Introduction to Nervous SystemChapter 1

The human body consists of numerous tissues and organs, which are diverse in structure and function, yet they function together and in harmony for the well-being of the body as a whole. There has to be some kind of influence that monitors and controls the working of different parts of the body. The overwhelming role in directing the activities of the body rests with the nervous system. Neuroanatomy is the study of the structural aspects of the nervous system. It cannot be emphasized too strongly that the study of structure is meaningless unless correlated with function.

The nervous system may be divided into the central nervous system (CNS), made up of the brain and spinal cord, the peripheral nervous system (PNS), consisting of the peripheral nerves and the ganglia associated with them (Figures 1.1 and 1.2, Table 1.1). The brain consists of the cerebrum, diencephalon, midbrain, pons, cerebellum and medulla oblongata. The midbrain, pons, and medulla oblongata together form the brainstem. The medulla oblongata is continuous below with the spinal cord (Figure 1.2).

The nervous system is made up, predominantly, of tissue that has the special property of being able to conduct impulses rapidly from one part of the body to another. The specialized cells that constitute the functional units of the nervous system are called neurons. Within the brain and spinal cord, neurons are supported by a special kind of connective tissue that is called neuroglia.

A neuron consists of a cell body that gives off a number of processes called neurites (Figures 1.3A and B).

The cell body is also called the soma or perikaryon. The cytoplasm contains a large central nucleus (usually with a prominent nucleolus), numerous mitochondria, lysosomes and Golgi complex (Figure 1.3B). The cytoplasm also shows the presence of a granular material that stains intensely with basic dyes called Nissl substance (also called Nissl bodies or granules) (Figure 1.3C). These bodies are rough endoplasmic reticulum (Figure 1.3B).

The processes arising from the cell body of a neuron are called neurites. These are of two kinds. Most neurons give off a number of short branching processes called dendrites and one longer process called an axon. The differences between axon and dendrite are summarized in Table 1.2 (Figure 1.3C).

The cytoplasm of neurons is in constant motion. Movement of various materials occurs through axons. This axoplasmic flow takes place both away from and towards the cell body. Axoplasmic transport of tracer substances introduced experimentally can help trace neuronal connections.

Neurons are classified based on:

Axons (and some dendrites, which resemble axons in structure) constitute what are commonly called nerve fibres. The bundles of nerve fibres found in CNS are called as tracts, while the bundles of nerve fibres found in PNS are called peripheral nerves.

Each nerve fibre has a central core formed by the axon. This core is called the axis cylinder. The plasma membrane surrounding the axis cylinder is the axolemma. The axis cylinder is surrounded by a myelin sheath. This sheath is in the form of short segments that are separated at short intervals called the nodes of Ranvier. The part of the nerve fibre between two consecutive nodes is the internode. Each segment of the myelin sheath is formed by one Schwann cell.

Peripheral nerve fibres are separated from circulating blood by a blood–nerve barrier. Capillaries in nerves are nonfenestrated and their endothelial cells are united by tight junctions. There is a continuous basal lamina around the capillary. The blood-nerve barrier is reinforced by cell layers present in the perineurium.

Peripheral nerves are classified in many ways.

Details of diameter and conduction velocity in the peripheral nerves with examples are given in Table 1.4.

The nature of myelin sheath is best understood by considering the mode of its formation (Figures 1.6A to E). An axon lying near a Schwann cell invaginates into the cytoplasm of the Schwann cell. In this process, the axon comes to be suspended by a fold of the cell membrane of the Schwann cell. This fold is called the mesaxon.

There are some axons, which are devoid of myelin sheaths and examples include postganglionic autonomic fibres and fibres carrying “slow”, burning pain. The nonmyelinated fibres are also surrounded by Schwann cells. These unmyelinated axons invaginate into the cytoplasm of Schwann cells, but the mesaxon does not spiral around them (Figure 1.8). Another difference is that several such axons may invaginate into the cytoplasm of a single Schwann cell.

A reflex action is defined as an immediate, involuntary motor response of the muscles in response to a specific sensory stimulus. For example, if the skin of the sole of a sleeping person is scratched, the leg is reflexly drawn up.

Monosynaptic: The stimulus applied to a muscle or a tendon is carried by a unipolar neuron which terminates by synapsing with an anterior horn cell supplying the muscle (Figure 1.9). Here, there are only two neurons—one afferent and the other efferent. As only one synapse is involved, the reflex is monosynaptic.

Polysynaptic: Some reflexes are made up of three (or more) neurons as shown in Figure 1.10. The central process of the dorsal nerve root ganglion cell ends by synapsing with a neuron lying in the posterior grey column. This neuron has a short axon that ends by synapsing with an anterior horn cell. Such a reflex is said to be polysynaptic.

In addition to neurons, the nervous system contains several types of supporting cells called neuroglia (Flowchart 1.2).

(Origin: Greek. Neuro = nerve + bio = life + taxis = arrangement; literally, a law governing the arrangement of neuronal cell bodies and their fibres during life).

Nervous tissue within the central nervous system, till recently, used to be considered as post-mitotic, i.e. neurons are incapable of regeneration. However, recent research has identified cells which are capable of forming new neurons as well as glial cells in the subventricular zone of lateral ventricle and in the hippocampal gyrus. These areas are known as adult neurogenic zone. These cells which are called neural stem cells are capable of self-renewal and show plasticity.

