The Nervous System: Neural Tissue

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The Nervous SystemNeural Tissue

• Master controlling /communicating system of the
  body.
• 3 overlapping functions (1) Sensory input (2)
  Integration (3) Motor output.
• Neuron

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Organization of the Nervous System

• CNS-integrating/command center of nervous system.
• PNS-spinal,cranial nervesfunctional
  subdivisions----afferent(sensory),
  efferent(motor)
• Fibers-somatic(SA,SE)visceral(VA,VE)

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Organization of Nervous System (contd)

• The motor division has 2 main parts(1) Somatic
  nervous system (voluntary/involuntary)(2)
  Autonomic nervous system (visceral
  motor)functional subdivisions are sympathetic/
  parasympathetic (opposite effects on
  viscera-stimulaton/inhibition)

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Histology of Nervous Tissue

• Neuron-excitable nerve cells that transmit
  electrical signals
• Supporting cells-surround and wrap neuronsboth
  cell types (neurons/supportive) are bases for
  CNS/PNS

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Histology of Nervous Tissue (Neuroglia)

• Nonnervous supporting cells
• Six types-4 in CNS, 2 in PNS, each has unique
  function
• Scaffold neurons
• Chemical production guides young neurons to
  proper connections promote health/growth.

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CNS Supportive Cells

• Astrocytes- most numerous versatile, radiating
  processes anchor neurons to capillaries (form
  BBB) chemical control (K, recycle neurotrans.)
• Microglia- Ovoid cells, monitor neuron health,
  macrophage.
• Ependymal cells- range in shape from squamous to
  columnar, line central cavities of CNS, circulate
  CSF.
• Oligodendrocytes- producers of myelin sheaths.

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PNS Supportive Cells

• Satellite cells (amphicytes)-surround neuron soma
  within gangliaregulate nutrient/waste product
  exchange between soma and ECF.
• Schwann cells (neurolemmocytes)-surround and
  form myelin sheaths (functionally similar to
  oligodendrocytes)vital to peripheral nerve fiber
  regeneration.

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Neurons

• Structural unit of nervous system
• Have extreme longevity
• Amitotic exceptions are olfactory hippocampal.
• High metabolic rate, require ample supply of
  glucose oxygen.

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Neurons (contd)

• Large, complex cells
• Soma, processes
• 3 functional components input region, conducting
  component, secretory component.

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Neurons (contd)Cell Body

• Soma or perikaryon transparent, spherical
  nucleus (biosynthetic center) with conspicuous
  nucleolus lack centrioles.
• Free ribosomes, RER (Nissl bodies), Golgi
  apparatus arcs around nucleus mitochondria,
  neurotubules, neurofibrils CNS soma (nuclei),
  PNS soma (ganglia).

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Neurons (contd)Processes

• CNS contain soma and processes, PNS contain
  mostly processesbundles of processes in CNS
  called tracts, nerves in PNS.
• Dendrites-short, tapering branching extensions
  receptive regionsdendritic spine point of
  synapse.

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Neurons (contd)Processes

• Axon arises from hillock long axon is a nerve
  fiber each neuron possesses 1 axon collaterals,
  telodendria (terminal branches) motor neuron
  impulse triggered at hillock, terminal represents
  secretory component axolemma
• Axoplasmic transport is anterograde and
  retrograde

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Neurons (contd)Myelin sheath and Neurilemma

• Myelin protects and electrically insulates fibers
  and hastens impulses(myelinated 150 m/s vs.
  unmyelinated 1m/s )Schwann cells
  neurilemmanodes of Ranvier (collaterals
  arise)white matter (myelinated fibers), gray
  matter(soma unmyelinated fibers)

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Classification of Neurons Structural

• Bipolar-single dendrite unmyelinated axon
  rarespecial senses.
• Unipolar-continuous dendritic/axonal processes
  PNS sensory neurons/myelinated neurons.
• Multipolar-Most common (99) all skeletal muscle
  motor neurons myelinated axons.

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Classification of NeuronsFunctional

• Sensory(afferent)-Unipolar, soma located in
  sensory ganglia outside CNS only most distal
  parts act as impulse receptor sites.
• Motor (efferent)-Carry impulses away from CNS to
  effector organs (muscles/glands) multipolar,
  soma located in CNS.
• Interneurons-Lie between motor and sensory
  neuronsconfined within CNS comprise 99 of
  neurons of body.

