The Nervous System

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The Nervous System

• Nancy G. Morris
• Volunteer State Community College
• Campbell Chapter 48

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Nervous System Endocrine System
Complexity More structurally complex can integrate vast amounts of information stimulate a wide range of responses Less structurally complex evolved from the nervous system
Structure System of neurons that branch throughout the body Endocrine glands secrete hormones into bloodstream where they are carried to the target organ
Communication Neurons conduct electrical signals directly to and from specific targets allows fine pin-point control transmission is hormonal acetylcholine, epinepherin, norepinephrin, etc. Hormones circulate as chemical messengers via the bloodstream most cells are exposed but only target cells with receptors respond
Response Time Fast transmission of nerve impulses up to 100 m/sec May take minutes, hours or days for hormones to be produced, diffuse to target organ, for response to occur
Effect Acts at the cellular level Immediate and short-lived Acts a the cellular level Occur over time and are long-lived
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Organization of nervous systems

• There is great diversity among animals.
• All phyla have a nervous system except sponges.
• In the Hydras nerve net, impulses are conducted
  in both directions causing movement of entire
  body.
• Some cnidarians echinoderms have modified nerve
  nets with rudimentary centralization.

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Organization of nervous systems

• Cephalization evolutionary trend for
  concentration of sensory feeding organs on the
  anterior end of a moving animal (Bilaterial
  symmetry).
• Most bilateral animals have a PNS a CNS (brain
  one or more nerve cords).
• Flatworms have a simple brain containing large
  interneurons.
• Annelids arthropods have a well-defined ventral
  nerve cord prominent brain. Often coordinate
  ganglia in each segment to coordinate action.
• Cephalopods have the most sophisticated
  invertebrate nervous system containing a large
  brain giant axons.

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Fig. 48.13 Diversity in Nervous Systems
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Three overlapping functions of the Nervous System

• Sensory input is the conduction of signals from
  sensory receptors to integration centers of the
  nervous system.
• Integration is a process by which information
  from sensory receptors is interpreted
  associated with appropriate responses of the
  body.
• Motor output is the conduction of signals from
  the processing center to effector cells (muscle
  gland cells) that actually carry out the bodys
  response to stimuli.

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Fig. 48.1 Overview Vertebrate Nervous System
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Three majors classes of neurons

• Sensory neurons convey information about the
  external internal environments from sensory
  receptors to CNS most synapse with interneurons.
• Interneurons integrate sensory input and motor
  input located within the CNS synapse only with
  other neurons
• Motor neurons convey impulses form the CNS to
  effector cells

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Fig. 48.4 Structural Diversity of Neurons
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A single neuron on the surface of a
microprocessor. A cm3 of the human brain will
contain more than 50 million neurons.
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• Signals are conducted by nerves with many axons
  coming from many different neurons surrounded by
  connective tissue, the perineurium.
• Found in both parts of the nervous system
• 1) Central Nervous System (CNS) comprised of
  brain spinal cord responsible for integration
  of sensory input associating stimuli with
  appropriate motor output
• 2) Peripheral Nervous System (PNS) consists of
  a network of nerves extending into different
  parts of the body that carry sensory input to the
  CNS motor output away form the CNS

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Composition of Nervous System

• The Nervous System contains two types of cells
• 1) Neurons cells specialized for transmitting
  chemical electrical signals form one location
  to another
• 2) Glia or supporting cells structurally
  reinforce, protect, insulate, generally assist
  neurons

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Neurons

• Possess a large cell body located either in the
  CNS or a ganglion
• Possess two fingerlike extensions (processes)
  that conduct messages
• 1) Dendrites convey signals to the neurons cell
  body Numerous, short, extensively branched to
  increase surface area
• 2) Axons conduct impulses away from the cell
  body. Single, long process

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Fig. 48.2 Structure of Vertebrate Neuron
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AXONS

• Vertebrate axons in the PNS are wrapped in
  concentric layers of Schwann cells, which form an
  insulating myelin sheath.
• In the CNS, the myelin sheath is formed by
  ogliodendrites.
• Extend from the neuron cell body to many branches
  (arborization of the axon) which are tipped with
  synaptic terminals that release neurotransmitters.

