December 31, 2015

Brain Stem

by Sharadsaini on

Brain Stem

The brain stem consists of the medulla oblongata (or medulla), the pons and the midbrain. The three brain areas each contain cranial nerve nuclei, and the fourth ventricle lies partly in the pons and partly in the medulla. The brain stem may occasionally be referred to as the ‘bulb’ in such terms as the ‘corticobulbar’ tract.

The medulla is around 3 cm long in adult humans and widens rostrally. It is continuous with the spinal cord from just belowthe foramen magnum, at the level of the upper rootlet of the first cranial nerve, and extends through to the lower  (caudal) border of the pons. The medulla lies on the basilar part of the occipital bone, and is obscured from view by the cerebellum. Externally, the spinal cord and medulla appear to merge imperceptibly, but internal examination reveals extensive reorganization of white and gray matter at the junction. In the medulla the central canal widens into the fourth ventricle.

From the ventral aspect, the central median fissure appears as a central groove, which is a continuation of that of the spinal cord. The progress of the fissure is interrupted by the decussation (crossing over) of the fiber tracts of the corticospinal tract, where they cross over at the pyramid of the medulla to form the lateral corticospinal tract. Lateral to the pyramids on each side is the olive, made up of a convoluted mass of gray matter called the inferior olivary nucleus. The olive is separated from the pyramids by the rootlets of the hypoglossal nerve (XII). Rootlets of the vagus (X) and the cranial accessory (XI) nerves arise lateral to the olive, the latter two being united with the spinal accessory nerve (XI). The facial (VII) and vestibulocochlear (VIII) nerves arise at the border between the lateral medulla and the pons.

The pons is about 2.5 cm in length. Its name is Latin for ‘bridge’, since it appears to connect the cerebellar emispheres though this is not actually the case. Ventrally, the pons is a sort of relay station, where cerebral cortex fibers terminate ipsilaterally on pontine nuclei, whose axons become the contralateral middle cerebellar peduncles. Thus the ventral (or basal) pons is a sort of massive synaptic junction that connects each cerebral hemisphere with the contralateral cerebellar hemisphere. Functionally, this system maximizes efficiency of voluntary movement.

The ventral surface of the midbrain extends rostrally from the pons to the mamillary bodies, which mark the caudal  border of the diencephalon. On either side are prominent swellings called the crus cerebri (basis pedunculi). These are made up of the fiber tracts of the descending pyramidal motor system, and fibers from the cortex to the pons (corticopontine fibers). Although not shown here, the midbrain is penetrated by several small blood vessels in the floor of the interpeduncular  fossa, and the area has been named the posterior perforated substance because of these blood vessels. The oculomotor nerve (III) to the eye leaves the brain through the cavernous venous sinus from each side of the interpeduncular fossa. The optic chiasm and optic nerves, together with the diencephalic tuber cinereum are exposed on the ventral surface. brain stem

Gray Matter

by Sharadsaini on

Gray Matter

Gray Matter

The gray matter of the cord is butterfly shaped, with the so-called dorsal (posterior) horns forming the upper wings of the butterfly shape. These are linked by a thin gray commissure in which lies the central canal. In the thoracic and upper lumbar segments the gray matter extends on both sides to form lateral horns. The lower wings of the butterfly shape are formed by the ventral (anterior) horns of the gray matter. (The size of the gray matter is greatest at egments that innervate the most skeletal muscle. These are the cervical and lumbosacral, which innervate upper and lower limb muscles, respectively.)

Structurally, the gray matter is composed of neuronal cell nuclei, their processes, neuroglia  and blood vessels. The overall arrangement of the gray matter of the cord was systematized by Rexed, who proposed the generally accepted laminar arrangement, commonly referred to as the cytoarchitectonic organization of the spinal cord. The gray matter is divided arbitrarily into nine visually distinct laminae, labeled I through IX, and an area X, which surrounds the central canal. Most laminae are present throughout the cord, but VI, for example, is apparently absent from T4 to L2.

