Nervous Tissue

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Nervous Tissue

• Dr. Michael P. Gillespie

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Nervous System

• The nervous system is an intricate, highly
  organized network of billions of neurons and even
  more neuroglia.
• The nervous system has a mass of only 2 kg (4.5
  lb), which comprises approximately 3 of total
  body weight.

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Structures of the Nervous System (CNS)

• Brain (100 billion neurons)
• Spinal cord (100 million neurons)

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Structures of the Nervous System (PNS)

• Spinal nerves (31 pairs)
• Cranial nerves (12 pairs)
• Ganglia (Masses of primarily neuron cell bodies)
• Enteric plexuses (networks of neurons in the GI
  tract)
• Sensory receptors (dendrites of sensory neurons)

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Functions of the Nervous System

• Sensory function afferent neurons
• Sensory receptors detect internal and external
  stimuli
• Integrative function interneurons
• The nervous system processes sensory information
  and coordinates responses. It perceives stimuli.
• Motor function efferent neurons
• The cells contacted by these neurons are called
  effectors (muscles and glands)

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Organization of the Nervous System

• Central nervous system
• Brain
• Spinal cord

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Organization of the Nervous System

• Peripheral nervous system
• Cranial nerves and their branches
• Spinal nerves and their branches
• Ganglia
• Sensory receptors
• Somatic nervous system
• Autonomic nervous system
• Enteric nervous system

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Somatic Nervous System (SNS)

• Sensory neurons.
• Motor neurons located in skeletal muscles.
• The motor responses can be voluntarily
  controlled therefore this part of the PNS is
  voluntary.

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Autonomic Nervous System (ANS)

• Sensory neurons from the autonomic sensory
  receptors in the viscera.
• Motor neurons located in smooth muscle, cardiac
  muscle and glands.
• These motor responses are NOT under conscious
  control Therefore this part of the PNS is
  involuntary.

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ANS Continued

• The motor portion of the ANS consists of
  sympathetic and parasympathetic divisions.
• Both divisions typically have opposing actions.

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Enteric Nervous System (ENS)

• The brain of the gut.
• Functions independently of the ANS and CNS, but
  communicates with it as well.
• Enteric motor units govern contraction of the GI
  tract.
• Involuntary.

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Types of Nervous Tissue Cells

• Neurons.
• Sensing.
• Thinking.
• Remembering.
• Controlling muscular activity.
• Regulating glandular secretions.
• Neuroglia.
• Support, nourish, and protect neurons.

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Neurons

• Have the ability to produce action potentials or
  impulses (electrical excitability) in response to
  a stimulus.
• An action potential is an electrical signal that
  propagates from one point to the next along the
  plasma membrane of a neuron.
• A stimulus is any change in the environment that
  is strong enough to initiate an action potential.

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Parts of a Neuron

• Cell Body
• Dendrites
• Axon

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Parts of a Neuron (Cell Body)

• Cell body (perikaryon or soma).
• Contains the nucleus surrounded by cytoplasm
  which contains the organelles.
• Clusters of rough ER called Nissl bodies (produce
  proteins to grow and repair damaged nerves)

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Parts of a Neuron (Nerve Fiber)

• Nerve fiber any neuronal process that emerges
  from the cell body of a neuron.
• Dendrites
• Axon

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Parts of a Neuron (Dendrites)

• Dendrites ( little trees).
• The receiving (input) portion of a neuron.
• Short, tapering, and highly branched.

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Parts of a Neuron (Axon)

• Axon ( axis).
• Each nerve contains a single axon.
• The axon propagates nerve impulses toward another
  neuron, muscle fiber, or gland cell.
• Long, thin, cylindrical projection that often
  joins the cell body at a cone-shaped elevation
  called the axon hillock ( small hill).
• The part of the axon closest to the hillock is
  the initial segment.
• The junction between the axon hillock and the
  initial segment is the trigger zone (nerve
  impulses arise here).
• The cytoplasm of the axon is the axoplasm and is
  surrounded by a plasma membrane known as the
  axolemma (lemma sheath).

