Nervous System

The nervous system detects environmental
changes that impact the body, then works
in tandem with the endocrine system to
respond to such events.
– It is responsible for all our behaviors,
memories, and movement.
• It is able to accomplish all these functions because
of the excitable characteristic of nervous tissue,
which allows for the generation of nerve impulses
(called action potentials).

Everything done in the nervous system involves
3 fundamental steps

A sensory function detects internal and external
stimuli.
2. An interpretation is made (analysis).
3. A motor response occurs (reaction)

Sensory system (PNS)

input; sense changes in the
internal and external environment through sensory
receptors
 sensory neurons serve this function

Interpretation system (CNS)

process; interpret the
sensory information, store some aspects, and make
decisions association or interneurons serve this function

Motor response (PNS)

output; respond to the stimuli by
initiating the appropriate action in effectors (muscles and
glands)
 motor neurons serve this function

CNS

Most signals that stimulate muscles to contract and
glands to secrete originate in

PNS is further divided into

A somatic nervous
system (SNS)
– An autonomic
nervous system
(ANS)
– An enteric nervous
system (ENS)

SNS consists of

Somatic sensory (afferent) neurons that convey
information from sensory receptors in the head, body
wall and limbs towards the CNS.
– Somatic motor (efferent) neurons that conduct
impulses away from the CNS towards the skeletal
muscles under voluntary control in the periphery.
– Interneurons are any neurons that conduct impulses
between afferent and efferent neurons within the
CNS.

The ANS consists of

Sensory neurons that convey information from autonomic
sensory receptors located primarily in visceral organs like
the stomach or lungs to the CNS.
2. Motor neurons under involuntary control conduct nerve
impulses from the CNS to smooth muscle, cardiac muscle,
and glands. The motor part of the ANS consists of two
branches which usually have opposing actions:
• the sympathetic division
• the parasympathetic division

Enteric Nervous System (ENS)

The operation of the ENS, the “brain of the
gut”, involuntarily controls GI propulsion,
and acid and hormonal secretions.
• Once considered part of the ANS, the ENS
consists of over 100 million neurons in
enteric plexuses that extend most of the
length of the GI tract.

2 Main Types of Nerve Cells

Neuron – specialized cells of the nervous system that
transmit signals throughout the body;
 “thinking” cells of brain
 has property of electrical excitability
(the ability to respond to a stimulus)
Neuroglia (glial cells) - play a major role in support and
nutrition of the brain, but they do not manipulate
information
maintain the internal environment so that neurons can
do their jobs

Dendrites

Receiving end of neuron
Short, highly branched structures that receive
signals and conduct these impulses toward the
cell body Contain
numerous
receptor sites for binding chemical messengers from other cells

Cell Body / Soma / Perikaryon

Nucleus surrounded by cytoplasm
Contains organelles such as lysosomes, mitochondria,
Golgi complexes, and Nissl bodies
 Nissl bodies = membranous sacs of rough ER of a neuron
 they are the primary site of protein synthesis
Neurofibrils form the
cytoskeleton
No mitotic apparatus is
present (what does this infer?)

Axon

Conducts Impulses AWAY from Cell Body to Other
Cells

axon hillock

where axon joins cell body

initial segment

beginning
of the axon

trigger zone

junction
between axon hillock and
initial segment Axon starts off
as a single fiber
but may branch
off into

Axon Terminals

Fine processes or divisions at end of axon.
Highly branched - interact with the dendritic tree
of neurons “downstream”
Axon Terminals
Some axon terminals
swell into bulb shaped
structures = synaptic
end bulbs

Structural Classification

Based on the number of processes (axons or dendrites) extending
from the cell body

Multipolar neurons

have several dendrites and
only one axon and are located throughout
the brain and spinal cord.
– The vast majority of the
neurons in the human body
are multipolar

Bipolar neurons

have one main dendrite and
one axon.
– They are used to convey the special senses of sight,
smell, hearing and balance.
As such, they are found
in the retina of the eye, the
inner ear, and the olfactory
(olfact = to smell) area of
the brain

Unipolar (pseudounipolar) neurons

contain one
process which extends from the body
and divides into a central
branch that functions as an axon
and as a dendritic root.
– Unipolar structure is often
employed for sensory
neurons that convey touch
and stretching information
from the extremities

functional classification

based
on electrophysiological properties (excitatory or
inhibitory) and the direction in which the AP is
conveyed with respect to the CNS.