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Membrane Potentials

• Depolarization-inside becomes less neg.
• Hyperpolarization-inside becomes more neg.
• Action potentials

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Generation of AP

• (1) Resting stage voltage-gated channels closed
• (2) Depolarizing phase Na permeability
  increases
• (3) Repolarizing phaseKpermeability increases
• (4) Undershoot-K permeability persists

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The Synapse

• Electrical-very rapid less common gap
  junctionsembryonic nerve tissue jerky eye
  movements eventually replaced by chemical
• Chemical-presynaptic terminals vesicles
  cleftpostsynaptic membrane (receptors)

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Neurotransmitters

• Acetylcholine-excitatory to skeletal muscles
• Biogenic Amines-norepinephrine (E or I), dopamine
  (E or I), Serotonin (I)emotional behavior
  biological clock
• Amino acids-GABA (E), Glutamate (E), Glycine (I)

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Classification of Neurons(contd)Receptors

• Exteroceptors- External environment information
  touch, temperature, pressure complex special
  senses (somatic sensory neurons)
• Proprioceptors- Gauge somatic movement (somatic
  sensory neurons)
• Interoceptors- Gauge digestive, respiratory,
  cardiovascular systems deep pressure sensations
  (visceral sensory neurons).

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Brain Organization

• Telencephalon-cerebral hemispheres
• Diencephalon-thalamus, hypothalamus, epithalamus
• Mesencephalon-midbrain
• Metencephalon-pons, cerebellum
• Myelencephalon-medulla oblongata

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Cranial Meninges CSF

• Dura mater- Falx cerebri, tentorium cerebelli,
  falx cerebelli
• Arachnoid
• Pia mater
• Choroid plexus-combination of ependymal cells
  permeable capillaries

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Brain Ventricles

• Lateral ventricles
• Interventricular foramen
• Third ventricle
• Cerebral aqueduct
• Fourth ventricle

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Cerebral hemispheres

• 80 of total brain mass
• Gyri,sulci
• Fissures (longitudinal, transverse)
• Lobes Frontal, Temporal, Parietal, Occipital,
  and Insula

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Cerebral Cortex Motor Sensory Areas

• Precentral gyrus-primary motor cortex (pyramidal
  cells)
• Postcentral gyrus-primary sensory cortex
• Occipital lobe-visual cortex
• Temporal lobe-auditory/olfactory cortex
• Insula/portions of frontal lobe gustatory cortex
  

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The Cerebral (Basal)Nuclei

• Amygdaloid nucleus-Limbic system component
• Corpus striatum(lentiform nucleus, caudate
  nucleus)-Subconscious adjustment/modification of
  voluntary motor commands

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Limbic System

• Hippocampus-involved in learning/long term memory
• Amygdala
• Cingulate gyrus
• Fornix

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The Thalamus

• Makes up 80 of diencephalon bilateral masses
  adhered by intermediate mass
• Anterior nuclei-hypothalamus
• Pulvinar,lateral dorsal posterior nuclei-Project
  visual/auditory information to visual/auditory
  cortices.
• Mediates sensation, motor activities.

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The Hypothalamus

• Mammillary bodies
• Infundibulum
• Functions
• Controls autonomic functions
• Sets appetite thirst drives
• Homeostasis
• Emotional response
• Sleep-wake cycles

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The Epithalamus

• Pineal gland-melotonin
• Choroid plexus

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Mesencephalon (Midbrain)

• Cerebral peduncles
• Cerebral aqueduct
• Corpora quadrigemina-sensory nuclei
• Superior collicus-visual input
• Inferior colliculus-auditory input
• Red nucleus, substantia nigra

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The Pons

• Brain stem region wedged between midbrain
  medulla
• Cerebellar peduncles

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The Cerebellum

• Accounts for 10 of total brain mass
• Cerebellar hemispheres
• Vermis
• Folia
• Primary fissure (anterior,posterior lobes)
• Cortex contains Purkinje cells
• Arbor vitae-internal white matter

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Medulla Oblongata

• Pyramids
• Decusssation point
• Visceral motor nuclei for cardiovascular,
  respiratory rhythmicity others (hiccuping,
  swallowing, etc,)

