Nervous System

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Nervous System

• AP Biology Chapter 48

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The Neuron

• Another term for a nerve cell
• Basic structural unit of the nervous system
• Can be very long
• Fraction of an inch to several feet

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Parts of a Neuron

• Cell Body
• Dendrites
• Receive stimuli
• Axon
• Transmits the nerve impulse
• Myelin sheath
• Schwann cells
• insulation
• Nodes of Ranvier

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The Neuron
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Myelin Sheath
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Nerve Impulse Direction

• The nerve impulse always travels FROM the
  dendrites TO the ends of the axon.

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Basic Vertebrate Nervous System
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3 Groups of Neurons

• Sensory Neurons
• Receive initial stimulus
• Afferent or receptor neurons
• Examples
• Sensory neurons in the eye are stimulated by light

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3 Groups of Neurons

• Motor Neurons
• Stimulate effector cells
• Effector cells
• Target cells that produce a response
• Example Muscle cells

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3 Groups of Neurons

• Interneurons
• Also called connector or associative neurons
• Location
• Brain
• Spinal cord
• Job
• Integrators
• Evaluate nerve impulses from sensory neurons and
  send impulses to motor neurons for an appropriate
  response

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The Transmission of the Nerve Impulse

• Transmission occurs as a result of polarization
  across a neurons membrane
• Polarization difference in electrical charge
  which exists between the inside and outside of a
  membrane

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The Transmission of the Nerve Impulse Set up
of a neuron

• An unstimulated neuron is said to be polarized
• The inside of the membrane is negative
• The outside of the membrane is positive

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The Transmission of the Nerve Impulse - Set up
of a neuron

• How is polarization maintained across the
  neurons membrane?
• An excess of sodium (Na) ions exists on the
  outside of the membrane
• An excess of potassium (K) ions exists on the
  inside of the membrane
• A certain amount of Na and K are always
  leaking to one side or the other, but Na/K
  Pumps embedded in the membrane actively restore
  ions to the appropriate side

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The Transmission of the Nerve Impulse - Set up
of a neuron

• Na and K are NOT the only ions associated with
  neurons. There are others as well and THESE are
  actually what make the charge difference from
  inside to outside the membrane exist
• Inside the neurons membrane are large,
  negatively charged molecules such as proteins
• It is these molecules that contribute to the
  overall negative charge on the inside of the cell
  membrane
• The are so large that they cannot move across the
  membrane easily, so they stay inside

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Membrane Potential - Definition

• Voltage measured across a membrane
• Difference between the internal and external
  concentrations of solution (ions)

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Events of the Nerve Impulse

• Resting Potential
• The unstimulated, polarized state of a neuron
• About 70 millivolts membrane potential (charge
  difference existing across the membrane)

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Events of the Nerve Impulse

• Action Potential
• A complete depolarization
• In response to a stimulus, structures called
  gated ion channels suddenly open
• This permits the Na ions on the outside to rush
  inside

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Events of the Nerve Impulse

• Because these Na ions rush in, the cell becomes
  depolarized
• Membrane potential at 0mv.
• If the stimulus is strong enough (above a certain
  threshold level) even more ion gates will open
  and even more Na will come in to the cell
• Positive feedback
• Membrane potential 30 millivolts

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Events of the Nerve Impulse

• The action potential in turn stimulates
  neighboring Na gates further down the neuron to
  open
• This is how the action potential travels down the
  length of the neuron

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Events of the Nerve Impulse

• Note that action potential is an ALL or NOTHING
  event
• If the stimulus fails to produce depolarization
  that exceeds the threshold value, then NO action
  potential results
• If the threshold value is met, then complete
  depolarization occurs

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Events of the Nerve Impulse

• Repolarization
• The original polarization is restored
• The inflow of Na ions, in turn, stimulates
  another type of gated ion channel to open
• This channel allows K to move OUT of the cell
• This movement of positive ions to the outside
  causes the original poarization to return, but
• The K are on the OUTSIDE now!
• The Na are on the inside!
• OPPOSITE of resting potential
• The Na gates close soon after the K gates open

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Events of the Nerve Impulse

• Hyperpolarization
• Undershoot
• When more K has moved out of the cell than is
  actually needed to repolarize it
• Membrane potential is 80 millivolts

