The Nervous System Ch. 12

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The Nervous SystemCh. 12 13

• Lindsey Bily
• Austin High School
• Anatomy Physiology

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The Nervous System

• Made up of the brain, spinal cord and nerves.
• The purpose of the nervous system is to detect
  changes in internal and external environment,
  evaluate the information, and possibly respond by
  causing changes in the muscles or glands.
• Divided into the Central and Peripheral Nervous
  System.

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Divisions of the Nervous System

• Central Nervous System brain and spinal cord and
  nerves that lie completely within the brain and
  spinal cord.
• Peripheral Nervous System nerve tissues that lie
  in the outer regions of the nervous system.
• Cranial Nerves nerves that originate in the
  brain.
• Spinal Nerves nerves that originate in the
  spinal cord.

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Central and Peripheral Nervous System

• ?CNS
• PNS ?

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Afferent and Efferent Divisions

• Obviously, signals go to the brain and back out
  of it, so we need nerves to send messages both
  ways.
• Afferent Division incoming or sensory pathways
  (usually blue in diagrams).
• Efferent Division outgoing or motor pathways
  (usually red in diagrams).

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Somatic and Autonomic Nervous System

• Somatic Nervous System carry information to the
  skeletal muscle cells. Voluntary.
• Autonomic Nervous System carry information to
  the smooth and cardiac muscles, and glands.
  Involuntary.
• Sympathetic fight or flight
• Parasympathetic normal resting activities rest
  and repair.

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Cells of the Nervous System

• Glia or Glial cells do not conduct information
  but support the function of neurons.
• Neurons excitable cells that conduct nerve
  impulses.

?Neurons Glial Cells (gray) ?
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Glial Cells

• Glia literally means glue.
• There are about 900 billion glial cells in the
  body. 9 times the number of stars in the Milky
  Way.
• Unlike neurons, glial cells can divide throughout
  their life.
• Susceptible to cancer due to their ability to
  divide. Most brain cancers are due to glial
  cells.

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Types of Glial Cells

• Astrocytes Stars of the Nervous System They
  get glucose from the blood and feed it to the
  neurons. Help to form the Blood Brain Barrier
  (BBB).
• Microglia In CNS. They are usually small and
  stationary, but enlarge and move around when they
  are needed to eat (phagocytosis) microorganisms
  and cellular debris during inflamed or
  degenerating nerve tissue.
• Ependymal cells form thin sheets that line fluid
  filled cavities in the brain and spinal cord.
  Similar to epithelial cells.
• Oligodendrocytes similar to astrocytes but have
  fewer branches. Help to hold nerve fibers
  together and produce the fatty myelin sheath
  around the nerve fibers in the CNS.
• Schwann Cells found only in the PNS. Serve the
  same role as oligodendrocytes.

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Types of Glial Cells

• ?Motor neuron (red) astrocytes (green)
• Microglia (green) ?

Ependymal Cells
Oligodendrocytes Schwann
Cells
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Schwann Cells

• Many Schwann Cells wrap themselves around a
  single neuron.
• Myelin is a white fatty substance that insulates
  the neuron like plastic on a wire.
• The microscopic gaps between Schwann Cells are
  called Nodes of Ranvier.
• Very important for nerve impulse conduction.
• Cells with myelin are called white fibers and
  gray fibers when they are non-myelinated.

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Blood Brain Barrier

• Formed by the astrocytes that wrap their feet
  around the capillaries in the brain.
• Regulates the passage of ions and molecules into
  and out of the brain.
• Water, oxygen, carbon dioxide, glucose and small
  lipid soluble molecules such as alcohol can cross
  the barrier easily.
• Ions (Na and K) are regulated because they
  could disrupt nerve impulses.
• Must be taken into consideration when developing
  drug treatments for brain disorders.
• Ex. Parkinsons need dopamine but it cannot pass
  BBB. They are given L-dopa which can pass and is
  made into dopamine by the brain cells.