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Synapse

• The synapse is the site of communication between
  two neurons or between a neuron and an effector
  cell.
• Synaptic end bulbs and varicosities contain
  synaptic vesicles that store a chemical
  neurotransmitter.

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Axonal Transport

• Slow axonal transport.
• 1-5 mm per day.
• Travels in one direction only from cell body
  toward axon terminals.
• Fast axonal transport.
• 200 400 mm per day.
• Uses proteins to move materials.
• Travels in both directions.

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Structural Diversity of Neurons

• The cell body diameter can range in size from 5
  micrometers (µm) (slightly smaller than a RBC) up
  to 135 µm (barely visible to the naked eye).
• Dendritic branching patterns vary.
• Axon length varies greatly as well. Some neurons
  have no axon, some are very short, and some run
  all the way from the toes to the lowest part of
  the brain.

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Classification of Neurons

• Both Structural and Functional features are used
  to classify neurons.

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Structural Classifications of Neurons

• Structurally, neurons are classified according to
  the number of processes extending from the cell
  body.
• 3 Structural Classes
• Multipolar neurons
• Bipolar neurons
• Unipolar neurons

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Multipolar Neurons

• One axon and several dendrites.
• Most neurons of the brain and spinal cord are of
  this type.

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Bipolar Neurons

• Bipolar neurons.
• One axon and one main dendrite.
• Retina of the eye, inner ear, and the olfactory
  areas of the brain.

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Unipolar Neurons

• Unipolar neurons.
• The axon and the dendrite fuse into a single
  process that divides into two branches.
• The dendrites monitor a sensory stimulus such as
  touch, pressure, pain, heat, or stretching.
• Called psuedounipolar neurons.

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Functional Classification of Neurons

• Functionally, neurons are classified according to
  the direction in which the nerve impulse (action
  potential) is conveyed with respect to the CNS.
• 3 Functional Classes
• Sensory or afferent neurons
• Motor of efferent neurons
• Interneurons or association neurons

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Sensory (Afferent) Neurons

• Either contain sensory receptors or are located
  adjacent to sensory receptors that are separate
  cells.
• Conveyed into the CNS through cranial or spinal
  nerves.
• Most are unipolar.

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Motor (Efferent) Neurons

• Away from the CNS to effectors (muscles and
  glands).
• Most are multipolar.

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Interneurons (Association Neurons)

• Mainly located within the CNS between sensory and
  motor neurons.
• They process sensory information and elicit a
  motor response.
• Most are multipolar.

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Neuroglia

• Half the volume of the CNS.
• Generally, they are smaller than neurons, but 5
  to 50 times more numerous.
• They can multiply and divide.
• Gliomas brain tumors derived from glia.

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Types of Neuroglia

• CNS
• Astrocytes
• Oligodendrocytes
• Microglia
• Ependymal cells
• PNS
• Schwann cells
• Satellite cells

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Astrocytes

• Star shaped cells with many processes.
• Largest and most numerous of the neuroglia.

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Astrocytes

• Functions
• Support neurons.
• Processes wrap around capillaries to create a
  blood-brain barrier.
• Regulate growth, migration and interconnection
  among neurons in the embryo.
• Maintain chemical environment for impulse
  transmission
• Influence formation of neural synapses.

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Astrocytes
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Astrocytes
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Astrocytes
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Oligodendrocytes

• Similar to astrocytes, but smaller with fewer
  processes.
• Function
• Form and maintain the myelin sheath around the
  CNS axons.

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Oligodendrocytes
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Microglia

• Small cells with slender processes giving off
  numerous spine like projections.
• Function
• Phagocytes.

