Chapter 12 topics. Anatomical Divisions of the Nervous System 5/3/16. Organization of Nervous system Neural tissue. Central Nervous System (CNS)

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1 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) 1

2 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 2

3 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 3

4 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 4

5 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 5

6 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 (O 2, CO 2) ) 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 6

7 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 O 2, CO 2 nutrients neurotransmitter 7

8 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 8

9 Neurophysiology Transmembrane potential Changes in transmembrane potential opening of channels change in permeability two types of changes in potential graded potentials action potentials 9

10 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 10

11 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 11

12 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 12

13 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 13

14 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 14

15 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 15

16 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 16

17 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 17

18 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 18

19 Presynaptic facilitation and inhibition Presynaptic inhibition Presynaptic inhibition & facilitation Presynaptic inhibition Presynaptic facilitation 19

20 Rate of AP Generation Determines intensity of stimulus The h Frequency of AP s, the h intensity of the sensation 20