Download Nervous System

Nervous System
Functions and Divisions of
the nervous system
 The nervous system has three basic
functions:
 Sensory input: gathering information.
 Integration: interpreting sensory input and
what to do with it.
 Motor output: causes a response by
activating effector organs (muscles and
glands).
Dividing the nervous
system
 The nervous system can be subdivided
into two parts:
 The central nervous system:
 The brain and spinal cord
 The peripheral nervous system:
 All other neural units outside the brain and
spinal cord
Further subdividing the
PNS
 The peripheral nervous system is further
subdivided in these two parts:
 Sensory/afferent division:
 Consists of nerve fibers called axons that send
impulses to the CNS from receptors throughout the
body.
 Sensory fibers sending impulses from muscles are called
somatic afferent fibers.
 Fibers sending impulses from visceral organs are called
the visceral afferent fibers.
 Motor/efferent division: transmits
information from CNS to effectors like
muscles and glands that allow them to
respond.
Subdividing the motor
division
 The motor division of the PNS is also
subdivided into two parts:
 Somatic nervous system: voluntary, allows
conscious control of muscles.
 Autonomic nervous system: Involuntary,
responsible for control muscles and other
organs that we cannot voluntarily control,
like the heart.
Subdividing the ANS
 Sympathetic division: “fight or flight”
reactions. (Decrease digestion speed,
increase heart rate, etc.)
 Parasympathetic division: involves organ
stimulation when body is at rest. (slow
heart beat, normal digestive rates,
normal breathing, etc.)
Sympathetic v.s.
parasympathetic response
 Sympathetic Structure
 Rate increased
 Force increased
Heart
Heart

Lungs
Broncial muscle relaxed
 Pupil dilation
Eye
 Motility reduced Intestine
 Sphincter closed Bladder

Decreased urine secretion Kidneys
Parasympathetic
Rate decreased
Force decreased
Broncial muscle contracted
Pupil constriction
Digestion increased
Sphincter relaxed
Increased urine secretion
Histology of Nervous
Tissue
 Neurolgia (also called glial cells) are
smaller cells that are closely associated
with the nervous system.
 There are six types of glial cells in the
nervous system, four in the CNS and two
the PNS.
CNS Neuroglia
 The glial cells of the CNS include:
 Astrocytes: most abundant, looks star
shaped in the cell. Braces neuron and
anchors it to its food and oxygen supply
line (capillaries)
 Microglia: oval shaped cells with a thorny
branches. Can become a type of
macrophage to fight foreign invaders.
 Ependymal cells: squamous shaped with
many cilia. Helps move ceuribrospinal
fluid (which acts as a cushion) around the
brain and spinal cord.
 Oligodendrocytes: Branched like
astrocytes, but with less branches. They
form myelin sheathes, which are used for
insulation.
Neuroglia in PNS
 The PNS contains two types of glial cells:
 Satellite cells: Serves most of the same
functions of astrocytes in the CNS.
 Schwann cells: bundles and forms myelin
sheathes around large nerve bundles,
similar to oligodendrocytes.
Neurons
 Structural unit of the nervous system.
 Stats:
 Conducts nerve impulses
 Incredibly long life: can last 100 years or more with
good nutrition.
 They are amitotic (can’t divide). Very few places
where neurons can regenerate (must come from
stem cells)
 High metabolic rate. Needs a lot of oxygen and
nutrients.
Neuron: Cell Body
 Contains a spherical nucleus, and a
perikaryon (the body of the neuron).
 The neuron’s protein making machinery
comes from ribosomes and rough ER.
 The rough ER in a neuron is called a
Nissl bodies.
 Neurofilaments: give structural integrity to
the cell body.
 Neuron clusters located in the CNS are
called nuclei.
 Neuron clusters located in the PNS are
called ganglia.
 Neurofibrils (neurofilament) and
microtubules are found in the neuron to
help hold the cells shape.
The Process (tail part)
 Extends from the body of the neuron.
 The processes are called tracts in the
CNS.
 The processes are called nerves in the
PNS.
Dendrites
 Dendrites are branching extensions that
are found on the neuron and are
designed to pick up chemical messages.
The chemical messages are sent
towards the cell body (in other words,
dendrites are receivers)
The axon
 The axon is designed to generate an
action potential within the neuron. This is
where the “impulse” part of a nerve
impulse comes from.
 Impulses are carried down the “tail”
called a nerve fiber until it reaches an
axon terminal which releases chemicals
that signal the next neuron.
Neuron Classification
 Most neurons are classified by either
structure or function.
Structural Classification
 Unipolar-have a single short process that
from the cell body will divide into a Tshape to form a proximal and distal
process.
