7. Nervous Tissue, Overview of the Nervous System.

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1 7. Nervous Tissue, Overview of the Nervous System. This chapter touches on the essentials of nervous system and nervous tissue in the context ANHB Cells of nervous tissue : Neurons and glia. The functionality of nervous tissue rests largely on its ability of its cells (neurons) to generate, transmit and process information. Information gathered from body or its surroundings is described as sensations. Information which directs body tissues into activity is described as being motor in nature. Despite the wide variety of types of sensations, motor commands and modulatory activities, the cells of nervous tissue rely upon a single basic electrical mechanism to convey information. A comprehensive discussion of this mechanism is beyond the scope of this unit. The key points underlying this mechanism are : A complex movement of ions across the cell membrane results in a potential (voltage) difference between the interior and exterior of the cell membrane. This constant difference is called the resting membrane potential. The interior is negative compared to the exterior, and the magnitude of the difference is approximately 70 millivolts. Resting membrane potential is generally expressed as 70 mv. Resting membrane potentials are observed in cells other than neurons as well. We confine our discussion to neurons. The cell membrane of a neuron or its parts can be excited by a number of physical or chemical factors around it. When so stimulated, the cell membrane exhibits a series of electrical events. This begins with a momentary reversal of polarity ( depolarisation ) over a small area of the cell membrane the interior becomes positive compared to the exterior. Such a change is called depolarisation. Once generated, this reversal travels over successive parts of the cell membrane. As a new area of the cell membrane is depolarised, the previously depolarised part reverts to the resting state ( repolarised ). Thus there is a wave of depolarisation that travels along the cell surface. This is described as an action potential. Since action potentials are seen in other excitable cells like muscle cells as well, in case of a neuron it is also known as a nerve impulse. Besides such excitable cells or neurons, nervous tissue has a huge population of supporting cells that hold neurons together and have a variety of specialised subgroups and functions. These cells are called neuroglia (literally, nerve glue ). A detailed account of neuroglial cells is beyond the scope of this unit. Neuron : Body and processes. Receiving and transmitting information requires long extensions ( processes ) of the cytoplasm, and neurons have them in plenty. These processes may be just a fraction of a millimetre in length, while the longest processes in some neurons can be as much as a metre in length. The main mass of cytoplasm, from which these processes arise, is the body of the neuron, also called the soma. (The plural of soma is somata ). It contains the nucleus of the neuron. The bodies of neurons range from a few microns to few tens of microns. The processes of a neuron are of two types those which bring nerve impulses towards the neuron body, and those that carry them away from the neuron body. In general, those of the first type are the dendrites ( branches, due to their appearance). The second type of process is the axon. It is obvious that the bodies have to support processes far longer than their sizes. The maintenance machinery of a neuron is all in the body, from which products have to be transported over great distances to the ends of long processes. With this elementary understanding, let us see how the neuron copes with these functions in histological terms (fig. 1). Soma Axon Dendrites Fig. 1. A typical neuron. The axon is shown folded to accommodate its enormous length in the picture. 1

2 The neuron body. A neuron is a highly active cell. Its body has to produce a number of substances to be transported; and generation of electrical activity is an energy-intensive affair. The nucleus of a neuron is euchromatic, with a prominent nucleolus. In keeping with the activities, the cytoplasm is rich in ribosomes and rer. These organelles fill up almost all the cytoplasm as basophilic granules, collectively known as Nissl granules or Nissl substance. The cytoplasm is also replete with mitochondria (not shown in fig. 2). The appearance of the nucleus and the presence of Nissl substance makes larger neurons very easy to recognise in histological sections. The nucleus of a large neuron itself is comparable to some of the smaller cells in size. The prominent nucleolus is quite obvious as well. Clumps of Nissl substance stand out with their blue colour in the cytoplasm. It is worth remembering that most histological sections are 5 to 8 µm in thickness. A large neuron is often 40 or even more µm in diameter. Fig. 2. Neuron body, dendrites and a part of the axon. Dendrites. Dendrites are short, thick processes. In most neurons they indeed look like trees with branches. Nissl substance extends into the dendrites. Some neurons types have extremely complex dendritic trees. The cell membranes of the neuron body and the dendrites have similar physiological properties. Dendrites are arranged in three dimensions around the body of a neuron. In a given section we often see barely two or three dendrites in one section. Axons. The axon is a single, long process. Close to the origin of an axon from the neuron body, the body itself shows an elevation, called the axon hillock. The axon hillock lacks Nissl substance. Axons have few branches if at all, along most of their length. At its end, an axon may show a branching pattern, known as terminal arborisation. (Arbor = tree, arborisation = tree formation). It is a matter of chance to see the origin of the (single) axon of a large neuron in the plane of one section. Functioning of dendrites and axons. The sheer number of dendrites against the single axon gives us an insight into their functioning. Dendrites can collect information from a vast number of sources (other neurons), add it up and pass it along the body to the axon. An axon, in general, transmits what it receives. Functioning of dendrites and axons. The sheer number of dendrites against the single axon gives us an insight into their functioning. Dendrites can collect information from a vast number of sources, add it up and pass it along the body to the axon. An axon, in general, transmits the final outcome without modification. The structure of some neurons is an exception to the description above. Sensory neurons have long processes which bring information towards their somata. These processes ought to be called dendrites, but structurally and functionally they resemble axons. The key point here is that all such long processes are called nerve fibres. Nerve fibres conduct electrical changes at a phenomenal speed the fastest fibres do so at the speed of some 30 metres per second. The slowest fibres carry impulses at speeds of roughly 2 metres per second. Consider that 30m/sec amounts to 108 kmph close to the country highway speed limit! Even 2 m/sec equals over 7 kmph. In terms of the dimensions of our body, a nerve impulse takes just a tiny fraction of a second to reach from one point to another. The nervous system is indeed a high-speed control system. The next question we ask is, what makes some fibres conduct at high speeds, whereas some fibres conduct at low speeds. The answer lies in a further specialisation around the nerve fibres. Next page 2

