Nervous System

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Chapter 9

• Nervous System

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Organs

• Brain
• Spinal Cord
• Nerves

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Functions

• Orientation of body to external and internal
  environment
• Coordinate and control body functions
• Learning memory intellect
• Instinctual behavior

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Neuron Structure
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Neuron Structure

• Dendrites receive information to transmitted
  down the axon
• Nissil Body Similar to rough endoplasmic
  reticulum makes neurotransmitters
• Neurofibrils network of fine threads
• Axon Hillock conducts nerve impulses away from
  cell body
• Axon Contains microtubules to deliver
  neurotransmitter down to Axon terminal
• Schwann Cell (neurolemmocytes) cells that
  surround nerve membrane
• Myelin Sheath insulation for message
• Nodes of Ranvier spaces between schwann cells

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Myelinated vs Unmyelinated

• Myelinated
• Found in CNS
• White matter
• Allows for faster transmission of messages
• Schwann cells wrap around the axon and their cell
  membranes contain a lipid myelin.
• Unmyelinated
• Found in CNS
• Grey matter
• Slower message transmission

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Classification of Neurons

• Bipolar
• Two nerve fibers
• One is the axon and the other is dendrite
• Found in eyes, nose, and ears.
• Unipolar
• Cell body is in the middle of the cell (outside
  the main track)
• Messages do not go through cell body
• One nerve fiber extending to the peripheral organ
  and the other going to brain or spinal cord

• Cell Bodies form Ganglia
• Pain, Touch Temperature
• Multipolar
• Many dendrites off of the cell body
• Most common type
• Motor Neurons

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Classification of Neurons

• Neurons can also be either sensory, motor, or
  interneurons.
• Sensory Neurons
• Carry impulses away from their source
• Some have specialized receptor ends at the tips
  of dendrites
• They can also be associated with receptor cells
  in the skin or other sensory organs
• Most are unipolar, some are bipolar
• Interneurons
• Found in brain and spinal cord
• Multipolar
• Link two neurons
• Relay messages from sensory neurons
• Motor Neurons
• Multipolar
• Carry impulse out to effector organs (muscle,
  gland, etc)

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Support and Protection
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Support and Protection

• Neuroglial cells provide support and protection
  for neurons
• Types of Neuroglial cells
• Schwann cells
• Microglial cells
• Oligodendrocytes
• Astrocytes
• Ependymal Cells

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Support and Protection

• Microglial Cells
• Found throughout the CNS
• Phagocytize bacteria and cellular debris
• Multiple Sclerosis Viral infections cause
  Microglial cells to attack the nervous system

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Support and Protection

• Schwann Cells
• Support cells found in the Peripheral Nervous
  System (PNS)
• Wrap around the a single segment of an axon
• Create Myelin Sheath
• Clean up PNS debris

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Support and Protection

• Oligodendrocytes
• Found in the CNS
• Similar to Schwann cells, but wraps around
  several axons
• Do not form neurilemmal sheaths

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Support and Protection

• Astrocytes
• Found between neurons and blood vessels in CNS
  (Blood brain Barrier)
• Help regulate concentrations of nutrients
• Form scar tissue following injury to CNS

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Support and Protection

• Ependymal Cells
• Form an epithelial like membrane
• Covers parts of brain
• Lines ventricle (spaces) within the brain and
  spinal cord

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Nerve Impulse Cell Membrane Potential

• Ions are distributed on the inside and outside of
  cells
• Ions pass through ion channels that open and
  close when necessary
• Polarized membrane electrically charged membrane
• Potassium ions can move more easily across the
  membrane

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Nerve Impulse Resting Potential

• Active Transport moves ions to the inside or to
  the outside of the cell
• Potential difference Difference in electrical
  charge between two regions
• Inside the cell has a large concentration of
  Potassium (K)
• Also there is a large concentration of negative
  ions (SO42-, PO43-) and proteins
• Outside the cell has a large concentration of
  Sodium (Na)
• During Resting Potential
• Potassium Easily diffuses out of cell
• Sodium cannot easily diffuse into the cell
• Inside of cell slightly negative and outside of
  cell slightly positive
• Active transport Na/K pumps maintain the
  concentrations inside and outside of cell

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Nerve Impulse Changes in Potentials

• Nerves cells respond to changes (excitable)
• Amount of change in a potential is related to the
  intensity of the stimulus given to the neuron
• Summation If additional stimulation arrives
  before the effect of the previous stimulation
  subsides the intensity increases.
• Threshold Potential many subthreshold
  potentials combine (summation) to reach the
  threshold.

