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

Fig 13.3 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

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

Ependyma is a cellular layer that lines brain ventricles and the central canal of the spinal cord

Figure 13.6

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

Figure 13.7 Satellite cells surround neuron cell bodies in peripheral ganglia

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Satellite Cells and Peripheral Neurons

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

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Fig 13.8

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)Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

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 sulfatide-rich 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.