Lamina

nuclei

Lamina I is at the apex of the dorsal horn, and contains the posterior marginal nucleus. These cells respond to thermal and other noxious stimuli, and receive axosomatic connections from lamina II. Near the apex, in lamina II, is the substantia gelatinosa, which is found throughout the length of the cord, and which receives touch, temperature and pain afferents, as well as inputs from descending fibers. Both I and II are rich in substance P, considered to be an excitatory neurotransmitter of pain impulses, in opioid receptors and the enkephalin.

Nucleus

Nucleus

Ventral to the substantia gelatinosa, extending through III and IV, is the largest dorsal horn nucleus, the nucleus proprius, which also exists at all cord levels. This receives inputs concerning movement, position, vibration and two-point discrimination from the dorsal white column. The nucleus reticularis is present in the broad lamina V, which is divided into medial and lateral zones, except in thoracic segments. Lamina VI, seen only at cord enlargements, receives group I muscle afferents in its medial zone, and descending spinal terminations in its lateral zone. Lamina VII contains the nucleus dorsalis of Clark (Clark’s column), a group of relatively large multipolar or oval nerve cells that extends from C8 through L3 or L4. Most of the cells respond to stimulation of muscle and tendon spindles. Layer VIII is a zone of heterogeneous cells most prominent from T1 through L2 or L3, associated with autonomic function.

Lamina IX is situated in the anterior or ventral horn of the gray matter, and contains clusters of large, motor nerve cells. The larger cells send out a efferent motoneuron axons, which innervate the extrafusal skeletal muscle fibers, while smaller cells send out g motoneuron axons, which innervate the intrafusal spindle fibers.

 

Meninges

by Sharadsaini on

Meninges

The nervous system consists of two main divisions: TheCentral Nervous System (CNS), consisting of brain and spinal cord,and the peripheral nervous system, consisting of cranial and spinal nerves, and their associated gangliaThree membranes surround both spinal cord and brain: dura mater, arachnoid mater, and pia mater. The dura mater is a tough, fibrous coat that encloses the spinal column and cauda equina, which is abundle of nerve roots from the lumbar, sacral and coccygeal spinal nerves. The dura mater runs rostrally and is continuous beyond the foramen magnum with the dural meninges, which cover the brain. Caudally, the dura ends on the filum terminale at the level of the lower end of the second sacral vertebra. The dura is separated from the walls of the vertebral canal by the extradural space, which contains the internal vertebral venous plexus. The dura extends along the nerve roots and is continuous with the connective tissue that surrounds the spinal nerves. The inner surface of the dura is in direct contact with the arachnoid mater.

                                 The arachnoid mater is a relatively fragile, impermeable layer that covers the spinal cord, the brain and spinal nerve roots, and is separated from the pia by the wide subarachnoid space, which is filled with cerebrospinal fluid. The pia mater is a highly vascularized membrane closely apposed to the spinal cord. It thickens on each side between the nerve roots to form lateral supports, anchored to the arachnoid, which suspend the spinal cord securely in the center of the dural sheath.

                               The spinal cord is an approximately cylindrical column, continuous with the medulla oblongata, that extends in adults from the foramen magnum to the lower border of the first lumbar vertebra. Structurally, the cord contains central gray matter, roughly H-shaped, consisting of the anterior and posterior horns and joined by a thin commissure containing the central canal, which is connected to the fourth ventricle. The gray matter is surrounded by white matter, which consists mainly of ascending and descending tracts, and has been divided arbitrarily into anterior, lateral, and posterior columns. The individual tracts will be dealt with in more detail later.

                               In the peripheral nervous system, there 12 pairs of cranial nerves, which leave the brain through foramina (apertures) in the skull, and 31 pairs of spinal nerves, which leave the spinal cord through vertebral foramina. There are eight cervical, 12 thoracic, five lumbar, five sacral, and one coccygeal pair of spinal nerves. The spinal nerves are linked to the cord by dorsal (posterior) nerve roots, which carry afferent
nerves into the CNS, and ventral (anterior) nerve roots, which carry efferent nerves away from the CNS. Afferent fibers are also called sensory fibers, and their cell bodies are situated in the swellings or ganglia on the dorsal roots.

meninges