of vessels to the heart 4) Pupils dilate 5) Blood vessels of the skin and viscera constrict 6) Rapid and deeper breathing occurs 7) Bronchioles dilate to allow faster movement of air in and out of the lungs. 8) Blood sugar level rises as liver glycogen is converted to glucose to supply the body's additional energy needs. 9) The medullae of the adrenal glands are stimulated to produce epinephrine and norepinephrine, hormones that intensify and prolong the sympathetic effects noted previously. 10) Processes that are not essential for meeting the stress situation are inhibited. 11) Digestion stops - Neurotransmitters for the sympathetic nervous system – acetylcholine, epinephrine, and norepinephrine Divided into 2 groups: 1. Cholinergic fibers (acetylcholine) - produce short-lived & local responses 2. Adrenergic fibers release norepinephrine. Parasympathetic Nervous System: - Secretion and Receptors of the Parasympathetic Nervous System 3 A&P2 Review Sheet Kimberly Denise Vigee’, N.D. * Acetylcholine = Nicotinic and Muscarinic receptors * Nicotinic receptors - produce actions similar to that of nicotine * Muscarinic receptors - produce actions similar to that of the muscarine, toxic mushroom. - Parasympathetic nervous system = Energy conservation System * Exact opposite reactions of the sympathetic nervous system * Example: digestion & GI tract dominate over sympathetic stimulation. Nervous Tissue is composed of 2 Types of Cells: Neuroglia and Neurons - This tissue is derived from the ectoderm. Recap: Have you forgotten the embryonic germ layers? Embryonic Germ Layers: Develop the following Outer Ectoderm Epidermis Nervous Tissue Sense organs Middle Mesoderm Connective Tissue Muscle Tissue Bone Urogenital System Circulatory System Inner Endoderm GI Tract Lungs and associated structures 4 A&P2 Review Sheet Kimberly Denise Vigee’, N.D. *** Student should make a list comparing the characteristics of neuroglia and neurons! 1. Neuroglia (Glial Cells) – “nerve glue” Neuro = “nerve” Glia = “glue” - Non conductive portion of nervous tissue - Provide a support and protective system for the neurons - Significantly more numerous than neurons - Mitotic - 6 Glial Cell Types * Mneumonic for 6 different cell types: SOS: Make Every Attempt 1.Astrocytes – Appearance: Star shaped; numerous radiating processes with bulbous ends for attachment Functions: Binds blood vessels to nerves, regulate the composition of fluid around neurons 2. Ependymal cells – Appearance - Columnar cells with cilia Functions - Activate role in formation and circulation of cerebrospinal fluid 3. Microglia - Appearance: modified macrophages Functions: Protection, become mobile and phagocytotic in response to inflammation 4. Oligodendrocytes - Appearance: processes that wrap around axons Function: Form myelin sheaths around axons in the CNS 5. Schwann cells - Appearance: wraps around an axon in the PNS Function: Form myelin sheath around axons in the PNS, activate role in nerve fiber regeneration 6. Satellite cells - Appearance: similar to schwann cells Function: Support nerve cell bodies within ganglia Note: Some authorities do not consider schwann cells and satellite cells to be neuroglia because they are located in the PNS, rather than the CNS like the others. 5 A&P2 Review Sheet Kimberly Denise Vigee’, N.D. Note: The blood-brain barrier * What makes up the blood brain barrier? Neuroglia, particularly astrocytes, form a wall around the outside of the blood vessels in the nervous system. * Substances that pass through the BBB: Water, oxygen, carbon dioxide, caffeine, alcohol, nicotine, heroine * Substances that do not pass through the BBB: Toxins, pathogens, proteins, antibodies and certain drugs i.e. penicillin 2. Neurons (Nerve cells) - Structural and functional portion of the nervous system. - Conductive and excitable portion of nervous tissue - Neurons can communicate with other neurons, with muscles, and with glands. Each neuron is thought to make synaptic contact with at least 100,000 other neurons. - Highly specialized and amitotic (possesses no centrioles), if a neuron is destroyed, it cannot be replaced. - Neurons receive and integrate signals at one location and transmit an action potential at another location. Where do they receive, integrate and transmit theses action potentials? Cell body, axon, and dendrites. - 3 Basic parts of a Neuron: 1. Cell body or Soma - Located centrally - Has a nucleus with at least one nucleolus - Contain many of the typical cytoplasmic organelles, except centrioles = amitotic. - Cell body is the main nutritional and metabolic region of the neuron, it receives signals from other cells and sends them toward the axon 2. Dendrites (1 or more) - Usually, but not always, dendrites are multiple, short branching processes extended from the soma. This extensive branching from the dendrites increase the surface area to receive signals from other neurons. - Dendrites receive signals coming in from other cells and send them toward the axon - They are also called afferent processes because they transmit impulses to the neuron cell body 6 A&P2 Review Sheet Kimberly Denise Vigee’, N.D. - The dendrites and the cell body constitute both the receptive and integrative regions of the neuron. In contrast, the axon is the transmitting or conducting region of neuron. 3. Axon (only 1) - Cytoplasmic extensions that project from the soma (cell body) - It is called an efferent process because it carries impulses away from cell body - Axons can be short or it can be very long. In general, the longest axons are associated with the largest cell bodies. - Each neuron has a single axon, which arises from the cell body at a region called the axon hillock. Remember that dendrites are multiple. - Axon can branch, forming axon collaterals - At their ends, axons can branch into thousands of axon terminals (AKA telodendria) - The action potential is generated at the axon hillock and conducted along the axon until it reaches the axon terminals. - Synpatic end bulbs are enlarged and possess vesicles that store neurotransmitters. - Some axons are covered with an insulating material called a myelin sheath. Myelin is a phospholipid that is white in color. The CNS possesses both white matter and gray matter. White matter is myelinated and gray matter is unmyelinated. - Nodes of Ranvier – an interruption in myelination on the axon - Depending on the location of the axon, myelin can be produced by 2 different cells: 1. PNS axon – schwann cells produce myelin PNS axon (only) contains a neurilemma. The neurilemma is formed by the process of the cytoplasm, nucleus, and outer membrane of the schwann cells tightly covering around the myelin and axon itself at the nodes of ranvier. * Neurilemmas play a very important role in regeneration of nerve fiber. 2. CNS axon - oligodendrites produce the myelin but there is no neurilemma. * Since a CNS axon has no neurilemma this is why the nerve fibers in the CNS can’t regenerate. * See figure 16.3a, b & 16.6 for the 3 basic structures of neuron, i.e. cell body, dendrites, and axon 7 A&P2 Review Sheet Kimberly Denise Vigee’, N.D. (3) Functional Classifications of Neurons 1. Afferent (Sensory) - Transmits impulses from peripheral sense receptors to the central nervous system - Possess really long dendrites to reach many areas 2. Efferent (Motor) - Transmits impulses from the central nervous system to effectors organs such as muscles and glands - Possess short dendrites b/c it only needs to reach a specific area. 3. Association (Interneurons) - Transmits impulses from afferent neurons to efferent neurons (3) Structural Classification of Neurons: 1. Multipolar Neurons - several dendrites and 1 axon; found in brain & spinal cord. i.e. used in thinking 2. Bipolar Neurons - 1 dendrite and 1 axon; i.e. used in sensory organs (hearing taste, etc.) 3. Unipolar Neurons - only 1 process extending from 1 cell body; i.e. used in afferent fibers Nerve Impulse or Nerve Action Potential - The functional characteristics of neurons are: 1. Excitability- the ability to respond to a stimulus. 2. Conductivity- the ability to transmit an impulse from one point to another * These 2 functions are the result of the structures on the cell membrane. - A nerve impulse depends on the movement of sodium, potassium and other ions between interstitial fluid and intracellular fluid. - Cell At Rest = Resting Membrane * A resting cell has a nonconducting membrane (resting membrane). 