Download Nervous Systems

THE NERVOUS SYSTEM
THE NERVOUS SYSTEM
• Overall Function
• COMMUNICATION
• Works with the
endocrine system
in regulating body
functioning, but
the nervous
system is
specialized for
SPEED
NEURONS
• A neuron is the functional unit of the nervous
system
• Neurons are specialized for transmitting signals
from one location in the body to another
• Neurons consist of a large cell body (contain a
nucleus and other organelles), and neuronal
processes
• Axons
• Conduct messages AWAY from cell body
• Dendrites
• Conducts messages TOWARD cell body
NEURON STRUCTURE
PARTS OF THE NEURON
• Cell body: this is where most of the neuron’s organelles
(including the nucleus) are located
• Dendrites: highly branched extensions from the cell
body that RECEIVE signals from other neurons
• Axon: a large extension from the cell body that
TRANSMITS signals to other neurons or “effector” cells
• Axon hillock: where the axon joins the cell body
• Myelin sheath: a fatty layer of cells that “insulates” the
axon (not present in most invertebrates)
• Synaptic terminal: the branching ends of the axon that
release a “neurotransmitter” to send a message
• Synapse: the space between the synaptic terminal and
the effector cell
SUPPORTING CELLS OF THE NERVOUS
SYSTEM
• Glia is the term given to the many cells that
support the neurons in the nervous system
• Astrocytes: provide structural support for
neurons in the CNS. They also regulate
extracellular ion concentrations (important
when we talk about membrane
potentials)
• Oligodendrocytes (in the CNS) and
Schwann cells (in the PNS): responsible for
creating the myelin sheath on the axon
ORGANIZATION OF THE NERVOUS SYSTEM
• Organisms have different types of nervous systems based
on their complexities
• The simplest organisms will have a web-like arrangement
of nerves throughout the body the act as a nerve net
• These organisms are able to react to stimuli, but do not
show any higher activity
• Example: Hydra
• A little more complicated organism also have bundled
fiber-like extensions of neurons called nerves, along with
nerve nets
• This allows nerve nets to control more complex
movements
• Example: Sea star
ORGANIZATION OF THE NERVOUS SYSTEM:
MORE COMPLICATED ORGANISMS
• Central Nervous
System (CNS)
• Consists of brain and
spinal chord
• In more primitive
organisms, this could
include a cluster of
neurons (called
ganglia) along a
ventral nerve and a
brain
• Peripheral Nervous
System (PNS)
• Consists of all of the
peripheral nerves
that connect with the
CNS
CENTRAL AND PERIPHERAL
NERVOUS SYSTEMS
• The central nervous
system consists of the
brain and spinal cord
• This is where integration
occurs
• Made of interneurons
• The peripheral nervous
system consists of the
nerve cells that
communicate signals
between the CNS and
the rest of the body
• Sensory neurons
• Carry info from the sensory
receptors to the brain
• Motor neurons
• Carry info from the brain to
effector cells (to do
whatever the brain said!)
