Chapter 2 Nerve Cells and Nerve Impulses

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Chapter 2Nerve Cells and Nerve Impulses
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Animal Cells

• Membrane
• separates the inside of the cell from the
  outside environment
• comprised of two layers of lipids with proteins
  embedded

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Animal Cells

• Nucleus refers to the structure that contains the
  chromosomes
• Mitochondria perform metabolic activities and
  provide energy that the cell requires.
• Ribosomes Sites at which the cell synthesizes
  new protein molecules
• Endoplasmic reticulum Transports newly
  synthesized proteins

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The Human Nervous System2 Kinds of Cells

• Neurons
• Approx. 100 billion in brain
• Receive and transmit info
• Behavior depends upon their communication
• Glia
• 10X the number of neurons
• Support neural communication

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Neuroanatomy Handout 1 The Motor Neuron

• A motor neuron
• has its soma in the spinal cord
• receives excitation from other neurons
• conducts impulses along its axon to a muscle or
  gland
• is the largest of the nerve cells

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Neuroanatomy Handout 1 The Motor Neuron

• Neurons are similar to other cells of the body
• All neurons have a cell body (soma, A)
• responsible for the metabolic work of the neuron
• surrounded by cell membrane (A1)
• Containing a nucleus (A2), mitochondria (A3),
  ribosomes (A4), endoplasmic reticulum (A5)

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Neuroanatomy Handout 1 The Motor Neuron

• Neurons are different from other cells of the
  body because they have distinctive shape and
  function

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Neuroanatomy Handout 1 The Motor Neuron

• The 4 major components of a motor neuron
• Soma/Cell body
• Dendrites
• Axon
• Presynaptic terminals

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Neuroanatomy Handout 1 The Motor Neuron

• Dendrites (B)- branching fibers responsible for
  receiving information from other neurons
• Dendritic spines (B1) further branch out and
  increase the surface area of the dendrite

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Neuroanatomy Handout 1 The Motor Neuron

• Axon (C) - thin fiber responsible for sending
  impulses to other neurons, glands, or muscles
• Some neurons are covered with an insulating
  material called the myelin sheath (D) with
  interruptions in the sheath known as nodes of
  Ranvier (C2).
• Axon hillock (C1) bulge in the cell body where
  axon begins

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Neuroanatomy Handout 1 The Motor Neuron

• Presynaptic terminals (E) refer to the end points
  of an axon responsible for releasing chemicals
  (neurotransmitters) to communicate with other
  neurons

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Neuroanatomy Handout 1 The Motor Neuron

• Axons from other neurons (F) converge on
  receiving neuron
• Synapse gap between neurons
• Postsynaptic neuron (G) and dendrite (G1)

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Sensory and Motor Neurons

• A motor neuron receives excitation from other
  neurons and conducts impulses along its axon to a
  muscle or gland
• It carries information from the brain to the
  perimeter of the body

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Sensory and Motor Neurons

• A sensory neuron is specialized at one end to be
  highly sensitive to a particular type of
  stimulation (touch, temperature, odor etc.)
• It carries information from the perimeter of the
  body to the brain

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Other Cells of the Nervous System

• Terms used to describe the neuron include the
  following
• Afferent axon - refers to bringing information
  into a structure.
• Efferent axon - refers to carrying information
  away from a structure.
• Interneurons or Intrinsic neurons are those whose
  dendrites and axons are completely contained
  within a structure.

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Other Cells of the Nervous System

• Glia are the other major component of the nervous
  system and include the following
• Astrocytes help synchronize the activity of the
  axon by wrapping around the presynaptic terminal
  and taking up chemicals released by the axon.
• Microglia - remove waste material and other
  microorganisms that could prove harmful to the
  neuron.

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The Cells of the Nervous System

• Oligodendrocytes Schwann cells- build the
  myelin sheath that surrounds the axon of some
  neurons.
• Radial glia- guide the migration of neurons and
  the growth of their axons and dendrites during
  embryonic development.

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The Cells of the Nervous System

• Spaniard Santiago Ramon y Cajal (1852-1934) was
  the first to demonstrate that neurons do not
  touch one another.
• With this understanding came new ideas about how
  neurons communicate.

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The Nerve Impulse

• A nerve impulse is the electrical message that is
  transmitted down the axon of a neuron.
• The impulse is regenerated at points along the
  axon.
• The speed of nerve impulses ranges from
  approximately 1 m/s to 100 m/s.

