CH. 12

Nervous Tissue

Overview of the Nervous System

Objectives List the structures and basic functions of the nervous

system. Describe the organization of the nervous system.

Structures

Nervous system Smallest and most

complex system Billions of neurons Includes:

Brain Cranial nerves Nerves – bundle of axons Spinal cord – connects to

brain Spinal nerves

Functions

Sensory – detect internal and external stimuli Sensory neurons carry information to brain

Integrative – analyzes and stores information; makes appropriate responses Perception – conscious awareness of sensory stimuli Interneurons – participate in integration

Function

Motor – causes movement or gland secretion in response to stimuli Motor neurons – carry information to muscle or gland

(effectors)

CNS – Central NS

PNS – Peripheral NS

Contains brain and spinal cord (axial)

Processes sensory info

Source of thoughts, emotions, memories

All tissue outside the CNS (appendicular)

Includes cranial nerves, branches, spinal nerves, sensory receptors

Divided further SNS – somatic NS (body) ANS – autonomic NS (self) ENS – enteric NS

(intestines)

Nervous System

Nervous System

Peripheral NS Branches

Somatic NS – voluntary Consists of:

Sensory neurons in head, body, limbs, vision, hearing, taste, and smell that send messages to CNS

Motor neurons that conduct messages from CNS to skeletal muscles only

Autonomic NS – involuntary Consists of:

Sensory neurons in organs that send messages to CNS Motor neurons that conduct messages from CNS to

smooth muscle, cardiac muscle, and glands

Sympathetic divisionParasympathetic division

Supports exercise or emergency actions

“fight or flight”Ex: increases heart

rate

“rest and digest” Ex: decreases heart

rate

Usually work in opposing actions

ANS Branches

ENS

“brain of the gut”Controls all activities associated with

digestion and the gastrointestinal (GI) tractInvoluntary

Overall Organization

Organization

Review

What are the components of the CNS and PNS?

What kinds of problems would result from damage of sensory neurons, interneurons, and motor neurons?

What are the components and functions of the SNS, ANS, and ENS?

Which subdivisions of the PNS control voluntary actions? Involuntary actions?

Histology of Nervous Tissue

Objectives Contrast the histological characteristics and the

functions of neurons. Contrast the functions of neuroglia.

Neurons Neuroglia

Provide unique functions

Sensing, thinking, remembering, controlling muscle activity, regulating glandular secretions

Support, nourish, and protect the neurons

Maintain homeostasis in the interstitial fluid that bathes them

Neurons vs. Neuroglia

Neurons

Vocabulary: Neuron – nerve cell Electrical excitability

the ability to respond to a stimulus and convert it into an action potential

Stimulus any change in the environment that is strong enough to

initiate an action potential Action potential – nerve impulse

An electrical signal that propogates (travels) along the surface of the membrane of a neuron

Can travel up to 280 mph

Parts of a Neuron

Three parts Cell body

Main part of the cell Includes organelles, nucleus, and cytoplasm

Dendrites Receiving parts of the neuron Short, tapered, and highly branched

Axon Transmitting parts of the neuron Long, thin, cylindrical

Parts of a Neuron

Parts of a Neuron

Synapse – site of communication between 2 neurons or a neuron and an effector cell

Synaptic end bulb – swollen end of an axon where synaptic vesicles hold neurotransmitters

Neural Diversity

Multipolar neurons Several dendrites, one axon Found in brain and spinal cord

Bipolar neurons One main dendrite, one axon Eye, ear, olfactory of brain

Unipolar neurons Axon and dendrite fuse at beginning and then branch Occurs as an embryo

Neural Diversity

Others

Purkinje cells – cerebellum

Pyramidal cells – cerebral cortex of brain

Neuroglia

Actively participate in nervous tissue functioning

Do not generate action potentials

Can multiply and divide – neurons cannot

Types of Neuroglia

CNS Astrocytes – create blood-brain barrier, strength Oligodendrocytes – create myelin sheath around CNS

axons Microglia – remove cellular debris during neural

development Ependymal cells – assist with circulation of cerebrospinal

fluidPNS

Schwann cells – create myelin sheath around PNS axons Satellite cells – support, regulate exchange of materials

Types of Neuroglia

Types of Neuroglia

Myelination

Myelin sheath – multilayered lipid and protein covering around some axons

Provides insulationIncreases speed of nerve impulseIf a cell has myelin we say that it is

myelinatedGaps in the myelin sheath are called nodes of

Ranvier

Review

Describe the parts of a neuron and the functions of each.

Give examples of the structural diversity of neurons.

Give examples of the different types of neuroglia. Where are each found? What do they do?

What is myelin?

Electrical Signals in Neurons

Objectives Describe the cellular properties that permit

communication among neurons and effectors. Compare the basic type of ion channels, and explain

how they relate to action potentials and graded potentials.

Describe the factors that maintain a resting membrane potential.