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Blood Brain Barrier
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Multiple Sclerosis

• Myelin disorder of the oligodendrocytes.
• Loss of myelin and destruction of the
  oligodendrocytes.
• Hard plaquelike lesions replace the myelin and
  causes inflammation.
• Impaired nerve conduction, loss of coordination,
  visual impairment and speech disturbances.
• Most common in women 20-40.
• Caused by autoimmunity or a viral infection.

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Neurons

• We have about 100 billion neurons. This is only
  about 10 of all the nervous system cells in the
  brain.
• Neurons are also called nerve fibers.
• Parts of the neuron
• Cell body
• Dendrites branch off from the cell body. Means
  tree. They receive stimuli and conduct
  electrical signals towards the cell body and/or
  axon.
• Axon a single process that comes off the cell
  body via the axon hillock. They conduct impulses
  away from the cell.

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Neurons
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Classification of Neurons

• Multipolar one axon, several dendrites. Most
  neurons in the brain and spinal cord.
• Bipolar one axon and one highly branched
  dendrite. Least common type of neuron, found in
  retina, inner ear, and olfactory pathway (nasal).
• Unipolar or Psuedounipolar single process
  extending from the cell body. Always sensory
  neurons that conduct information to the CNS.

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Classification of Neurons

• Multipolar
• Bipolar
• Unipolar

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Classification of Neurons

• Afferent (sensory) neurons Transmit impulses to
  the CNS.
• Efferent (motor) neurons transmit impulses away
  from CNS towards or to muscles or glands.
• Interneurons Transmit impulses from afferent
  neurons to efferent neurons. Lie completely
  within the CNS.

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Reflex Arc

• Neurons are often arranged in a pattern called a
  reflex arc. Its a signal conduction route.
• Most common form is a 3-neuron arc (sensory?
  interneuron? motor)
• 2-neuron arc (sensory ? motor)
• Synapse place where nerve information is passed
  from one neuron to another. Passed from the
  synaptic knobs of one neuron to the dendrites of
  the other.

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Reflex Arc
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Nerves and Tracts

• Nerves bundles of nerve fibers in the Peripheral
  nervous system held together by several layers of
  connective tissue. (ex. Sciatic nerve)
• Tracts bundles of nerve fibers in the central
  nervous system. (ex. Corticospinal tract)
• White matter myelinated nerve fibers
• Gray matter unmyelinated nerve fibers and cell
  bodies

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Repair of Nerve Fibers

• Mature neurons cannot divide, damaged neurons
  cannot be replaced.
• They can sometimes repair themselves in the PNS
  if the damage is not too severe.
• 1. After the damage has occurred, the distal
  portion of the axon degenerates.
• 2. macrophages move in and remove the debris.
• 3. the neurolemma (nerve sheath formed by Schwann
  Cells) forms a tunnel from the point of injury to
  the effector.
• 4. new Schwann cells grow within the tunnel to
  support axon growth.

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Repair of Nerve Fibers

• The skeletal muscle that is innervated to the
  damaged nerve atrophies as it is not being
  stimulated.
• If the damaged axon doesnt repair itself,
  sometimes a nearby healthy neuron will establish
  a connection with the muscle.
• One damaged axon in a single neuron can shut down
  an entire nerve pathway if not repaired.

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CNS Repair

• Cells in the CNS hardly ever repair themselves.
  They lack a neurolemma to build a tunnel and
  astrocytes fill in damaged areas and form scar
  tissue.
• Most spinal cord injuries involve crushing or
  bruising of the nerves.
• Inflammation after the accident causes more
  damage to surrounding nerves.
• Early treatment of the antiinflammatory drug,
  methylprednisolone is prescribed within 8 hours
  of injury to reduce the swelling.

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Nerve Impulses

• Neurons exhibit excitability and conductivity.
• A nerve impulse is a wave of electrical
  fluctuation that travels along the plasma
  membrane.
• Membrane potential Cells have slightly more (-)
  charges inside the cell than on the outside
  (extracellular fluid is more ).
• This difference in ion concentration across the
  plasma membrane has potential energy.