OTHER DIVISIONS OF THE NERVOUS
SYSTEM
• Autonomic Nervous
System
• Regulates internal
environment (digestion,
cardiovascular,
excretion and hormone
release
• Called the
“involuntary” nervous
system
• Three parts:
• Sympathetic
• Parasympathetic
• Enteric
• Somatic Nervous System
• Carries signals to and
from the skeletal
muscles
• Responds to external
stimuli
• Called the “voluntary”
nervous system
AUTONOMIC NERVOUS SYSTEM
• Sympathetic: corresponds to increased arousal or
energy output (fight or flight response)
• Increased heart rate
• Dilate blood vessels and respiratory passages
• Convert glycogen to glucose
• Release epinephrine (adrenaline)
• Inhibits digestion
• Parasympathetic: corresponds to self-maintenance
and relaxation (“rest” and “digestion”)
• Opposite of sympathetic nervous system
• Enteric: network of neurons responsible for digestion
(digestive tract, pancreas, and gallbladder)
INFORMATION PROCESSING
• Regardless of the complexity of the
nervous system, there are 3 general
stages to information processing:
• Sensory input
• Integration
• Motor output/effect
COMMUNICATION LINES
Stimulus
(input)
Receptors
(sensory neurons)
Integrators
(interneurons)
motor neurons
Effectors
(muscles, glands)
Response
(output)
MAJOR NERVOUS SYSTEM PROCESSES
• Input
• The conduction of signals from sensory neurons to
integration centers in the nervous system
• Detect external stimuli (light, sound, heat, smell,
touch, taste)
• Detect internal conditions (blood pressure,
blood CO2 levels, muscle tension)
• Integration
• The process by which the information from the
environmental stimulation of the sensory receptors
is sent and interpreted by interneurons in the CNS
• The complexity of the CNS has to do with the
amount of connections between interneurons
MAJOR NERVOUS SYSTEM PROCESSES
• Motor Output
• The conduction of signals from the processing
center of the CNS to the motor neurons which
communicate with muscle cells or gland cells
(effector cells) that actually carry out the body’s
responses to stimuli
ACTION POTENTIALS: HOW THE
NERVES CONDUCT SIGNALS
• In order to actually TRANSMIT a signal, the voltage (charge)
across the membrane (membrane potential) has to change
• A signal will cause the ion channels to open, letting some
of the ions (Na+, K+) through, trying to achieve
EQUILIBRIUM
• This depolarizes the membrane
• This causes the signal to be passed along the neuron,
which is known as an ACTION POTENTIAL (like a wave of
electricity)
RESTING POTENTIAL:
NOT TRANSMITTING A SIGNAL
• Resting Potential: charge difference across the
plasma membrane of a neuron when not
transmitting signals
• Fluid just outside cell is more positively charged than
fluid inside because of large negatively charged
proteins in the cytoplasm
• Potassium (K+): Higher inside than outside
• Sodium (Na+): Higher outside than inside
• Potential is measured in millivolts
• Resting potential is usually about -60mV to -80mV
(inside of the membrane is “-” and outside is “+”)
RESTING POTENTIAL
• The resting potential of a neuron creates an ionic
gradient
• Remember the concentration gradient in the H+ pump
to make ATP
• There are many open potassium ion channels in the
plasma membrane and few sodium ion channels
(ungated)
• This causes a net flow of Na+ and K+ across the
membrane
• This is what creates the voltage (flow of ions)
• To maintain the levels of Na+ and K+, the cells utilize
the sodium-potassium pump (remember active
transport)
GATED ION CHANNELS
• Neurons also have 3 gated ion channels
(controls the flow of ions)
• Stretch-gated ion channels: sense
stretching of the cell and cause the gates
to open
• Ligand-gated ion channels: open or close
when a specific chemical binds to the
channel
• Voltage-gated ion channels: open or
close when the membrane potential
changes
ACTION POTENTIALS:
TRANSMITTING A SIGNAL
• Depending on external stimuli, gated ion channels
can open or close
• Some stimuli can cause a hyperpolarization which makes
the membrane potential of the cell greater than resting
potential
• Example: opening K+ gated channels allows the movement of
K+ out of the cell (remember: at rest K+ is more concentrated
