Nervous System and
Nervous Tissue
Master control and communication
Functions (system level and cell level)
 Sensory input – monitoring stimuli
 Integration – interpretation of sensory input
 Motor output – response to stimuli
PNS
Dendrites: input
CNS
Cell body: integration
Axon: output
PNS
Central nervous system (CNS)
Form: Brain and spinal cord
Function: Integration and command center
Peripheral nervous system (PNS)
Form: Paired spinal and cranial nerves
Function: Carries messages to and from the
spinal cord and brain
Central Nervous
System
Peripheral Nervous
System
Sensory
(afferent)
Motor (efferent)
Somatic
(voluntary)
Sympathetic
(Action! Go!)
Autonomic
(involuntary)
Parasympathetic
(Stop! )
INPUTS: Sensory (afferent) division
 Sensory afferent fibers – from skin, skeletal
muscles, and joints to the brain
 Visceral afferent fibers – from visceral organs
to the brain
OUTPUTS: Motor (efferent) division
 Transmits impulses from the CNS to effector
organs
Somatic nervous system (SNS)
 Conscious control of skeletal muscles
Autonomic nervous system (ANS)
 Regulates involuntary muscle (smooth and
cardiac) and glands
▪ Sympathetic (Stimulates = Go!)
▪ Parasympathetic (Conserves = Stop!)
Neurons
 Transmit electrical signals
Neuroglia (“nerve glue”)
 Supporting cells
Neuroglia in the CNS
Neuroglia in the PNS
 Astrocytes
Satellite cells
 Microglia
Schwann cells
 Ependymal cells
 Oligodendrocytes

Structural units of the nervous system

Long-lived (100+ years)

Amitotic (no centrioles = can’t divide)

High metabolic rate (glucose gobblers!)
Sensory (afferent)
 transmit impulses toward the CNS
Motor (efferent)
 transmit impulses away from the CNS
Interneurons (association neurons)
 shuttle signals through CNS pathways
Neuron cell body
Dendrites
Cell body
Dendritic
spine
(a)
Nissl bodies
Axon hillock
(b)
Impulse
direction
Node of Ranvier
Schwann cell
Axon terminals
(secretory component)

Contains nucleus and nucleolus

Major biosynthetic center

Focal point for the outgrowth of neuronal
processes (dendrites and axons)
 Axon hillock – where axons arise
Dendrites
 Numerous
 Short and tapering
 Diffusely branched
 Contain “spines” where synapses form
Axons
 One per cell
 Long (up to 4 ft. in length)
 Form synapses at terminals (release neurotransmitters)
 Anterograde and retrograde transport (out and back!)

Provide a supportive scaffolding for neurons

Segregate and insulate neurons

Guide young neurons to the proper connections

Promote health and growth

Help regulate neurotransmitter levels

Phagocytosis

Most abundant and versatile

Cling to neurons and synaptic endings

Cover capillaries (blood-brain barrier)

Support and brace neurons

Guide migration of young neurons

Control the chemical environment

Monitor health of neurons

Transform into macrophages to remove cellular
debris, microbes and dead neurons
NOTE: Normal immune system cells can’t enter CNS
Shape: squamous to columnar
(often ciliated)
Location: Line the central cavities
of the brain and spinal column
Function: Circulate cerebrospinal
fluid

Wrap CNS axons like a jelly roll

Form insulating myelin sheath
Schwann cells
 Surround axons of the PNS
 Form insulating myelin sheath
Satellite cells
 Surround neuron cell bodies
Nodes of Ranvier
Myelin Sheath

White, fatty sheath protects long axons

Electrically insulates fibers

Increases the speed of nerve impulses
Neurilemma

remaining nucleus and cytoplasm of a
Schwann cell

Both myelinated and unmyelinated fibers are
present

Oligodendrocytes insulate up to 60 axons each
White matter: dense collections of myelinated
fibers
Gray matter: mostly soma and unmyelinated
fibers

Electrical impulses carried along the length
of axons

Always the same regardless of stimulus

Based on changes in ion concentrations
across plasma membrane

This is HOW the nervous system functions
Voltage (V)
 potential energy from separation of charges (+ and -)
 For neurons, measured in millivolts
Current (I)
 the flow of electrical charge between two points
Insulator
 substance with high electrical resistance
 Think myelin sheath!
Passive (leakage) channels: always open
Voltage-gated channels: open and close in
response to membrane potential
Ligand-gated (chemically gated) channels: open
when a specific neurotransmitter binds
Mechanically gated channels: open and close in
response to physical forces
When gated channels are open:
 Ions move along electro-chemical gradients
▪ Takes into account charge differences
▪ Takes into account concentration differences
 An electrical current is created
 Voltage changes across the membrane
Resting membrane potential (–70 mV)
 The inside of a cell membrane has more negative charges than
outside the membrane
 Major differences are in Na+ and K+
Depolarization
 the inside of the membrane becomes less
negative
Hyperpolarization
 the inside of the membrane becomes more
negative than the resting potential
Repolarization
 the membrane returns to its resting membrane
potential

Principal means of neural communication

A brief reversal of membrane polarity

All or nothing event

Maintain their strength over distance

Generated only by muscle cells and neurons
1.
Resting state
2.
Depolarization
3.
Repolarization
4.
Hyperpolarization
5.
Return to resting
potential

Na+ and K+ GATED channels are closed

Each Na+ channel has two voltage-regulated gates
 Activation gates
 Inactivation gates

Na+ permeability increases; membrane potential reverses

Na+ gates are opened, but K+ gates are closed

Threshold: critical level of depolarization (-55 to -50 mV)