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Nerve Impulse Action Potential

• At the threshold permeability suddenly changes
  and sodium channels open allowing sodium to flow
  in. (Depolarization)
• The negative charge on the inside attracts the
  positive sodium
• Potassium gates open and K ions move out of the
  cell.
• The outside of the cell membrane becomes
  negatively charged.
• As potassium floods out the inside becomes
  negative again and outside becomes positive
  (repolarization)
• Rapid repolarization and depolarization occurs in
  one-thousandth of a second this is an action
  potential.

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Nerve Impulse Conduction

• Nerve Impulse series of action potentials
  across a nerve fiber.
• Unmyelinated nerves conduct impulses across their
  entire surface.
• Myelinated nerves conduct impulse only in the
  nodes of Ranvier causing the impulse to jump from
  node to node
• Speed of impulse is in proportion to the diameter
  of the nerve fiber.
• Greater the diameter the greater the speed
• Exmyelinated motor fiber associated with a
  skeletal muscle can travel at 120 m/sec
• Ex unmyelinated sensory fiber that is thin .5
  m/sec

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Nerve Impulse All or none response

• Nerve fiber responds it will respond completely
• This occurs when a threshold potential is reached

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The Synapse
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The Synapse

• Junction between two communicating neurons
• Gap between two neurons is called the synaptic
  cleft
• Synaptic transmission process of crossing the
  synapse
• Synaptic foot contains neurotransmitters in
  vesicles
• Action potentials travel down the neuron
• When action potential reaches synaptic knob there
  is an increase in permeability to Calcium ions.
• Calcium flows in and in response some synaptic
  vesicles move toward the end of the synaptic foot
  merge with cell membrane of the neuron.
• Neurotransmitters are released and cross the gap
  and stimulate receptors on the next neuron.

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The Synapse

• Inhibitory Action
• Some neurontransmitters decrease neuron membrane
  permeability Na
• Lessens the chance that threshold will be reached
• Excitatory
• Some neurotransmitters increase neuron membrane
  permeability to Na
• Increases chance that threshold will be reached

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Neurotransmitters

• Neurotransmitters pass signal from neuron to
  neuron
• Some neurons release one type while other release
  two to three types.
• Common neurotransmitters
• Acetylcholine skeletal muscle, excitatory
• Monoamines epinephrine and nor-epinephrine,
  excitatory
• Dopamine inhibitory
• Glycine inhibitory
• Neurotransmitters when released will either
  stimulate the next neuron, be broken down by
  enzymes such as acetylcholinesterase (breaks down
  acetylcholine), or are brought back into synaptic
  knob.

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How the impulses are processed

• The nervous system responds to impulse in four
  ways
• Neuronal Pools
• Central nervous system
• Groups of neuron with specific characteristics
• Process information based on the characteristics
  
• Facilitation
• Subthreshold stimulation from a neuronal pool
  that causes neuron to be more excitable
• Convergence
• Neurons arriving at the neuronal pool from
  different parts of the nervous system.
• Allows for nervous system to respond to a variety
  of information process it and respond to it.
• Divergence
• Single neuron diverges into several different
  neurons.
• Ex. Allows for a more forceful contraction of a
  skeletal muscle.

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Reflex Arcs
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Reflex Arcs

• Reflex arcs are automatic subconscious nerve
  pathways that an impulse follows.
• Begins at a receptor which is at the end of a
  sensory neuron.
• The message is transported to an interneuron
  within the CNS (reflex center)
• These connect to motor neurons which are
  connected to structures called effectors.

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Types of Reflexes
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Types of Reflexes

• Actions such as coughing, sneezing, digestion,
  and sneezing are reflexes.
• Knee Jerk reflex
• Also known as the patellar tendon reflex
• Helps maintain an upright position
• Uses two neurons sensory and motor
• Striking the patellar ligament, which is located
  below the patella stimulates stretch receptors.
• These receptors send a message to the spinal cord
  which forms a synapse with a motor neuron.
• The message is then sent to quadracep femoris
  muscle and the muscle contracts extending the leg
  

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Types of Reflexes

• Withdrawal Reflex
• Touching a part of your body such as the finger
  to something it senses as pain
• Sensory receptor sends message to spinal cord.
• There it connects with an interneuron which
  transfers the message to a motor neuron.
• The motor neuron effects the bicep brachii muscle
  and causes it to contract.
• This pulls the finger away.