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Transmission of the impulse

• Must cross the synapse, the gap between a
  synaptic terminal and a target cell (either
  another neuron or an effector cell).
• Neurotransmitters are chemicals that cross the
  synapse to relay the impulse
• Table 48.1 acetylcholine, norepinephrine,
  dopamine, serotonin, neuropeptides endorphines

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Neurons are arranged in circuits

• Simple circuit synapse between sensory neurons
  motor neurons, resulting in a simple reflex.
• Complex circuit such as those associated with
  most behaviors, involve integration by
  interneurons in the CNS
• Convergent circuits
• Divergent circuits
• Reverberating circuits (memory storage)

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Figure 48.3The knee-jerk reflex
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Coordination by cluster

• Nerve cell bodies are often arranged into
  clusters these clusters allow coordination of
  activities by only a art of the nervous system
• A nucleus is a cluster of nerve cell bodies
  within the brain
• A ganglion is a cluster of nerve cell bodies in
  the peripheral nervous system

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Supporting cells

• Do not conduct impulses
• Outnumber neurons by 10- 50- fold
• Several types of glia cells
• 1) astrocytes encircle capillaries of the brain
• 2) oligodendrocytes form the myelin sheaths that
  insulate the CNS nerve processes
• 3) Schwann cells form the insulating myelin
  sheath around axons in the PNS

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Myelination of neurons

• Occurs when Schwann cells or oligodenrocytes grow
  around an axon so their plasma membranes form
  concentric layers
• Provides electrical insulation
• Increases speed of nerve impulse propagation
• In MS, myelin sheaths deteriorate causing a
  disruption of nerve impulse transmission
  consequent loss of coordination

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Nature of Neural Signals

• Signal transmission along a neuron depends on
  voltages created by ionic fluxes across neuron
  plasma membranes.
• Membrane potentials arise from differences in ion
  concentrations between a cells contents and the
  extracellular fluid.
• All cells have an electrical potential or voltage
  across their plasma membrane.
• The charge outside is designated as zero, so the
  minus sign indicates that the cytoplasm inside is
  negatively charged compared to the extracellular
  fluid.

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Nature of Neural Signals

• Ion channel integral transmembrane protein that
  allows a specific ion to cross the membrane.
• May be passive all the time or it may be gated,
  requiring stimulus to change into an open
  conformation.
• Is selective for a specific ion, such as Na, K,
  and Cl-
• A shift in ionic gradients is prevented by
  sodium-potassium pumps which maintain the
  concentration gradient.

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Action Potential

• A rapid change in the membrane potential of an
  excitable cell, caused by stimulus-triggered
  selective opening closing of voltage-gated ion
  channels.
• There are four stages.

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Four stages of an Action Potential

• 1) resting stage no channels open.
• 2) depolarizing phase membrane reverses
  polarity (cell interior becomes relative to
  exterior) the Na activation gates open, Na
  rushes in, potassium gates remain closed.
• 3) repolarizing phase - returns the membrane
  potential to resting level inactivation gates
  close Na channels K channels open.
• 4) undershoot phase membrane potential is
  temporarily more negative than the resting stage
  (hyperpolarized) Na channels remain closed but
  K channels remain open since the inactivation
  gates have not had time to respond to
  repolarization of the membrane.

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Figure 48.9Role of gated ion channels in the
action potential
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Action Potential.

• Refractory Period occurs during the undershoot
  phase.
• During this period, the neuron is insensitive to
  depolarizing stimuli.
• Limits the maximum rate at which action
  potentials can be stimulated in a neuron.
• Action potentials are all-or-none events. The
  nervous system distinguishes between strong
  weak stimuli based on the frequency of action
  potentials generated.

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Action potential travel

• Action potentials travel along the axon because
  they are self-propagating.
• A neuron is stimulated at its dendrites or cell
  body and the action potential travels along the
  axon.
• The signal travels in a perpendicular direction
  along the axon regenerating the action potential.
  