A synapse transmits an impulse only in one direction. The two elements taking part in a synapse can, therefore, be spoken of as presynaptic and postsynaptic (Figure 1.13). In some areas several neurons may take part in forming complex synapses encapsulated by neuroglial cells to form synaptic glomeruli (Figure 1.14).

The transmission of impulses through synapses involves the release of chemical substances called neurotransmitters into the synaptic cleft. Depending on the neurotransmitter (excitatory of inhibitory), the postsynaptic neuron becomes depolarized or hyperpolarized.

Each skeletal muscle fibre receives its own direct innervation. The site where the nerve ending comes into intimate contact with the muscle fibre is a neuromuscular junction. In this junction, axon terminals are lodged in grooves in the sarcolemma covering the sole plate (Figure 1.16). Acetylcholine is released when nerve 10impulses reach the neuromuscular junction. It initiates a wave of depolarization in the sarcolemma resulting in contraction of the entire muscle fibre.

The peripheral endings of afferent nerve fibres make contact with receptors that respond to various kinds of sensory stimuli.

At the time when the nervous system begins to develop, the embryo is in the form of a three-layered disc, i.e. the gastrula (Figures 1.20 and 1.21).

The brain develops from the enlarged cranial part of the neural tube (Figure 1.23A). At about the end of 4th week, the cavity of the developing brain shows three dilatations (Figure 1.23B). Craniocaudally, these are the prosencephalon (forebrain vesicle), mesencephalon 14(midbrain vesicle), and rhombencephalon (hindbrain vesicle). The prosencephalon becomes subdivided into the telencephalon and the diencephalon (Figure 1.23C). The telencephalon consists of right and left telencephalic vesicles. The rhombencephalon also becomes subdivided into a cranial part, the metencephalon, and a caudal part, the myelencephalon. The parts of the brain that are developed from each of these divisions of the neural tube are shown in Figure 1.23D and Flowchart 1.3.

The prosencephalon, mesencephalon, and rhombencephalon are at first arranged craniocaudally (Figure 1.25A). Their relative position is greatly altered by the appearance of a number of flexures. These are:

Each of the subdivisions of the developing brain encloses a part of the original cavity of the neural tube (Figure 1.26).

At the time when the neural plate is being formed, some cells at the junction between the neural plate and the rest of the ectoderm become specialized (on either side) to form the primordia of the neural crest (Figures 1.22B and C). With the separation of the neural tube from the surface ectoderm, the cells of the neural crest appear as groups of cells lying along the dorsolateral sides of the neural tube (Figure 1.22D). The neural crest cells soon become free (by losing the property of cell-to-cell adhesiveness). They migrate to distant places throughout the body. In 17subsequent development, several important structures are derived from the neural crest. These include some neurons of sensory and autonomic ganglia, Schwann cells, and the pia mater and the arachnoid mater. Many other derivatives of the neural crest are recognized in widespread tissues (Figure 1.28).

Diagnosing a neurological disease involves a thorough history-taking and physical examination aided by an array of basic to sophisticated investigations so as to anatomically localize the lesion and also to know its pathology. The investigative techniques used in the diagnosis of neurological disorders vary from plain radiography of skull and vertebral column to complex MR tractography. The neuroimaging modalities are classified based on the technique used as given Flowchart 1.4.

The basic principle in plain radiography is that the X-rays incident on bone, soft tissue or fluid/air get absorbed to a different extent and the emergent beam after such absorption reacts differently with the chemical on the X-ray plate. So, bone produces a dense white shadow, air produces a black shadow and the soft tissue produces varying shades of grey.

In this investigation, a radio-opaque dye is injected into the spinal subarachnoid space after lumbar puncture.

A radio-opaque dye is injected into the blood vessels supplying the brain, namely the internal carotid artery and the vertebral artery. This is followed by taking serial radiographs of skull to show the arterial, the capillary and the venous phases of flow of the dye which helps to visualize the normal anatomy of the arterial system (Figures 1.29 and 1.30).

This technique uses a collimated beam of X-rays which is passed circumferentially around a transverse slice of head and multiple detectors around the slice capture the emerging X-rays to produce multiple images.18

19These are then put together with the help of a computer to get an axial tomogram (Figures 1.31A and B).

This investigation uses the principle of nuclear magnetic resonance which states that the atoms of a tissue/substance oscillate and release energy when subjected to a strong magnetic field. This energy is captured in the form of an image (T1 weighted image). Later when the oscillating atoms are subjected to radiofrequency waves, the direction of oscillation changes and releases energy in another direction and a diametrically opposite image is produced (T2 weighted image).

This is based on Doppler effect in which the frequency of the ultrasound waves gets changed when they strike a moving object such as the blood flowing in a vessel.

Positron emitting radioisotopes such as ¹⁵O and ¹⁸F are used in this study. When brain tissue is scanned after administration of these isotope containing substances, high metabolic areas with more blood flow or neurons with higher glucose intake will show up as hot spots. The advantage over CT is PET gives information about function because of neuronal activity or blood flow.

This imaging technique is based on the principle of diffusion weighted imaging (DWI). Water molecules in live tissues are in constant motion due to the thermal energy carried by them, and this is called as Brownian motion. In white matter of the brain, this motion is along the longitudinal axis of the white fibres because the axolemma limits their perpendicular motion. MR signals catch these motions and different computer algorithms are used to reconstruct the fibre tracts called as diffusion tensor imaging (DTI) or MR tractography (Figure 1.32).20