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Microglia
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Ependymal Cells

• Cuboidal to columnar cells.
• Possess microvilli and cilia.
• Functions
• Produce cerebrospinal fluid (CSF)
• Assist in circulation of CSF
• Possibly monitor CSF

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Ependymal Cells
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CNS Neuroglia
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Schwann Cells

• Encircle PNS axons to forma sheath around them.
• One Schwann cell per axon.
• Function
• Form myelin sheath around PNS neurons
• Assist in axon regeneration

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Schwann Cells
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Myelination

• The myelin sheath is a lipid and protein
  covering. It is produced by the neuroglia.
• The sheath electrically insulates the axon of a
  neuron.
• The sheath increases the speed of nerve impulse
  conduction.
• The amount of myelin increases from birth on.
• Axons without a covering are unmyelinated. Axons
  with a covering are myelinated.

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Myelination Continued

• Two types of neuroglial cells produce
  myelination.
• Schwann cells located in the PNS.
• Oligodendrocytes located in the CNS.

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Neurolemma (Sheath of Schwann)

• The neurolemma (sheath of Schwann) is the outer
  nucleated cytoplasmic layer of the Schwann cell.
• It encloses the myelin sheath.
• It is only found around the axons of the PNS.
• If the axon is injured, the neurolemma forms a
  regeneration tube that guides and stimulates
  re-growth of the axon.

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Nodes of Ranvier

• The nodes of Ranvier are gaps in the myelin
  sheath at intervals along the axon.
• Each Schwann cell wraps one axon segment between
  two nodes.
• The electrical impulse jumps from node to node to
  speed up the propagation
• Nodes of Ranvier are present in the CNS, but
  fewer in number.

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Demyelination

• Demyelination is the loss or destruction of the
  myelin sheaths around axons.
• It occurs as the result of disorders such as
  multiple sclerosis or Tay-Sachs disease.
• Radiation and chemotherapy can also damage the
  myelin sheath.
• Demyelination can deteriorate the affected nerves.

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Collections of Nervous Tissue

• Neuronal cell bodies are grouped in clusters.
• Axons of neurons are grouped in bundles.
• Nervous tissue is grouped in gray and white
  matter.

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Clusters of Neuronal Cell Bodies

• Ganglion cluster of neuronal cell bodies in the
  PNS.
• Associated with the cranial and spinal nerves.
• Nucleus cluster of neuronal cell bodies in the
  CNS.

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Bundles of Axons

• Nerve a bundle of axons in the PNS.
• Cranial nerves connect the brain to the
  periphery.
• Spinal nerves connect the spinal cord to the
  periphery.
• Tract a bundle of axons in the CNS.
• Tracts interconnect neurons in the spinal cord
  and brain.

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Gray and White Matter

• The white matter consists of aggregations of
  primarily myelinated and some unmyelinated axons.
  (Myelin is whitish in color)
• The gray matter consists of neuronal cell bodies,
  dendrites, unmyelinated axons, axon terminals,
  and neuroglia. (Nissl bodies impart a gray color)

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Electrical Signals in Neurons

• Neurons are electrically excitable and
  communicate with one another using 2 types of
  electrical signals.
• Graded potentials (short distance communication).
• Action potentials ((long distance communication).
• The plasma membrane exhibits a membrane
  potential. The membrane potential is an
  electrical voltage difference across the membrane.

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Electrical Signals in Neurons

• The voltage is termed the resting membrane
  potential.
• The flow of charged particles across the membrane
  is called current.
• In living cells, the flow of ions constitutes the
  electrical current.

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Ion Channels

• The plasma membrane contains many different kinds
  of ion channels.
• The lipid bilayer of the plasma membrane is a
  good electrical insulator.
• The main paths for flow of current across the
  membrane are ion channels.

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Ion Channels

• When ion channels are open, they allow specific
  ions to move across the plasma membrane down
  their electrochemical gradient.
• Ions move from greater areas of concentration to
  lesser areas of concentration.
• Positively charged cations move towards a
  negatively charged area and negatively charged
  anions move towards a positively charged area.
• As they move, they change the membrane potential.

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Ion Channel Gates

• Ion channels open and close due to the presence
  of gates.
• The gate is part of a channel protein that can
  seal the channel pore shut or move aside to open
  the pore.