inside the cell)
• Increases membrane potential to -92 mV (losing “+” out of cell)
• Some stimuli can cause a depolarization which makes the
membrane potential of the cell less than resting potential
• Example: opening Na+ gated channels allows the movement
of Na+ into the cell (remember: at rest Na+ is more
concentrated outside the cell
• Decreases membrane potential to +62 mV (gaining “+” in cell)
ACTION POTENTIALS:
TRANSMITTING A SIGNAL
• A change in membrane potential is called a
graded potential
• Action potentials are either ALL or NOTHING
• Either there is enough change in the voltage to
pass the message along, or there isn’t
• The neuron either “fires” or it doesn’t fire
• In order to “fire”, the membrane potential must hit
a threshold (the membrane voltage that sets the
reaction)
• If the threshold is reached, then the neuron
undergoes an action potential (these are what
carries a signal along the axon)
ALL OR NOTHING
• All action potentials are the same size
• If stimulation is below threshold level, no action potential
occurs
• If it is above threshold level, cell is always depolarized to the
same level
• Action potential is initiated at the axon hillock and travels
down the axon to the axon terminal
STRUCTURE OF A NEURON
dendrites
INPUT ZONE
cell body
axon
TRIGGER ZONE
CONDUCTING ZONE
OUPUT ZONE
axon
endings
ACTION POTENTIAL
• Step 1: Neuron is in the resting potential, the gatedion channels are closed
• Step 2: A stimulus causes some Na+ ion channels to
open allowing Na+ to diffuse through the
membrane. This causes the membrane to be
depolarized. The depolarization causes even more
Na+ ion channels to open (positive feedback) until
a threshold is reached in the membrane potential
• Step 3: Once the threshold is reached, positive
feedback progresses at a rapid rate to create an
action potential (the voltage that allows the
membrane to conduct the signal)
ACTION POTENTIAL
• Step 4: After the action potential is reached, the
Na+ gates close, preventing the influx of any more
Na+ ion. At the same time, the K+ ion channels
open. This allows the K+ ions to diffuse out of the
membrane (high concentration of K+ inside the
membrane compared to outside). This release of
K+ ions rapidly lowers the membrane potential.
• Step 5: As the membrane potential lowers, it falls a
little below the resting potential, undershoot The K+
ion channels close and the membrane eventually
returns to its resting potential
STEPS IN THE ACTION POTENTIAL
• An action potential is very quick (each one
only takes 1-2 milliseconds
• After an action potential, it takes a little bit
of time to return all of the Na+ and K+
concentrations to their original levels
• Na+ / K+ pumps the Na+ and K+ back to
original positions
• During this time, a second action potential
cannot by initiated (refractory period)
RECORDING OF ACTION
action potential
POTENTIAL
Membrane potential (millivolts)
+20
0
-20
threshold
-40
resting
membrane
potential
-70
0
1
2
3
4
Time (milliseconds)
5
Figure 34.6b
Page 583
TRANSMITTING SIGNAL ALONG AXON
• Transmitting the signal
• In order to propagate the signal, the membrane potential
must be depolarized along the length of the axon
• To make this occur, when the Na+ is being let into the cell
(depolarization) in one part of the axon, it creates an
electric current that causes depolarization in an adjacent
area
• Behind the zone of depolarization is where the membrane is
returning to resting potential (repolarization)
• The refractory period prevent the action potential from
being sent “backwards” along the neuron
ACTION POTENTIAL
1
Na+
Na+
K+
K+
K+
2
Na+
K+ K+
K+
K+
Na+
Na+
Na+
Na+
3
Na+
Na+
4
Figure 34.5d
Page 583
SPEED OF CONDUCTION
• In general, the speed of a signal along an axon is
dependent on a few things
• The smaller the axon diameter, the slower the speed of
signal conduction
• Simple invertebrates (worms) may have conduction speeds of
centimeters/second
• Larger axon diameters allow increased speed of signal
conduction
• Complex invertebrates (squid or octopi) have conduction
speeds of about 100 meters/second
• In the vertebrate axon, there is a myelin sheath which
