Transcript Nervous System Notes

Anatomy and Physiology
The Nervous System
Functions of the Nervous System
1. Sensory Function –
senses certain
changes (stimuli) both
within the body
(internal environment)
and outside the body
(external
environment)
Nervous System Functions
2. Integrative Function – analyzes the
sensory information, stores some things
and makes decisions about behavior
Nervous System Functions
3. Motor Functions –
may respond to
stimuli by causing
muscle contractions
or glandular
secretions
Nervous System Divisions
Consists of 2 parts:
1. The Central Nervous System (CNS)
2. The Peripheral Nervous System (PNS)
The Central Nervous System
• Consists of the brain
and spinal cord
• Here, sensory
information is
integrated, thought
and emotions are
generated and
memories are formed
and stored
CNS Continued
• Most nerve impulses
that cause muscles to
contract or glands to
make secretions
begin here
The Peripheral Nervous System
• The CNS is
connected to sensory
receptors, muscles
and glands by the
PNS
• Consists of cranial
nerves that arise from
the brain and spinal
nerves that emerge
from the spinal cord
PNS Continued
• The input components are called sensory
or afferent neurons
• They send information from sensory
receptors to the brain
• The output components are called motor
or efferent neurons
• They send messages from the CNS to
muscles or glands
PNS Divisions
1. Somatic Nervous System – this part is
voluntary, and contains sensory neurons for
sensations you are consciously aware of and
usually sends messages to skeletal muscles
2. Autonomic Nervous System – receives
messages from parts of the body that you are
not aware of and sends impulses to smooth
muscle, cardiac muscle and glands
(involuntary)
Nervous System Cell Types
1. Neurons – message
sending cells
2. Neuroglia – do not
send messages, but
support the cells that
do
Neuroglia
• Literally means “nerve glue”
• Usually smaller than neurons and 5-50
times more numerous
• Unlike neurons, they can multiply and
divide in mature nervous tissue
Types of Neuroglia - Astrocytes
• Star-shaped cells with
many processes
• Metabolize
neurotransmitters,
maintain K balance,
and maintains a link
between neurons and
blood vessels
Oligodendrocytes
• Most common
neuroglia in the CNS
• Make the myelin
sheath
Microglia
• Small, phagocytic
neuroglia
• They remove
microbes and debris
Ependymal Cells
• Range from cuboid to
columnar, many are
ciliated
• Make cerebrospinal
fluid and circulate it
Neurolemmocytes
• AKA Schwann cells
• Make the myelin
sheath in the PNS
Satellite Cells
• Support neurons in
ganglia (clusters on
cell bodies in the
PNS)
Myelination
• Myelin – a molecule
composed of lipid and
protein (appears
white)
• Myelin Sheath - a
layer of myelin
surrounding an axon
- It insulates the axon
and increases the
speed of the nerve
impulse
Neurons
• Some send messages a few mm, others
are the longest cells in the body
• Nerve impulses travels at speeds varying
from 1 to 100 m/sec
• Synapse – the functional contact area
between two neurons or a neuron and
muscle (or gland)
Parts of a Neuron
• Most have 3 parts
1. Cell body
2. Dendrites
3. Axons
Cell Body
• Usually contain most
normal organelles
• Chromatophilic
Substance – orderly
arrangement of rough
ER, makes protein to
repair damaged
axons
• Neurofibrils – form
the cytoskeleton
Dendrites
• Input portion of a
neuron
• Usually they’re
processes with many
short branches
• Usually they are not
myelinated
Axon
• Sends messages to
another neuron,
muscle or gland
• Joins the cell body at
a cone-shaped area
called the axon
hillock
• Nerve impulses begin
at the first segment,
known as the Trigger
Zone
More About Axons
•
•
•
•
Cytoplasm is called axoplasm
Cell membrane is the axolemma
Branches are called axon collaterals