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Types of Ion Channels

• Leakage channels
• Ligand-gated channel
• Mechanically gated channel
• Voltage gated channel

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Leakage Channels

• Leakage channels gates randomly alternate
  between open and closed positions.
• More potassium ion (K) leakage channels than
  sodium (Na) leakage channels.
• The potassium ion leakage channels are leakier
  than the sodium ion leakage channels.

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Ligand-gated Channel

• Ligand-gated channels open and close in
  response to a specific chemical stimulus.
• Neurotransmitters, hormones, and certain ions can
  act as the chemical stimulus that opens or closes
  these channels.

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Mechanically Gated Channel

• Mechanically gated channels opens or closes in
  response to mechanical stimulation.
• Vibration, touch, pressure, or tissue stretching
  can all distort the channel from its resting
  position, opening the gate.

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Voltage-gated Channel

• Voltage-gated channels opens in response to a
  change in membrane potential (voltage).
• These channels participate in the generation and
  conduction of action potentials.

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Gradients

• Concentration Gradient A difference in the
  concentration of a chemical from one place to
  another.
• Electrochemical Gradient The combination of the
  effects of the concentration gradient and the
  membrane potential.

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Transport Across the Membrane

• Passive Transport does not require cellular
  energy.
• Substances move down their concentration or
  electrochemical gradients using only their own
  kinetic energy.
• Active Transport requires cellular energy in
  the form of ATP.

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3 Types of Passive Transport

• Diffusion through the lipid bilayer.
• Diffusion through membrane channels.
• Facilitated diffusion.

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Diffusion

• Materials diffuse from areas of high
  concentration to areas of low concentration.
• The move down their concentration gradient.
• Equilibrium molecules are mixed uniformly
  throughout the solution.

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Factors Influencing Diffusion

• Steepness of the concentration gradient.
• Temperature.
• Mass of the diffusing substance,
• Surface area.
• Diffusion distance.

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Resting Membrane Potential

• The resting membrane potential occurs due to a
  buildup of negative ions in the cytosol along the
  inside of the membrane and positive ions in the
  extracellular fluid along the outside of the
  membrane.
• The potential energy is measured in millivolts
  (mV).

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Resting Membrane Potential

• In neurons, the resting membrane potential ranges
  from 40 to 90 mV. Typically 70 mV.
• The minus sign indicates that the inside of the
  cell is negative compared to the outside.
• A cell that exhibits a membrane potential is
  polarized.
• The potential exists because of a small buildup
  of negative ions in the cytosol along the inside
  of the membrane and positive ions in the
  extracellular fluid along the membrane.

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Electrochemical Gradient

• An electrical difference and a concentration
  difference across the membrane.

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Factors Producing the Resting Membrane Potential

• Unequal distribution of ions in the ECF and
  cytosol.
• Inability of most anions to leave the cell.
• Electrogenic nature of the Na/K ATPases.

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Unequal distribution of ions in the ECF and
cytosol.

• ECF is rich in Na and CL- ions.
• Cytosol has the cation K and the dominant anions
  are phosphates attached to ATP and amino acids in
  proteins.
• The plasma membrane has more K leakage channels
  than Na leakage channels.

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Inability of most anions to leave the cell.

• The anions are attached to large nondiffusable
  molecules such as ATP and large proteins.

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Electrogenic nature of the Na/K ATPases.

• Membrane permeability to Na is very low because
  there are very few sodium leakage channels.
• Sodium ions do slowly diffuse into the cell,
  which would eventually destroy the resting
  membrane potential.
• Na/K ATPases pump sodium back out of the cell
  and bring potassium back in.
• They pump out 3 Na for every 2 K they bring in.

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Graded Potentials

• A graded potential is a small deviation from the
  resting membrane potential.
• It makes the membrane either more polarized (more
  negative inside) or less polarized (less negative
  inside).
• Most graded potentials occur in the dendrites or
  cell body.

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Graded Potentials

• Hyperpolarizing graded potential make the
  membrane more polarized (inside more negative).
• Depolarizing graded potential make the membrane
  less polarized (inside less negative).
• Graded potentials occur when ligand-gated or
  mechanically gated channels open or close.
• Mechanically gated and ligand-gated channels are
  present in sensory neurons.
• Ligand-gated channels are present in interneurons
  and motor neurons.

