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Chapter 12 topics
• Organization of Nervous system • Neural tissue – Histology – Function
Anatomical Divisions of the Nervous System • Central Nervous System (CNS) – Brain – Spinal cord
• Peripheral Nervous System (PNS) – Nerves (peripheral nerves)
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Functional Divisions of the Nervous System • Afferent division (sensory) – Receptors - many types – Sensory neurons
• Efferent division (motor) – Motor neurons – Effectors • Organs/tissues – Subdivisions exist…
Subdivisions of efferent (motor) division • Somatic nervous system (SNS) • Autonomic nervous system (ANS) • Each has separate effectors
Neural Tissue
Neurons, supporting cells, and their functions
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Neurons • Major functional cell of nervous system • Vary in form • vary in location • Amitotic…lack centrioles
Neuron Structure
• Axon collaterals • Axoplasmic transport: anterograde & retrograde
Synapse: where two cells communicate • Presynaptic cells – Usually neurons – Synaptic vesicles – Neurotransmitter
• Postsynaptic cells – Neurons – Muscle fibers – Gland cells
• Synaptic knob: Reabsorption and reassembly of neurotransmitter breakdown products
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Classification of Neurons: Structural
• • • •
Anaxonic - brain, sense organs, poorly understood Bipolar - sensory - eye, ear, nasal mucosa Unipolar - sensory - PNS Multipolar - interneurons and motor neurons
Functional classification of neurons • 1- Sensory • 2- Motor • 3- Interneurons
Sensory neurons • two types – somatic sensory – visceral sensory
• Begin at a receptor and • end in CNS • Receptors monitor changes – Exteroreceptors: outside body – Interoceptors: internal environment – Proprioceptors: body position
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Motor neurons • two types – somatic motor – visceral motor
• Begin in the CNS • end at effectors • Effectors – What are the five effectors?
Interneurons • also called association neurons • mostly in CNS • situated between sensory and motor neurons • roles include – Reflex mediation, memory, planning, learning, etc.
Neuroglia • Cells that – Support neurons – Protect neurons
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Ependymal cells • Line CNS spaces – Central canal of spinal cord – Ventricles in brain
• 3 functions: • secrete cerebrospinal fluid (CSF) in ventricles • monitor CSF composition – FYI – these are called tancytes
• circulate CSF – ciliated in ventricles
Astrocytes • maintain blood-brain barrier • provide framework for CNS • repair damaged neural tissue – stabilize area • regulate interstitial fluid concentrations of – nutrients – Ions: Na, K – respiratory gases (O2, CO2) )
• regulate blood flow in CNS capillaries • enhance synaptic function • absorb and recycle neurotransmitters • may form scar tissue following CNS injury
Oligodendrocytes • myelinate axons in CNS • provide structural organization of CNS
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Myelination: refers to the wrapping of axons with cell membrnae • electrically insulates fibers • increases rate of impulse conduction along axolemma • results in nodes and internodes
Microglia • form from blood stem cells that give rise to macrophages • phagocytosis – debris – waste – pathogens
Satellite cells • PNS • located in ganglia • regulate environment around PNS neurons – O2, CO2 – nutrients – neurotransmitter
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Schwann cells • myelinate most all axons in PNS • participate in repair
Axons: Myelinated vs. Unmyelinated
Neural Response to Injuries • Occurs in PNS only • CNS – Too many axons – Astrocytes produce dense scar tissue – Growth inhibiting factors secreted by astrocytes
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Neurophysiology
Transmembrane potential
Changes in transmembrane potential • opening of channels • change in permeability • two types of changes in potential – graded potentials – action potentials
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Graded potentials • On dendrites and somas • What channels are present here?
