Nervous System

Chapter 1

Nervous System 

Introduction

The nervous system has three basic functions :

1. Sensory 

2. Integrative 

3. Motor

• The sensory function senses changes within and outside the body.
• Integrative function analyses the sensory information, stores some of it and makes decisions regarding appropriate actions
• Motor function responds to stimuli by initiating muscular contractions and glandular secretions.

• The two principal divisions of the nervous system are the Central Nervous System (CNS) and the Peripheral Nervous System (PNS). The CNS consists of the brain and spinal cord. The PNS consist of cranial nerves coming from the brain and spinal nerve coming from the spinal cord. The PNS is further subdivided into Somatic Nervous System (SNS) and Autonomic Nervous System (ANS). The SNS consists of sensory neurons, which convey information from cutaneous and special sense receptors from the head, body wall and limbs to the CNS. Motor neuron conducts impulses to skeletal muscles only. This portion of SNS is voluntary.

• The ANS consists of sensory neurons which convey information from receptors in the viscera to the CNS. Motor neurons from CNS conduct impulses to smooth muscle, cardiac muscle and glands. ANS is involuntary in nature. ANS is further subdivided into two branches; sympathetic division and parasympathetic division.

Neuroglia and Neurons 

Neuroglia

Neuroglia fills up about half the CNS. They are generally smaller than neurons and are more numerous in number. There are six types of neuroglia. Four types, i.e., viz. osteocytes, oligodendrocytes, microglia and ependymal cells are found in CNS. Remaining two types, i.e., neurolemmocites or Schwann cells and satellite cells exist in PNS.

•  Osteocytes: They participate in the metabolism of neurotransmitters, maintain proper balance of potassium ions, participate in brain development, help to form the blood-brain barrier and provide a link between neurons and blood vessels.
• Oligodendrocytes: These are the most common glial cells in CNS. They form supporting network by twining around neurons and produce a lipid and protein wrapping called a myelin sheath.
•  Microglia: These are small, phagocytic neuralgia derived from monocytes. They protect the CNS from disease by engulfing infective microbes and clearing away debris from dead cells. 

•  Ependymal cells: These are epithelial cells. They line the fluid-filled ventricles, cavities within the brain and the central canal. They form the cerebrospinal fluid and assist its circulation.

•  Neurodermatitis or Schwann cells: They produce myelin sheaths around PNS neurons. 

Neurons 

Neurons consist of three main parts: 

1. Cell body

2. Dendrites and 

3. Axon 

 The cell body contains a nucleus surrounded by cytoplasm that includes lysosomes mitochondria and Golgi complex. Other structures are characteristic of neurons: chromatophilic substance and neurofibrils. The chromatophilic substance (Nussle bodies) is an orderly arrangement of rough endoplasmic reticulum. Newly synthesized proteins replace the old ones and are used for growth of neurons and regeneration of damaged peripheral.

Classification of Neurons

Structurally neurons are classified into three subtypes:

 1. Unipolar neurons: They have just one process extending from the cell body and are always sensory neurons. The singular process branches off into two pathways; one having dendrites is sensory in nature and another having axon is motor in nature.

2. Bipolar neurons: They have one main dendrite and one axon. They are found in the retina of the eye, inner ear, and olfactory area of the brain.

3. Multipolar neurons: They have several dendrites and one axon. Most of them are located in the brain or spinal cord. 

Functionally neurons are classified as:

 1. Sensory or afferent neurons: They transmit sensory nerve impulses from receptors in the skin, sense organs, muscles, joints and viscera into the CNS. 

2. Motor or efferent neurons: They convey motor nerve impulses from the CNS to effector organs like muscles or glands.

• All other neurons are termed as association neurons or interneurons. Most neurons in the body, probably 90 per cent, are association neurons. Spinal and cranial nerves contain fibres in seven functional categories; four sensory and three motor.
• General somatic sensory neurons convey impulses for senses of pain, temperature, touch, vibration; and pressure from skin, muscles, and joints.
• Special somatic sensory neurons relay impulses for the special senses of vision, hearing and balance.
• General visceral sensory neurons are autonomic neurons that convey information from the viscera, such as distention of organs and chemical conditions within the body. 
• Special visceral sensory neurons convey impulses for the special senses of taste and smell. 