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Graded Potentials

• Graded potentials are graded because they vary in
  amplitude (size) depending on the strength of the
  stimulus.
• The amplitude varies depending upon how many
  channels are open and how long they are open.
• The opening and closing of channels produces a
  flow of current that is localized.

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Graded Potentials

• The charge spreads a short distance and dies out
  (decremental conduction).
• The charge can become stronger and last longer by
  adding with other graded potentials (Summation).

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Types of Graded Potentials

• Post-synaptic potentials a graded potential
  that occurs in the dendrites or cell body of a
  neuron in response to a neurotransmitter.
• Receptor potentials and generator potentials
  graded potentials that occur in sensory receptors
  and sensory neurons.

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Action Potentials

• An action potential or impulse is a sequence of
  events that decrease and reverse the membrane
  potential and eventually restore it to its
  resting state.
• Depolarizing phase the resting membrane
  potential becomes less negative, reaches zero,
  and then becomes positive.
• Repolarizing phase restores the resting
  membrane potential to -70 mV.

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Threshold

• Threshold depolarization reaches a certain
  level (about 55 mV), voltage gated channels
  open.
• A weak stimulus that does not bring the membrane
  to threshold is called a sub-threshold stimulus.
• A stimulus that is just strong enough to
  depolarize a membrane is called a threshold
  stimulus.
• Several action potentials will from in response
  to a supra-threshold stimulus.
• Action potentials arise according to an all or
  none principal.

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Depolarizing Phase

• A depolarizing graded potential or some other
  stimulus causes the membrane to reach threshold.
• Voltage-gated ion channels open rapidly.
• The inflow of positive Na ions changes the
  membrane potential from 55mv to 30 mV.
• K channels remain largely closed.
• About 20,000 Na enter through the gates.
  Millions are present in the surrounding fluid.
• Na/K pumps bail them out.

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Repolarizing Phase

• While Na channels are opening during
  depolarization, K channels remain largely
  closed.
• The closing of Na channels and the slow opening
  of K channels allows for repolarization.
• K channels allow outflow of K ions.

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Refractory Period

• The refractory period is the period of time after
  an action potential begins during which an
  excitable cell cannot generate another action
  potential.
• Absolute refractory period a second action
  potential cannot be initiated, even with a very
  strong stimulus.
• Relative refractory period an action potential
  can be initiated, but only with a larger than
  normal stimulus.

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Propagation of Nerve Impulses

• Unlike the graded potential, the impulse in the
  action potential is not detrimental (it does not
  die out).
• The impulse must travel from the trigger zone to
  the axon terminals.
• This process is known as propagation or
  conduction.
• The impulse spreads along the membrane.
• As Na ions flow in, they trigger depolarization
  which opens Na channels in adjacent segments of
  the membrane.

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2 Types of Propagation

• Continuous Conduction step by step
  depolarization and repolarization of each segment
  of the plasma membrane.
• Saltatory Conduction a special mode of action
  potential propagation along myelinated axons.
• The action potential leaps from one Node of
  Ranvier to the next.

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Continuous and Saltatory Conduction

• Few ion channels are present where there is
  myelin.
• Nodes of Ranvier areas where there is no myelin
  contain many ion channels.
• The impulse jumps from node to node.
• This speeds up the propagation of the impulse.
• This is a more energy efficient mode of
  conduction.

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Neurotoxins Local Anesthetics

• Neurotoxins produce poisonous effects upon the
  nervous system.
• Local anesthetics are drugs that block pain and
  other somatic sensations.
• These both act by blocking the opening of
  voltage-gated Na channels and preventing
  propagation of nerve impulses.

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Factors That Affect Speed of Propagation

• 1. Amount of myelination - Myelinated axons
  conduct impulses faster than unmyelinated ones.
• 2. Axon diameter - Larger diameter axons
  propagate impulses faster than smaller ones.
• 3. Temperature Axons propagate action
  potentials at lower speeds when cooled.

