Chapter 12– Nervous Tissue

Ch. 12 Nervous Tissue– Study Guide Critically read Chapter 12 pp. 442-461 before 12.5 Synapses. Comprehend Terminology (those in bold in the textbook) within the reading scope above Study-- Figure questions, Think About It questions, and Before You Go On (section-ending) questions (within the reading scope above) . Before You Go On Questions. Do end-of-chapter questions— Testing Your Recall— 1-4, 7, 11-17 True or False– 1-4, 6, 8 10-2 2 2

12.1 Overview of the nervous system

§ Maintaining internal coordination by two body systems Homeostasis? How? Endocrine and nervous systems-- Endocrine sys. – slower chemical messengers (hormones) delivered to the bloodstream Example-- insulin Nervous sys. – quicker chemical and electrical means Example– in cold environment, vasoconstriction/shivering

§ Subdivisions of Nervous System Two major ANATOMICAL subdivisions: Central nervous system (CNS) brain and spinal cord enclosed in bony coverings Peripheral nervous system (PNS)– all nervous tissue outside the CNS; made up of: Nerves-- bundles of axons in connective tissue; emerge from the CNS; carry signals Ganglia-- knotlike swellings in nerves Fig. @12.1

Part of PNS (next slide)

§ Functional divisions of PNS (SAME) Sensory (Afferent) divisions (receptors to CNS)– carry signals to the CNS somatic division—Ex. visceral sensory division—Ex. Motor (Efferent) division (CNS to effectors) somatic motor division Effectors: skeletal muscles visceral motor division (also called ANS) Effectors: glands and cardiac/smooth muscles sympathetic division/parasympathetic division Fig. 12.2

PNS CNS Brain Sensory division Spinal cord Motor division Somatic Visceral motor division Visceral sensory division Somatic motor division Sympathetic division Parasympathetic division

12.2 Properties of neurons

§ Universal properties of neurons Nerves made up of nerve cells (neurons); neurons’ properties include: Excitability ability to respond to stimuli by producing action potential Conductivity produce traveling electrical signals Secretion Where? Why? What is secreted?

§ Functional Classes of Neurons Sensory (afferent) neurons detect changes in body and external environment Ex. Interneurons (association neurons) Confine ENTIRELY in CNS 90% of our neurons are interneurons process, store and retrieve information Motor (efferent) neuron send signals out to muscles and gland cells (effectors carry out body responses) Fig. 12.3

Fig.12.3 Three Classes of Neurons Example--Detecting your own pulse at wrist

§ Neuron Cell body = soma Nucleus Dendrites (1-many) Function Axon (single; nerve fiber) Fig. 12.4 c-d-e

Neurofibrils Axon (d)

Figure 12.4d

Schwann cell nucleus Axoplasm Axolemma Neurilemma Myelin sheath (c)

§ Variation in Neuron Structure– No. of processes from the soma: (Fig. 16.34) Multipolar neuron most common Bipolar neuron one dendrite/one axon Unipolar neuron Ex. sensory from skin to spinal cord directly Anaxonic neuron many dendrites/no axon Ex. help in visual processes

12.3 Supportive cells (Neuroglia)

§ Introduction How many?! Neurons are outnumbered by neuroglia (1:50) in the nervous sys. Functions- protect the neurons and help them function Example– in the fetus, guide young migrating neurons to their destinations

§ 4 Types of Neuroglial Cells (CNS) Astrocytes (star-shaped) most abundant glial cells - form framework of CNS contribute to blood-brain barrier and regulate composition of brain tissue fluid Oligodendrocytes form myelin sheaths in CNS; distinguish these from Schwann cells Ependymal cells (epithelial cells) line ventricles of the brain and central canal of the spinal cord; produce CSF Microglia formed from monocytes; engulf invading microbes in areas of infection, trauma or stroke Fig. 12.6

Neuroglial Cells of CNS

§ 2 Types of Neuroglial Cells (PNS) Schwann cells -- myelinate fibers of PNS; assist in the regeneration of damaged fibers Satellite cells – surround cell bodies in ganglia; regulate the chemical environment of the neurons

