Nervous Systems

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Chapter 49

• Nervous Systems

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Nervous systems consist of circuits of neurons
and supporting cells

• The simplest animals with nervous systems, the
  cnidarians, have neurons arranged in nerve nets
• A nerve net is a series of interconnected nerve
  cells
• More complex animals have nerves
• Nerves are bundles that consist of the axons of
  multiple nerve cells

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Nerve nets
Fig. 49-2a
Radial nerve
Nerve ring
Nerve net
(a) Hydra (cnidarian)
(b) Sea star (echinoderm)
Characteristic of animals with radial symmetry.
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• Bilaterally symmetrical animals exhibit
  cephalization
• Cephalization is the clustering of sensory organs
  at the front end of the body
• Relatively simple cephalized animals, such as
  flatworms, have a central nervous system (CNS)
• The CNS consists of a brain and longitudinal
  nerve cords

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Fig. 49-2
Eyespot
Brain
Brain
Radial nerve
Nerve cords
Ventral nerve cord
Nerve ring
Transverse nerve
Nerve net
Segmental ganglia
(a) Hydra (cnidarian)
(b) Sea star (echinoderm)
(d) Leech (annelid)
(c) Planarian (flatworm)
Brain
Brain
Ganglia
Anterior nerve ring
Spinal cord (dorsal nerve cord)
Ventral nerve cord
Brain
Sensory ganglia
Longitudinal nerve cords
Ganglia
Segmental ganglia
(e) Insect (arthropod)
(h) Salamander (vertebrate)
(f) Chiton (mollusc)
(g) Squid (mollusc)
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• Nervous system organization usually correlates
  with lifestyle
• Sessile molluscs (e.g., clams and chitons) have
  simple systems, whereas more complex molluscs
  (e.g., octopuses and squids) have more
  sophisticated systems

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• In vertebrates
• The CNS is composed of the brain and spinal cord
• The peripheral nervous system (PNS) is composed
  of nerves and ganglia

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Fig. 49-4
Peripheral nervous system (PNS)
Central nervous system (CNS)
Brain
Cranial nerves
Spinal cord
Ganglia outside CNS
Spinal nerves
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Organization of the Vertebrate Nervous System

• The spinal cord conveys information from the
  brain to the PNS
• The spinal cord also produces reflexes
  independently of the brain
• A reflex is the bodys automatic response to a
  stimulus
• For example, a doctor uses a mallet to trigger a
  knee-jerk reflex

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Fig. 49-3
Cell body of sensory neuron in dorsal
root ganglion
Gray matter
Quadriceps muscle
White matter
Hamstring muscle
Spinal cord (cross section)
Sensory neuron
Motor neuron
Interneuron
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• The central canal of the spinal cord and the
  ventricles of the brain are hollow and filled
  with cerebrospinal fluid
• The cerebrospinal fluid is filtered from blood
  and functions to cushion the brain and spinal
  cord

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• The brain and spinal cord contain
• Gray matter, which consists of neuron cell
  bodies, dendrites, and unmyelinated axons
• White matter, which consists of bundles of
  myelinated axons

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Fig. 49-5
Gray matter
White matter
Ventricles
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The Peripheral Nervous System

• The PNS transmits information to and from the CNS
  and regulates movement and the internal
  environment
• In the PNS, afferent neurons transmit information
  to the CNS and efferent neurons transmit
  information away from the CNS

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• Cranial nerves originate in the brain and mostly
  terminate in organs of the head and upper body
• Spinal nerves originate in the spinal cord and
  extend to parts of the body below the head

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Fig. 49-7-2
PNS
Efferent neurons
Afferent (sensory) neurons
Motor system
Autonomic nervous system
Hearing
Enteric division
Sympathetic division
Parasympathetic division
Locomotion
Hormone action
Circulation
Gas exchange
Digestion
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• The PNS has two functional components the motor
  system and the autonomic nervous system
• The motor system carries signals to skeletal
  muscles and is voluntary
• The autonomic nervous system regulates the
  internal environment in an involuntary manner

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• The autonomic nervous system has sympathetic,
  parasympathetic, and enteric divisions
• The sympathetic and parasympathetic divisions
  have antagonistic effects on target organs

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• The sympathetic division correlates with the
  fight-or-flight response
• The parasympathetic division promotes a return to
  rest and digest
• The enteric division controls activity of the
  digestive tract, pancreas, and gallbladder