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The Cranial Nerves

• Components of PNS
• 12 pairs
• Positioned along longitudinal axis
• Olfactory (I)-Special sensory (smell)
• Optic (II)-Special sensory (vision)
• Oculomotor (III)-Motor, eye movements
• Trochlear (IV)-Motor, eye movements

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Cranial Nerves (contd)

• Trigeminal (V)-Mixed, maxillary/mandibular
  branches
• Abducens (VI)-Motor, eye movements
• Facial (VII)-Mixed
• Vestibulocochlear (VIII)-Special sensory, hearing
• Glossopharyngeal (IX)-Mixed
• Vagus (X)-Mixed
• Accessory (XI)-Motor
• Hypoglossal (XII)-Motor, tongue movements

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The Spinal Cord and Spinal Nerves

• Spinal cord extends from foramen magnum to level
  of first or second lumbar vertebra ( 42 cm long,
  1.8 cm thick)
• Major reflex center, ascending descending
  tracts.

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Gross Anatomy of Spinal Cord

• Posterior, anterior median sulci
• Cervical, lumbar enlargements
• Conus medullaris
• Filum terminale
• Dorsal,ventral root ganglia
• Spinal nerve (31 pairs)
• Cauda equina

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Spinal Meninges

• Three layers Dura mater, arachnoid, pia mater
• Continuous with cranial meninges
• Cerebrospinal fluid
• Epidural space

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Spinal meninges (contd)The dura mater

• Outermost covering of spinal cord and brain
• Fuse at margins of foramen magnum
• Coccygeal ligament merges with components of
  filum terminale

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Spinal meninges (contd)Arachnoid

• Subdural space
• Arachnoid-middle meningeal layer, simple squamous
  epithelium
• Subarachnoid space-arachnoid trabeculae
  (collagen, elastin fibers)

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Spinal Meninges (contd)The pia mater

• Innermost layer
• Anterior, posterior spinal arteries
• Spinal cord surface consist of astrocytes that
  reinforce pia mater in place
• Denticulate ligament
• Filum terminale

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Cross-Sectional Anatomy of the Spinal CordGray
Matter Spinal Roots

• Posterior (dorsal) gray horn-somatic visceral
  sensory neurons (interneurons)
• Anterior(ventral)gray horn-somatic motor control
• Lateral gray horn- located in thoracic/superior
  lumbar segments contain visceral motor nuclei.
• Ventral root
• Dorsal root
• Gray commissures

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Cross Sectional Anatomy of the Spinal CordWhite
Matter

• Anterior, posterior white columns (funiculi)
• Anterior white commissure
• Lateral white columns
• Columns contain ascending, descending tracts

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Spinal Nerves

• 31 pairs (cervicals precede adjacent vertebra)
  1st cervical spinal nerve is between the skull
  the atlasC1-C8 thoracics procede adjacent
  vertebra
• Epineurium-collagen fibrous sheathcontinuous
  with dura at intervertebral foramina
• Perineurium-surround fascicles
• Endoneurium-surround individual axons

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Peripheral Distribution of Spinal Nerves

• Spinal nerves are formed by fusion of ventral
  dorsal roots.
• White ramus
• Gray ramus
• Dorsal ramus
• Ventral ramus
• Dermatomes

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Nerve Plexuses

• Convergence of ventral rami of adjacent spinal
  nerves producing a series of compound nerve
  trunks.
• Major plexuses Cervical, brachial, lumbar,
  sacral.

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Cervical PlexusC1-C5

• Buried deep under sternocleidomastoid formed by
  ventral rami of 1st four cervical nerves.
• Innervate neck muscles phrenic is major nerve
  cutaneal branches (superficial) motor branches
  (deep).

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Brachial PlexusC5-T1

• Larger,more complex than cervical, situated
  partly in the neck axilla gives rise to nerves
  that innervate upper limb organizational
  sequence roots,trunks, divisions, cords.
• Nerves axillary, radial, musculocutaneous,
  median, ulnar.

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Lumbar PlexusT12-L4

• Arises from 1st four lumbar spinal nerves and
  lies within psoas major muscle
• Nerves Femoral, obturator, iliohypogastric,
  ilioinguinal, genitofemoral.

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Sacral PlexusL4-S4

• Nerves Gluteal, Sciatic (tibialis, peroneal),
• Pudendal