development of nervous system-hem

8/9/2019 Development of Nervous System-hem

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(1) Formation of the neural plate,

(2) Folding of the plate to form the neural tube.

(3) Bulging and bending of the tube to form a curved

configuration of five brain vesicles attached to a straighttubular spinal cord.


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The neural plate, from which the nervoussystem develops, is formed by a thickening ofthe ectoderm during the third week of

gestation in humans.

Through a process called neural induction,


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Growth of the neural plate is accelerated along itslateral edges,

causing the edges to curve toward each other,

eventually fuse, and form an open neural tubewith both cranial and caudal openings calledneuropores.

Closure of both neuropores is normally completeby the fourth week.


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Preferential growth at three nodes along the cranialportion of the sealed neural tube causes intermittentbulging of tissue, formation of three brain vesicles

Prosencephalon

Mesencephalon and

Rhombencephalon


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Subsequent growth and division of the three vesiclesleads to the formation of five major brain regions:

(1) Telencephalon,

(2) Diencephalon

(regions 1 and 2 are collectively called the forebrain),

(3) Mesencephalon (midbrain),

(4) Metencephalon (pons and the overlying cerebellum),

(5) Myelencephalon (medulla).


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The spinal cord retains its originalconfiguration as a relatively uniform tubularstructure with the generation of slightenlargements at cervical and lumbar

levels.


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The closed lumen of the neural tube is altered

in shape as a result of enlargement of the threebrain vesicles, particularly the telencephalon.

As the cerebral hemispheres of the

telencephalon increase in size, two lateralexpansions of the lumen form the first andsecond (lateral ) ventricles, connected to eachother and to the third ventricle by a thin, Y-shaped midline channel called the foramen ofMonro.


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Tissue surrounding the lateral ventricles developsinto:

(1) primary motor and sensory cortex, which providesconscious motor control and sensory perception;

(2) limbic structures, which establish mood andemotion; and

(3) basal ganglia, which contain relay nuclei for motor

pathways.


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Two structures develop from the diencephalon andsurround the third ventricle:

The Thalamus, which contains sensory relaynuclei,and

The Hypothalamus,

which provides neurohormones controllingreproductive and other behaviors


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.A narrow cerebral aqueduct extends rostrally from thethird ventricle through brain stem structures (midbrain,pons, and medulla), areas that contain ascending,descending, and reflex pathways for motor and sensorycircuits, as well as nuclei controlling vegetativefunctions.

The fourth ventricle emerges from the cerebral aqueductand is flattened between the caudal portion of the brainstem and the overlying cerebellum, an area involved in

fine motor control.


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The peripheral nervous system arises from neural crestcells located along the lateral edges of the neural plate .

These cells bud off and become separated from theneural tube as it forms.

Some groups of neural crest cells migrate to form a

linear array of dorsal root ganglia located adjacent to thelateral borders of the spinal cord. These cells function assensory neurons.


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Other groups of neural crest cells migrate to equivalentpositions along the medulla and pons, where they form

sensory ganglia associated with the cranial nerves, andstill other groups form motor ganglia associated withboth sympathetic and parasympathetic components ofthe autonomic nervous system.


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At the spinal level of the sensory axis, somatosensorycell bodies reside in dorsal root ganglia, and their fibersexit through spinal nerves to innervate skin and skeletalmuscles of the body.

Through projections of the cranial nerves, sensory cellbodies residing in cranial nerve ganglia innervateanalogous structures in the head and neck region plus

organs of the special senses of vision, taste, hearing,balance, and smell. The only areas not innervated bysensory fibers are bone and nervous tissue itself.


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After entering appropriate regions of the spinal cordor brain stem, proximal sensory fibers are classified

as sensory tracts,

Many sensory tracts cross the midbrain of the neuralaxis and project to the contralateral cerebral

hemisphere.


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The output or motor axis of the nervous system

has two major divisions:

One is involved in conscious control of skeletal musclecontraction and involves the corticospinal tract;

The other operates subconsciously either throughbasic motor reflexes of skeletal muscle orchestrated bysubcortical motor areas such as the cerebellum,basal ganglia, and brain stem nuclei, or throughautonomic nervous system control of smooth muscle.


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development of nervous system-hem

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