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Fig. 49-8a
Parasympathetic division
Sympathetic division
Action on target organs
Action on target organs
Dilates pupil of eye
Constricts pupil of eye
Inhibits salivary gland secretion
Stimulates salivary gland secretion
Sympathetic ganglia
Constricts bronchi in lungs
Cervical
Slows heart
Stimulates activity of stomach and intestines
Stimulates activity of pancreas
Stimulates gallbladder
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Fig. 49-8b
Parasympathetic division
Sympathetic division
Relaxes bronchi in lungs
Accelerates heart
Inhibits activity of stomach and intestines
STRESS
Thoracic
Inhibits activity of pancreas
Stimulates glucose release from liver inhibits
gallbladder
Lumbar
Stimulates adrenal medulla
Promotes emptying of bladder
Inhibits emptying of bladder
Sacral
Promotes ejaculation and vaginal contractions
Promotes erection of genitals
Synapse
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Neurotransmitters

• Parasympathetic acetylcholine
• Sympathetic norepinephrine or acetylcholine
  depending on location of ganglia

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• Cerebrum integrates sensory and motor
  information, thinking (cortex)
• Brainstem regulates involuntary responses
  (breathing, heart rate, digestion)
• Cerebellum balance and movement
• Thalamus sorts and relays information to
  cerebrum
• Hypothalamus homeostatic regulation, secretes
  hormones
• Pituitary gland secretes hormones

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What is lateralization of the cerebrum?

• Lateralization means that some functions are
  carried out exclusively on one side of the brain
  (e.g., speech, which is on the left side of the
  brain in most people). 
• Left side of cerebrum controls right side of body
  and vice versa.

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Left Hemisphere Right Hemisphere
Speech Music and art appreciation, drawing ability
Movement of the right side of the body Movement of the left side of the body
Sensation on the right side of the body Sensation on the left side of the body
Vision in the right half of the "visual field" Vision in the left half of the "visual field
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• The corpus callosum in involved with
  communication between the hemispheres.

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Fig. 49-15
Frontal lobe
Parietal lobe
Somatosensory cortex
Motor cortex
Somatosensory association area
Speech
Frontal association area
Taste
Reading
Speech
Hearing
Visual association area
Smell
Auditory association area
Vision
Temporal lobe
Occipital lobe
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Studying the Brain
http//video.nationalgeographic.com/video/science/
health-human-body-sci/human-body/brain-bank-sci/
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Phineas Gage A Rod Went Through His Skull
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Memory and Learning

• Learning can occur when neurons make new
  connections or when the strength of existing
  neural connections changes
• Glutamate is often involved.

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Fig. 49-20a
Ca2
Na
Mg2
Glutamate
NMDA receptor (closed)
NMDA receptor (open)
Stored AMPA receptor
(a) Synapse prior to long-term potentiation (LTP)
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Fig. 49-20b
1
3
2
(b) Establishing LTP
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Fig. 49-20c
3
4
1
2
(c) Synapse exhibiting LTP
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Nervous system disorders can be explained in
molecular terms

• Disorders of the nervous system include
  schizophrenia, depression, addiction, Alzheimers
  disease, and Parkinsons disease
• Genetic and environmental factors contribute to
  diseases of the nervous system

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Drug Addiction and the Brain Reward System

• The brains reward system rewards motivation with
  pleasure
• Some drugs are addictive because they increase
  activity of the brains reward system
• These drugs include cocaine, amphetamine, heroin,
  alcohol, and tobacco
• Drug addiction is characterized by compulsive
  consumption and an inability to control intake

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• Addictive drugs enhance the activity of the
  dopamine pathway
• Drug addiction leads to long-lasting changes in
  the reward circuitry that cause craving for the
  drug

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Fig. 49-22
Nicotine stimulates dopamine- releasing VTA
neuron.
Opium and heroin decrease activity of
inhibitory neuron.
Cocaine and amphetamines block removal of
dopamine.
Cerebral neuron of reward pathway
Reward system response
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Stem CellBased Therapy

• Unlike the PNS, the CNS cannot fully repair
  itself
• However, it was recently discovered that the
  adult human brain contains stem cells that can
  differentiate into mature neurons
• Induction of stem cell differentiation and
  transplantation of cultured stem cells are
  potential methods for replacing neurons lost to
  trauma or disease