Graded Potentials • changes in potential • dendrites/somas • local – Effect decreases w/ distance
• depolarizing or hyperpolarizing… • vary in strength… • do not propagate
Action Potential
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Action potentials • Why do VG sodium channels open? • threshold voltage • all or none • propagated • initiated at initial segment • travel down axolemma
Action Potentials • refractory periods – Absolute – Relative
• absolute refractory period – Time of depolarization and sodium channel inactivation
• relative refractory period – Stronger than normal stimulus is required to generate an action potential
Propagation of AP’s Continuous
Saltatory
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FYI Axon types: diameter, propagation speed, and information • Type A fibers – Large, myelinated, fast 268 mph
• Type B fibers – Medium, myelinated 40 mph
• Type C fibers – Small, unmyelinated 2 mph
• Type A – Position, balance, delicate touch, somatic motor commands
• Type B & C – Temperature, pain, touch, – Visceral motor commands
Synaptic Activity • Electrical • Chemical
FYI Electrical synapses • uncommon • gap junctions – connexons
• A few locations in CNS & PNS – vestibular nuclei – eye – ciliary ganglia
• AP’s always elicit AP’s
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Chemical synapse • Presynaptic neurons store neurotransmitter in synaptic vesicles • Presynaptic neurons release neurotransmitters in response to action potentials • Neurotransmitter types – Excitatory neurotransmitters: depolarizing effect – Inhibitory neurotransmitters: hyperpolarizing effect
• Synaptic cleft • Postsynaptic neuron
Cholinergic synapse • • • •
Example of a chemical synapse use Ach Ach opens chemically-gated Na channels Enzymes inactivate neurotransmitters – For example, AchE inactivates Ach
Cholinergic synapse summary
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many other neurotransmitters exist • Norepinephrine (NE) – Usually excitatory
• FYI Dopamine – Excitatory or inhibitory – Cocaine inhibits dopamine removal
• FYI Serotonin – Attention/emotional states
• FYI Gamma-aminobutyric acid (GABA) – Inhibitory – Copmmon: 20% of brain synapses
• FYI Nitric Oxide (NO) • FYI Carbon monoxide (CO)
FYI
FYI
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Neuromodulators • also released by axon terminals • alter synaptic activity by changing – rate of NT release – postsynaptic cell response
• have long term effects – multi-step responses
• may be released alone OR with a neurotransmitter • FYI examples – Opioids – Neuropeptides
Mechanisms of neurotransmitter and neuromodulator function • 1. Direct effect
A second mechanism • 2. Indirect effects via G proteins
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A third mechanism • 3. Indirect effect via intracellular enzymes
cAMP effects from Basic Neurochemistry: Molecular, Cellular and Medical Aspects. 6th edition. By Duman and Nestler • FYI slide • regulation of the general metabolic state of the target neurons • modulatory effects on neurotransmitter synthesis, storage, release and receptor sensitivity; • cytoskeletal organization and structure; • neuronal growth and differentiation. • long-term actions of neurotransmitters that are mediated through alterations in neuronal gene expression.
Information processing • Multiple synapses
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Postsynaptic Potentials • Develop on postsynaptic membranes when ion channels open • are graded potentials • may be – excitatory post-synaptic potentials (EPSPs) or – inhibitory post-synaptic potentials (IPSPs) – depends on which ions diffuse
EPSPs and IPSPs
Summation of PSPs (they can be added) • may occur one after the other • together, multiple EPSPs may push initial segment to threshold • Temporal summation
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Summation of PSPs: • may occur simultaneously in separate locations • together may push initial segment to threshold • Spatial summation
Postsynaptic Potentials • are additive • neuromodulators and hormones promote • facilitation of neurons or • inhibition of neurons
Facilitation • Neurons can brought closer to threshold by some factor that effects – Receptors – Na channels
• examples – nicotine – caffeine
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Presynaptic facilitation and inhibition
Presynaptic inhibition
Presynaptic inhibition & facilitation Presynaptic inhibition
Presynaptic facilitation
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Rate of AP Generation • Determines intensity of stimulus • The h Frequency of AP’s, the h intensity of the sensation
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