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Classification of Nerve Fibers

• A fibers.
• Largest diameter.
• Myelinated.
• Convey touch, pressure, position, thermal
  sensation.

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Classification of Nerve Fibers

• B fibers.
• Smaller diameter than A fibers.
• Myelinated.
• Conduct impulses from the viscera to the brain
  and spinal cord (part of the ANS).

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Classification of Nerve Fibers

• C fibers.
• Smallest diameter.
• Unmyelinated.
• Conduct some sensory impulses and pain impulses
  from the viscera.
• Stimulate the heart, smooth muscle, and glands
  (part of ANS).

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Encoding Intensity of a Stimulus

• A light touch feels different than a firmer touch
  because of the frequency of impulses.
• The number of sensory neurons recruited
  (activated) also determines the intensity of the
  stimulus.

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Signal Transmission at Synapses

• Presynaptic neuron the neuron sending the
  signal.
• Postsynaptic neuron the neuron receiving the
  message.
• Axodendritic from axon to dendrite.
• Axosomatic from axon to soma.
• Axoaxonic from axon to axon.

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Types of Synapses

• Electrical synapse
• Chemical synapse

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Electrical Synapses

• Action potentials conduct directly between
  adjacent cells through gap junctions.

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Electrical Synapses

• Tubular connexons act as tunnels to connect the
  cytosol of the two cells.
• Advantages.
• Faster communication than a chemical synapse.
• Synchronization they can synchronize the
  activity of a group of neurons or muscle fibers.
  In the heart and visceral smooth muscle this
  results in coordinated contraction of these
  muscle fibers.

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Chemical Synapses

• The plasma membranes of a presynaptic and
  postsynaptic neuron in a chemical synapse do not
  touch one another directly.
• The space between the neurons is called a
  synaptic cleft which is filled with interstitial
  fluid.
• A neurotransmitter must diffuse through the
  interstitial fluid in the cleft and bind to
  receptors on the postsynaptic neuron.
• The synaptic delay is about 0.5 msec.

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Removal of Neurotransmitter

• Diffusion.
• Enzymatic degradation.
• Uptake by cells.
• Into the cells that released them (reuptake).
• Into neighboring glial cells (uptake).

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Spatial and Temporal Summation of Postsynaptic
Potentials

• A typical neuron in the CNS receives input from
  1000 to 10,000 synapses.
• Integration of these inputs is known as summation.

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Spatial and Temporal Summation of Postsynaptic
Potentials

• Spatial summation summation results from
  buildup of neurotransmitter released by several
  presynaptic end bulbs.
• Temporal summation summation results from
  buildup of neurotransmitter released by a single
  presynaptic end bulb 2 or more times in rapid
  succession.

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Neural Circuits

• Diverging circuit single presynaptic neuron
  influences several postsynaptic neurons (i.e.
  muscle fibers or gland cells).
• Converging circuit several presynaptic neruons
  influence a single post-synaptic neuron (results
  in a stronger signal).

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Neural Circuits

• Reverberating circuit Branches from later
  neurons stimulate earlier ones (may last for
  seconds to hours) (breathing, coordinated
  muscular activities, waking up, short-term
  memory).
• Parallel after-discharge circuit a presynaptic
  neuron stimulates a group of neurons that all
  interact with a common postsynaptic cell (quick
  stream of impulses) (mathematical calculations).

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Neural Circuits
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Neurogenesis in the CNS

• Birth of new neurons.
• From undifferentiated stem cells.
• Epidermal growth factor stimulates growth of
  neurons and astrocytes.
• Minimal new growth occurs in the CNS.
• Inhibition from glial cells.
• Myelin in the CNS.

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Damage and Repair in the PNS

• Axons and dendrites may undergo repair if the
  cell body is intact, if the Schwann cells are
  functional, and if scar tissue does not form too
  quickly.
• Wallerian degeneration.
• Schwann cells adjacent to the site of injury grow
  torwards one another and form a regeneration tube.

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