Sensory or afferent neurons

convey APs into the CNS
through cranial or spinal nerves. Most are unipolar.

Responsible for sensing a stimulus and sending information about the stimulus to the CNS Information about vision, sound, touch, pain, smell, temperature, position,& pressure

– Motor or efferent neurons

convey APs away from
the CNS to effectors (muscles and glands) to initiate
an action (motor
output)
Most are multipolar
Convey APs away from
CNS to Effectors

Interneurons or Association Neurons

A neuron which forms a connection between
two or more neurons
 Mainly located within the CNS
between sensory and motor neurons
 Integrate (process) incoming sensory
information from sensory neurons and
then elicit a motor response by
activating the appropriate motor
neurons
 Most are multipolar
 Most abundant type of neuron in
our bodies

electrochemical impulses (brain cells)

Within individual neurons,
messages are sent relying on
electrical charges from ions such
as Na+, K+, and others. Between
neurons, messages are relayed
using chemical transmitters

Synapse

Site of communication between two neurons or
between a neuron and another effector cell

Synaptic cleft

the gap
between the pre and post-
synaptic cells

Synaptic End Bulbs Contain

Synaptic vesicles - tiny membrane-
enclosed sacs that store packets of
neurotransmitters
May contain two or even three types
of neurotransmitters, each with
different effects on the postsynaptic
cell

Neurotransmitters

signaling molecules used at
synapses to pass an
excitatory or inhibitory signal
from a neuron to its target

Synaptic Transmission

Electrical impulses or action potentials (AP)
cannot propagate across a synaptic cleft.
Instead, neurotransmitters are used to
communicate
at the synapse, and re-establish the AP in the postsynaptic cell.

Neuron Transport

Substances synthesized or recycled in the
neuron cell body are needed in the axon or at
the axon terminals. Two types of transport
systems carry materials from the cell body to
the axon terminals and back

Slow axonal transport

Always anterograde
o Moves enzymes, cytoskeletal components, and new
axoplasm down the axon during repair and regeneration
of damaged axons
o Damaged nerve fibers regenerate at a speed governed
by slow axonal transport

Fast axonal transport

Fast anterograde transport
o Organelles, enzymes, synaptic vesicles and small
molecules
o Fast retrograde transport
o For recycled materials and pathogens -rabies, herpes
simplex, tetanus, polio viruses

Neuroglia / glial cells

Do not generate nerve impulses Glial cells function to
serve, protect, and
support neurons!

Oligodendrocytes

Creates myelin
sheaths around the
axons of neurons in
the CNS Provide a structural
framework

Microglial Cell

Removes cell debris,
wastes, and pathogens
by phagocytosis

Promote repair in the
CNS

Ependymal Cells

Ciliated cells which line
the ventricles in the brain
and the central canal of
spinal cord
Form a structure called
the “choroid plexus”
Assist in producing,
circulating, and monitoring
cerebrospinal fluid (CSF)

Satellite Cells

Surround neuron cell bodies
in ganglia
Regulate exchange of
materials (O2, CO2, and
nutrients) between neuronal
cell bodies and ISF
Regulate neurotransmitter
levels around neurons in
ganglia

Schwann Cells

Produce myelin
Surround axons in PNS and
myelinate them
Begin formation of myelin
sheath during fetal development
Participate in repair process after
injury

Myelination

Process of forming a myelin sheath which...
 electrically insulates
the axon
 increases the speed of
electric conduction,
thereby enabling nerves
to receive and interpret
messages from the brain
at maximum speed

formed by oligodendrocytes in CNS

Demyelination

Loss of Myelin
Damaged regions of myelin become
hardened scars called “scleroses” that
interfere with the transmission of
nerve impulses.
Due to scleroses, messages are
passed along at less than normal
speeds.
Results in paralysis, loss of
sensation, or loss of vision depending
on part of NS affected

Nodes of Ranvier

Unmyelinated gaps between myelin sheaths surrounding
an axon
 Each Schwann cell wraps one axon
segment between two nodes of Ranvier
 Myelinated nodes are about 1 mm in
length and have up to 100 layers
Amount of myelin greatly
increases from birth to maturity
which increases the speed of
nerve conduction

