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The Nervous System
It is comprised
of 3 basic
components




Brain
Spinal cord
Nerves
General Functions of the
Nervous System

Sensation


Integration


Monitors changes/events occurring in and outside the body. Such changes
are known as stimuli and the cells that monitor them are sensory receptors.
The parallel processing and interpretation of sensory information to
determine the appropriate response (Conscious or subconscious)
Reaction

Motor output or the activation of effectors such as muscles or glands
(typically via the release of neurotransmitters).
Organization of the Nervous System
2 big initial divisions:
1.
Central Nervous System


2.
The brain + the spinal cord
The center of integration and
control
Peripheral Nervous System


The nervous system outside of
the brain and spinal cord
Consists of:


31 Spinal nerves

Carry info to and from
the spinal cord
12 Cranial nerves

Carry info to and from
the brain
What is the basic functional unit of
the nervous system?







Neuron!
What does the neuron do?
Carries messages throughout
the body
How does it carry the messages?
By conducting electrical signals
What are these signals called?
Nerve impulses!
Neuron Anatomy
Cell Body

Three parts to a NEURON:
1.
Cell body:
Large, central portion of
the neuron where all
organelles are located.


What is its job?
Interpret incoming signals
Neuron Anatomy
2. Dendrites:
Short, highly branched
fibers
What is its job?
 Carries impulses toward
the cell body
 Referred to as the
afferent process

Dendrites
Neuron Anatomy
3. Axon:
Long, slightly branched fiber
What is its job?
 Carry impulses away from the cell body
 Referred to as the efferent process

Neuron Anatomy
Three parts of the Axon:
A.
Axon hillock: site of initiation of an action potential
(point where axon and cell body meet)
B.
B. Axon fiber: the main portion of the axon
C.
C. Axon terminal:branched end of the axon (point of
communication with other cells)
The Axon
Axon Hillock
fiber
Axon Terminals
Neuron Anatomy
Additional Parts of a Neuron
Structure of a Typical Neuron
Dendrite
Axon terminal
Cell body
Nodes of
Ranvier
Axon
Nucleus
Myelin sheath
Neuron anatomy
What do you remember?
1
dendrites
6
Myelin
sheath
2
nucleus
Direction
of impulse
3
Cell body
5axon
Nodes of
Ranvier4
Axon
7
terminals
3 Types of Neurons
A. Sensory Neurons: Receive incoming stimuli

Five types of sensory neurons:
o
o
o
o
o
Thermo-receptors
Mechano-receptors
Chemo-receptors
Photo-receptors
Pain-receptors
Thermo-receptors

Location:




Skin
Hypothalamus
Body Core
Function:


Sensation of hot and cold
Detects change in body
core temp.
Mechano-receptors

Location:




Skin
Skeletal muscle
Inner ear
Function:





Touch
Pressure
Muscle movement
Motion
Sound
Chemo-receptors

Location:
Nose
 Tongue (taste buds)
 Blood vessels


Function:
Smell
 Taste
 Detects levels of CO2 in
blood

Photo-receptors

Location:


Eyes
Function:

Allow vision thru detection of light
Pain-receptors

Location:


Everywhere, except the
brain
Function:
Sensation of pain
 Detects chemicals
released by damaged
cells

Three Types of Neurons
B. Motor Neurons:
Carry impulses to muscles and glands
Cause a response to some stimuli
C. Interneurons:
Connect sensory and motor neurons
Allow for quick response (reflex action)
Three types of neurons
Structural diversity in neurons

Multipolar- many dendrites, one axon


Bipolar- one dendrite, one axon


Most neurons in CNS
Sensory organs
Unipolar- sensory

Axon termini extend into CNS
Neurons

What is grey matter?
 Collective
cell bodies and
dendrites of all neurons

What is white matter?
 Myelinated
nerve fibers
 Axons of all neurons
 Can be approximately
one meter in length
White matter
Gray matter
What is a Nerve?





Bundle of axons
held together by
connective tissue.
What color is
a nerve?
White!
Why?
Because axons
are white matter
and they compose
nerves
How are nerves held together?
Connective tissue
 What is this connective
tissue called?
 Neuroglial cells (nerve glue)
 Approximately half of the
volume of the brain is composed
of neuroglial cells
 Most brain tumors develop in mesoglial cells – NOT
neurons

What do neuroglial cells do?



Support the axons
Insulate the electrical impulses
Like electrical tape insulates
electric wires this prevents “leaking”
of electric signals
Four types of neuroglia in CNS

Oligodendrocytes
 Myelinating

Astrocytes
 Connects

neurons and blood vessels together
Microglia
 Phagocytes

cells
(from bone marrow)
Ependymal cells
 Line
ventricles of brain; produce cerebrospinal fluid
(CSF)
Neuroglia of the PNS

Schwann cells
Myelinating cells
 Help direct axon
regeneration


Satellite cells
Support, protection,
regulation of molecular
exchange
 “Filter out” other stimuli

The Nerve Impulse

Resting potential – the charge that exists across a
neuron’s membrane while at rest.
 -70
mV.
 This is the starting point for an action potential.
The Nerve Impulse

A nerve signal or action potential is an
electrochemical message of neurons.
 An
all-or-none phenomenon – either the fiber is
conducting an action potential or it is not.