8 A&P2 Review Sheet Kimberly Denise Vigee’, N.D. * The resting membrane does not allow (impermeable to) passive diffusion of 2 ions: sodium and potassium. * In order to move ions in and out of the cell, an active pump is required. The pump is referred to as the Sodium – Potassium Pump. This pump maintains the concentrations of sodium and potassium on the inside and outside of the cell * The result of the Sodium – Potassium Pump: cell membrane becomes polarized with more sodium ions (positively charged) on the outside of the cell and more potassium ions (positively charged) on the inside of the cell. The inside of the cell also contains proteins and negatively charged ions. * Final end result = The inside of the cell is now more negative (-70 millivolts) than the outside of the cell. The differences in charges from the 2 sides of the resting membrane is referred to as the Resting Membrane Potential. - Stimulation of a Neuron: * A stimulus is a physical, chemical, or electrical event that alters the neuron cell membrane and reduces its polarization for a brief time. * Sequence of Events: 1. Depolarization: When a stimulus is applied, the membrane now becomes permeable to sodium (positively charged), allowing it into the cell and altering the cell charge from more negative to more positive. - Movement of sodium into the cell is driven by a concentration gradient and an electrical gradient. Remember the inside of the cell is negative, which attracts the positively charged sodium ions. Note: If the stimulus is weak, the membrane is only slightly permeable and the inward movement of sodium is offset by an outward movement of potassium. : Threshold Stimulus – the minimal stimulus necessary to initiate an action potential : Subthreshold Stimulus – a weaker stimulus that does not cause sufficient depolarization to elicit an action potential - This process is termed depolarization because at rest the inside of the cell in more negative than the outside. 2 extremes in charge (polar extremes): positive and negative. When the positive sodium rushes in, the membrane is now at a lesser extreme (depolarized) Note: Some anesthetics produce their effects by inhibiting the diffusion of sodium into the cell membrane and blocking the initiation and conduction of nerve impulses. 2. Repolarization – 9 A&P2 Review Sheet Kimberly Denise Vigee’, N.D. - Sodium ions are trapped in the cell because the sodium channels are closed (impermeable membrane). Next the potassium channels open and the membrane becomes permeable to potassium (positively charged) allowing potassium to move outside the cell. - What has happened? The membrane is polar again. The inside of the cell is -70 millivolts Conduction of an Impulse Along a Neuron - Once a sufficient stimulus is applied and the action potential has been generated, it must propagate down the entire length of the neuron. The nerve impulse will then go to either another neuron or to an effector (muscle or gland). * The sequence of events listed above represent actions of an unmyelinated axon. Because myelin is an insulating substance, it inhibits the flow of current. - Saltatory Conduction: * In myelinated axons, depolarization occurs only at the places where there is no myelin – Nodes of Ranvier. The action potential jumps from node to node. This action of jumping is referred to as “Saltatory Conduction”. * Note: Saltatory conduction occurs only in unmyelinated portion of axons. - Refractory Period: * Refractory period - the period of time in which a point on the cell membrane is “recovering” from depolarization. While the membrane is permeable to sodium ions, it can’t respond to a second stimulus, no matter how strong. - All or None Principle * Just like muscle fibers, nerve fibers obey the “all or none principle”. * If a sufficiently strong stimulus is applied an action potential is generated and it propagates down the entire length of the neuron at maximum strength and speed for the existing conditions. * A stronger stimulus does not increase the strength of the action potential or change the rate of conduction. * A weaker stimulus (subthreshold) will not conduct an action potential. Speed of a Nerve Impulse 10 A&P2 Review Sheet Kimberly Denise Vigee’, N.D. - The speed is determined by the size, type, myelin and physiological condition of the nerve fiber. Example: the larger the diameter the greater the speed; sciatic nerve is the largest. Conduction Across Synapses - Synapse - a junction between cells * Neuromuscular Junction - Impulses from a neuron to a muscle fiber (cell) * Neuroglandular Junction - Impulses from a neuron to glandular cell - Structures of Synapse: 1. Synaptic End Bulb - tiny bulges at the end of presynaptic neuron 2. Synaptic Vesicles - small sacs within the synaptic end bulb containing neurotransmitters. 