Axon terminals, synaptic end bulbs (or
varicosities), synaptic vesicles and
neurotransmitters
Nerve Fibers
• A general term for any neuronal process
(dendrite or axon)
• Nerve – a bundle of many nerve fibers
that follow the same path in the PNS
• Most include bundles of both sensory and
motor fibers
• In the PNS, cell bodies usually cluster
together to form ganglia
Axonal Transport
• Things must be transported from the cell body to
the axon
• Slow Axonal Transport – moves materials 15mm per day
- Moves things only from the cell body toward the
axon terminals (supplies axon with new
axoplasm)
- Fast Axonal Transport – moves materials 200400 mm per day
- Moves various organelles and materials in both
directions
Structural Classification of Neurons
• Multipolar Neurons – usually have
several dendrites and one main axon
• Most neurons in the CNS are this type
Bipolar Neurons
• Have one main axon and one main
dendrite
• Found in retina of eye, inner ear, and
olfactory area of the brain
Unipolar Neurons
• Have just one process and are always sensory
neurons
• They begin in the embryo as a bipolar neuron,
but the axon and dendrite fuse into a single
process
• Trigger zone is where dendrites meet the axon
Functional Classification
• Look at #1-7 on page 339
Gray and White Matter
• White Matter – refers to collections of
myelinated processes from many neurons
- Myelin appears whitish in appearance
• Gray Matter - contains neuron cell
bodies, dendrites, axon terminals, or
bundles of unmyelinated axons and
neuroglia
- Appears gray because of a lack of myelin
Gray Matter in the Spinal Cord
Gray and White Matter in the Brain
Neurophysiology
•
Communication among neurons and
from neurons to muscle and glands
depends on two basic properties of the
plasma membrane of excitable cells
(neurons an muscle fibers)
1. Membrane Potential
2. Ion Channels
Membrane Potential
• Like most cells of the body, excitable cells
have a Membrane Potential – an
electrical voltage difference across the
membrane
• That difference can change suddenly,
causing graded and action potentials
(nerve impulses)
• Current – the flow of charge
Ion Channels
• Graded and action potentials occur
because the plasma membranes contain a
variety of ion channels
• Ion Channels – openings in the cell
membrane that open and close in
response to specific stimuli
• The phospholipid bilayer is a good
insulator, so the main path for current to
flow is through these channels
Action and Graded Potentials In a
Nutshell
• Action Potentials – Provide
communication over both long and short
distances
• Graded Potentials – Provide
communication of short distances only
Ion Channel Types
• See Page 341-342
Resting Membrane Potential
• Occurs because of a small buildup of negative
charges just inside the cytoplasm and an equal
build up of positive charges in the ICF just
outside the surface of the membrane
• The separated charges form potential energy,
measured in millivolts (mV)
• The greater the difference in charge across the
membrane, the larger the membrane
potential(voltage)
Resting Membrane Potential
• Resting membrane potentials usually
range from -90mV to -40mV
• The “-” indicates that the negative charge
is on the inside of the cell
• The normal accepted value for the resting
membrane potential is -70mV
• A cell that exhibits a membrane potential
is said to be polarized
Factors that Contribute to the
Resting Membrane Potential
1. Unequal distribution of ions across the
plasma membrane. (ECF is rich in Na+
and Cl- ions the cytoplasm is high in K+)
2. Relative permeability of the plasma
membrane to Na+ and K+.
- In a resting neuron or muscle fiber, the
permeability of the plasma membrane is
50 to 100 times greater to K+ than Na+.
Why are these things important?
