Introduction to the Central Nervous System

To the naked eye the brain and spinal cord appear a white and pinkish grey gelatinous mass covered in blood vessels and layers of membranes called Meninges . Top

The brain is contained within the skull and the two cerebral hemispheres (the cerebrum), one on each side, are the largest structures.   The cerebrum is responsible for the control of willed movements and a variety of functions including the processing of sensory messages, thinking, perceiving, understanding language, and speech.

The next largest visible structure within the skull is the cerebellum at the back of the brain. This is a solid organ arching across the back of the brainstem.  The cerebellum has a major function in the planning, execution and coordination of movements.

The cerebral hemispheres and cerebellum are both attached to a central structure, the brainstem, which is a rostral (headwards) extension of the spinal cord.

The brainstem is divided anatomically into three sections, medulla, pons and midbrain.

The midbrain connects with both cerebral hemispheres through the two cerebral peduncles which carry nerve fibres from each hemisphere to and from the brainstem, cerebellum and spinal cord.  

Diagram of a lateral view of the brain and top of spinal cord.

The brainstem has many functions including the control of breathing, blood pressure, eye movements, and balance, and the sleep/waking cycle; in addition most of the cranial nerves have their origin within the brainstem.

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The spinal cord runs caudally (downwards) from the medulla through the foramen magnum of the skull and is divided into segments which correspond with the segments of the vertebral column.

Each segment gives rise to a spinal nerve on each side; the spinal nerves reach the tissues of the body by passing through spaces (inter-vertebral foramina) between the vertebrae on each side.

The spinal nerves that originate in the neck, chest, abdomen and pelvic regions of the vertebral column and are called cervical, thoracic, lumbar and sacral nerves respectively. 

In humans there are 8 cervical, 12 thoracic, 5 lumbar and 5 sacral nerves.

The spinal cord ends opposite the 2nd lumbar vertebra, where it is sometimes named the Conus Medullaris, and the lower lumbar and sacral spinal nerves remain within the meninges and the vertebral column until they exit in their respective foramina.

The cauda equina (horse's tail) is the name given to these spinal nerves in the lowest part of the vertebral column

Dermatomes and Myotomes.

Each spinal nerve innervates skin and muscle in the body. The sensory nerves carry information into the spinal cord; the messages they carry are conducted in an afferent direction (towards to spinal cord).

The area of skin from which information is relayed through a single spinal nerve is called a dermatome.   Similarly the muscle groups that are innervated by one spinal nerve is called a myotome.

Other nerve fibres pass to skeletal muscles and cause them to develop force and shorten (contract). These are the axons of alpha-motoneurones which pass from the spinal cord directly to the skeletal muscle fibres. The direction of conduction is efferent, i.e. from the CNS to the muscles.

These afferent and efferent spinal nerves communicate with structures in the brain using bundles of nerve fibres, called tracts, that travel up or down the cord. Ascending nerve tracts carry sensory information from the spinal cord to the brain. Descending nerve tracts carry messages from the brain to neurones in the spinal cord.

Diagram of Vertebral Column, Spinal Cord and the Spinal Nerves.

Anatomical Definitions are required to describe where different structures are in relation to each other.

• The rostral-caudal axis refers to the length of the CNS, rostral being the head end, and caudal the tail end.
• Dorsal refers to the back, and ventral refers to the front of the body.

Anatomists and radiologist use a range of other terms such as superior, inferior, coronal, sagittal, etc to describe the type of section of the brain that they are describing. Essentially these terms describe the direction of the cut into the tissue, and the meaning of these terms can be found here: Some Anatomical Terms

In the earliest stages of development the embryo consists of a ball of cells, within which three layers can be easily identified.

1. The outer layer, Ectoderm, will develop into the skin and central nervous system.
2. The innermost layer, Endoderm, will form the lining of the gastro-intestinal tract.
3. Between these is the Mesoderm, which will form the muscles, bones and many of the organs of the body.

The earliest sign of the nervous system is the development of the neural plate on the dorsal surface of the ectoderm, behind a column of cells known as the Notochord. The notochord is present in all vertebrates and is essential for inducing the development of the neural plate.

