NOTES: CH 48 Neurons, Synapses, and Signaling

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NOTES CH 48Neurons, Synapses, and Signaling
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? A nervous system has three overlapping
functions

• 1) SENSORY INPUT signals from sensory receptors
  to integration centers
• 2) INTEGRATION information from sensory
  receptors is interpreted and associated with
  appropriate responses
•  
• 3) MOTOR OUTPUT conduction of signals from the
  integration center to effector cells (muscle
  cells or gland cells)

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• CENTRAL NERVOUS SYSTEM (CNS)
• ? integration center
• ? brain and spinal cord

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• PERIPHERAL NERVOUS
• SYSTEM (PNS)
• ? made up of nerves
• (ropelike bundles
• of neurons)
• ? nerves communicate
• motor and sensory
• signals to and from CNS
• and rest of body

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Two Main Classes of Cells

• 1) NEURONS
• ? functional unit of the nervous system
• ? transmits signals from one location to another
• ? made up of cell body, dendrites, axon
• ? many axons are enclosed by an insulating layer
  called the MYELIN SHEATH
• ? include sensory neurons,
• interneurons,
• motor neurons

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• 2) GLIAL CELLS (GLIA) - SUPPORTING CELLS
• ? 10 to 50 times more numerous than neurons
• ? provide structure protect, insulate, assist
  neurons
• ? example Schwann cells and oligodendrocytes
  form myelin sheaths in the PNS and CNS,
  respectively

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MYELIN SHEATH

• ? produced by Schwann cells in the peripheral
  nervous system
• ? gaps between successive Schwann cells are
  called NODES OF RANVIER.
• the 10 term!!! ?

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NODES OF RANVIER!

• word 10 on my list!!!
• 1) Okazaki fragments
• 2) plasmodesmata
• 3) ???????
• 4) ???????
• 5) ???????
• 6) rubisco
• 7) oxaloacetate
• 8) islets of Langerhans
• 9) Batesian mimicry
• 10) nodes of Ranvier

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• 2) GLIA (SUPPORTING CELLS)
• ? example astrocytes responsible for
  blood-brain barrier

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Astrocyte
Nerve cells
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ACTION POTENTIALS NERVE IMPULSES

• all cells have an electrical charge difference
  across their plasma membranes that is, they are
  POLARIZED.
• ? this voltage is called the MEMBRANE POTENTIAL
  (usually 50 to 100 mV)
•  ? inside of cell is negative relative to outside
•  ? arises from differences in ionic
  concentrations inside and outside cell

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• A- group of anions
• including proteins,
• amino acids, sulfate,
• phosphate, etc. large
• molecules that cannot
• cross the membrane
• and therefore provide
• a pool of neg. charge
• that remains in the
• cell

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How is this potential maintained?

• ? the sodium-potassium pump uses ATP to maintain
  the ionic gradients across the membrane
• (3 Na out 2 K in)

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• ? the resting potential
• of a nerve cell is approx.
• 70 mV
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• ? neurons have special
• ion channels (GATED
• ION CHANNELS) that allow the cell
• to change its membrane potential
• (a.k.a. excitable cells)

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• ? when a stimulus reaches a neuron, it causes the
  opening of gated ion channels
• (e.g. light ? photoreceptors in the eye sound
  waves/vibrations ? hair cells in inner ear)

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• ? HYPERPOLARIZATION membrane potential becomes
  more negative (K channel opens increased
  outflow of K)
•  
• ? DEPOLARIZATION membrane potential becomes
  less negative
• (Na channel opens increased inflow
• of Na)
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• If the level of depolarization reaches the
  THRESHOLD POTENTIAL, an ACTION POTENTIAL is
  triggered.

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• ACTION POTENTIALS (APs)
• ? the nerve impulse
•  
• ? all-or-none event magnitude is independent of
  the strength of the stimulus
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5 Phases of an A.P. 1) Resting state 2)
Depolarizing phase 3) Rising phase of A.P. 4)
Falling phase of AP (repolarizing phase) 5)
Undershoot
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• during the undershoot, both Na channel gates
  are closed if a second depolarizing stimulus
  arrives during this time, the neuron will NOT
  respond (REFRACTORY PERIOD)
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• ? strong stimuli result in greater frequency of
  action potentials than weaker stimuli

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• How do action potentials travel along an axon?
• ? the strong depolarization of one action
  potential assures that the neighboring region of
  the neuron will be depolarized above threshold,
  triggering a new action potential, and so on

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Saltatory Conduction
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• ? SYNAPSE junction between a neuron and another
  cell found between
• -2 neurons
• -sensory receptor
• sensory neuron
• -motor neuron muscle cell
• -neuron gland cell

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Motor Neuron and Muscle Cell
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• ? Presynaptic cell transmitting cell
• ? Postsynaptic cell receiving cell

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• Electrical Synapses allow action potentials to
  spread directly from pre- to postsynaptic cell
• connected by gap junctions (intercellular
  channels that allow local ion currents)
• Most synapses are
• Chemical Synapses cells are separated by a
  synaptic cleft, so cells are not electrically
  coupled a series of events converts
• elec. signal ? chem.signal ? elec.signal
• HOW???...

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• NEUROTRANSMITTERS intercellular messengers
  released into synaptic cleft when synaptic
  vesicles fuse with presynaptic membrane
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• ? specific receptors for neurotransmitters
  project from postsynaptic membrane most
  receptors are coupled with ion channels
•  
• ? neurotransmitters are quickly broken down by
  enzymes so that the stimulus ends

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• ? the electrical charge caused by the binding of
  neurotransmitter to the receptor can be
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• EPSP (Excitatory Postsynaptic Potential)
  membrane potential is moved closer to threshold
  (depolarization)
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• IPSP (Inhibitory Postsynaptic Potential)
  membrane potential is hyperpolarized (more
  negative)

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• ? most single EPSPs are not strong enough to
  generate an action potential
• ? when several EPSPs occur close together or
  simultaneously, they have an additive effect on
  the postsynaptic potential SUMMATION
• -temporal vs. spatial

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Examples of neurotransmitters
epin. norep. also function as hormones fight
or flight response
dop. ser. both affect sleep, mood, attention,
learning LSD mescaline bind to ser. dop.
receptors
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Neurotransmitters Ach

• ? ACETYLCHOLINE triggers skeletal muscle fibers
  to contract
• ? so, how does a muscle contraction stop???

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Neurotransmitters Ach

• ? a muscle contraction ceases when the
  acetylcholine in the synapse of the neuromuscular
  junction is broken down by the enzyme..
• wait for it.

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ACETYLCHOLINESTERASE!!

• Its term 4!!!!!
• ? ACETYLCHOLINESTERASE the enzyme the breaks
  down the neurotransmitter acetylcholine.

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ACETYLCHOLINESTERASE!

• word 4 on my list!!!
• 1) Okazaki fragments
• 2) plasmodesmata
• 3) ????????
• 4) acetylcholinesterase
• 5) ????????
• 6) rubisco
• 7) oxaloacetate
• 8) islets of Langerhans
• 9) Batesian mimicry
• 10) nodes of Ranvier