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Meninges

• Membranes found beneath the skull and vertebrae
  that protect the brain and spinal cord

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Meninges
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Meninges

• Three Layers
• Dura Mater
• Outer most layer
• White fibrous connective tissue
• Forms internal periosteum of skull
• Terminates at end of spinal cord
• In spinal cord not directly attached to vertebrae
  there is a space call the epidural space

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Meninges

• Arachnoid Mater
• Weblike membrane
• Lacks blood vessels
• Spreads over the brain and spinal cord without
  dipping into the spaces
• Between the arachnoid and pia mater is
  subarachnoid space.
• Contains CSF (cerebral spinal fluid

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Meninges

• Pia Mater
• Very thin covering that sits on the brain and
  spinal cord
• Contains blood vessels and nerves that nourish
  the brain and spinal cord
• Follows the contours of the brain and spinal cord

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Subdural Hematoma

• A severe head injury may break blood vessels
  associated with the brain.
• A hematoma forms increasing the amount of
  pressure on the brain

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Spinal Cord

• Consists of 31 segment, each with a pair of
  nerves
• Two parts
• Cervical Enlargement
• Supplies nerves to the upper limbs
• Lumbar Enlargement
• Supplies nerves to the lower limbs

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Cross Section of Spinal Cord

• Ganglion a grouping of cell bodies outside the
  CNS
• Thalmus (in brain) will relay messages down the
  columns of white matter
• Messages come in through the dorsal root ganglia
  and leave through the ventral root ganglia
• Central Canal contains cerebral spinal fluid

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Spinal Cord Functions

• Provides a pathway for messages traveling to
  various regions of the body
• Spinal cord functions in many reflexes (spinal
  reflexes)

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Brain
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Brain
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Structure of the Brain

• The brain can separated into 5 Parts
• Telencephalon
• Frontal Lobe
• Parietal Lobe
• Occipital Lobe
• Temporal Lobe
• Insula
• Corpus Callosum
• Diencephalon
• Thalamus
• Pineal Gland
• Hypothalamus
• Pituitary

• Mesencephalon
• Corpora Quadragemina
• Metencephalon
• Pons
• Cerebellum
• Myencephalon
• Medulla Oblongata

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Function of the Brain Telencephalon

• Frontal Lobe
• Planning and execution of movement
• Critical thinking
• Speech
• Parietal Lobe
• Skin sensations
• Temporal Lobe
• Hearing
• Occipital Lobe
• Vision
• Insula
• Memory
• Corpus Callosum
• Relays messages between the right and left
  hemisphere

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Diencephalon

• Thalamus
• Relays Center
• Pineal Gland
• Produces melatonin
• Pituitary
• Produces a variety of hormones that regulate
  various parts of the body.

• Hypothalamus
• Heart rate and blood pressure
• Body temperature
• Water and electrolyte balance
• Control of hunger and body weight
• Control of movements and gland secretion in
  stomach and intestines
• Control of pituitary secretion
• Sleep and wakefulness
• Pleasure center

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Mesencephalon and Metencephalon

• Mesencephalon
• Corpora quadragemina
• Visual and auditory reflex center
• Metencephalon
• Pons
• Sensory impulses from periphery to higher brain
  centers.
• Regulates depth and rate of breathing
• Cerebellum
• Motor coordination

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Myencephalon

• Medulla Oblongata
• Cardiac Center
• Alters heart rate
• Vasomotor Center
• Blood Pressure
• Respiratory Center
• Regulate rate, rhythm and depth of breathing
• Works with Pons

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Bovine Spongiform Encephalopathy

• Transmissible disease that slowly wastes away the
  brain
• The human form primarily affects younger people
• Very hard to detect until it is too late
• Most cases result in death many years after
  contracting the disease.
• Is caught from eating infected meat.
• Commonly known as Mad Cow

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Alzheimers

• Most common form of dementia
• Fatal
• Loss of memory and ability to control certain
  functions
• Can no longer help self
• Caused by genetic and non genetic factors
• Plaques within the brain cause cells to die which
  leads to dementia
• Treatments
• 3 medications have been approved to delay
  temporarily the effects of the plaques.
• No cure yet

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Epilepsy

• Epilepsy is a brief disruption in electrical
  charges
• Seizures vary from 1-4 second disruption
  (blanking out) to Grand Mole (extremely severe)
• Medications Barbituates such as depakote are
  used to decrease or lessen the effect of
  seizures.
• Stimulation of Vagus nerve and cutting of Corpus
  Callosum are used if medications dont work.

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