Grey and White Matter

In the brain or spinal cord, some regions look white whereas others appear grey. White matter refers to aggregations of myelinated processes from many neurons. The white colour is due to myelin. The grey matter consists of neurons cell bodies, dendrites, axon terminals or bundles of unmyelinated axons and neuralgia. It is greyish because it does not contain myelin.

Neurophysiology  

• Neurons are excitable cells. A normal excitable cell exhibits a membrane potential, indicating voltage difference across the membrane. In an unexcited cell, the potential is termed as resting membrane potential'. On excitation, a nerve cell gets depolarized, causing 'action potential'. Generation of action potential indicates that a cell is transmitting an impulse. Transmission of the impulse is due to movement of ions across the nerve cell membrane. The principal ions involved in the movement are sodium (Na+) and potassium (K+). In the resting state, there is a continual tendency for these ions to diffuse along their concentration gradients, i.e. Outwards and Nat inwards. On stimulation, Nations enter into neurons from the external fluid, K ions come outwards resulting into action potential. 

• Depolarization is very rapid, occurring in a few milliseconds. It passes from the point of stimulation in one direction only. Following depolarization of a nerve cell, it gets depolarized gradually. The time for the cell to get depolarized is termed as refractory period during which restimulation of a cell is not possible. During this stage action of the sodium pump expels sodium ions from the cell in exchange for potassium.

Neurotransmission  

• There is no anatomical continuity between two neurons. The gap between them is termed as synapse. At its free end the axon of a neuron breaks up into small branches termed as synaptic knobs. They are in close proximity to the dendrites of the next neurons. The space between them is called as synaptic cleft. At the ends of synaptic knobs there are spherical synaptic vesicles which contain neurotransmitters. On stimulation of a nerve, synaptic vesicles empty specific neurotransmitters into the synaptic cleft. There are various kinds of neurotransmitters. Some are excitatory in nature while some are inhibitory in nature. The neurotransmitters interact with specific receptors to cause specialized biological effects. Substances like acetylcholine, noradrenaline, dopamine, serotonin (5-hydroxytryptamine), enkephalins, raminobutyric acid (GABA) exist as neurotransmitters

• The specialized structures present on the surface of effector cells are termed as receptors. They are responsible for causing the biological effects. There are different types of receptors for various neurotransmitters. Each type of receptor mediates certain sets of biological activities. The major receptors for acetylcholine are (a) Muscarinic and (b) Nicotinic. Muscarinic receptors are present on smooth muscles while nicotinic receptors are present on skeletal muscles and ganglia. Major receptors for noradrenaline and adrenaline are a and B. Noradrenaline primarily acts on a-receptors while adrenaline primarily acts on B-receptors. While a-receptors are mainly located in blood vessels, B-receptors are mainly located on the heart.

Nerve Endings

• At the end sensory nerves lose myelin sheath and get divided into fine branching filaments. The filaments are stimulated by touch, pain, heat or cold and the impulse is transmitted to the brain. The motor nerves divide into filaments and terminate in minute pads termed as motor end plates. At this point they lose myelin sheath and every muscle is stimulated through a single motor and plate. One motor neuron may have several motor end-plates. The connection between the nerve and the muscle is termed as neuromuscular junction. (See Fig. 1.5). Acetylcholine is the neurotransmitter in the neuromuscular junction. The group of muscle fibres and the motor end-plates is collectively termed as a motor unit.

Membranes Covering the CNS 

• The brain and spinal cord are completely surrounded by three membranes lying between the skull and brain and also between the vertebrae and the spinal cord. The layers are termed as meninges. They are named as:

1.Dura mater 

2. Arachnoids mater

3. Pia mater

• The dura and arachnoids maters are separated by a potential space, the subdural space. The arachnoids and pia maters are separated by the subarachnoid space, containing cerebrospinal fluid.  