§ Myelin Insulating layer around a nerve fiber; analogy– the rubber insulation on a wire In CNS, Each oligodendrocyte myelinate several fibers (Fig. 12.7a) In PNS, The ___________ cell wraps the nerve fiber outermost coil is called neurilemma containing bulging body of the Schwann cell (nucleus and most of its cytoplasm) (Fig. 12.7b)

Myelin Sheath in CNS Figure 12.7b

Myelin Sheath in PNS Schwann cell Axon Myelin sheath Neurilemma

Myelin Sheath in PNS Node of Ranvier (gaps)-- between Schwann cells (also in CNS) Internodes– from one gap to the next

§ Unmyelinated nerve fibers Locations? CNS, PNS, both (circle one) One Schwann cell harbors ______ small fibers The Schwann cell– folds once around each fiber Fig. 12.8

Unmyelinated nerve fibers Schwann cell Basal lamina

§ Myelination and Speed of Nerve Signal Diameter of fiber and presence of myelin large fibers have more surface area for signal conduction Speeds small, unmyelinated fibers = 0.5 - 2.0 m/sec small, myelinated fibers = 3 - 15.0 m/sec large, myelinated fibers = up to 120 m/sec Functions slow signals supply the stomach and dilate pupil fast signals supply skeletal muscles and transport sensory signals for vision and balance

12.4A Electrophysiology of neurons KEY issues– How does a neuron generate an electrical signal? Cellular mechanisms for producing electrical potential and currents

§ Electrical Potentials & Currents Electrical potential – a difference in the concentration of charged particles between one point and another Electrical current– flow of charged particles from one point to another Living cells have electrical potentials (are polarized) resting membrane potential is -70 mV with a negative charge on the _______ of membrane; why? (next slide)

§ Resting Membrane Potential of all cells (RMP; -70 mV) Factors contribute to RMP: unequal distribution of electrolytes in ECF & ICF Diffusion of ions down their conc. gradient Selective permeability of the cell mem. Cations and anions attract to each other Details-- Membrane very permeable to K+ Membrane much less permeable to Na+ Cytoplasmic anions can not escape— EX. Proteins, phosphates etc. Fig. 12.11

Na+ more concentrated in the ECF K+ more concentrated in the ICF Fig. 12.11--Ion basis of the resting membrane potential Negative charge (-70 mV) Na+ more concentrated in the ECF K+ more concentrated in the ICF

§ When a neuron is stimulated Local potentials– changes in membrane potential when a neuron is stimulated Causes– How? Results– depolarization Ionic bases-- Na+ rushes into/out of (circle one) the cell Na+ diffuses for short distance inside membrane producing a local potential Fig. 12.12 and X

For example– a chemical (pain signal; ligand) stimulates a neuron

Local potential-- Local, graded, and decremental Resting mem. Potential (-70 mV) Time Magnitude of stimulus Stimuli A B C D

§ Action potentials if stimulus is strong (Fig. 12.13) Threshold reached Depolarization (sodium channels open) Repolarization (Sodium channels close and K+ gates fully open) Hyperpolarization Resting membrane potential restores 12-38

Figure 12.13a

§ Action potentials vs. local potentials (Table 12.5) ? Graded + reversible Decremental

Ionic base

§ The refractory period of the action potential (AP) Period of resistance to stimulation for another AP Absolute refractory period as long as Na+ gates are open no stimulus will trigger AP Relative refractory period as long as K+ gates are open only especially strong stimulus will trigger new AP

12.4B--Conduction of a nerve signal in an unmyelinated fiber

Where are ion gates? First action potential occurs at? The next action potential occurs at? Chain reactions continue until _____

12.4C--Conduction of a nerve signal in a myelinated fiber

§ Saltatory conduction (Fig. 12.17a-b) Most (99%) of the voltage-regulated ion gates are at the _______________. Slow but nondecremental At the internodes– nerve signals travel very ______ (diffusion of ions) and decremental. Most of the axon is covered with myelin (internodes) nerve signal is faster at 120m/sec (than unmyelinated ones (up to 2 m/sec)

Action potentials occurs only at the _________________ It is called saltatory conduction meaning _____________.