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Chapter 10 The Nervous System
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Introduction Types of neural tissue: 1. Neurons – react to changes around them & send impulses 2. Neuroglia – support tissue with a variety of functions Functions of nervous system: 1. Sensory – use sensory neurons to gather info. inside & outside the body 2. Motor – use motor neurons to help the body react to stimuli 3. Integrative – integrate signals from sensory & motor neurons to produce thought, memory, etc.
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Divisions of the Nervous System Central Nervous System (CNS) – consists of the brain & spinal cord Peripheral Nervous System (PNS) – consist of nerves that connect the CNS to other body parts
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Structure of a Neuron Dendrites – pick up impulses Cell body – contains cell parts Axon – sends impulses Schwann cells – wrap around the axon Myelin – lipid covering formed by Schwann cells; speeds rate of impulse Axon terminals – end of axon
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Structure of A Neuron Axon hillock – slight elevation where axon originates Node of Ranvier – gap in myelin
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Structure of A Neuron Neurofibrils – network of fine threads that extend into the axon; for support Nissl bodies – consist of rough ER Neurilemmal sheath – formed by the cytoplasm & nucleus of the Schwann cell that remain on the outside
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Direction of Impulse Impulse always travels from dendrites, through cell body, & down axon Axon synapses w/next neuron or an effector (muscle or gland)
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Structural Classification of Neurons Bipolar – has 2 processes from the cell body, 1 at each end; in sense organs Unipolar – has 1 process from c.b. that divides into 2; in PNS Multipolar – have many processes from c.b; in CNS and motor neurons.
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Functional Classification of Neurons Sensory (afferent) – unipolar & carry impulses from body parts to brain or s.c. Interneurons (association neurons) – multipolar & in CNS; form links b/t other neurons Motor (efferent) – multipolar & carry impulses from brain or s.c. to muscle or gland
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Types of Neuroglia Support tissue w/a variety of functions: 1. Astrocytes –star-shaped; found b/t neurons & b.v.; support, transport & communication b/t nerves & b.v. Transport glucose to Neuron and store glycogen Separate neurons from each other.
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Types of Neuroglia 2.Microglia – small w/few processes; found throughout CNS; support & phagocytosis of harmful sub- stances
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Types of Neuroglia 3. Oligodendrocytes – resemble astrocytes but w/fewer processes; form myelin sheath in CNS
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Types of Neuroglia 4. Ependyma – columnar & cuboidal shaped cells; form inner lining of brain & s.c.; provide a layer for diffusion to occur
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Types of Neuroglia
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Cell suicide Microglia can destroy cells that are old &/or damaged A – healthy neuron B – neuron being destroyed & DNA breaking apart C – microglia removing debris
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Nerve Impulse Cartoon Impulse Animation
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Resting Potential A resting neuron is one not sending an impulse & is in resting potential The cell membrane of this neuron is polarized b/c of an un= distribution of ions on either side Outside the neuron – greater concentration of Na+ ions Inside the neuron – greater concentration of K+ ions & negatively charged proteins
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Resting Potential K+ leak out of K+ channels at a slow rate leaving behind negatively charged proteins This makes the charge on the inside of the membrane negative The voltage meter (next pg.) shows a charge of -70 mv & refers to the charge of a neuron in resting potential
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Resting Potential
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Movement of Ions Ions follow the laws of diffusion (movement from high to low concentrations) when moving thru membranes Ions enter & leave the membrane thru channels or gates that are specific for that ion
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Ion Channels 3 types –Passive- always open –Ligand gated- opened by a chemical compound. (neurotransmitter) –Voltage gated- opened in response to a change in electric potential.
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Resting Potential The charge outside the cell is positive b/c: 1. the high concentration of Na+ ions 2. the movement of K+ ions to the outside
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Resting Potential Animation
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Sodium Potassium Pump Membrane protein used for the active transport of Na+ and K+ across membrane. Requires ATP Removes 3 Na+ ions and accepts 2 K+ for every ATP molecule used. Maintains resting potential.