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The Nerve Impulse

• The resting potential state of the neuron prior
  to the sending of a nerve impulse
• Electrical gradient a difference in the
  electrical charge inside and outside of the cell
• At rest, the membrane is slightly negative with
  respect to the outside (approximately -70
  millivolts)

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Competing forces maintain a -70mV resting
potential

• Concentration gradient The difference in the
  distribution of ions between the inside and the
  outside of the membrane
• Sodium (Na) more abundant outside cell than
  inside (101)
• Potassium (K) more abundant inside cell than
  outside (201)
• Negatively charged proteins inside cell

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Cellular mechanisms of the resting potential

• Selective permeability of the membrane allows
  some molecules (e.g. water, oxygen) to pass more
  freely than others.
• Charged ions, like sodium (Na), potassium (K),
  calcium (Ca) and chloride (Cl-) pass through
  channels in the membrane.
• When the membrane is at rest
• Na channels are closed
• K channels are partially closed allowing the
  slow passage of sodium

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Cellular mechanisms of the resting potential

• The sodium-potassium pump puts three sodium ions
  out of the cell while drawing in two potassium
  ions.
• helps to maintain the electrical gradient
• The electrical gradient and the concentration
  gradient work to pull sodium ions into the cell.
• The electrical gradient tends to pull potassium
  ions into the cells.

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The resting potential allows a neuron to respond
quickly to a stimulus
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The Action Potential

• The resting potential (-70mV) remains stable
  until the neuron is stimulated.
• Electrical polarization the difference in the
  electrical charge between two places

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Two things can happen to a resting neuron

• Hyperpolarization increasing the difference
  (polarization) between the electrical charge of
  two places (less likely to fire)
• Depolarization refers to decreasing the
  polarization towards zero (more likely to fire)
• The threshold of excitement refers any
  stimulation beyond a certain level that results
  in a massive depolarization (action potential).

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The Action Potential

• An action potential is a rapid depolarization of
  the neuron.
• Stimulation of the neuron past the threshold of
  excitation triggers a nerve impulse, action
  potential, or firing
• -70mV can become 50mV

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The Nerve Impulse

• Voltage-activated channels are membrane channels
  whose permeability depends upon the voltage
  difference across the membrane.
• Sodium channels are voltage activated channels.
• When sodium channels are opened, positively
  charged sodium ions rush in and a subsequent
  nerve impulse occurs.

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The Nerve Impulse

• Scorpion venom attacks the
• nervous system by keeping
• sodium channels open and
• closing potassium channels
• Local anesthetic drugs block sodium channels and
  therefore prevent action potentials from
  occurring.
• Example Novocain
• General anesthetics open potassium channels wider
  than usual

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The Nerve Impulse

• The all-or-none law states that the amplitude and
  velocity of an action potential are independent
  of the intensity of the stimulus that initiated
  it.
• Action potentials are equal in intensity and
  speed within a given neuron.

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The Nerve Impulse

• A refractory period happens after an action
  potential occurs, during which time the neuron
  resists another action potential.
• The absolute refractory period the first part,
  when membrane cannot produce an action potential
• The relative refractory period the second part,
  when it takes a stronger than usual stimulus to
  trigger an action potential.

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The Nerve Impulse

• In a motor neuron, the action potential begins at
  the axon hillock (a swelling where the axon exits
  the soma).
• Propagation of the action potential is the term
  used to describe the transmission of the action
  potential down the axon.

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The Nerve Impulse

• The myelin sheath of axons are interrupted by
  short unmyelinated sections called nodes of
  Ranvier.
• At each node of Ranvier, the action potential is
  regenerated by a chain of positively charged ions
  pushed along by the previous segment.

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The Nerve Impulse

• Saltatory conduction the jumping of the action
  potential from node to node.
• Provides rapid conduction of impulses
• Conserves energy for the cell
• Multiple sclerosis disease in which myelin
  sheath is destroyed associated with poor muscle
  coordination

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The Nerve Impulse

• Not all neurons have lengthy axons.
• Local neurons have short axons, exchange
  information with only close neighbors, and do not
  produce action potentials.
• When stimulated, local neurons produce graded
  potentials which are membrane potentials that
  vary in magnitude and do not follow the
  all-or-none law,.
• A local neuron depolarizes or hyperpolarizes in
  proportion to the stimulation.