Download - Chapter 12– Nervous Tissue
Chapter 12– Nervous Tissue
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Ch. 12 Nervous Tissue– Study Guide
1. Critically read Chapter 12 pp. 442-461 before 12.5 Synapses.
2. Comprehend Terminology (those in bold in the textbook) within the reading scope above
3. 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.
4. Do end-of-chapter questions—– Testing Your Recall— 1-4, 7, 11-17– True or False– 1-4, 6, 8
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12.1 Overview of the nervous system
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§ Maintaining internal coordination by two body systems
1. Homeostasis? How?
2. Endocrine and nervous systems--A. Endocrine sys. – slower– chemical messengers (hormones) delivered
to the bloodstream– Example-- insulin
B. Nervous sys. – quicker– chemical and electrical means– Example– in cold environment,
vasoconstriction/shivering 12-4
§ Subdivisions of Nervous SystemTwo 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
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Part of PNS (next slide)
• Sensory (Afferent) divisions (receptors to CNS)– carry signals to the CNS– somatic division—Ex. – visceral sensory division—Ex.
• Motor (Efferent) division (CNS to effectors)1.somatic motor division
Effectors: skeletal muscles2.visceral motor division (also called ANS)
Effectors: glands and cardiac/smooth muscles• sympathetic division/parasympathetic
divisionFig. 12.2
§ Functional divisions of PNS (SAME)
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Brain
CNS PNS
Spinalcord
Sensorydivision
Motordivision
Visceralsensorydivision
Somaticsensorydivision
Visceralmotor
division
Somaticmotor
division
Sympatheticdivision
Parasympatheticdivision
12.2 Properties of neurons
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§ Universal properties of neuronsNerves 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?
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§ Functional Classes of Neurons1. Sensory (afferent) neurons
– detect changes in body and external environment– Ex.
2. Interneurons (association neurons)– Confine ENTIRELY in CNS– 90% of our neurons are interneurons– process, store and retrieve information
3. Motor (efferent) neuron– send signals out to muscles and gland cells
(effectors carry out body responses)– Ex. Fig. 12.3
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Fig.12.3 Three Classes of Neurons
Example--Detecting your own pulse at wrist
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§ Neuron• Cell body = soma
– Nucleus
• Dendrites (1-many)
– Function
• Axon (single; nerve fiber)
– Function
Fig. 12.4 c-d-e 12-13
Neurofibrils
Axon(d)
Figure 12.4d
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Myelin sheath
Axolemma
Axoplasm
Neurilemma
(c)
Schwann cellnucleus
§ 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
processes12-17
12.3 Supportive cells (Neuroglia)
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§ 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
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§ 4 Types of Neuroglial Cells (CNS)1. Astrocytes (star-shaped)
– most abundant glial cells - form framework of CNS– contribute to blood-brain barrier and regulate
composition of brain tissue fluid2. Oligodendrocytes form myelin sheaths in
CNS; distinguish these from Schwann cells3. Ependymal cells (epithelial cells) line
ventricles of the brain and central canal of the spinal cord; produce CSF
4. Microglia formed from monocytes; engulf invading microbes– in areas of infection, trauma or stroke
Fig. 12.612-21
Neuroglial Cells of CNS
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§ 2 Types of Neuroglial Cells (PNS)
1. Schwann cells -- myelinate fibers of PNS; assist in the regeneration of damaged fibers
2. Satellite cells – surround cell bodies in ganglia; regulate the chemical environment of the neurons
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§ 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) 12-24
Figure 12.7b
Myelin Sheath in CNS
Myelin Sheath in PNS
NeurilemmaMyelin sheath
Axon
Schwann cell
Myelin Sheath in PNSNode of Ranvier
(gaps)-- between Schwann cells (also in CNS)
Internodes–
from one gap to the next
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• Locations?CNS, PNS, both (circle one)
• One Schwann cell harbors ______ small fibers
• The Schwann cell– folds once around each fiber
Fig. 12.8
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§ Unmyelinated nerve fibers
Schwann cell
Basal lamina
Unmyelinatednerve fibers
§ 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 12-30
12.4A Electrophysiology of neurons
KEY issues–
1.How does a neuron generate an electrical signal?
2.Cellular mechanisms for producing electrical potential and currents
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§ Electrical Potentials & Currents1. Electrical potential – a difference in the
concentration of charged particles between one point and another
2. Electrical current– flow of charged particles from one point to another
3. Living cells have electrical potentials (are polarized)– resting membrane potential is -70 mV with a
negative charge on the _______ of membrane; why? (next slide)
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§ Resting Membrane Potential of all cells (RMP; -70 mV)
Factors contribute to RMP: unequal distribution of electrolytes in ECF & ICF
1. Diffusion of ions down their conc. gradient2. Selective permeability of the cell mem.3. 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. 12.1112-33
• Na+ more concentrated in the ECF
• K+ more concentrated in the ICF 12-34
Negative charge (-70 mV)
Fig. 12.11--Ion basis of the resting membrane potential
§ 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
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For example– a chemical (pain signal; ligand) stimulates a neuron
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Magnitude
of stimulus
Resting mem.Potential (-70 mV)Time
Stimuli A B C D
Local potential-- Local, graded, and decremental
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§ Action potentials if stimulus is strong (Fig. 12.13)
1. Threshold reached2. Depolarization
(sodium channels open)
3. Repolarization (Sodium channels close and K+ gates fully open)
4. Hyperpolarization5. Resting membrane
potential restores
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Figure 12.13a
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§ Action potentials vs. local potentials (Table 12.5)
Local Potentials Action Potentials
Local ?
Graded + reversible
?
Decremental ?
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Ionic base
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§ 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
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12.4B--Conduction of a nerve signal in an unmyelinated fiber
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• Where are ion gates?
• First action potential occurs at?
• The next action potential occurs at?
• Chain reactions continue until _____
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12.4C--Conduction of a nerve signal in a myelinated fiber
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§ Saltatory conduction (Fig. 12.17a-b)
A. Most (99%) of the voltage-regulated ion gates are at the _______________.– Slow but nondecremental
B. At the internodes– – nerve signals travel very ______ (diffusion
of ions) and decremental.
C. Most of the axon is covered with myelin (internodes)– nerve signal is faster at 120m/sec (than
unmyelinated ones (up to 2 m/sec)12-46
•Action potentials occurs only at the _________________•It is called saltatory conduction meaning _____________.
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