Neuronal cell bodies are often grouped together in clusters called

Ganglion = cluster of neuronal body cells in PNS

Nucleus = cluster of neuronal cell bodies in the CNS

Axons of neurons are usually group together in bundles

Nerve = bundle of axons in the PNS

Tract = bundle of axons in the CNS

Nerve tissue regions

White and gray matter

White matter

Formed from aggregations of myelinated axons of many neurons
Lipid part of myelin gives white appearance

Fiber Types

The characteristics of the neuronal axon define the
“fiber types”
 A fibers are large, fast (130 m/sec), myelinated
neurons that carry touch and pressure
sensations; many motor neurons are also of this
type.
 B fibers are of medium size and speed (15 m/sec)
and comprise myelinated visceral sensory &
autonomic preganglionic neurons.
 C fibers are the smallest and slowest (2 m/sec)
and comprise unmyelinated sensory and autonomic
motor neurons.

Neurotransmitters

Both excitatory and inhibitory neurotransmitters
are present in the CNS and PNS.
 The same neurotransmitter may be excitatory in
some locations and inhibitory in others.
 For example, acetylcholine (ACh) is a
common neurotransmitter released by many
PNS neurons (and some in the CNS). Ach is
excitatory at the NMJ but inhibitory at other
synapses.

Neurotransmitters

Many amino acids act as neurotransmitters:
 Glutamate is released by nearly all
excitatory neurons in the brain.
 GABA is an inhibitory neuro-
transmitter for 1/3 of all brain synapses.
• Valium is a GABA agonist that enhances
GABA’s depressive effects (causes
sedation).
 Other important small-molecule
neurotransmitters are listed.

Neurotransmitters effects can by modified by

Synthesis can be stimulated or inhibited.
 Release can be blocked or enhanced.
 Removal can be stimulated or blocked.
 The receptor site can be blocked or activated.
• An agonist is any chemical that enhances or
stimulates the effects at a given receptor.
• An antagonist is a chemical that blocks or
diminishes the effects at a given receptor

Postsynaptic Potentials

A neurotransmitter causes either an excitatory or
an inhibitory graded potential:
 Excitatory postsynaptic potential (EPSP)
causes a depolarization of the postsynaptic cell,
bringing it closer to threshold. Although a single
EPSP normally does not initiate a nerve impulse,
the postsynaptic cell does become more
excitable.
 Inhibitory postsynaptic potential (IPSP)
hyperpolarizes the postsynaptic cell taking it
farther from threshold.

Postsynaptic Potentials

Spatial summation occurs when postsynaptic
potentials arrive near the same location. Temporal
summation occurs when postsynaptic potentials
arrive close to the same time.
 Whether or not the
postsynaptic cell
reaches threshold
depends on the
net effect after
Summation of all
the postsynaptic

Neurotransmitter Clearance

If a neurotransmitter could linger in the synaptic
cleft, it would influence the postsynaptic neuron,
muscle fiber, or gland cell indefinitely – removal
of the neurotransmitter is essential for normal
function.
 Removal is accomplished by diffusion out of the
synaptic cleft, enzymatic degradation, and re-
uptake by cells.
• An example of a common neurotransmitter
inactivated through enzymatic degradation is
acetylcholine. The enzyme

Neural Circuits

\Integration is the process accomplished by the
post-synaptic neuron when it combines all
excitatory and inhibitory inputs and responds
accordingly.
 This process occurs over
and over as interneurons
are activated in higher
parts of the brain (such
as the thalamus and
cerebral cortex

A neuronal network may contain

thousands or
even millions of neurons.
 Types of circuits include diverging, converging,
reverberating, and parallel after-discharge.

Neural circuit -diverging circuit

a small number of neurons in
the brain stimulate a much larger number of neurons
in the spinal cord. A converging circuit is the
opposite

Neural circuit - reverberating circuit

impulses are sent back
through the circuit time and time again – used in
breathing, coordinated muscular activities, waking up,
and short-term memory

Parallel after-discharge circuits

involve a single
presynaptic cell that stimulates a group of neurons,
which then synapse with a common postsynaptic cell –
used in precise activities such as mathematical
calculations.