The Nervous System

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The Nervous System
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The Nervous System

• Overall Function
• COMMUNICATION
• Works with the endocrine system in regulating
  body functioning, but the nervous system is
  specialized for SPEED

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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

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Neuron Structure
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PARTS OF THE NEURON

• Cell body this is where most of the neurons
  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

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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

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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

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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

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Central and PeripheralNervous 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!)

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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

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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)

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Information processing

• Regardless of the complexity of the nervous
  system, there are 3 general stages to information
  processing
• Sensory input
• Integration
• Motor output/effect

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Communication Lines
Stimulus (input)
Receptors (sensory neurons)
Integrators (interneurons)
motor neurons
Effectors (muscles, glands)
Response (output)
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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

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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
  bodys responses to stimuli

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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)

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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 )

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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)

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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

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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)

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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 isnt
• The neuron either fires or it doesnt 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)

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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

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Structure of a Neuron
dendrites
INPUT ZONE
cell body
axon
OUPUT ZONE
TRIGGER ZONE
CONDUCTING ZONE
axon endings
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Action potential

• Step 1 Neuron is in the resting potential, the
  gated-ion 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)

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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

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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)

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Recording of Action Potential
action potential
20
0
-20
Membrane potential (millivolts)
threshold
-40
resting membrane potential
-70
0
2
3
5
1
4
Figure 34.6bPage 583
Time (milliseconds)
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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

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Action Potential
Na
1
2
Na
Na
K
K
K
K
K
K
K
Na
Na
Na
Na
3
4
Na
Na
Figure 34.5dPage 583
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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)

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Communication between neurons
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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)

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Chemical Synapse

• Gap between axon terminal of one neuron and
  dendrite of adjacent neuron
• Action potential in axon ending of presynaptic
  cell causes voltage-gated calcium channels to
  open
• Flow of calcium into presynaptic cell causes
  release of neurotransmitter into synaptic cleft

plasma membrane of axon ending of presynapic cell
plasma membrane of postsynapic cell
synaptic vesicle
synaptic cleft
membrane receptor
Figure 34.7aPage 584
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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

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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

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Ion Gates Open
neurotransmitter
ions
receptor for neurotransmitter
gated channel protein
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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

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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

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Nerve

• A bundle of axons enclosed within a connective
  tissue sheath

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Reflexes

• Automatic movements made in response to stimuli
• In the simplest reflex arcs, sensory neurons
  synapse directly on motor neurons interneurons
  in CNS arent involved.
• Most reflexes involve an interneuron

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Stretch Reflex
STIMULUS Biceps stretches.
sensory neuron
motor neuron
Response Biceps contracts.
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Structure of the Spinal Cord
spinal cord
ganglion
nerve
meninges (protective coverings)
vertebra
Figure 34.19aPage 593
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Divisions of Brain
Division
Main Parts
Forebrain
Cerebrum
Olfactory lobes
Thalamus
Hypothalamus
Limbic system
Pituitary gland
Pineal gland
Midbrain
Tectum
Hindbrain
Pons
Cerebellum
Medulla oblongata
anterior end of the spiral cord
Figure 34.20Page 594
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Cerebrospinal Fluid

• Surrounds the spinal cord
• Fills ventricles within the brain
• Blood-brain barrier controls which solutes enter
  the cerebrospinal fluid

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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

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Lobes of the Cerebrum
Primary somatosensory cortex
Primary motor cortex
Parietal
Frontal
Occipital
Temporal
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Limbic System

• Controls emotions and has role in memory

(olfactory tract)
cingulate gyrus
thalamus
amygdala
hypothalamus
hippocampus
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Other Parts of the Brain

• Cerebellum - Controls muscle coordination and
  posture
• Medulla oblongata- Controls heart rate
  breathing rate

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Variations in Nervous Systems Among Animals
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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