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Figure 48.7Propagation of the action potential
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Action potential travel

• Saltatory conduction the action potential
  jumps from one node of Ranvier to the next,
  skipping myelinated regions of membranes
• Figure 48.11

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Figure 48.11 Saltatory Conduction
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Communication

• Communication between cells happens across the
  synapse
• Synapse tiny gap between a synaptic terminal of
  an axon a signal- receiving portion of another
  neuron or effector cell
• Presynaptic cell is the transmitting cell
  postsynaptic cell is the receiving cell
• There are two types of synapses
• 1) electrical
• 2) chemical

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Electrical Synapses

• Allow action potentials to spread directly from
  pre- to postsynaptic cells via gap junctions
  (intercellular channels)
• Allow impulse travel without delay or loss of
  signal strength
• Less common than chemical synapses
• Common in crustaceans

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Chemical Synapses

• Synaptic vesicles containing thousands of
  neurotransmitter molecules are present in the
  cytoplasm of the synaptic terminal of the
  presynaptic neuron
• Chemical synapses allow transmission in one
  direction only
• Receptors for neurotransmitters are located only
  on postsynaptic membranes

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Figure 48.12 A Chemical Synapse
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• One neuron may receive information from thousands
  of synapses. Some synapses are excitatory,
  others are inhibitory (Fig. 48.11).
• The same neurotransmitter can produce different
  effects on different types of cells
• (Table 48.2)

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Figure 48.13 Integration of multiple synaptic
inputs
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Numerous synaptic terminals communicating with a
single postsynaptic cell (SEM).
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Fig. 48.17 Functional hierarchy of the
peripheral nervous system
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Figure 48.18Parasympathetic maintains normal
activity- slows down- acetylcholineSympathetic
-flight or fight- speeds up- norepinephrine
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Integration Control
CNS PNS Autonomic Somatic NS
NS (internal organs- (muscles,
skin, viscera) joints) Parasympath
etic Sympathetic (normal activity)
(flight or fight)
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CNS Meninges

• Meninges series of 3 membrane layers which
  surround brain spinal cord
• 1) dura mater (outer)
• 2) arachnoid membrane
• 3) pia mater (inner)

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CNS Spinal Cord

• connecting mechanism between the body the brain
• part of the CNS
• enclosed in vertebral column
• 31 pairs of spinal nerves
• fluid-filled cavity cerebrospinal fluid

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Fig. 48.16The nervous system of a vertebrate31
pairs of spinal nerves each containing one
sensory neuron (afferent dorsal route) one
motor neuron (efferent ventral route)
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Vertebrate Brain

• 100 billion neurons in human brain
• 10 billion in the cerebral cortex
• weights about 3 lbs
• composed of gray matter (cell bodies) white
  matter (axons dendrites)
• developed from the 3 bulges at the anterior end
  of the dorsal tubular nerve cord hind midfore
  brain)

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Fig.48.19 Embryonic
Development of the Vertebrate Brain
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Fig.48.19 Embryonic Development of the brain
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Major parts of the human brain.
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Vertebrate Brain (Hindbrain)

• Medulla (upper spinal cord)
• center for respiratory, cardiac function
  vomiting, sweating, gastric secretion, heartbeat
• Cerebellum regulate controls bodily muscular
  contractions coordination, balance, equilibrium
• Pons bridge between two halves of the
  cerebellum carries fibers that coordinate
  activity of muscles on two sides of the body

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Vertebrate Brain (Mid Fore)

• Midbrain relay center visual auditory
  reflexes
• Thalamus relay center for sensory impulses
  going to cerebrum control center for external
  manifestations of emotion (laughing, crying,
  etc.)
• Hypothalamus (floor of thalmus) regulates
  hunger, thirst, body temp, CHO fat metabolism,
  blood pressure, sleep regulates the pitiutary

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Vertebrate Brain (Forebrain)

• Cerebral hemispheres Cerebrum Controls
  learned behavior memory makes up about 80 of
  brain mass
• 1) frontal (speech, motor cortex)
• 2) parietal (taste, reading,)
• 3) temporal (smell, hearing)
• 4) occipital (vision)
• Corpus Collosum junction between two
  hemispheres

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Figure 48.24 Structure and Functional areas of
the cerebrum
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Figure 48.26 Mapping Language areas of the
cerebral cortex.
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Fig. 49.3Sensory receptors in human skin
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Figure 49.16 Neural pathways for vision
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Sensory transduction by a taste receptor.