NERVOUS SYSTEM I' The Neuron A' General components drawings

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The Nervous System
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I. The Neuron

• A. General components

Direction of Impulse
Cell Body
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• B. Sensory nerves - transmit messages to spinal
  cord or brain ganglia outside the spinal cord
• C. Motor nerves - transmit to an effector, e. g.,
  a muscle, a gland, etc. nerve bodies inside the
  spinal cord

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II. Accessory Cells

• A. Neuroglia - half of bulk of N.S.1. Occurrence
  (non-neural dendritic processes and fill spaces -
  function not well understood)
• 2. Types a. Oligodendroglia (Schwann or
  neurilemmal cells) - wrap around nerves and
  secrete mylein
• b. Microglia - phagocytes that remove debris

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• c. Astroglia - starlike and function in nerve
  impulse and connect nerves with blood vessels

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III. Differentiation of the Nervous System

• A. Neural tube1. Germinal layer - mitotic
  cells
• 2. Mantle layer - from germinal layer and some
  are neuroblasts which differentiate into neurons
  or spongioblasts -gt neuroglia
• 3. Marginal layer - nerve fibers from mantle
  layer
• 4. Alar and basal platesAlar becomes
  sensoryBasal becomes motor

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1. Germinal layer - mitotic cells 2. Mantle
layer - from germinal layer and some are
neuroblasts which differentiate into neurons or
spongioblasts -gt neuroglia 3. Marginal layer
- nerve fibers from mantle layer 4. Alar and
basal platesAlar becomes sensoryBasal becomes
motor
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B. Motor nerve development

• 1. Axons - that sprout from basal plate to
  muscles
• 2. Preganglionic fibers - fibers sprouting from
  basal plate to neuroblasts of autonomic ganglia
• 3. Postganglionic fibers - fibers sprouting from
  neuroblasts in autonomic glanglia to smooth
  muscle or glands

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C. Sensory nerve development

• 1. Neural crestsPaired neural crests that
  grow to sense organs and brain or nerve
  cordNerve body external to the nerve cord

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2. Exceptions to external nerve body

• a. Olfactory nerves - sprout from olfactory
  epithelia and thus ganglia in olfactory epithelia
• b. Optic nerves - sprout from embryonic retina -
  nerve body in retina. Actually optic cup
  develops as evagination of brain and "optic
  nerve" is a misnomer.
• c. Propioreceptors - neuroblasts within alar
  plate of midbrain sprout processes that grow out
  to muscles. Mesencephalic nucleus of trigeminal
  nerve

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IV. Spinal Cord

• A. Sheaths meninges
• 1. Meninx primitiva - most fishes

Leptomeninx - inner vascular area Amphibians,
reptiles, birds
Dura mater - outer layer Amphibians, reptiles,
birds
Arachnoid Pia mater Mammals Mamm
als
Dura mater - outer layer Mammals
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B. Spinal nerves

• 1. Early chordates
• a. Dorsal and ventral roots did not
  uniteb. Dorsal roots were mixed functionc. No
  dorsal root gangliad. Sensory bodies first
  aggregating into ganglia were bipolar
• 2. Jawless fish - Dorsal and ventral roots
  separate
• 3. Fish and tetrapods - Dorsal and ventral roots
  united

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4. General plan of mammals
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5. Fiber Components

• NERVESSENSORY FUNCTION
• GSA General Somatic Afferent Cutaneous receptors
  - touch, pain, temp, pressure Stretch -
  muscles, tend.?
• GVA General Visceral Afferent Viscera MOTOR
  FUNCTION
• SE - Somatic Efferent Myotomal muscle
• GVE - General Visceral Efferent Smooth cardiac
  muscle (Autonomic Nervous System) and glands
  

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V. The Brain

• A. Forebrain - Telencephalon
• 1. Rhinencephalon - primarily the olfactory
  lobes

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2. Cerebral hemispheres - relatively recent
acquisition

• a. Fish - primitive hemispheres - paleostriatum
  - connects with rhinencephalon - hence olfaction
  very important
• b. Amphibians - primarily paleostriatum - more
  sensory fibers project into it - more sensory
  stimuli

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Cerebral Hemispheres (cont.)

• c. Reptiles - more nuclei - neostriatum
  -hemispheres bulge laterally and have tracts from
  thalamus - Increased sensory input
• d. Birds - even more nuclei - hyperstriatum -
  much of sterotypical behavior is centered here -
  nest building, courtship, etc.