3. Synaptic Cleft - a fluid filled place that separates pre & post synaptic neurons 4. Postsynaptic Membrane - recipient - Conducting a nerve impulse across a synapse = Synaptic Transmission: Series of Events in Synaptic Transmission: 1. In neuron #1 (presynaptic neuron) the nerve impulse propagates down the entire axon until it reaches the synaptic end bulb. 2. After the nerve impulse reaches the synaptic end bulb, calcium enters the cell and mediates the release the neurotransmitters into the synaptic cleft. 3. The neurotransmitters diffuse across the cleft and react with the receptors on the neuron #2 (post synaptic neuron) opening ion channels. * Note: To prevent prolonged reactions with the postsynaptic receptors, the neurotransmitters are inactivated by enzymes. Example of a neurotransmitter = acetylcholine Example of a enzyme for acetylcholine = acetylcholineresterase - Synaptic Transmission may either be excitatory or inhibitory: 1. Excitatory Transmission - the neurotransmitter receptor reaction on the postsynaptic membrane depolarizes (sodium influx) the membrane and initiates an action potential. This is called excitation or stimulation. 2. Inhibitory Transmission- the reaction between the neurotransmitter and the receptor opens potassium channels in the membrane so that potassium diffuses out of the cell, but has no effect on the sodium channels. This makes the inside of the membrane even more negative than it is in resting condition. It hyperpolarizes the membrane, which makes it more difficult to generate an action potential. This is called inhibition. 11 A&P2 Review Sheet Kimberly Denise Vigee’, N.D. - 2 Types of Synapses: 1. Electrical Synapses : Nerve impulses pass from one neuron to another through small, tubular, protein structures called Gap Junctions (bridges) found between visceral (smooth) and cardiac muscle fibers. 2. Chemical Synapses : When the impulse reaches the presynaptic axonal end bulb, calcium enters the cell and mediates neurotransmitter release. Neurotransmitters diffuse across the synaptic cleft and attach to postsynaptic membrane receptors, opening ion channels. : At a chemical synapse, the conduction travels from a presynaptic axon to a postsynaptic cell – because only synaptic end bulbs of presynaptic neurons can release neurotransmitters. They can not back up - it would seriously disrupt homeostasis. Note: An electrical synapse can go both ways. Neurotransmitters - Common Neurotransmitters found in both the CNS and PNS: 1. Acetylcholine – generally excitatory Found in skeletal neuromuscular junctions and in many autonomic nervous system synapses. 2. Norepinephrine - excitatory or inhibitory. Found in visceral and cardiac muscle. Cocaine and amphetamines exaggerate the effects. 3. Epinephrine - excitatory or inhibitory. Found in pathways concerned with behavior and mood. 4. Dopamine - excitatory Found in pathways that regulate emotional responses. Decreased levels of this neurotransmitter in Parkinson’s Disease. - Neurotransmitters found in the central nervous system only include: 1. Serotonin - inhibitory Found in pathways that regulate temperature, sensory, perception, mood, and onset of sleep. 2. Gamma-aminobutric acid (GABA) - inhibitory Inhibits excessive discharge of neurons. 12 A&P2 Review Sheet Kimberly Denise Vigee’, N.D. 3. Endorphins and enkephalins - inhibitory Inhibits release of sensory pain neurotransmitters. Opiates mimic the effects of these peptides. Reflex Arc - Structural unit of the nervous system = neuron - Functional unit of the nervous system = reflex arc - A reflex arc is a type of conduction pathway = One - Way Impulse Conduction - Automatic and involuntary - The simplest arc consists of 2 neurons, but most have multiple neurons. - 5 Components of the Reflex Arc: 1. Sensory Receptor 2. Sensory Neuron 3. Center 4. Motor Neuron 5. Effector (muscle or gland) Animations For the Nervous System http://faculty.washington.edu/chudler/introb.html Nerve Synapse: http://camel2.conncoll.edu/academics/zoology/courses/zoo202/Nervous/synapse.html Glial Cells – Read the Page – It’s really Funny!!! http://faculty.washington.edu/chudler/glia.html Saltatory Conduction http://www.blackwellscience.com/matthews/actionp.html Reflex Arc: http://www.sumanasinc.com/webcontent/anisamples/neurobiology/reflexarcs.html 13 A&P2 Review Sheet Kimberly Denise Vigee’, N.D. Myelinated vs. Unmyelinated Axons: 1. http://www.getbodysmart.com/ap/nervoussystem/cellphys/actpot/figure2.htm 2. http://www.getbodysmart.com/ap/nervoussystem/cellphys/actpot/figure4.htm 14