At the lateral edges of the neural plate, ridges appear that grow and fold towards each other to form a tube, the neural tube. The cells that lead this development are called Neural crest cells (C), and when they have completed their role in forming the neural tube, they go on to form some more specialised parts of the nervous system including the dorsal root ganglia, the autonomic nervous system and the adrenal medulla (D).

Thus the Neural tube is formed from the primitive Ectoderm of the embryo, and its formation depends on the presence of the notochord; the notochord is also important in the development of primitive nerve cells in the ventral half of the neural tube into motoneurones. The neural crest is important in ensuring the closure of the neural tube. This process starts in the middle section of the embryo, and the closure of the neural tube progresses towards the head and tail ends simultaneously. In humans this process is normally completed when the embryo is about 28 days old.

Diagram of the Neural Plate and the formation of the Neural Tube.

As the neural tube is forming, the central region closes first, and for a short time, the neural tube is open both at both ends, and the openings are called neuropores, these close completely around 28 days in the human.

The diagram shows the cranial and caudal neuropores before closure of the neural tube. Once closed, the neural tube becomes segmented, and each segment is called a somite.

After closure, the cranial end of the neural tube develops into primitive brain which can be divided into four distinct regions:

The spinal cord is segmented with 8 cervical, 12 thoracic, 5 lumbar and 5 sacral segments in humans. When the caudal neuropore fails to close, the deficiency is usually in the lumbo-sacral segments of the spinal cord.

The cranial end of the neural tube develops into massive structures in the adult human:

• the prosencephalon, which expands enormously to become the cerebral hemispheres, basal ganglia, thalamus and hypothalamus
• the brainstem, consisting of the mesencephalon (midbrain) and the rhombencephalon, which becomes the hindbrain to which the cerebellum is attached.

Diagram of the Embryological Origins of the Central Nervous System.

The brainstem is divided into three levels, the medulla (immediately above the spinal cord), pons and midbrain, through which nerve cells pass from the cerebral hemispheres to the spinal cord.

The medulla and pons of the embryo are sometimes called the hindbrain or rhombencephalon; just as the midbrain is sometimes called the mesencephalon.

The brainstem can be likened to a stalk that has some specialised appendages attached to it. One such large appendage, the cerebellum, bridges over the back of the pons, connected to it by a cerebellar peduncle on each side. 

The cerebellar peduncles are composed of masses of nerve fibres that allow communication between the cerebellum with the spinal cord and the rest of the brain. Here the central canal is expanded as the fourth ventricle.

At the top of the brainstem, the ventral side of the midbrain consists of the cerebral peduncles, one on each side; these are the stalks which support and connect lower levels of the nervous system with the forebrain - the cerebral hemispheres and their central nuclei.

The cerebral peduncles (crura) contain nerve fibres that connect lower levels of the nervous system with the large nuclei in the centre of each cerebral hemisphere (the thalamus and basal ganglia) and the cerebral cortex. Each cerebral peduncle also contains a dark pigmented band of neurones - the substantia nigra, which is of importance in the control of voluntary movement.

On the dorsal surface the midbrain, behind the aqueduct (the analogue of central canal) exist four colliculi (small bumps- literally 'small hills'- on the surface of the tissue), two on each side, with specialised functions related to eye movements and hearing.

The two appendages that develop at the level of the thalamus are the optic cups that develop into the retinas and optic nerves of each eye. The Optic Nerve connects the retina and a specialised area of the thalamus - the lateral geniculate body or nucleus -  which  is easily visible and concerned with the processing of the visual image.

The thalamus and the area immediately below it, called the hypothalamus, are sometimes referred to as the diencephalon.  The Optic Chiasma is an X-shaped structure where some optic nerve fibres from the retinas cross over to the opposite side of the brain, and can be seen clearly at the front end of the hypothalamus in the upper diagram opposite.

The forebrain or telencephalon consists of bilateral expansions at the front of the midbrain, and form the cerebral hemispheres and thalamus of the adult.

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To the naked eye the brain and spinal cord appear a white and pinkish grey gelatinous mass, but when opened with a knife, there is a clear separation between two types of matter- grey and white.

The white matter consists of the myelinated axons of neurones, whereas the grey matter consists largely of neuronal cell boides and axonal terminals.

In the cerebral hemispheres the gray matter is on the outside of the brain and the white matter is inside.

This arrangement is reversed within the spinal cord, whose surface is white, consisting of axons passing along its length and connecting it to the brain.

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