Neurons and Nervous Systems

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Neurons and Nervous Systems

• Chapter 39

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Outline

• Invertebrate Nervous Organization
• Vertebrate Nervous Organization
• Human Nervous System
• Neurons
• Action Potentials
• Synapse Structure and Function
• Brain and Spinal Cord
• Peripheral Nervous System
• Autonomic System

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Evolution of the Nervous System
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Evolution of the Nervous System
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Invertebrate Nervous Organization

• Hydras have a nerve net composed of neurons in
  contact with one another and contractile
  epitheliomuscular cells.
• Planarians have a ladderlike nervous system.
• Cephalization has occurred as evidenced by a
  concentration of ganglia and sensory receptors in
  the head.
• Annelids, Arthropods and Molluscs are complex
  animals with true nervous systems.

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Vertebrate Nervous Organization

• Central nervous system develops from an embryonic
  dorsal tubular nerve cord.
• Cephalization and bilateral symmetry result in
  several paired sensory receptors.
• Vertebrate brain is organized into three areas.
• Hindbrain
• Midbrain
• Forebrain

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Human Nervous System

• Nervous system has three specific functions.
• Receiving sensory input.
• Performing integration.
• Generating motor output.

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Human Nervous System

• In humans, the central nervous system (CNS)
  includes the brain and spinal cord that lie in
  the midline of the body.
• The peripheral nervous system (PNS) lies outside
  the CNS, and contains cranial nerves and spinal
  nerves.

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Peripheral Nervous System

• Somatic system controls the skeletal muscles.
• Autonomic system controls the smooth muscles,
  cardiac muscles, and glands.
• Divided into sympathetic division and
  parasympathetic division.

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Nervous Tissue

• Neurons
• Cell body contains nucleus.
• Dendrites receive signals from sensory receptors.
• Axon conducts nerve impulses.
• Any long axon is also called a nerve fiber.
• Covered by myelin sheath.

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Types of Neurons

• Motor Neurons take nerve impulses from the CNS to
  muscles or glands.
• Sensory Neurons take impulses from sensory
  receptors to CNS.
• Interneurons convey nerve impulses between
  various parts of the CNS.

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Resting Potential

• Resting Potential is the membrane potential when
  the axon is not conducting an impulse.
• Inside of neuron is more negative than the
  outside -65 mV.
• In part due to the action of the sodium-potassium
  pump.

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Action Potential

• Gated channel protein allows sodium to pass
  through the membrane, while another allows
  potassium to pass through.
• An action potential is generated only after the
  occurrence of a threshold value.
• A minimum change in polarity across the
  axomembrane is required to generate an action
  potential.

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Propagation of Action Potentials

• In myelinated fibers, an action potential at one
  neurofibril node causes an action potential at
  the next node.
• Saltatory Conduction
• Conduction of a nerve impulse is an
  all-or-nothing event.
• Intensity is determined by how many impulses are
  generated within a given time span.

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Synapse Structure and Function
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Synapse Structure and Function
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Transmission Across a Synapse

• Region of close proximity is a synapse.
• Small gap between is the synaptic cleft.
• Transmission across a synapse is carried out by
  neurotransmitters.
• Sudden rise in calcium stimulates synaptic
  vesicles to merge with the presynaptic membrane,
  and neurotransmitter molecules are released into
  the synaptic cleft.

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Synaptic Integration

• A single neuron is on the receiving end of many
  excitatory and inhibitory signals.
• Integration is the summing of excitatory and
  inhibitory signals.

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CNS Brain and Spinal Cord

• Spinal cord and brain are wrapped in three
  protective membranes, meninges.
• Spaces between meninges are filled with
  cerebrospinal fluid.
• Fluid is contained in central canal of the spinal
  cord and within the ventricles of the brain.

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Spinal Cord

• The spinal cord has two main functions.
• Center for many reflex actions.
• Means of communication between the brain and
  spinal nerves.
• Cell bodies and short unmyelinated fibers give
  the gray matter its color.
• Myelinated long fibers of interneurons running in
  tracts give white matter its color.

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The Brain

• Cerebrum is the largest portion of the brain in
  humans.
• Communicates with, and coordinates the activities
  of, the other parts of the brain.
• Divided into left and right cerebral hemispheres.
• Longitudinal fissure divides the hemispheres.

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Lobes of Cerebral Hemisphere
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Cerebral Cortex

• Cerebral cortex is a thin but highly convoluted
  outer layer of gray matter that covers the
  cerebral hemispheres.
• Contains motor areas and sensory areas as well as
  association areas.
• Primary motor area is in the frontal lobe just
  ventral to central sulcus.
• Primary somatosensory area is just dorsal to
  central sulcus.

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Cerebrum

• Much of the rest of the cerebrum is composed of
  white matter.
• Descending tracts communicate with lower brain
  centers. Ascending tracts send sensory
  information to primary somatosensory area.
• Basal nuclei integrate motor commands, ensuring
  proper muscle groups are activated or inhibited.

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Diencephalon

• Hypothalamus and thalamus are in the
  diencephalon, a region encircling the third
  ventricle.
• Hypothalamus forms floor of the third ventricle.
• Thalamus consists of two masses of gray matter
  located in the sides and roof of the third
  ventricle.
• Pineal gland secretes melatonin, and is located
  in the diencephalon.

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Cerebellum

• Cerebellum is separated from the brain stem by
  the fourth ventricle.
• Receives sensory input from the eyes, ears,
  joints, and muscles.
• Ends motor impulses by way of the brain stem to
  the skeletal muscles.

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Brain Stem

• The brain stem contains the midbrain, the pons,
  and the medulla oblangata.
• Midbrain acts as a relay station for tracts
  passing between the cerebrum and the spinal cord
  or cerebellum.
• Pons helps regulate breathing and head movements.
• Medulla oblongata contains reflex centers for
  vomiting, coughing, sneezing, hiccuping, and
  swallowing.

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Limbic System

• Complex network of tracts and nuclei that
  incorporates medial portions of the cerebral
  lobes, subcortical basal nuclei, and
  dicenephalon.
• Blends higher mental functions and primitive
  emotions into a whole.
• Hippocampus and amygdala are significant
  structures.

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Limbic System
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Peripheral Nervous System

• Peripheral nervous system (PNS) lies outside the
  CNS.
• Nerves are bundles of axons.
• Cranial nerves take impulses to and/or from the
  brain, and spinal nerves take impulses from the
  spinal cord.

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Autonomic System

• Autonomic system of PNS regulates activity of
  cardiac and smooth muscle, and glands.
• Divided into sympathetic and parasympathetic
  divisions. Both
• Function automatically and usually in an
  involuntary manner.
• Innervate all internal organs.
• Utilize two neurons and one ganglion for each
  impulse.

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Sympathetic and Parasympathetic Divisions

• Sympathetic division is especially important
  during fight or flight responses.
• Accelerates heartbeat and dilates bronchi.
• Parasympathetic division promotes all internal
  responses associated with a relaxed state.
• Promotes digestion and retards heartbeat.

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Review

• Invertebrate Nervous Organization
• Vertebrate Nervous Organization
• Human Nervous System
• Neurons
• Action Potentials
• Synapse Structure and Function
• Brain and Spinal Cord
• Peripheral Nervous System
• Autonomic System

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