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Membrane Potentials

• A membrane is polarized if it has a membrane
  potential.
• We can measure the potential difference between
  the two sides of the polarized membrane in
  (Vvolts or mV millivolts).
• -70 mV tells us that the difference in charge is
  70 mV and that the inside of the cell is negative
  (-).
• 30 mV tells us that the difference in charge is
  30 mV and the inside of the cell is positive ().

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Membrane Potential
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Resting Potential

• A neuron is resting when it is not conducting
  nerve impulses.
• Stays about -70mV (RMP-resting membrane
  potential)
• There are no gates or they are closed to not
  allow anions (-) in or out of the cell.
• Cations () Na and Kcan move in and out of the
  cell through gates.

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Resting Potential

• K gates are usually open and Na gates are
  usually closed.
• There are also Na and K pumps that are active
  transport mechanisms.
• Pumps 3 Na out for every 2 K in and at
  different rates.
• If 100 K are pumped inside the cell, 150 Na are
  pumped out.
• This maintains a difference in charges inside
  and out of the cell. Slightly more positive
  outside the cell.

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Resting Potential

• Very little Na diffuses through the membrane.
  The pump maintains a imbalance of ions inside and
  outside of the cell.

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Local Potentials

• The resting membrane potentials (RMP) of neurons
  can fluctuate due to certain stimuli.
• Local Potential slight change in the RMP.
• Stimulus-gated Na channels open in response to a
  sensory stimulus or stimulus from another neuron
  (excitation)
• When they open, more Na rushes into the cell,
  causing it to become more . Depolarization
  (movement of the membrane potential to zero mV).

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Local Potentials

• Stimulus-gated K channels open during
  inhibition. Causing the outside of the cell to
  become more . Hyperpolarization (movement of
  the membrane potential away from zero mV.) Now we
  are below the RMP.
• Local potentials are graded potentials meaning
  they can be large or small depending on the
  strength of the stimulus. They are also isolated
  to a particular location on the plasma membrane
  and do not travel down the axon.

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Local Potentials

• Small depolarizations or hyperpolarizations
  applied to certain dendrites on a neuron.

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Action Potentials

• Action Potential is the membrane potential of a
  neuron that is conducting an impulse. Also called
  a nerve impulse.
• There are 6 steps in conducting an action
  potential.
• http//www.metope.org/neuron/

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Action Potentials

• An adequate stimulus must be applied and the
  stimulus-gated Na channels will open to allow
  Na in (depolarization).
• If the level of depolarization surpasses the
  threshold potential (usually -59 mV)
  voltage-gated Na channels will open allowing
  MORE Na in the cell.
• As more Na comes inside, the voltage inside the
  cell gets closer and closer to 0 mV and will
  continue to 30 mV. Means we now have more ions
  in the cell than outside of the cell.
• Voltage-gated Na channels only stay open for
  about 1 millisecond before they close. Action
  potentials are all-or-none, either they will
  occur or not at all.
• Once the peak of the action potential is reached
  , it starts to move back to -70 mV (resting
  potential). This is called repolarization. The
  reaching of the threshold potential causes
  voltage-gated K channels to open as well, but
  they are slow to respond. So they dont open
  until the 30 mV potential is reached. Then K
  pours out and Na goes back out.
• Because so much K pours out of the cell, the
  voltage goes past -70 mV for a brief period of
  hyperpolarization, but then it gets back to the
  resting state.

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Refractory Period

• Brief period during which a local area on the
  axons membrane can not be restimulated.
• Absolute refractory period ½ a millisecond after
  the threshold potential is surpassed. Axon will
  not respond to any stimulus.
• Relative refractory period few milliseconds
  after the absolute refractory period. Can only
  be stimulated if the stimulus is really strong.
• A strong stimulus causes more action potentials
  vs. a weak stimulus. However, the strength of
  each action potential is the same.