 Usually associated with sensory
receptors.
 Bipolar- have two processes. Has an
axon and dendrite extending from
different parts of the neuron.
 Found only in special sensory
organs like the retina and the
olfactory canal.
 Multipolar- have three or more
processes: One axon, and the rest
dendrites. More than 99% of neurons
are multipolar.
 Major neuron type
of the CNS.
Classification Based on
Function
 Sensory (afferent) neurons- Transmits
nerve impulses toward the CNS. Mostly
unipolar neurons.
 Motor (efferent) neurons- carries
impulses away from the CNS towards the
effector organs.
 Usually multipolar.
 Usually Located in the CNS.
 Interneurons (association neurons)- act
as shuttle and pathways for impulses to
travel.
 Most all are multipolar.
Membrane Potentials
 Neurons are highly irritable. Their
response to a stimulus is an action
potential (a.k.a. nerve impulse).
Voltage, Resistance, and
Current
 The measure of potential energy
generated is called voltage.
 The difference in voltage between two
points is called potential difference.
 The flow of electrical charge is called
current.
 Resistance is the hindrance of electrical
flow.
Ohm’s Law
 Ohm’s law shows the relationship
between current, voltage and resistance.
 Current = Voltage/Resistance
 Less resistance equals greater current.
 Greater voltage equals greater current.
Roles of membrane ion
channels
 Membrane ion channels are large
proteins found in nerve cells. They are
designed to regulate the flow of ions
within a neuron.
Channel types:
 Nongated channels- always open.
 Gated channels- change shape in response to
a certain stimulus.
 Chemical gated channels- respond to a
certain chemical stimulus (neurotransmitters).
 Voltage gated channels- change in response
to membrane potential.
 Mechanically gated channels- opens in
response to a physical deformation in the
protein.
Resting membrane
potential
 Resting potential in a neuron represents
the standing charge in a neuron when at
rest.
 RP in a neuron- -40 mV to -90 mV.
 Most common ion pumps in neurons: the
sodium-potassium pump.
 The negative sign means that there is a
negative charge within the cell.
Sodium-potassium pumps
 Sodium-potassium pumps help stabilize
membrane potential by maintaining the
proper levels of sodium and potassium
within the cell.
Membrane potential acts
as a signal
 Neurons use change in their membrane
potential to receive, integrate, and send
information.
 Changes in membrane potential can be
caused by:
 1. anything that alters the ion concentration
on either side of the cell membrane.
 2. anything that changes membrane
permeability.
Depolarization and
hyperpolarization
 These terms are relative to the resting
membrane potential within the neuron.
 Depolarization- becoming less negative.
Moving closer to zero. i.e.- -70 mV to -65mV.
(think of it as becoming more positive)
 Hyperpolarization- Becoming more negative.
i.e. moving from -70 mV to -75 mV.
Graded potentials
 Graded potentials- short lived localized
changes in membrane potential. Caused
by either hyperpolarization or
depolarization.
Action Potentials
 A.k.a. nerve impulse
 A brief depolarization of the neuron
membrane followed by a quick
hyperpolarization.
 Unlike a graded potential, Action
potentials don’t change in strength over a
distance.
Generation of an action
potential
 Generating an action potential occurs in four
stages:
 1. Resting State: all Na+ and K+ gates closed.
 2. Depolarization: Na+ gates open.
 3. Repolarization: Na+ channels begin to
inactivate; K+ channels open.
 4. Some K+ channels stay open and Na
Channels reset.
 Action potentials must be propagated
(sent or transmitted) down the entire
length of the neuron.
 When A Na+ channel closes during AP
generation, no new Na+ is coming in,
because it stays depolarized, the neuron
will always send the AP away from it’s
origin.
 An AP is self propagating, so when it
starts, it will continues without any
outside assistance.
All or Nothing
Phenomenon
 In or for an AP to occur a
certain threshold charge
(around 15 to 20 mV or
depolarization) must
occur. If it is less than
that an AP will not occur.
 There is no “almost” AP.
It either completely
happens, or does not
happen at all.
 Once stimulated, an AP will not be
affected by a stimulus strength. Once
the AP is fired, there is no change in
intensity.
Absolute Refractory
Period
 Simply put, it is the period in an AP
where Na+ channels reset.
 This guarantees that an AP is all or
nothing.
Relative refractory period
 This period follows the absolute
refractory period.
 Na channels return to original states
 Some K channels still open
 Repolarization occurs.
 Stimulus for a new AP at this time is
much higher due to the neuron trying to
rebalance itself.
