Electrical Signals in Animals

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Electrical Signals in Animals

• Chapter 45

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Nervous Systems

• Consist of circuits of neurons and supporting
  cells
• All animals except sponges have some type of
  nervous system
• Differ in the way that the neurons are organized
  into circuits
• The simplest animals with
• nervous systems, the cnidarians
• Have neurons arranged in nerve nets

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Nervous Systems

• More complex animals contain nerve nets as well
  as nerves
• Bundles of fiber-like extensions of neurons
• Sea stars have a nerve net in each arm
• Connected by radial nerves to a central nerve ring

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Nervous Systems

• Cephalization evolved with greater complexity in
  nervous systems
• Clustering of neurons in a brain near the
  anterior end in worms
• Small brain, longitudinal nerve chords
  constitute the simplest central nervous system

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Nervous Systems

• Annelids and arthropods have more complicated
  brains
• Have segmentally arranged clusters of neurons
  called ganglia
• These ganglia connect to the
• CNS
• And make
• up a peripheral
• nervous system
• (PNS)

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Nervous Systems

• Nervous systems in mollusks
• Correlate with the animals lifestyles
• Sessile mollusks have simple systems
• While more complex mollusks have more
  sophisticated systems
• Can complete complicated tasks

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Nervous Systems

• In vertebrates
• The central nervous system consists of a brain
  and dorsal spinal cord
• The PNS connects to the CNS

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Neuron Structure

• The ability of the neuron to receive and transmit
  information depends on their structure. Consists
  of
• Cell body- where most of the organelles are
  housed
• Numerous dendrites- highly branched extensions
  that receive signals from other neurons
• Axon- larger extension that transmits signals,
  may be covered with a myelin sheath

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Dendrite

• Receives electrical signals from the axons of
  adjacent cells.
• Axon then sends the signal to the dendrites of
  other neurons
• Cell body, or soma, which includes the nucleus,
  integrates the incoming signals and generates an
  outgoing signal in the axon

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Neurons

• Neurons have a wide variety of shapes
• That reflect their input and output interactions
• Reflects the number of synapses it has with other
  neurons

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Glia

• Glia are supporting cells
• That are essential for the structural integrity
  of the nervous system and for the normal
  functioning of neurons
• In the CNS astrocytes regulate extracellular ion
  concentrations

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• Oligodendrocytes (in the CNS) and Schwann cells
  (in the PNS)
• Are glia that form the myelin sheaths around the
  axons of many vertebrate neurons
• Act as insulators
• Multiple Sclerosis is deterioration of the myelin
  sheath

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Resting Potential of a Cell

• Ion pumps and ion channels maintain the resting
  potential of a neuron
• The resting potential
• Is the membrane potential of a neuron that is not
  transmitting signals
• Across its plasma membrane, every cell has a
  voltage
• Called a membrane potential
• The inside of a cell is negative
• Relative to the outside

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Resting Potential of a Cell

• The concentration of Na is higher in the
  extracellular fluid than in the cytosol
• While the opposite is true for K
• A neuron that is not transmitting signals
• Contains many open K channels and fewer open Na
  channels in its plasma membrane
• The diffusion of K and Na through these
  channels
• Leads to a separation of charges across the
  membrane, producing the resting potential

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Gated Ion Channels

• Ungated ion channels are always open, results in
  the resting potential of the cell
• Gated ion channels open or close
• In response to membrane stretch or the binding of
  a specific ligand
• In response to a change in the membrane potential
• Neurons have gated ion channels
• Responsible for generating the signals of the
  nervous system

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Starting An Action Potential?

• An action potential is a rapid, temporary change
  in a membrane potential
• Has three phases depolarization, repolarization,
  and the undershoot
• The initial event is a rapid depolarization of
  the membrane
• Membrane potential must shift from its resting
  potential of 70 mV to about 55 mV

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• If the threshold potential is reached, channels
  in the axon membrane open and ions rush into the
  axon, following their electrochemical gradients
• The current flow causes further depolarization
• When the membrane potential reaches about 40 mV
  the membrane experiences a rapid repolarization
  as ions flow out of the axon

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• The repolarization event results in the membrane
  becoming more negative than the resting potential
• Called the undershoot
• All phases occur in about a millisecond

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Regeneration of the Action Potential

• An action potential can travel long distances
• By regenerating itself along the axon
• At the site where the action potential is
  generated
• An electrical current depolarizes the neighboring
  region of the axon membrane

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Regeneration of the Action Potential
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Action Potentials

• The speed of an action potential
• Increases with the diameter of an axon
• In vertebrates, axons are myelinated
• Also causing the speed of an action potential to
  increase
• Causes the membranes to have a simulated wider
  width
• Action potentials in myelinated axons
• Jump between the nodes of Ranvier in a process
  called saltatory conduction

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Synapses

• Neurons communicate with other cells at synapses
• In an electrical synapse
• Electrical current flows directly from one cell
  to another via a gap junction
• Synchronize the activity of neurons responsible
  for rapid responses (flight response)
• The vast majority of synapses
• Are chemical synapses
• Not as fast as electrical synapsis

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Chemical Synapses

• In a chemical synapse, a presynaptic neuron
• Releases chemical neurotransmitters, which are
  stored in the synaptic terminal

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Synapses

• The interface between two neurons is called a
  synapse.
• Just inside the synapse, the axon contains
  synaptic vesicles that serve as storage sites for
  neurotransmitters
• The sending cell is called the presynaptic neuron
  and the receiving cell is called the postsynaptic
  neuron

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What Do Neurotransmitters Do?