Ventricles of the Brain  

Dura Mater: Arachnoids Mater Pons varoli of spinal cord 18+ lumbar vertebra Medulla oblongata Spinal cord Central canal of spinal cord Termination Filum terminale Coccyx Sacrum Fig. 1.6: The Meninges Covering the Brain and Spinal Cord It consists of two layers of fibrous tissue. The outer layer is on the inner surface of skull bones and the inner layer provides protective covering for the brain. Venous blood from the brain drains into venous sinuses between the layers of dura mater. Spinal dura mater forms a loose sheath round the spinal cord.

Arachnoids Mater: Pons varoli of spinal cord 18+ lumbar vertebra Medulla oblongata Spinal cord Central canal of spinal cord Termination Filum terminale Coccyx Sacrum Fig. 1.6: The Meninges Covering the Brain and Spinal Cord It consists of two layers of fibrous tissue. The outer layer is on the inner surface of skull bones and the inner layer provides protective covering for the brain. Venous blood from the brain drains into venous sinuses between the layers of dura mater. Spinal dura mater forms a loose sheath round the spinal cord. It is a delicate serous membrane between the dura and pia maters. It is separated from the dura mater by the subdural space, and from pia mater by the subarachnoid space, containing cerebrospinal fluid. It continues to envelop the spinal cord and ends by merging with the dura mater at the level of second sacral vertebra.

Pia Mater: It is a fine connective tissue containing many minute blood vessels. It closely covers the brain including convolutions and each fissure. It continues to cover the spinal cord also.

Cerebrospinal Fluid (CSF)  

Within the brain there are four irregular-shaped cavities called as ventricles of the brain:

(a)The Lateral Ventricles: These cavities lie within the cerebral hemispheres, one on each side of the median plane just below the corpus callosum. They are separated from each other by a thin membrane, the septum lucidum, and are lined with ciliated epithelium. They are connected to the third ventricle by interventricular foramina.

 (b)Third Ventricle: It is a cavity situated below the lateral ventricles between the two parts of the thalamus. It is connected to the fourth ventricle by a canal, the cerebral aqueduct or aqueduct of the midbrain.

(c)Fourth Ventricle It is a cavity situated below and behind the third ventricle, between the cerebellum and pons varolli.

Pons varolli: It is continuous below as the central canal of this spinal cord and communicates with the subarachnoid space. Cerebrospinal fluid enters the subarachnoid space through these openings and through the open distal end of the central canal of the spinal cord. 

Brain 

It is the largest part of the brain and occupies anterior and middle part of the cranial cavity. It is divided by a deep cleft into right and left cerebral hemispheres. Deep within the brain, the hemispheres are connected by a mass of nerve fibres called corpus callosum. The superficial part of the cerebrum is composed of nerve cell body, forming cerebral cortex. 

The cerebral cortex shows many enfolding of varying depth. The exposed areas of the fold are called as gyro or convolutions and are separated by sulci or fissure. The convolutions help in increasing surface area of the cerebrum.

Each hemisphere of the cerebrum is divided into the following lobes:

Frontal 

Parietal 

Temporal 

Occipital

 Within the cerebrum the lobes are connected by masses of nerve fibres, or tracks, which constitute white matter of the brain. Various afferent (incoming) and efferent (outgoing) fibres linking the different parts of the brain and spinal cord are as follows:

• Arcuate (association) fibres: These fibres connect different parts of a cerebral hemisphere by extending from one gyro to another.

• Commissural fibres These fibres connect corresponding areas of the two cerebral hemispheres. The largest commissural is called as Corpus callosum.

• Projection fibres: These fibres connect the cerebral cortex with grey matter of lower parts of the brain and with the spinal cord. Internal capsule is one of the projection fibres. It lies deep within the brain between the basal ganglia and the thalamus. All nerve impulses passing to and from the cerebral cortex are carried by fibres that form the internal capsule. Motor fibres within the internal capsule form the pyramidal tracts (corticospinal tracts) that decussate at the medulla oblongata.

