Cells of the Nervous System

1. Neuroblast :: immature neurons which become the structural and functional cells of the central  nervous system; a neuron would only persist if a functional synapse is formed; classification:

2. Spongioblast :: develop into the connective tissue of the CNS called neuroglia.

BRAIN VESICLES

• Primary :: prosencephalon, mesencephalon, rhombencephalon

• Secondary- telencephalon, diencephalon, mesencephalon, metencephalon,  myelencephalon

BRAIN FLEXURES

• Cephalic flexure :: Ventral cranial flexure which occurs in the midbrain

• Cervical flexure :: A gradual ventral bend between the hindbrain and the spinal cord

• Pontine flexure :: A slight dorsal bending in the rhombencephalon

Inverted arrangement :: of white and gray matter is brought about by unequal growth of the basal and alar regions of the brain vesicles.

Spinal cord :: from the posterior portion of the neural tube (where there is an underlying notochord)

Meninges :: covering of the CNS; composed of pia mater, arachnoid, and dura mater; derived from  neural crest cells

Notochord :: gives rise to the nucleus pulposus, a gel-like substance surrounded by annulus  fibrosus found between vertebrae

Ventricles :: spaces wherein CSF-secreting choroid plexuses are found

The peripheral nervous system (PNS) :: consists of cranial and spinal nerves.

Nerve fibers :: within peripheral nerves may be classified as afferent (sensory) or efferent (motor) and as somatic (innervating skin and skeletal muscle) or visceral (innervating vessels and viscera).

The visceral efferent (autonomic) :: pathway involves two neurons: 1] a preganglionic neuron that originates in the CNS and 2] a postganglionic neuron l__ocated entirely in the PNS__. The glial cell of the PNS is the neurolemmocyte (Schwann cell).

Somites:

• Mesoderm blocks :: located just lateral to the notochord, which induced somite development.

• A pair of somites :: develop for every vertebra, plus a half dozen somite pairs in the head.

• Number of somites in an embryo is indicative of age, individual somites develop chronologically, in craniocaudal order.

Somites develop as follows:

— mesoderm, designated paraxial mesoderm*,* accumulates on each side of the notochord progressing from rostral to caudal over time, transverse fissures divide the paraxial mesoderm into blocks

— each block becomes a somite (epithelioid cells within a somite block re-orient 90°, from  transverse to the notochord to longitudinal)

— head (occipital) somites develop from proliferation of local mesenchyme lateral to the cranial  end of the notochord

— rostral to the notochord, mesenchyme forms less-developed somites, called somitomeres; these migrate into pharyngeal arches and form muscles of the jaw, face, pharynx, & larynx.

Each somite differentiates into three regions:

• Sclerotome (ventromedial region) :: gives rise to vertebrae, ribs, and endochondral bones at the base of the skull.

• Dermatome (lateral region) :: gives rise to the dermis of skin

• Myotome (intermediate region) :: gives rise to skeletal muscles of the body

The early embryo is flat, but the vertebrate body plan features a cylindrical theme—various cylindrical structures (derivatives of the gut, neural tube, notochord, etc.) enclosed within a  cylindrical body.

Head Process Formation:

• The cranial end :: of the embryo grows dorsally and forward so that it projects above the region originally in front of the embryo.

• The cylindrical head :: process elongates by additional growth from its base (located in front of the primitive node). Consequently, the most anterior part of the embryo is the oldest. The elongation incorporates the most anterior half-dozen somites into the future head.

• Within the head process, the endoderm is reflected ventrally upon itself, forming a blind-ended ***foregut ***(future pharynx).

Tail Fold Formation

• At the caudal end of the embryo, a cylindrical tail fold is formed in a manner similar to that of the head process.

• Folded endoderm encloses a blind hindgut.

Lateral Body Folds:

• As the head process elongates upward & forward, a subcephalic pocket (space) is formed ventral to the head process, between the head process and extra-embryonic tissue. The bilateral margins of this pocket are lateral body folds— which constitute the continuity between the elevated embryo and the relatively flat extra-embryonic tissue.

• Similar folds exist caudally in association with the tail process.

• As the embryo grows and is elevated dorsally, lateral body folds adduct and join together ventrally, establishing a tubular embryo separated from flattened extra-embryonic tissue.

• Progressing caudally from the head process and cranially from the tail fold, ventral fusion of  lateral body folds stops at the umbilicus— leaving a ventral opening in the body wall that allows  vessels and the yolk sac and allantois to enter the embryo (and communicate with the gut)

• Ventral fusion :: of lateral body folds distinguishes the embryo from extra-embryonic tissue (fetal  membranes):

— embryonic coelom :: (future body cavities of the trunk) is distinguished from extra-embryonic coelom within fetal membranes.

— somatopleure :: (somatic mesoderm + ectoderm) that forms body wall is distinguished from that forming fetal membranes (chorion and amnion).

— splanchnopleure :: (splanchnic mesoderm + endoderm) merges bilaterally to form gut and mesentery, differentiated from extra-embryonic yolk sac (and allantois).

Pharyngeal Arches:

In the head region, dorso-ventral arches demarcated by grooves (clefts) appear. The arches are called pharyngeal arches and they are bounded internally by pharyngeal pouches. Each arch contains a vessel (aortic arch). Within each arch, ectomesenchyme (derived from  neural crest) gives rise to bone and fascia. Myotomes of somitomeres migrate to pharyngeal  arches to provide skeletal musculature.

Each arch is innervated by one cranial nerve.

Only the first three pharyngeal arches are externally evident in mammals. The first arch develops into upper and lower jaws and muscles of mastication. The second gives rise to hyoid bones and muscles of the face. The remaining pharyngeal arches form hyoid bones, larynx and associated muscles. Each arch is innervated by a particular cranial nerve.

The pharynx (foregut) develops five bilateral diverticula that internally demarcate the pharyngeal arches. These pharyngeal pouches develop into auditory tubes, parathyroid glands, thymus, etc. Flexures:

The tube-shaped embryo undergoes three flexures that make it C-shaped. The first occurs in the future midbrain region, the second in the future neck region, and the third occurs in the tail region.

Development of the Spinal Cord

Malformations of the Central Nervous System

Malformations of the Brain

1. Cerebellar hypoplasia and atrophy :: failure of the cerebellum to develop due to destruction of the cerebellar cortex. Generally caused by viral infections like feline panleukopenia in cats and bovine virus diarrhea infection in calves**.**

2. Cerebellar abiotrophy :: premature degeneration of Purkinje cell layer of the cerebellar cortex.

3. Hydrocephalus :: accumulation of the excessive amount of cerebrospinal fluid in the cranial cavity.

4. Hydranencephaly :: thin-walled and greatly enlarged lateral ventricle filled with CSF.

Malformations of the Spinal Cord

1. Myelodysplasia :: a general term for a malformation of the spinal cord. Originates during the neurula stage.

a. Hypoplasia (aplasia) :: reduced or absence of development of one or more segments of the spinal cord.

b. Hydromyelia :: dilation of the central canal due to excess accumulation of CSF.

c. Syringomyelia :: abnormal cavitation of the spinal cord.

d. Diplomyelia :: 2 spinal cords develop beside each other usually in one set or meninges and in one vertebral canal.

e. Diastematomyelia :: 2 spinal cords develop with a partition between them. Usually in separate vertebral canals and have separate meninges.

2. Myeloschisis :: cleft in the neural tube cause by failure of this part to close during neurulation.

3. Meningocoele :: protrusion of the meninges through an opening in the vertebral arches to form a cyst beneath the skin.

4. Meningomyelocoele :: like menigocoele but both the meninges and the spinal cord protrude.