12-1

Nervous Tissue

Chapter 12

12-2

Nervous Tissue

• Overview of the nervous system

• Nerve cells (neurons)• Supportive cells

(neuroglia)• Electrophysiology of

neurons• Synapses• Neural integration

12-3

Overview of Nervous System

• Master Control of the Body– Endocrine and nervous system maintain

internal coordination• endocrine = _____________________________ • nervous

– _______________________ receive information– brain and spinal cord determine responses– brain and spinal cord __________________________

_____________________________________________

12-4

Subdivisions of Nervous System

Two major subdivisions• _________________________– brain and spinal cord

enclosed in bony coverings• _________________________– nerve = _________________

_______________________– _____________ = swelling

of ____________ in a nerve

12-5

• Sensory (____________) divisions (receptors to CNS)– visceral sensory and somatic

sensory division• Motor (_______________) division

(CNS to effectors)– visceral motor division (ANS)

effectors: cardiac, smooth muscle, glands

• sympathetic division (action)

• parasympathetic division (digestion)

– somatic motor division effectors: skeletal muscle

Functional Divisions of PNS

12-6

Fundamental Types of Neurons• Sensory (afferent) neurons

– __________________________________________________

– Transmit info to brain/spinal cord

• ___________________________– In between sensory and

motor pathways in CNS– 90% of neurons are

interneurons– _________________________

_________________________• ___________________________

– send signals to muscles and glands

– organs respond called effectors

12-7

Properties of Neurons

• ____________________________– ability to respond to changes (stimuli)

in/out of body

• Conductivity– _______________________________

• ____________________________– ____________________________________

12-8

Structure of a Neuron

• _________________________ _________________________– single, central nucleus – cytoskeleton • microtubules and

neurofibrils (bundles of actin filaments)• RER Nissl bodies

• _________________________– ______________________

• Singe axon (nerve fiber) arising from axon hillock for rapid conduction– _______________________

_______________________

12-9

Variation in Neural Structure• ____________________

– most common– many dendrites/one axon

• Bipolar neuron– ______________________– ______________________

• ____________________– sensory from skin and

organs to spinal cord

• Anaxonic neuron– many dendrites/no axon– help in visual processes

12-10

Types of Neuroglial Cells • ___________________________________________– wraps around nerve fibers

• ___________________________________________• ___________________________________________– in areas of infection, trauma or stroke

12-11

Types of Neuroglial Cells • ________________________________

– _________________________________________________– regulate composition of brain tissue fluid– convert glucose to lactate to feed neurons– secrete nerve growth factor promoting synapse formation– electrical influence on synaptic signaling– sclerosis – damaged neurons replace by hardened mass of

astrocytes• ________________________________________

12-12

Myelin

• ________________________________________– formed from wrappings of plasma membrane• 20% protein and 80 % lipid (looks white)

– all myelination completed by late adolescence• In PNS, hundreds of layers wrap axon– ____________________________________________– covered by basal lamina and endoneurium

• Gaps between myelin segments = _________________

12-13

Speed of Nerve Signal

• Diameter of fiber and presence of myelin• ______________________________________________

• 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

12-14

Regeneration of Peripheral Nerves• Soma and neurilemmal tube intact• Stranded end of axon and myelin

sheath degenerate– cell soma swells, ER breaks up

and some cells die• Axon stump puts out several

sprouts• _____________________________

_____________________________– schwann cells produce nerve

growth factors

12-15

Local Potentials • Local disturbances in

membrane potential – ___________________

___________________– depolarization (opens

gated Na+ channels) decreases potential across cell membrane• Na+ rushes in • Na+ diffuses for

short distance inside membrane producing a change in voltage called a local potential

12-16

Local Potentials 2

• ___________________________– vary in magnitude with stimulus strength– get weaker the farther they spread– are reversible as K+ diffuses out of cell– ____________________________________

____________________________________

12-17

Action Potentials

• __________________________________________ __________________________________________

• If threshold potential (-55mV) is reached voltage-gated Na+ channels open (Na+ enters causing depolarization)

• Slow K+ gates fully open• K+ exits repolarizing the cell• __________________________________________– excessive exiting of K+

12-19

The Refractory Period• Resists stimulation• ________________________– as long as Na+ gates are open– _________________________

• ________________________– as long as K+ gates are open– __________________________

__________________________

• Refractory period occurs to a small patch of membrane at one time (quickly recovers)

12-20

Impulse Conduction - Unmyelinated Fibers

12-21

_______________________________

• Voltage-gated channels needed for APs– fewer than 25 per m2 in myelin-covered regions – up to 12,000 per m2 in nodes of Ranvier

• Fast Na+ diffusion occurs between nodes

12-22

Saltatory Conduction

• Notice how the action potentials jump from node of Ranvier to node of Ranvier.

12-26

Chemical Synapse Structure

• ______________________ have synaptic vesicles with neurotransmitter and ____________________ have receptors

12-27

Types of Neurotransmitters• _______________________________– formed from acetic acid and choline

• Amino acid neurotransmitters• _______________________________– replace –COOH in amino acids with another

functional group– ______________________ (epi, NE and dopamine)– indolamines (serotonin and histamine)

• ________________

12-29

Synaptic Transmission

3 kinds of synapses with different actions• _______________________________ = ACh• Inhibitory GABA-ergic synapse = GABA• ________________________________ = NE

Synaptic delay (.5 msec) – time from arrival of nerve signal at synapse to

start of AP in postsynaptic cell

12-30

Neural Integration• More synapses a neuron has the greater its

information-processing capability– cerebral cortex 40,000 synapses• estimated to contain 100 trillion synapses

• Chemical synapses are decision-making components – ______________________________________

12-31

Postsynaptic Potentials- EPSP• __________________________________

_____________________– a positive voltage change causing

postsynaptic cell _____________________• result from Na+ flowing into the cell

– glutamate and aspartate are excitatory neurotransmitters

• ACh and norepinephrine may excite or inhibit depending on cell

12-32

Postsynaptic Potentials- IPSP

• ___________________________________ postsynaptic cell to be less likely to fire (hyperpolarize)

• result of Cl- flowing into the cell or K+ leaving the cell

– glycine and GABA are inhibitory neurotransmitters

• ACh and norepinephrine may excite or inhibit depending upon cell