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Cerebral Hemispheres (cont.)

• e. Mammals -Complex striata with basal ganglia
  and play a greater role in motor
  activitiesCortex - greatly expanded
• 1. Discrimination
• 2. memory stored3. data analyzed and responses
  are employed4. initiation of voluntary motor
  responses

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Corpus callosum - connects halves of the
hemispheres
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Evolution of Vertebrate Cerebral Hemispheres Pal
lium - secondary olfactory information and
visual relay from thalamus Subpallium -
limbic system (receives stimuli and relays to
ANS Striatum or basal ganglia - coordination of
motor function
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3. Diencephalon of the Forebrain

• a. Optic chiasma - x of optic fibers
• b. Pituitary - on stalk infundibulum
• c. Hypothalamus - homeostasis

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• d. Epithalamus - roof of diencephalon sense
  organs pineal (photoreceptive) and
  theparapineal
• e. Thalamus - location of synapse of all sensory
  pathways before entering telencephalon and relays
  sensory impulse to hemispheres

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• f. Third ventricle - connects with ventricles of
  each hemisphere and serves as aquaduct for
  cerebrospinal fluid

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B. Midbrain - Mesencephalon

• 1. Optic lobes - especially large in birds - roof
  Tectum
• 2. Auditory lobes - do not bulge on surface in
  fish - also receive stimuli from the membranous
  labryinth vibrations

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• 3. Tegmentum - thickened basal plate with motor
  neurons and continuous with tegmentum of hind
  brain
• 4. Cerebral aqueduct aqueduct of sylvius -
  constriction of midbrain ventricle behind optic
  lobes

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C. Hindbrain - Rhombencephalon

• 1. Myelencephalon a. Medulla oblongata
• b. Fourth ventricle

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• 2. Metencephalon a. Cerebellum - size
  correlated with complexitity of motor activity
  larger in higher verts. b. Tegmentum -
  connects midbrain with hindbrain c. Pons -
  connects halves of hindbrain

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Vertebrate Cerebella
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D. Choroid plexuses

• Cavity with vascularization - ependyma
• Capillaries of the pia mater or leptomeninx
  constantly seep cerebrospinal fluid into it. It
  supplies brain because circulation cannot
  penetrate the main tissue.

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V. Cranial Nerves

• A. General patterns
• 10 cranial nerves in anamniotes, 12 in amniotes .
  Actually 11 and 13, respectively. O   terminal
  - old Branchiomeric nerve I olfactory II op
  tic - not a nerve but a brain tract III
  oculomotor IV trochlear V trigeminus VI
  abduscens VII facial VIII otic
  (auditory or vestibuloacoustic) IX glossopharyn
  geal X vagus XI spinal accessory XII
  hypoglossalNot in fishes or modern amphibians
  - but all 12 in fossil amphibians

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B. Sensory group

• O Terminal - sensory but function poorly
  understood
• I Olfactory - sensory for olfaction
• II Optic - sensory for vision
• VIII Auditory or Vestibuloacoustic - sensory
  for hearing and balance (innervates semicircular
  canals)

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C. Somatic motor
XI Spinal Accessory - nerve to trapezius,
cleidomastoid, and sternomastoid muscles

• III Oculomotor - motor to superior, inferior,
  and lateral rectus muscles of eye, and ANS fibers
  to iris and lens of eye.
• IV Trochlear - motor to superior oblique muscle
  of eye
• VI Abduscens - motor to external rectus muscle
  of eye and nictitating membrane muscles of
  nonmammalian vertebrates
• XII Hypoglossal - motor to hypobranchial muscles
  of neck region

IV Trochlear
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D. Mixed motor - sensory

• V Trigeminus - motor nerve to muscles of
  mandibular arch, i.e., masseter, temporal, and
  digastric
• Sensory to skin of head, teeth, and anterior 2/3
  of tongue
• VII Facial - mixed nerve to muscles of hyoid
  archSensory to taste buds
• IX Glossopharyngeal - ANS motor to parotid
  salivary glands, Sensory from taste buds and
  lateral line system of fish
• X Vagus - Visceral motor and sensory to
  postcranial renal systemSensory to skin of ear,
  lateral line system and gills of fishANS fibers
  to heart and digestive tract

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Mixed Motor and Sensory Nerves
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Autonomic Control
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