Nervous Tissue
CH. 12
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)
Nervous System
CNS – Central NS
PNS – Peripheral NS
 All tissue outside the
 Contains brain and
spinal cord (axial)
 Processes sensory info
 Source of thoughts,
emotions, memories
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
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

ANS Branches
Sympathetic division
Parasympathetic division
 Supports exercise or
 “rest and digest”
emergency actions
 “fight or flight”
 Ex: increases heart rate
 Ex: decreases heart
rate
 Usually work in
opposing actions
ENS
 “brain of the gut”
 Controls all activities associated with digestion and
the gastrointestinal (GI) tract
 Involuntary
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 vs. Neuroglia
Neurons
Neuroglia
 Provide unique
 Support, nourish, and
functions
 Sensing, thinking,
remembering,
controlling muscle
activity, regulating
glandular secretions
protect the neurons
 Maintain homeostasis
in the interstitial fluid
that bathes them
Neurons
 Vocabulary:
 Neuron – nerve cell
 Electrical excitability


Stimulus


the ability to respond to a stimulus and convert it into an action
potential
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 fluid
 PNS
 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 insulation
Increases speed of nerve impulse
If a cell has myelin we say that it is myelinated
Gaps 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 – difference
 Electrochemical gradient – difference in charge and
concentration
 Positive cations move toward negative areas,
negative anions move toward positive areas
 Ion location can be controlled with gates that can
open or close the pore
Ion Channels
 Leakage channels
 Voltage-gated channels
 Ligand-gated channels
 Mechanically 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 open
 Na+ rushes into cell
 K+ channels open
 K+ flows out of 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
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 increases
 Potential goes from +30 to -70
Depol. and Repol.
Painkillers
 Local anesthetics – block pain and other body
sensations
 Novacaine or Lidocaine
 Block the opening of Na+ channels
 The 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.
2.
3.
4.
5.
Nerve impulse arrives
Depolarizing phase
opens Ca++ channels,
Ca++ flows in
Ca++ causes release of
NT from vesicles
NT binds to receptors
on postsynaptic neuron
Action potential
continues (Na+, K+, Cl)
Review
 What are the differences between a chemical and
electrical synapse?