Across its plasma membrane, every cell has a
voltage called a membrane potential.
The inside of a cell is negative relative to the
outside.
The Nerve Impulse

Neuron at rest – active transport channels in the
neuron’s plasma membrane pump:
ions (Na+) out of the cell.
 Potassium ions (K+) into the cell.
 Sodium

More sodium is moved out; less potassium is moved
in.
 Result
is a negative charge inside the cell.
 Cell membrane is now polarized.
Sodium-Potassium Exchange Pump

Na+ flows into the
cell during an action
potential, it must be
pumped out using
sodium pumps so
that the action
potential will
continue.
The Nerve Impulse

A nerve impulse starts when pressure or other
sensory inputs disturb a neuron’s plasma membrane,
causing sodium channels on a dendrite to open.
 Sodium
ions flood into the neuron and the membrane
is depolarized – more positive inside than outside.
The Nerve Impulse

This moving local reversal of voltage is called an action
potential.



After the action potential has passed, the voltage gated
channels snap closed and the resting potential is restored.


A very rapid and brief depolarization of the cell membrane.
Membrane potential changes from -70 mV to +35 mV.
The membrane potential quickly returns to -70 mV during the
repolarization phase.
An action potential is a brief all-or-none depolarization of a
neuron’s plasma membrane.


Carries information along axons.
An action potential is self-propagating – once started it continues to
the end.
Synapses: Junctions Between Nerves



Eventually, the impulse
reaches the end of the axon.
Neurons do not make direct
contact with each other.
The junction between the
axon of one neuron and the
dendrite of the next is
called a synapse .
Synaptic Pathways



Presynaptic neurons bring action potentials toward
the synapse.
Postsynaptic neurons carry action potentials away
from the synapse.
A synaptic cleft is the small gap between the two
neurons.
Neurotransmitters

Chemical messengers
called
neurotransmitters
carry the message of
the nerve impulse
across the synapse.
Neurotransmitters

Neurotransmitters are released into the synapse
and bind with receptors on the postsynaptic cell
membrane, which cause ion channels to open in the
new cell.
Acetylcholine – Example
Neurotransmitter
Reflex Arc

A simple reflex produces a very fast motor response to a
stimulus because the sensory neuron bringing information
about the stimulus passes the information directly to the motor
neuron.
Reflex Arc


Usually, there are interneurons between sensory and motor
neurons.
An interneuron may connect two neurons on the same side of
the spinal cord, or on opposite sides.
The Central Nervous System
 Meninges
– are membranes that protect the brain and
spinal cord
 Dura
mater (outermost layer)
 Arachnoid membrane ( middle layer)
 Pia mater (innermost layer)
The Central Nervous System




Cerebrospinal Fluid (CSF)
Located between the arachnoid mater and pia mater
is an area called the subarachnoid space
Continuously secreted from specialized cells (ependymal
cells) in the choroid plexus in ventricles
Functions: Physical and chemical protection of the CNS
45
Spinal Cord
Slender structure that is continuous with the brain
Descends into the vertebral canal and ends around the
level of the first or second lumbar vertebra.
Function of the spinal cord is to carry sensory
information to and from the brain
 31 spinal segments:
 8 cervical segments
 12 thoracic segments
 5 lumbar segments
 5 sacral segments
 1 coccygeal segment
46
Spinal Cord - Ascending and
Descending Tracts
 Ascending tracts - carry
sensory information up to the
brain
 Descending tracts - carry
motor information down from
the brain to muscles and
glands
The Brain
Four Parts:




47
Cerebrum
Diencephalons
Brain stem
Cerebellum
The Brain - Cerebrum
48
 Largest part of the
brain
 Two halves cerebral
hemispheres
 Thick bundle of nerve
fibers called the
corpus callosum
connect the two
hemispheres
 Lobes
 Frontal
 Parietal
 Temporal
 Occipital
 Cortex
 Ventricles
The Brain - Diencephalons
49
Located between the cerebral hemispheres
and is superior to the brain stem
 Thalamus - relay
station for sensory
information that
heads to the
cerebral cortex for
interpretation
 Hypothalamus maintains balance
by regulating many
vital activities such
as heart rate, blood
pressure, and
breathing rate.
The Brain - Brain stem
50
Connects the cerebrum to the spinal cord
Midbrain - controls
both visual and
auditory reflexes
Pons - regulates
breathing
Medulla oblongata
- controls many vital
activities such as
heart rate, blood
pressure, and
breathing
The Brain - Cerebellum
51
 Inferior to the
occipital lobes of
the cerebrum
 Posterior to the
pons and medulla
oblongata
 Coordinates
complex skeletal
muscle
contractions that
are needed for
body
movements
Diseases and Disorders of the Nervous
System

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Cerebrovascular Accident: CVA
Commonly called a stroke
The result of a ruptured blood
vessel supplying (cerebral
hemorrhage) a region of the
brain or a vessel is obstructed by
a clot.
Brain tissue supplied with
oxygen from that blood source
dies, swelling occurs in the brain
due to leaking of blood from
vessels.
Loss of some functions or death
may result
This is due often to elevated
blood pressure or hypertension.
Diseases and Disorders of the Nervous
System

Epilepsy: This disease is due to
random, mis-firing of neurons
within the brain affecting
sensory and motor regions of
the brain. Ranging in effects
from sleep-like state of
consciousness (narcolepsy),
muscle paralysis and spasms
(Petit mal and Grand mal
seizures). Still not understood
why this disease occurs.
However in some cases it can
result from brain trauma or
injury.