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Events of the Nerve Impulse

• Refractory Period
• When the membrane is polarized but the Na and K
  are on the wrong sides of the membrane
• During this time, the neuron will NOT fire
• How to restore Na and K to proper positions?
• Na/K pumps
• Once the Na and K are restored, the refractory
  period is over and the neuron may respond to
  another stimulus

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Events of the Nerve Impulse

• It is the number of action potentials per second
  that indicates a strong stimulus NOT the
  amplitude of the action potential

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The Myelin Sheath and Nodes of Ranvier

• Purpose
• Allows the action potential to literally JUMP
  from node to node rather than traveling the
  entire length of the axon
• Called saltatory conduction
• Saltatory means jumping
• It speeds the movement of the impulse

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Transmission of the Nerve Impulse from Neuron to
Neuron

• Synapse
• The gap that separates adjacent neurons
• OR the gap that separates a neuron from its
  effector cell
• Presynaptic membrane
• Membrane before the synapse - sending
• Postsynaptic membrane
• Membrane after the synapse receiving
• May be an effector cell (ex. Muscle cell) or
  another neuron

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Transmission across the synapse

• Electrical
• Involves gap junctions
• Small tubes of cytoplasm that connect adjacent
  cells
• The electrical impulse would essentially be
  physically connected between one neuron and the
  next
• Chemical
• One neuron sends chemicals across the synapse to
  the next neuron
• Most animals use this method

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Chemical transmission of the nerve impulse

• The action potential reaches the end of the axon
• This stimulates Calcium ion gates to open at the
  end of the axon
• Calcium ions enter the cell at the end of the axon

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Chemical transmission of the nerve impulse

• Calcium ions stimulate structures called synaptic
  vesicles to merge with the cell membrane at the
  ends of the axon
• Synaptic vesicles contain a chemical called a
  neurotransmitter
• The neurotransmitter is released into the synapse
  (the space between cells)

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Chemical transmission of the nerve impulse

• The neurotransmitter travels across the synapse
  and binds with receptors on the surface of the
  postsynaptic cell membrane
• Different receptors exist for different
  neurotransmitters

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Chemical transmission of the nerve impulse

• The Post synaptic membrane is either excited or
  inhibited
• Depends on the kind of neurotransmitter and the
  kind of membrane receptors present
• Excitation
• If Na gates open, the membrane will become
  depoarized
• An action potential would be generated and the
  transmission would continue
• Inhibition
• If the K gates open, the membrane will become
  MORE polarized (hyperpolarized) and it will be
  more difficult to generate an action potential

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Chemical transmission of the nerve impulse

• Neurtransmitter is degraded or recycled
• Enzymes in the synaptic cleft (synapse) break
  down the neurotransmitters that are stuck in the
  receptors
• This is essential or else the neuron would be
  permanently stuck in this excited (or inhibited)
  position
• Example
• Acetylcholine a neurotransmitter
• Cholinesterase enzyme that breaks down
  acetylcholine

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Common Neurotransmitters

• Acetylcholine
• Neuromuscular junctions
• Gaps between motor neurons and muscle cells
• Stimulates muscle cell contraction
• Others important neurotransmitters between
  neurons of the CNS
• Epinephrine
• Dopamine
• Serotonin

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Animal Nervous Systems
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Vertebrate Nervous System

• Two main parts
• Central Nervous System (CNS)
• Brain
• Spinal cord
• Peripheral Nervous System (PNS)
• Sensory neurons
• Transmit TO CNS
• Motor neurons
• Transmit FROM CNS to effectors

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Motor Neuron System

• Two main parts
• Somatic nervous system
• Directions contraction of skeletal muscles
• Autonomic nervous system
• Controls activities of organs and various
  involuntary muscles

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Motor Neuron System
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Autonomic Neuron System

• Two Main Parts
• Sympathetic nervous system
• Stimulation of activities that prepare the body
  for action
• Examples increased heart rate fight or flight
  situations
• Parasympathetic nervous system
• Stimulates tranquil functions
• Examples secretion of saliva digestive enzyme
  secrections
• In general these two systems target the same
  organs but work antagonistically one speeds up,
  the other slows down

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Reflex Arc

• Rapid involuntary stimulus
• Bypasses the brain
• Involves
• Sensory neuron
• Motor neuron
• Maybe an integrator neuron in the spinal cord,
  but not brain