Brain Stem  

• Mid brain: It is situated around the cerebral aqueduct between the cerebrum above and the Pons varolli below. It consists of groups of nerve cells and nerve fibres. It connects the cerebrum with lower parts of the brain and the spinal cord. The nerve cells act as relay stations for the ascending and descending nerve fibres.

• Pons varolli: It is situated in front of the cerebellum, below the midbrain and above the medulla oblongata. It consists of nerve fibres forming a bridge between the two hemispheres of the cerebellum, and of fibres passing between the higher levels of the brain and the spinal cord. Few groups of cells within the Pons act as relay stations. Some of them are associated with the cranial nerves. The nerve cells in the Pons lie deep and the nerve fibres are on the surface. 

• Medulla oblongata: It extends from the Pons varolli above and is continuous with the spinal cord below. It is shaped like a pyramid with its base upwards and it lies within the cranium above the foramen magnum. Its anterior and posterior surfaces are marked by central fissures. Some cells of medulla oblongata constitute relay stations for sensory nerves passing from the spinal cord to the cerebrum. Following vital centres which are associated with autonomic reflex activity lie in its deeper structure. 

1. Cardiac centre
2. Respiratory centre
3. Vasomotor centre
4. Reflex centres of vomiting, coughing, sneezing and swallowing.

Medulla oblongata has the following special feature:

• Decussating of the pyramids: Motor nerves descending from the motor area in the cerebrum to the spinal cord in the pyramidal or corticospinal tracts, cross from left side to right and vice versa. These tracts are the main pathways for impulses to voluntary, i.e. skeletal muscles.  

• Sensory decussating: Some of the sensory nerves ascending to the cerebrum from the spinal cord cross from left side to right and vice-versa in the medulla oblongata. Few other sensory nerves decussate at the level of spinal cord.

• The cardiac centre: It controls the rate and force of cardiac contraction. Sympathetic and parasympathetic nerve fibres starting from the medulla pass to the heart.

Spinal cord 

• It is the elongated and almost cylindrical part extending from brain just below medulla oblongata. It is suspended in the vertebral canal and is surrounded by meninges and cerebrospinal fluid.

• It extends from the first cervical vertebra to the lower border of first lumbar vertebra. It is approximately 45 cm long and about the thickness of the little finger. Except for the cranial nerves, the spinal cord is the nervous tissue link between the brain and the rest of the body. Motor nerves originating from the brain descends through the spinal cord and supply to various organs and tissues at appropriate levels of the cord. Sensory nerves from different organs and tissues enter and pass upwards to the brain via the spinal cord. Some activities like spinal reflexes are independent of the brain. In such cases motor action is decided and implemented at the level of spinal cord itself. In order to facilitate spinal reflexes, there are extensive neuron connections between sensory and motor neurons at the same or different levels in the spinal cord.

• The arrangement of grey matter in the spinal cord resembles the shape of the letter H, having two anterior, two posterior and two lateral columns. The area of grey matter lying transversely is the transverse commission and is pierced by the central canal. The canal extends from the fourth ventricle in the brain and contains cerebrospinal fluid. The spinal cord consists of the following cell bodies.

1. Sensory cells receiving impulses from the periphery of the body. 
2. Lower motor neurons transmitting impulses to the skeletal muscles. 
3. Spinal nerve (Mixed) Connector neurons, linking sensory and motor neurons at the same or different levels.

• Posterior columns of grey matter are composed of cell bodies which are stimulated by sensory impulses from the body. The nerve fibres of these cells form white matter and transmit the sensory impulses to the brain.
• Anterior columns of grey matter are composed of the cell bodies of the lower motor neurons; they are stimulated by the axons of the upper motor neurons or by the cell bodies of connector neurons. All sensory nerve fibres pass through posterior root ganglia. These ganglia promote onward movement of nerve impulses from the periphery.