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Conduction of the Action Potential

• The action potential causes an electrical current
  to flow down segments of the axons membrane.
• It will never move backward due to the refractory
  period of the membrane before the AP.
• In myelinated fibers, the myelin sheath prevents
  ion movement, so electrical changes only occur in
  the gaps between myelin (Nodes of Ranvier).
• The AP seems to leap from node to node. This is
  called saltatory conduction. (Latin-saltare-to
  leap)

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Conduction of the Action Potential

• The larger the diameter of the fiber, the faster
  it conducts impulses.
• Myelinated fibers conduct impulses faster than
  unmyelinated fibers.
• Fastest fibers innervate skeletal muscles and can
  fire impulses close to 300 mph.
• Slowest fibers, such as sensory receptors in the
  skin, conduct impulses at less than 1 mph.

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Saltatory Conduction
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Anesthetics

• Block pain.
• Inhibit the opening of Na channels so the nerve
  cannot conduct impulses.
• Bupivacaine (Marcaine) used in dental
  procedures.
• Procaine used to block signals in sensory
  pathways of the spinal cord.
• Benzocaine and phenol found in over the counter
  products that release pain associated with
  teething, sore throat pain, and other ailments.

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Synaptic Transmission

• Synapse is where signals are sent from one neuron
  to another (presynaptic neuron to the
  postsynaptic neuron)
• Types of Synapses
• Electrical occur where two cells are joined at
  gap junctions (cardiac muscle, some smooth
  muscle). The impulse goes from one plasma
  membrane to the other.
• Chemical Use chemicals (neurotransmitters) to
  send a signal from the pre- to the postsynaptic
  cell.

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Chemical Synapse

• Structures of the chemical synapse.
• 1. Synaptic knob- tiny bulge at the end of the
  presynaptic neurons axon. Contains numerous
  small sacs or vesicles that contain
  neurotransmitter.
• 2. synaptic cleft- space between the synaptic
  knob and the plasma membrane of the postsynaptic
  neuron, 1 millionth of an inch wide!
• 3. the plasma membrane of the postsynaptic
  neuron- has protein receptors embedded in which
  neurotransmitters bind.

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Types of Synapses
Electrical Synapse Chemical Synapse
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How Synaptic Transmission Occurs

• Action potentials cannot cross synaptic clefts
  even though the spaces are so tiny.
• Instead, neurotransmitters (NT) are released in
  cause a response in the postsynaptic neuron.
• Excitatory NT cause depolarization and inhibitory
  NT cause hyperpolarization.

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How Synaptic Transmission Occurs

1. Action potential (AP) reaches a synaptic knob,
   causing voltage-gated Ca 2 channels to open and
   allow Ca 2 to diffuse into the knob rapidly.
2. Increase in Ca2 causes NT to be released into
   the synaptic cleft.
3. The NT binds to receptors on the postsynaptic
   membrane which causes the ion gates to open.
4. The NT will either cause an excitatory
   postsynaptic potential (EPSP) or an inhibitory
   postsynaptic potential (IPSP).
5. Once the NT binds to the receptor its action is
   terminated.

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Neurotransmitters

• Neurotransmitters are how neurons talk to one
  another.
• Can be excitatory or inhibitory.
• Their affect is determined by the receptor, not
  the actual NT.

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Types of Neurotransmitters

• Acetylcholine- excites skeletal muscles but
  inhibits cardiac muscles.
• Amines- (seratonin, histamine, dopamine,
  epinephrine and norepinephrine). Affect learning,
  motor control, emotions, etc.
• Amino Acids- (glutamate, GABA, glycine). Some are
  excitatory and some are inhibitory in the CNS.
• Other small molecule transmitters- (nitric oxide
  NO and carbon monoxide CO).
• Neuropeptides- short strands of amino acids.
  Include enkephalins and endorphins. Inhibitory
  and block pain.

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Antidepressants

• Severe psychic depression occurs when there is a
  lack of norepinephrine, dopamine, serotonin and
  other amines.
• Antidepressants work several different ways.
• They may inhibit enzymes that are used to
  inactivate the NT.
• They may block the reuptake of the NT by the
  neuron, keeping them in the synapse longer.
• Cocaine blocks the reuptake of dopamine, giving a
  temporary feeling of well-being.