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Action Potential An abrupt change in the electrical potential across the cell membrane that occurs after a stimulus (a.k.a. nerve impulse): 1. Resting neuron stimulated (remember – a resting neuron is polarized) 2. Na+ channels open & Na+ move into membrane; charge inside cell becomes + (+30mv) & neuron is depolarized 3. Na+ channels close & K+ channels open; K+ move out & charge reverts back to negative (-70mv); cell is repolarized
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Resting Potential → Action Potential A)Resting potential (polarized) B)Action potential A.P. in the 1 st region stimulates adjacent region (de- polarized) C)1 st region repolarized
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Action Potential Animation
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Graphing Action Potential After repolarization a brief period of delay occurs when Na+ gates cannot temporarily open; called refractory period
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Graphing Action Potential Hyperpolarization when the cell becomes more negative than -70mv; depends on which ions are allowed to enter the cell, + or – ions (i.e. Cl- ions) Threshold – the minimum amt. of stimulus required to cause an action potential
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Impulse Conduction Saltatory conduction – impulse jumps from 1 node of Ranvier to another; why? Myelin covering – mostly lipids which prevent flow of ions channels - are located at nodes of Ranvier for ions to diffuse in & out Myelinated axons (white matter) - conduct impulses faster than unmyelinated axons (gray matter)
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Saltatory Conduction Animation Animation
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The Synapse Junction b/t 2 neurons Presynaptic neuron – occurs before the syapse Postsynaptic neuron – occurs after the synapse Synaptic knob – enlargement of axon terminal Synaptic vesicles – store ntm Synaptic cleft – space b/t neurons
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Actual Synapse
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Events at the Synapse Action potential travels down presynaptic neuron & arrives at synapse Synaptic knob becomes more permeable to Ca+ & they diffuse inward This causes vesicles to release ntm Ntm causes A.P. to enter postsynaptic neuron A.P. continues to travel down postsynaptic neuron
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The Synapse
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Types of Neurotransmitters The nervous system produces approx. 30 different types of ntm Some open ion channels, others close them Monoamines:Neuropeptides: - epinephrine- endorphins - norepinephrine- enkephalins - dopamine- substance P - serotoninAcetylcholine (ACh)
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Effects of Ntms Epinephrine & norepinephrine – hormones when released in blood, but ntm in the n.s.; stimulate autonomic n.s.; incr. HR, resp. rate, etc.; “fight-or-flight” response Dopamine – excitatory or inhibitory; create a sense of well-being; insufficient levels associated with Parkinson’s disease Serotonin – inhibitory; insufficient levels associated with insomnia Endorphins & enkephalins – generally inhibitory & influence mood; released under stress to reduce pain (blocks substance P) Substance P – excitatory; helps in perception of pain ACh – stimulates muscles to contract
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Synaptic Potentials Ion channels that respond to ntm are called chemically gated channels (as opposed to those that are voltage-gated & are involved in sending A.P.) Changes in chem. gated channels create local changes called synaptic potentials (a small, temporary change in the potential charge of a neuron) They allow one neuron to influence another
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The Synapse
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Synaptic Potentials 2 types: 1. Excitatory postsynaptic potential (EPSP) – occurs when the neuron is depolarized (or becomes less negative), but the charge is subthreshold (<+30mv). A true A.P. won’t occur, but will be more likely to occur if the neuron receives more subthreshold stimuli
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Synaptic Potentials 2. Inhibitory postsynaptic potential (IPSP) occurs when the neuron is hyperpolarized (or becomes more negative than -70mv). An A.P. will be less likely to occur. The type of ntm secreted will decide the effect that occurs.
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Effects of Ntm on Synaptic Potentials If a ntm opens Na+ channels & Na+ diffuse in, the membrane is depolarized (EPSP) If a ntm opens K+ channels & K+ diffuse out, the membrane is hyperpolarized (IPSP) A neuron can receive EPSP’s & IPSP’s simultaneously; the neuron responds to the algebraic sum of the + and - charges
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Synaptic Potential vs. Action Potential 2 differences: 1. P.S.P. are graded (depends on amt. of ntm) & their effect adds up (called summation) whereas A.P. are all-or-none 2. P.S.P. decr. in intensity w/incr. distance from synapse Facilitation – when a neuron receives subthreshold stimuli & gets closer to sending an A.P.
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Convergence vs. Divergence Convergence – impulses from 2 or more fibers converge on a single neuron (summation will occur) Divergence – when outgoing impulses are divided onto several branches of an axon
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Convergence vs. Divergence
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Importance of Ions Ca+ are needed for the release of ntm Ca+ are also needed to close Na+ channels Insufficient Ca+ levels result in channels remaining open & impulses repeatedly transmitted; results in tetany May occur in pregnancy (as fetus uses maternal Ca+), when diet lacks Ca+ or Vit D during dehydration
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Importance of Ions An incr. in extracellular K+ causes neuron to be less negative; threshold is reached sooner & neurons are very excitable; may result in convulsions A decr. in extracellular K+ causes neuron to be more neg.; does not allow an A.P. to occur & muscles may become paralyzed
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Resting Potential
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Action Potential
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Saltatory Conduction
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EPSP
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IPSP
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Convergence vs. Divergence
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