Once threshold is passed,action potential fires

Sodium inactivation gates close

Voltage-sensitive K+ gates open

K+ rushes out

Interior of the neuron
is negative again

Potassium gates remain open
 Excess K+ leaves cell

Membrane becomes hyperpolarized

Neuron is
insensitive to
stimuli until resting
potential is restored
Repolarization
 ONLY restores the electrical differences
across the membrane
 DOES NOT restore the resting ionic conditions
Sodium-potassium pump restores ionic
conditions
 More sodium outside
 More potassium inside
Sodium
channel
Na+
Potassium
channel
Activation
gates
Inactivation gate
Na+
1
K+
Na+
Resting state
K+
K+
4
2
Na+
Hyperpolarization
K+
3
Repolarization
Depolarization

http://outreach.mcb.harvard.edu/animations/actionpotential.swf

http://www.youtube.com/watch?v=SCasruJT-DU

http://bcs.whfreeman.com/thelifewire/content/chp44/4402s.swf

http://www.blackwellpublishing.com/matthews/channel.html
Absolute refractory period (NO WAY! NO HOW!)
 Neuron CANNOT generate an action potential
 Ensures that each action potential is separate event
 Enforces one-way transmission of nerve impulses
Relative refractory period (Well, maybe…)
 Threshold is elevated
 Only strong stimuli can generate action potentials
Membrane potential (mV))
Voltage
at 2 ms
+30
–70
Voltage
at 0 ms
(a) Time = 0 ms
Resting potential
Peak of action potential
Hyperpolarization
Voltage
at 4 ms
(b) Time = 2 ms
(c) Time = 4 ms
Stronger stimuli generate more frequent action potentials
Velocity determined by
 Axon diameter
▪ the larger the diameter, the faster the impulse
 Presence of a myelin sheath
▪ Myleinated neurons have much faster impulses
▪ Why? Node-jumping! (Saltatory conduction)
Cause: Autoimmune disease with symptoms appearing
in young adults (women at highest risk)
 UNKNOWN environmental and genetic factors
Symptoms: visual disturbances, weakness, loss
of muscular control, incontinence
Physiology
 Myelin sheaths in the CNS are destroyed, producing a
hardened lesion (scleroses)
 Shunting and short-circuiting of nerve impulses occurs
 Alternating periods of relapse and remission
Treatment
 Drugs that modify immune response
Prognosis
 Medications can prevent symptoms from worsening
 Reduce complications
 Reduce disability
 HOWEVER, not all drugs work long-term in all patients
Junction for cell  cell communication
 Neuron neuron
 Neuron  effector cell
Presynaptic neuron
 Conducts impulses toward the synapse
Postsynaptic neuron/cell
 Receives signal
 May/may not act on signal

Less common

Resemble gap junctions

Allow direct ion flow cell  cell
Important in the CNS
 Neural development
 Synchronization of activity
 Emotions and memory

Most common

Excitatory or inhibitory

Communication by neurotransmitters
 Presynaptic neuron releases neurotransmitter
 Postsynaptic neuron has membrane-bound receptors

Neurotransmitters must be recycled, removed or
degraded after release
Neurotransmitter
Ca2+
1
Axon terminal of
presynaptic neuron
Postsynaptic
membrane
Postsynaptic
membrane
5
Degraded
neurotransmitter
2
Ion channel
(closed)
Receptor
Ion channel open
vesicles
containing
Neurotransmitter
Synaptic
cleft
Na+
3
4
Ion channel closed
Ion channel (open)
NOTE: Ion channels are chemically gated, not voltage-gated

Acetylcholine (ACh)

Biogenic amines (dopamine, serotonin)

Amino acids (glutamate, GABA)

Peptides (endorphins, enkephalins)

Novel messengers
 ATP
 Nitric oxide (why Viagra works!)
 Carbon monoxide
Direct
 Alter ion channels
 Rapid response
 Important in sensory-motor coordination
 Ex.) ACh, GABA, glutamate
Indirect
 Work via second messengers and G-proteins
 Slower action
 Important in memory, learning, and autonomic
nervous system
 Ex.) dopamine, serotonin, norepinephrine
EPSP: excitatory postsynaptic potentials
 Cell is depolarized
 Ex.) glutatmate
IPSP: inhibitory postsynaptic potentials
 Cell is hyperpolarized
 Ex.) GABA
Will the postsynaptic cell fire? It depends on…
Which neurotransmitter is released
The amount of neurotransmitter released
The length of time the neurotransmitter is bound to
receptors
If threshold isn’t reached, no action potential
Spatial summation
 Multiple potentials arrive at the same time
 Number of IPSPs v. EPSPs determine if action
potential is generated
Temporal summation
 Multiple potentials arrive at different times
 Time intervals determine if action potential is
generated
NOTE: This is oversimplified. One neuron can receive inputs
from thousands of other neurons.
Depression
 Often linked to altered levels of serotonin
 Treated with SSRIs (selective serotonin reuptake
inhibitors)
 Provides greater signal from less neurotransmitter
WARNING: Suicide risk can actually increase in
some patients, particularly adolescents and young
adults.
Addiction
 Dopamine is essential in “reward” pathways
▪ Triggers pleasurable sensations
▪ Involved in both drug and alcohol addiction
 Glutamate is essential in memory pathways
▪ May trigger relapses
BoTox = botulinum toxin
 Works by blocking acetylcholine release at
neuromuscular junction
 Facial muscles can’t contract, wrinkles disappear
 Also used for many spastic disorders
Local anaesthesia
 Most block sodium channels, so action potentials
aren’t generated