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Nervous Tissue A. Nervous system divisions B. Functional anatomy of nervous tissue 1. Neuroglia a. Types of neuroglia b. Myelination 2. Neurons a. Parts of a neuron b. Classification of neurons 3. Gray and white matter C. Neurophysiology 1. Resting membrane potential 2. Ion channels 3. Action potential (impulse) a. Depolarization b. Repolarization c. Refractory period d. Propagation (conduction) of action potentials e. The all-or-none principle f. Saltatory conduction 4. Transmission at synapses a. Chemical synapses b. Excitatory and inhibitory postsynaptic potentials c. Spatial and temporal summation of PSPs d. Removal of neurotransmitter 5. Neuronal circuits The nervous system is the body's control center and communicates network. It serves three broad functions: 1. senses changes in the environment 2. integrates and interprets 3. responds Nervous Systems Divisions 1. central nervous system a. brain b. spinal cord 2. peripheral nervous system a. somatic division b. autonomic division (1) sympathetic vs (2) parasympathetic Neuroglia 1. 2. 3. 4. 5. astrocytes oligodendrocytes microglia ependyma neurolemmocytes (Schwann cells) 6. satellite cells Neuron 1. cell body a. nucleus b. nucleolus c. Nissl substance 2. dendrite 3. axon a. axon hillock b. trigger zone c. axon collaterals d. telodendrion e. end bulbs Myelination 1. PNS = neurolemmocytes CNS = oligodendrocytes 2. process 3. myelin sheath 4. neurolemma 5. nodes of Ranvier Structural Classification of Neurons 1. multipolar neuron 2. bipolar neuron 3. unipolar neuron a. central process b. receptor Functional Classification of Neurons 1. sensory (afferent) neurons 2. association neurons (interneurons) 3. motor (efferent) neurons Some Terminology 1. 2. 3. 4. 5. 6. nerve fiber nerve vs. tract tract ganglion vs. nucleus (center) gray matter Gray matter white matter White matter Neurophysiology Communication by neurons depends upon two basic properties of their cell membranes: 1. There is an electrical voltage, called the resting membrane potential, across the cell membrane. 2. Their cell membranes contain a variety of ion channels (pores) that may be open or closed. Resting Membrane Potential 1. build-up of ions 2. separation of charges = potential energy (mV) 3. membrane potential = -70 mV +++++ 4. polarized membrane +++++ ---------------- Two Main Factors Contribute to the RMP 1. distribution of ions across the cell membrane. a. extracellular fluid is rich in Na+ and Clb. intracellular fluid is rich in K+ and anions such as organophosphates and proteins 2. relative permeability of the cell membrane to Na+ and K+ a. moderately permeable to K+ and Clb. slightly permeable to Na+ c. impermeable to the intracellular anions Ion Channels(Pores) 1. non-gated (leakage) 2. gated (regulated) a. open in response to stimulus (chemical, voltage change, light, mechanical) b. excitable cells have them c. found in trigger zones How do chemically gated ion channels open? extracellular fluid Na+ Na+ ACh Na+ Na+ +++++++ Na+ Na+ extracellular fluid Na+ Na+ +++++++++++++ Na+ Na+ Na+ Na+ +++++++ +++++++++++++ cell membrane cell membrane Na+ ------- - - - - - - - - - - - -- ------- Na+ Na+ - - - - - - - - - - - -Na+ Na+ Na+ intracellular fluid CHEMICALLY-GATED Na+ CHANNEL closed intracellular fluid CHEMICALLY-GATED Na+ CHANNEL open Chemically gated channel How do voltage-gated channels open? extracellular fluid Na+ Na+ Voltage Na+ Na+ +++++++ Na+ Na+ extracellular fluid Na+ Na+ +++++++++++++ Na+ Na+ Na+ Na+ +++++++ +++++++++++++ cell membrane cell membrane Na+ ------- - - - - - - - - - - - -- ------- Na+ Na+ - - - - - - - - - - - -Na+ Na+ Na+ intracellular fluid VOLTAGE-GATED Na+ CHANNEL closed intracellular fluid VOLTAGE GATED Na+ CHANNEL open 3. Mechanically-gated channels, light-gated channels An Action Potential Action Potential-Depolarization 1. threshold stimulus 2. voltage-gated Na+ channels open 3. Na+ influx (-70 ---> +30 mV) _____________________________ 4. positive feedback Action Potential-Repolarization 1. 2. 3. 4. voltage-gated K+ channels open K+ efflux (-90 <--- +30 mV) hyperpolarization Na-K pumps restore ions Action Potential Summary Refractory Period 1. absolute 2. relative POSITIVE FEEDBACK OF AN ACTION POTENTIAL CONTROLLED CONDITION A stimulus or stress disrupts membrane homeostasis by causing a threshold depolarization NO RETURN TO HOMEOSTASIS Opening of the voltage-gated Na+ channels causes depolarization in adjacent membrane, opening more voltage-gated Na+ channels RECEPTOR The receptors in this case are voltage-gated Na+ channels in their resting state CONTROL CENTER The shape of the voltage-gated Na+ channel depends on membrane voltage change EFFECTORS Voltage-gated Na+ channels are also effectors. Threshold depolarization causes shape changes in the channel All of None Principle Each time an action potential is formed, it has a constant and maximum strength for the existing conditions. Continuous Conduction 1. trigger zone to synapse 2. propagation 3. one direction only Saltatory Conduction 1. 2. 3. 4. 5. myelin sheath nodes of Ranvier "jumping" impulse 0.5 vs 130 m/sec energy conservation Transmission at Synapses 1. "synapsis" means connection 2. synapses integrate and filter information 3. signals are transmitted or inhibited 4. presynaptic vs postsynaptic neurons Chemical Synapses 1. 2. 3. 4. 5. 6. 7. arrival of action potential Ca++ influx synaptic vesicle rupture NT release NT diffusion NT/receptor interaction postsynaptic potential Postsynaptic Potentials can be excitatory or inhibitory 1. excitatory (EPSP) a. facilitation b. summation (1) spatial (2) temporal 2. inhibitory (IPSP) a. hyperpolarization Postsynaptic Potentials Facilitation and Summation 1. spatial 2. temporal ___________ net effects 1. facilitation 2. summation (impulses) 3. inhibition (hyperpolarization) EXAMPLE OF SYNAPTIC INTEGRATION postsynaptic neuron excitatory presynaptic neuron inhibitory presynaptic neuron TYPES OF NEUROTRANSMITTERS Acetylcholine Most common neurotransmitter; In a class by itself chemically; Mostly excitatory, depending on location and function; Brain, spinal cord, neuromuscular and neuroglandular synapses of the periphery Excitatory amino acids Glutamate – 75% of excitatory synapses in brain Asparate – spinal cord Inhibitory amino acids Glycine – most common in spinal cord GABA (gamma amino butyric acid) – most common in brain Monoamines (biogenic amines) Catecholamines – norepinephrine, epinephrine, dopamine Other amines – serotonin, histamine Neuropeptides – substance P, enkephalines and endorphins, cholecystokinin Removal of Neurotransmitter from Synaptic Cleft 1. diffusion 2. enzymatic degradation 3. uptake into the cell Neuronal Circuits 1. 2. 3. 4. 5. simple series diverging converging reverberating parallel after-discharge