• See page 342-343 and Figure 12.8
Graded Potentials
• Uses chemically, mechanically or light
gated ion channels
• Graded Potentials are small deviations
from the resting membrane potential
caused by an appropriate stimulus
• These potentials are “graded,” which
means that it can vary in amplitude
• Useful only for short distance
communication
Action Potentials
• An action potential is a series of rapidly
occurring events that decrease and
eventually reverse the membrane potential
(depolarization), and then restore it to the
normal resting state (repolarization)
• Good for long distance communication
Ion Channels
• AP’s use voltage gated ion channels (2
kinds)
• The first kind opens and allows Na+ to
enter the cell, causing depolarization
• The K+ channels then open, allowing K+ to
flow out and repolarization to occur
• Threshold level – the level of
depolarization that brings about an action
potential (-55mV)
Action Potentials
• See page 345
Refractory Period
• The period of time in which an excitable
cell can not generate another action
potential
• Must repolarize before it can respond
again
• Only found in action potentials
The All-or-None Principle
• If depolarization reaches threshold
(-55mV), then an action potential arises, if
-55mV is not reached, nothing happens
• Each time an action potential occurs, it is
the same strength
Action vs Grades Potentials
1. Amplitude – graded potentials can vary in
amplitude, action potentials are all-or-none
2. Duration – graded potentials are much longer
(several milliseconds to several minutes) than
action potentials (1/2 to 2 milliseconds)
3. Channels – graded use chemically,
mechanically and light gated ion channels,
action potentials use voltage gated channels
Action vs Grades Potentials Cont’d
4. Location – GP’s arise mainly from
dendrites and the cell body (a few from
axons) while AP’s always begin at the
trigger zone of an axon
5. Propagation – GP’s travel short
distances, AP’s travel long or short
distances
6. Refractory Period – AP’s have a
refractory period, GP’s do not
Transmission at Synapses
• Presynaptic Neuron – the neuron
sending the signal
• Postsynaptic Neuron – the neuron
receiving the message
• There are two types of synapses; electrical
and chemical
Electrical Synapses
• At an electrical synapse, ionic current
spreads directly from one cell to another
through gap junctions
• Each gap junction contains a hundred or
so tubular protein structures called
connexons that form tunnels to connect
the cytosol of the two cells
• Common in smooth muscle, cardiac
muscle, and the developing embryo
Advantages of Electrical Synapses
1. Faster communication (connexons)
2. Can synchronize activity (coordinated
movements or contractions)
3. Can have two-way communication unlike
chemical synapses
Chemical Synapses
• Separated by a synaptic cleft and no
connexons
• Presynaptic neurons release
neurotransmitters into the synaptic cleft,
which diffuse to receptors on the
postsynaptic neuron
• Postsynaptic Delay – ½ msec; time
required for the processes at a chemical
synapse
Excitatory and Inhibitory
Postsynaptic Potentials
• Neurotransmitters act on the postsynaptic
neuron to create a postsynaptic graded
potential
• If it depolarizes the postsynaptic
membrane, it is excitatory because it
moves closer to threshold
• If it causes hyperpolarization, it moves
away from threshold and is called
inhibitory
Removal of Neurotransmitters
1. Diffusion out of synaptic cleft
2. Enzymatic degradation (think
acetylcholinesterase)
3. Uptake into cells (active transport)
Neurotransmitters
• Excitatory and inhibitory neurotransmitters
are present in both the CNS and PNS
• Some neurotransmitters are excitatory in
one location but inhibitory in others
• Agonist – a substance that enhances that
effect of a neurotransmitter
• Antagonist – a substance that inhibits
that effect of a neurotransmitter
Neuronal Circuits
• Neuronal Pools – billions of CNS neurons
organized into complicated patterns
• Neuronal pools are organized into patterns
called circuits
• Simple Series Circuit – a presynaptic neuron
stimulates only one neuron in a pool
• Diverging Circuit – a single presynaptic neuron
stimulates an increasing number of postsynaptic
neurons
Neuronal Pools Cont’d
• Converging Circuit – the post synaptic neuron
receives impulses from several different
presynaptic neurons
• Reverberating Circuit – one neuron, stimulate
another, it stimulates another, then another and
eventually an earlier neuron in the cycle is
stimulated again.
• Parallel After-Discharge Circuit – a single
presynaptic neuron stimulates several
postysynaptic neurons which stimulate a single
neruon