Transcript Central Nervous System

Chapter 12 : Anatomy of the Nervous
System
An Overview of the Nervous System
• NS includes all of the neural tissue in the body
• Two anatomical subdivisions: CNS and PNS
• Central nervous system
- brain and spinal cord
- integrating, processing, and coordinating
• Peripheral nervous system
- neural tissue outside the CNS
- provides sensory information to the CNS
- carries motor commands to peripheral tissues
Central Nervous System
• During development the CNS began as a mass of
neural tissue organized into a hollow tube
- as development continues, the central cavity decreases and
varies in size within the enclosed space
- called the central canal (within the spinal cord)
- ventricles are expanded chambers, continuous with the
central canal
• Cerebrospinal fluid (CSF) fills the central canal and
ventricles and surrounds the CNS
Peripheral Nervous System
The PNS is subdivided into 2 divisions:
• Afferent division brings sensory information to the
CNS
• Efferent division carries motor commands to muscles
and glands
- further divided into the somatic nervous system (SNS) and the
autonomic nervous system (ANS)
Afferent Division
• Begins at receptors that monitor specific
characteristics of the environment and include:
- dendrites, sensory processes of a neuron
- a specialized cell or cluster of cells
- complex sense organs – eye, nose, tongue, and ear
• Carries information from somatic and visceral sensory
receptors
– somatic include skeletal muscle, joints, and the skin
- visceral include smooth and cardiac muscle and glands
The Efferent Division
• Begins inside the CNS and ends at an effector
- a muscle cell, gland cell, or other specialized cell
• Efferent division includes the SNS and ANS
• SNS – controls skeletal muscle contraction
- voluntary under conscious control
- involuntary directed outside your control
• ANS or visceral motor system, regulates smooth and
cardiac muscle, and glandular activity
Cellular Organization in Neural Tissue
• Neural tissue contains 2 distinct cell types: nerve cells,
or neurons, and supporting cells, or neuroglia
• Neurons are responsible for the transfer and
processing of information in the nervous system
• Supporting cells, or neuroglia, isolate the neurons
Neuron
• Representative neuron has 4 main parts:
- cell body or soma with a perikaryon, region around the nucleus
- dendrites, branch from the soma and each branch has fine
processes called dendritic spines
- axon, elongated process attached to the soma
- synaptic terminals, end of the axon that communicates with
another cell
• The soma contains the organelles
- energy production and biosynthesis of organic molecules
A Review of Neuron Structure
Relationship of the 4 parts of a neuron (dendrites, cell
body, axon, and synaptic terminals);
- the functional activities of each part
- the normal direction of action potential conduction
Fig 13.3
Neuroglia
• Have many functions, including:
- provide framework for the neural tissue
- maintain the intercellular environment
- act as phagocytes
• 4 types: astrocytes, oligodendrocytes, microglia, and
ependymal cells
- distinguished on basis of size, intracellular organization, and
presence of specific cytoplasmic processess
• 100 billion neuroglia, or glial cells
- about 5 times the number of neurons
Fig 13.4
The Classification of Neuroglia
Astrocytes
• Largest and most numerous glial cells
• Variety of functions include:
- Controlling the interstitial environment
- Maintaining the blood-brain barrier
- Creating a 3-dimensional framework for the CNS
- Performing repairs in damaged neural tissue
- Guiding neuron development
Fig 13.5
Histology of Neural Tissue in the Spinal Cord of the CNS
Figure 13.6
Ependyma is a cellular layer that lines brain ventricles and the
central canal of the spinal cord
Neuroglia of the PNS
• PNS neuron cell bodies are usually clustered together in
masses called ganglia (sing. ganglion)
• Axons are bundled together and wrapped in CT, forming
peripheral nerves, or nerves
• Neuron cell bodies and axons are insulated from their
surroundings by processes of glial cells:
- satellite cells surround cell bodies in peripheral ganglia
- every peripheral axon (unmyelinated or myelinated) is covered by
Schwann cells or neurolemmocytes
- plasmalemma of an axon is called axolemma and the superficial
covering by Schwann cells is the neurilemma
Satellite Cells and Peripheral Neurons
Satellite cells surround neuron cell bodies in
peripheral ganglia
Figure 13.7
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a) a single
Schwann cell
forms the myelin
sheath around a
portion of a single
axon
b) a single
Schwann cell can
encircle several
unmyelinated
axons
Fig 13.8
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Representative Neuron
• Cell body or soma contains a relatively large, round
nucleus with a prominent nucleolus
- surrounding cytoplasm is the perikaryon (‘nucleus’)
- cytoskeleton of the perikaryon contains neurofilaments and
neurotubules
- neurofibrils, bundles of neurofilaments, are the cytoskeletal
elements that extend into dendrites and the axon
Representative Neuron
• Perikaryon contains organelles – provide energy and
biosynthetic activities
- numerous mitochondria and fixed and free ribosomes
- ribosomal clusters called chromatophilic substance or Nissl
bodies account for the gray color of the gray matter (cell
bodies)
• Most neurons lack the centrosome complex
- usually lose their centrioles during differentiation (incapable
of undergoing cell division)
Representative Neuron
• Neurilemma permeability of dendrites and cell body
can be changed by:
- chemical, mechanical, or electrical stimuli
• One primary function of glial cells is to limit the
number or types of stimuli affecting a neuron
- processes cover most of the surfaces of the cell body and
dendrites
- transmembrane potential resulting from unequal
distribution of ions across the neurilemma
• Axon or nerve fiber is a long cytoplasmic process capable
of propagating an action potential
- axon hillock connects the initial segment of the axon to the soma
- axoplasm or cytoplasm of the axon
- an axon may produce branches or collaterals
- main trunk and collaterals end at terminal arborizations or
telodendria
- terminal arborizations end in a synaptic terminal, part of a
synapse where a neuron contacts another neuron or effector
- terminal bouton or synaptic knob where one neuron synapses on
another
- axoplasmic transport, movement of organelles, nutrients,
synthesized molecules, and waste products
Fig 13.9
Anatomy of a Representative Neuron (Diagrammatic)
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Neuron Classification
• Anaxonic neurons – small with no anatomical clues to
distinguish the axon from their dendrites
- poorly undersood and found only in the CNS and in special sense
organs
• Bipolar neurons – number of fine dendrites that fuse to
form a single dendrite
- cell body lies between this dendrite and a single axon
- rare but important in sight, smell, and hearing
- axons are unmyelinated
Neuron Classification
• Pseudounipolar neurons – continuous dendritic and
axonal processes with the soma off to 1 side
- initial segment lies at the base of the dendritic branches
- rest of the process is an axon
- sensory neurons of the PNS are usually pseudounipolar with
axons that may be myelinated
• Multipolar neurons – several dendrites and a single axon
with one or more branches
- most common type in the CNS
- all of the motor neurons that control skeletal muscles with
myelinated axons
Figure 13.10
A Structural Classification of Neurons
Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Myelin Sheath Disorders
• Dysmyelinatin- Profound disturbance in the
formation and preservation of myelin so that
its proper functioning is never established.