Conduction Velocity
 How fast does an AP move?
 Impulses can typically travel around 100
m/s.
 The fastest nerve conduction velocity will
be found around neural pathways where
speed is essential (i.e. Brain, spinal cord)
especially in areas that govern reflexes.
 Slower conduction velocity it typically
found rooted closer to internal organs like
the intestines and blood vessels.
Rate of conduction
velocity
 Depends on two things:
 1. Axon diameter- Large axon= greater
speed, less resistance.
 2. degree of myelination- myelin sheaths act
as insulators. Insulators prevent charge
leakage. Thicker myelin sheaths increase
conduction speed.
Nerve Fiber Groups
 Group A: Fibers found in skin, skeleton and
muscles. Largest axons, and thick myelin
sheaths. Velocity: 150 m/s.
 Group B: autonomic nervous system. Found in
visceral organs and fibers, and smaller somatic
fibers. Thin myelin sheaths. Velocity: 15 m/s
 Group C: autonomic nervous system. Found in
visceral organs and fibers, and smaller somatic
fibers. Smallest diameter axon, and are
unmyelinated. Velocity: 1 m/s.
The Synapse
The Synapse
 Synapse are joints where neurons meet. This
a space that impulses must travel through to
reach another neuron.
 Axodendritic synapse: Synapse b/w an axon
and dendrite of another cell.
 Axosomic synapse: Synapses between, two
axons (axoaxonic), or two dendrites
(dendrodendritic), or a dendrite and a cell body
(dendrosomatic).
Electrical synapses
 Electrical synapses consists of gap
junctions found between cell bodies.
 Nerves connected this way are
considered to be electrically coupled.
 An important feature is that they provide
a way to synchronize interconnected
neurons.
Chemical synapses
 Chemical synapses specialize in the release
and reception of chemical neurotransmitters.
 A chemical synapse has two parts:
 A knoblike axon terminal of the presynaptic neuron
that contains synaptic vessels (tiny membrane
bound sacs)
 A neurotransmitter receptor region on the
membrane of the dendtrite or the cell body of the
post synaptic neuron.
Information Transfer across
a chemical synapse
 (Pass out picture)
 In case you lose it, it is on page 409.
Synaptic Delay
 Impulse can travel at speeds up to 150
m/s.
 Impulse across a chemical synapse
however is much slower. The delay,
called a synaptic delay, last between 0.3
and 5.0 ms. This means that the slowest
part of neural transmission is the gap
between neurons.
Question?
 Sex, drugs, and rock ‘n roll, eating till we
are fat and happy! Why does it feel so
good??
Answer
 Our brains are wired to reward us
whenever we do something that is for the
survival of our species.
 This reward system consists of
Dopamine-releasing neurons in areas of
the brain called the ventraltegumental
area (VTA for short) which consists of the
nucleus accumbens and the amygdala.
 When people have “relations” they
release a flurry of chemicals in the brain
including glutamate and norepinephrine,
which then cause the release of
dopamine, which makes you happy.
 Drugs like methamphetamine and
cocaine will stimulate the receptors in the
brain and tell it to release more
dopamine!
 When the body is given outside
neurotransmitters, it will make less of it’s
own.
 Once dopamine producing areas in the
brain become “tapped” they will no longer
produce dopamine, making those who
use it to need it to feel pleasure.
 However dopamine is not the only reason
that addicts stay addicted.
 Remember the glutamate we talked about
earlier?
 Glutamate, a neurotransmitter that is involved
in learning, will continue to signal and cause
more permanent changes in brain chemistry.
 This is why addicts tend to be reoffenders.
“Once an addict, always an addict” comes from
this information.
Neurotransmitters and
their receptors
 Neurotransmitters are basically the
“language” by which the neurons speak
to each other.
 Everything from happiness, rage,
sadness, lust, thought, and sleep are all
controlled by neurotransmitters.
Developmental Aspects of
Neurons
 Neuroblast- the neuron progenitor.
 The nervous system originates from a
dorsal neural tube and neural crest.
 The neural tube is the progenitor of the
CNS.
 A neuroblast will produce axons and
become amitotic and move into the
characteristic cell body-dendrite.
 The area that the axon develops from is
called the growth cone. The growth cone
branches out with branches called
filopodia that guide signals from the
surrounding environment to the cell body.
N-CAM
 Nerve cell adhesion molecules (N-CAM)
are designed to adhere neurons to a
single place.
 Neurotropins chemical signals that tell
neurons which way to line up.
Neuron Death
 When neurons die, either from lack of
oxygen or nutrients, new neurons do not
replace them. When neural tissue is
destroyed; it is gone forever.