Mini-Physics Lesson

Potential energy – energy stored in a system (the body) as a result of its position or chemical composition

Kinetic energy – energy being used for motion or force

Physics to Anatomy

Neurons are excitable because of a voltage difference across the membrane - potential

“Potential” will initiate an impulse that can travel through the nervous system

Graded potentials – used for short-distance communication

Action potentials – allow communication over short and long distance within the body

Potential

Membrane potential – electrical voltage difference across the membrane

Resting membrane potential – membrane potential in an excitable cell Like voltage stored in a battery If + and – ends connect, electrons flow in a current

Current Flow of charged particles For the body – these are ions instead of elecrons

Ion Channels

Gradient – differenceElectrochemical gradient – difference in

charge and concentrationPositive cations move toward negative areas,

negative anions move toward positive areasIon location can be controlled with gates that

can open or close the pore

Ion Channels

Leakage channelsVoltage-gated channelsLigand-gated channelsMechanically gated channels

Ion Channels

Leakage channels Randomly open and close most cells leak more potassium (K+) than sodium

(Na+) voltage-gated channels

Opens in response to a change in voltage (membrane potential)

Generate and conduct action potentials

Ion Channels

Ligand-gated channels Opens and closes in response to chemical stimulus

(nts, hormones, other ions – ligand molecule) Ex: Ach opens channels that allow Na+ and Ca2+ to

go in and K+ to go out Work in 2 ways

ligand molecule can open or close the channel itself by binding

Ligand molecule activates another chemical messenger to open the channel

Ion Chanenls

Mechanically gated Opens or closes in response to stimulation by:

Vibration: sound waves Pressure: touch Tissue stretching

The channel is physically disrupted and opens

Ion Channels

Resting Membrane Potential

Exists due to negative ions in cytosol (ICF) and equal positive ions in ECF

The greater the difference in charge, the larger the potential

Example: 5 Na+ on outside, 4 Cl- on inside – small potential 25 Na+ on outside, 4 Cl- on insdie – great potential

RMP

Most cells have potential between -40 to -90 mV; typical is -70mV

Minus sign means the cell is negative – not negative potential!

Any cell with potential is polarized The potential varies between +5 to -100 mV

How does the potential get there?

ECF Rich in Na+ and Cl-

ICF Rich in K+ Also has P-, amino acids,

RMP

Ion interaction There are many K+ leakage channels, so K+ diffuse

out + ions exit, inside becomes more negative - ions can’t leave because they are bound to molecules - charges attract the K+ back in toward the cell Few Na+ ions leak inward This would destroy the membrane potential, so there

are pumps that take care of this

Na+/K+ pump

To keep the RMP Pump out Na+ as it leaks in Pump in K+ as it leaks out 3 Na+ for each 2 K+ - this still maintains a negative

charge in the cell

RMP - Draw

Review

Define resting membrane potential.Describe each type of ion channel.Describe the cellular processes that create

the resting membrane potential.

Potentials

Objectives Describe what causes a graded potential. Understand the process for creating an action

potential. Explain depolarization and repolarization, including

the relationship between them.

Graded Potentials

When a stimulus causes a channel to open or close in an excitable cell

Causes more polarization (more - inside)Causes less polarization (less – inside)Hyperpolarized (much more – inside)Depolarized (much less – inside)Graded signals – vary in size

Spread out along plasma membrane and die out Only used for short distance communication

Generation of Action Potentials

Action potential (AP) or impulse – sequence of rapidly occurring events that take place in two phases Depolarizing phase – negative membrane potential

becomes Less negative Reaches 0 positive

Repolarizing phase – membrane potential is Restored to the resting state of -70 mV

Action Potential

Depol. vs. Repol.

Na+ channels openNa+ rushes into cell

All or none principle when depolarization reaches the threshold, the AP

occurs AP is always the same size Like dominos – no matter how hard you push the first

domino they will all fall or they won’t – same goes for the AP

K+ channels openK+ flows out of cell

Depolarizing Phase

Na+ channels open rapidly – Na+ rushes into the cell

Potential goes from -55 to +30 (inside is 30 more positive than the outside)

The more Na+ there is, the more channels open – this is a ____________-feedback mechanism

Repolarizing Phase

K+ channels open as the Na+ channels are closing

Na+ inflow slows, K+ outflow increasesPotential goes from +30 to -70

Depol. and Repol.

Painkillers

Local anesthetics – block pain and other body sensations

Novacaine or LidocaineBlock the opening of Na+ channelsThe nerve impulse can’t go past the blocked

region – CNS never gets the message that there is pain

Review

What causes a graded potential?Describe what happens to cause an action

potential.What happens during depolarization? What

does this cause?What happens during repolarization? What

does this cause?

Signal Transmission Synapses

Objectives Explain the events of signal transmission at a

chemical synapse.

Synaptic vocabulary

Presynaptic neuron – the one sending the signal

Postsynaptic neuron – the one receiving the signal

3 types Axodendritic – axon to dendrite Axosomatic – axon to body cell Axoaxonic – axon to axon

Electrical Synapse

Action potentials conduct between cells through gap junctions Tunnels connect the two cells together and the

electrical impulse can pass places:

Smooth muscle, cardiac muscle, developing embryo advantages:

Faster – don’t have to “jump the gap” Synchronization – allows groups of cell’s to work

together – heart beating

Electrical Synapse

Chemical Synapse

Pre- and post-synapses do not touch – separated by synaptic cleft

Transfer of signal must occur Pre-synaptic neuron turns electrical signal into

chemical signal (neurotransmitter) Post-synaptic neuron turns chemical signal into

electrical signal this transfer takes more time than electrical

signals

Chemical Synapse

1. Nerve impulse arrives2. Depolarizing phase

opens Ca++ channels, Ca++ flows in

3. Ca++ causes release of NT from vesicles

4. NT binds to receptors on postsynaptic neuron

5. Action potential continues (Na+, K+, Cl-)

Review

What are the differences between a chemical and electrical synapse?