These disorders are also termed
leukodystrophies, and almost all of them
manifest themselves early in life and are
genetically determined.
DYSMYELINATING DISEASES
(LEUKODYSTROPHIES)
Metachromatic Leukodystrophy
• Most common of these disorders
• Autosomal recessive disorder
• Both central and peripheral white matter
involved
• Predominantly a disease of infancy, but
juvenile and adult forms do exist
DYSMYELINATING DISEASES
(LEUKODYSTROPHIES)
Metachromatic Leukodystrophy
• Course is progressive, usually fatal in a few years
• Pathology is diffuse, confluent loss of myelin that is most
advanced in the cerebrum.
• Due to inborn error of metabolism in which arylsulfatase
A, although present, is enzymatically inactive. Leads to
breakdown of myelin and the accumulation of sulfatiderich lipids that appear as small globules of metachromatic
material in the white matter.
DYSMYELINATING DISEASES
(LEUKODYSTROPHIES)
Krabbe's Disease
Usually appears in early months of life and
progresses to death in one to two years
• Autosomal recessive caused by a deficiency of
galactocerebroside, B-galactosidase
• Expressed histologically by the presence of
perivascular aggregates of globoid cells
DYSMYELINATING DISEASES (LEUKODYSTROPHIES)
• Schilder's Disease (sudanophilic leukodystrophy)- Most of
these cases represent an X-linked recessive entity,
adrenoleukodystrophy, that cojoins an inborn error of lipids in
the adrenals and a disturbance in the preservation of myelin.
• Clinical symptoms (motor, sensory and cognitive) develop in
the first decade and progress insidiously.
• The central nervous system is depleted of myelin, and the
peripheral nervous system to a lesser degree.
• Alexander's Disease- characterized pathologically by lack of
formation of myelin and innumerable Rosenthal fibers.
Myelin Sheath Disorders
• Demyelinating- The myelin sheath, once
properly formed and funcioning, is
destrotyed by a disease process. The most
common disease in this category is multiple
sclerosis.
DEMYELINATING DISEASE
• Multiple Sclerosis
Clinical Course
– The main clinical feature is the dissemination of signs
and symptoms in time and space. The lesion can
occur anywhere in the white matter of the CNS,
almost at random, resulting in a variable clinical
presentation.
– Diplopia, numbness or weakness of an extremity, and
monocular blindness are common initial symptoms.
Largely a disease of young adults.
DEMYELINATING DISEASE
• Multiple Sclerosis
Clinical course variable but marked by
exacerbations (attacks followed by remissions.
Each cycle generally leaves further neurologic
deficit. May involve motor, sensory, cerebellar
functioning, etc., often in bizarre patterns.
DEMYELINATING DISEASE
• Multiple Sclerosis
– Important variant is progressive, non-remittent
spinal multiple sclerosis. Predominately in patients
over forty.
– No specific diagnostic tests. The CSF often shows
elevated gamma globulin and oligoclonal bands. Both
are valuable but not pathognomonic. Other helpful
tests include MRI and CT scans, and evoked
potentials.
Myelin Sheath Disorders
• Multiple Sclerosis
• Pathology
– Multiple plaques- these are sharply delineated, irregular
zones of total demyelination with initial preservation of
axons. The plaque follows no pattern of vascular or
anatomic distribution. They are most numerous in the
white matter of the cerebrum (periventricular), brain
stem, cerebellum and spinal cord (peripheral regions).
– Within the plaque, initially axons are preserved.
Microglial cells proliferate and phagocytize the myelin
debris. Later, a glial scar forms.
Myelin Sheath Disorders
• Multiple Sclerosis
Etiology
– Epidemiologists have shown a wide range in the incidence of the
disease. The frequency is "latitude related" and this geographic
relationship pertains to the individual's global location prior to the
age of 15
– Two main hypotheses concerning etiology:
• Viral: epidemiologic data compatible with this hypothesis. Viral particles
have been identified, but transmission experiments have been negative.
• Autoimmune: based on similarity to experimental allergic
encephalomyelitis. There are, however, significant differences between
the human disease and the animal model.