increases speed due to insulation
• There are gaps in the myelin sheath (Nodes of Ranvier),
where the depolarization can “jump” to. This greatly
increases conduction rate (about 120 meters/second)
COMMUNICATION
BETWEEN NEURONS
NEURON TO NEURON
COMMUNICATION
• As the action potential travels along the axon it
stops at the axon terminal (synaptic terminal)
• Action potentials do not travel between
different neurons
• Yet, it is still necessary to send the “signal” from
one neuron to the next
• To do this, there has to be a way to send a
signal across the space that exists between one
neuron and another (synaptic cleft or gap
junction)
CHEMICAL SYNAPSE
• Gap between axon terminal
of one neuron and dendrite
of adjacent neuron
plasma
membrane of
axon ending of
presynapic cell
• Action potential in axon
ending of presynaptic cell
causes voltage-gated
calcium channels to open
synaptic
vesicle
plasma
membrane of
postsynapic cell
• Flow of calcium into
presynaptic cell causes
release of neurotransmitter
into synaptic cleft
synaptic
cleft
membrane
receptor
Figure 34.7a
Page 584
NEUROTRANSMITTERS
• Neurotransmitters
are substances that
carry the “message”
across the synapse
• Important
neurotransmitters:
• Acetylcholine
(bridges gaps
between motor
neurons & muscle
cells),
• norepinephrine,
dopamine,
serotonin work in
CNS
SYNAPTIC TRANSMISSION
• Neurotransmitter diffuses
across cleft and binds to
receptors on membrane
of postsynaptic cell
• Binding of
neurotransmitter to
receptors opens ion
channels in the
membrane of
postsynaptic cell
ION GATES OPEN
neurotransmitter
ions
receptor for
neurotransmitter
gated channel
protein
SYNAPTIC TRANSMISSION
• Enzymes in synaptic cleft will degrade
neurotransmitters after action potential is initiated
on the post-synaptic cell. The neurotransmitters are
recycled after they are broken down.
• Example: Acetylcholine is broken down by the
enzyme acetylcholine esterase
INDIRECT SYNAPTIC TRANSMISSION
• The neurotransmitter does not bind directly
to an ion channel gate.
• Instead, it activates a signal transduction
pathway (Remember cell signaling . . .
again)
• Utilizes a second messenger (AMP to cAMP
. . . again)
• These signals take longer to activate, but
last for a longer period of time
axon
NERVE
myelin sheath
• A bundle of axons
enclosed within a
connective tissue
sheath
nerve
fascicle
REFLEXES
• Automatic movements made in response to
stimuli
• In the simplest reflex arcs, sensory neurons
synapse directly on motor neurons;
interneurons in CNS aren’t involved.
• Most reflexes involve an interneuron
STRETCH REFLEX
STIMULUS
Biceps
stretches.
sensory
neuron
motor
neuron
Response
Biceps
contracts.
STRUCTURE OF THE SPINAL CORD
spinal cord
ganglion
nerve
vertebra
meninges
(protective
coverings)
Figure 34.19a
Page 593
DIVISIONS
OF BRAIN
Division
Main Parts
Forebrain
Cerebrum
Olfactory lobes
Thalamus
Hypothalamus
Limbic system
Pituitary gland
Pineal gland
Tectum
Midbrain
Hindbrain
Pons
Cerebellum
Medulla oblongata
anterior end of the
spiral cord
Figure 34.20
Page 594
CEREBROSPINAL FLUID
• Surrounds the spinal
cord
• Fills ventricles within
the brain
• Blood-brain barrier
controls which
solutes enter the
cerebrospinal fluid
ANATOMY OF THE CEREBRUM
• Largest and most complex part of human brain
(Responsible for thinking & higher level
functions)
• Outer layer (cerebral cortex) is highly folded
• A longitudinal fissure divides cerebrum into left
and right hemispheres
• Corpus collosum connects the two hemispheres
LOBES OF THE CEREBRUM
Primary
somatosensory
cortex
Primary motor cortex
Frontal
Parietal
Occipital
Temporal
LIMBIC SYSTEM
• Controls emotions and has role in memory
(olfactory tract) cingulate gyrus thalamus
amygdala
hypothalamus
hippocampus
OTHER PARTS OF THE BRAIN
• Cerebellum Controls muscle
coordination and
posture
• Medulla oblongataControls heart rate
& breathing rate
VARIATIONS IN NERVOUS SYSTEMS
AMONG ANIMALS
EXAMPLE: PROBLEM WITH NERVOUS
SYSTEM
• Multiple Sclerosis:
• A condition in which nerve fibers lose
their myelin
• Slows conduction
• Symptoms include visual problems,
numbness, muscle weakness, and
fatigue