• There are several categories of
  neurotransmitters
• Many neurotransmitters function as ligands that
  bind to ligand-gated ion channels
• Cause the ion channels to open, generating a
  postsynaptic potential
• Causes the start of a new action potential

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The Vertebrate Nervous System

• The vertebrate nervous system is regionally
  specialized
• In all vertebrates, the nervous system
• Shows a high degree of cephalization and distinct
  CNS and PNS components

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The Central Nervous System

• The brain provides the integrative power
• That underlies the complex behavior of
  vertebrates
• The spinal cord integrates simple responses to
  certain kinds of stimuli
• And conveys information to and from the brain
• The central canal of the spinal cord and the four
  ventricles of the brain
• Are hollow, since they are derived from the
  dorsal embryonic nerve cord

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The Central Nervous System

• CNS consists of spinal chord and four ventricles
  in the brain
• Contain cerebrospinal fluid
• Assists in supply of nutrients and hormones to
  parts of the brain and in removal of wastes

• Grey matter contains mostly dendrites
• White matter contains long axons with great
  bundles of myelin sheaths

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The Peripheral Nervous System

• The PNS transmits information to and from the CNS
  and plays a large role in regulating a
  vertebrates movement and internal environment
• The cranial nerves originate in the brain
• And terminate mostly in organs of the head and
  upper body
• The spinal nerves originate in the spinal cord
• And extend to parts of the body below the head

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The Peripheral Nervous System

• The PNS can be divided into two functional
  components
• The somatic nervous system and the autonomic
  nervous system

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The Peripheral Nervous System

• The somatic nervous system
• Carries signals to skeletal muscles
• Often considered voluntary
• The autonomic nervous system
• Regulates the internal environment, in an
  involuntary manner
• Is divided into the sympathetic, parasympathetic,
  and enteric divisions

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Brain Structures in an Adult
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Parts of The Brain

• The brainstem consists of three parts
• The medulla oblongata, the pons, and the midbrain

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Brainstem

• The medulla oblongata
• Contains centers that control several visceral
  functions
• The pons
• Also participates in visceral functions
• The midbrain
• Contains centers for the receipt and integration
  of several types of sensory information

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The Cerebellum

• The cerebellum
• Is important for coordination and error checking
  during motor, perceptual, and cognitive functions
• The cerebellum
• Is also involved in
• learning and
• remembering motor
• skills

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Diancephalon

• The embryonic diencephalon develops into three
  adult brain regions
• The epithalamus, thalamus, and hypothalamus
• The epithalamus
• Includes the pineal
• gland and the
• choroid plexus
• Produce cerebro-
• spinal fluid

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Diancephalon

• The thalamus
• Is the main input center for sensory information
  going to the cerebrum and the main output center
  for motor information leaving the cerebrum
• The hypothalamus regulates
• Homeostasis
• Basic survival behaviors such as feeding,
  fighting, fleeing, and reproducing

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Cerebrum

• The cerebrum has right and left cerebral
  hemispheres
• That each consist of cerebral cortex overlying
  white matter and basal nuclei

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Cerebrum

• The basal nuclei
• Are important centers for planning and learning
  movement sequences
• In mammals
• The cerebral cortex has a convoluted surface
  called the neocortex
• Sensory information is analyzed , motor commands
  are issued, and language is generated
• Corpus callosum allows communication between
  hemispheres

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Cerebral Cortex

• In humans, the largest and most complex part of
  the brain
• Is the cerebral cortex, where sensory information
  is analyzed, motor commands are issued, and
  language is generated
• Controls voluntary movement and cognitive
  functions
• Each side of the cerebral cortex has four lobes
• Frontal, parietal, temporal, and occipital

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Cerebral Cortex
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Lateralization

• During brain development, in a process called
  lateralization
• Competing functions segregate and displace each
  other in the cortex of the left and right
  cerebral hemispheres
• The left hemisphere
• Becomes more adept at language, math, logical
  operations, and the processing of serial
  sequences
• The right hemisphere
• Is stronger at pattern recognition, nonverbal
  thinking, and emotional processing

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CNS Injuries

• Unlike the PNS, the mammalian CNS
• Cannot repair itself when damaged or assaulted by
  disease
• Current research on nerve cell development and
  stem cells
• May one day make it possible for physicians to
  repair or replace damaged neurons

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Diseases of the Nervous System

• About 1 of the worlds population suffers from
  schizophrenia
• Schizophrenia is characterized by
• Hallucinations, delusions, blunted emotions, and
  many other symptoms
• Available treatments have focused on
• Brain pathways that use dopamine as a
  neurotransmitter

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Diseases of the Nervous System

• Two broad forms of depressive illness are known
• Bipolar disorder and major depression
• Bipolar disorder is characterized by
• Manic (high-mood) and depressive (low-mood)
  phases
• In major depression
• Patients have a persistent low mood
• Treatments for these types of depression include
• A variety of drugs such as Prozac and lithium

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Diseases of the Nervous System

• Alzheimers disease (AD)
• Is a mental deterioration characterized by
  confusion, memory loss, and other symptoms
• AD is caused by the formation of
• Neurofibrillary tangles and senile plaques in the
  brain

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Diseases of the Nervous System

• Parkinsons disease is a motor disorder
• Caused by the death of dopamine-secreting neurons
  in the substantia nigra
• Characterized by difficulty in initiating
  movements, slowness of movement, and rigidity
• There is no cure for Parkinsons disease
• Although various approaches are used to manage
  the symptoms