3 Myelin sheath. Many nerve fibres have a fatty sheath around them. As a simple explanation we may say that it is an insulating sheath and helps in faster conduction of nerve impulses. In peripheral nerves, it is produced by supporting cells called Schwann cells. Broad processes of Schwann cells roll around a nerve fibre (fig. 3). NF NF NF Fig. 3. Schwann cell, nerve fibre and formation of myelin sheath. This is a diagrammatic representation, simplified to illustrate the basic process. Drawn with the nerve fibre in cross section. A. The Schwann cell rolls around a nerve fibre (NF). B. Myelin is deposited. C. Myelin sheath almost complete. Note the small amount of cytoplasm and the nucleus of the Schwann cell at the surface of the sheath. Note : Myelin is white. Here it shown in black for contrast. Indeed, in an osmium-stained cross section of a nerve, it is seen as black. Different nerve fibres have different diameters and the thickness of myelin sheath also differs among fibres. The proportions of their diameters shown here are just examples. As the this spiral rolling goes on, most of the cytoplasm of the Schwann cell is squeezed out, leaving the lipidladen cell membranes in contact. The outermost layer of the sheath contains the cytoplasm and the nucleus of the Schwann cells. In this process, one Schwann cell myelinates only a segment of the nerve fibre. Thus, in a myelinated fibre in the PNS, we see many such segments, with junctions between Schwann cells in between. These junctions are called nodes of Ranvier. When an electrical impulse is conducted along a myelinated nerve fibre, it jumps from node to node. Thicker the sheath, faster the conduction along a nerve fibre. Fig. 4. Transverse section of a nerve, stained with osmium tetroxide. Myelin is oxidised to a black colour, nerve fibres in the centre are not stained. Note the variation in the sizes of nerve fibres, thickness of myelin sheaths and the overall diameters of the myelinated fibres. The large black spots on the right are fat cells. Schwann cells are also associated with non-myelinated fibres. In this case, many nerve fibres can lie in grooves along a single Schwann cell. Fig. 5. Myelin sheath in a longitudinal section, with two nodes of Ranvier. (Nucleus of Schwann cell not shown). Schwann cells are present only in the peripheral nerves. In the central nervous system, myelination is done by a neuroglial cell type called oligodendrocyte. Even if some fibres in a bundle of nerve fibres are myelinated, the fatty material imparts a white colour to the entire bundle. Peripheral nerves, which are in fact bundles of nerve fibres, are white in colour (with some exceptions, where only nonmyelinated fibres are present). In the CNS, bundles of nerve fibres are usually called tracts which constitute the white matter in CNS. Collections of neuron bodies, on the other hand, have a grey appearance and form the grey matter of the CNS. Collections of neuron bodies outside the CNS are called ganglia (singular : ganglion). 3

4 Communication between neurons. Neurons communicate with each other through a novel mechanism. The structural basis of such communication is called a synapse (= coming together ). When processes of two neurons meet, there is a very fine gap between the two. This gap is called synaptic cleft, seen clearly only with an electron microscope. The terminal of the axon has a broad end with vesicles or packets of a chemical called a neurotransmitter. When an action potential arrives at the terminal of an axon, it causes release of the neurotransmitters molecules in the synaptic cleft. When the neurotransmitter reaches the membrane of the other neuron, it can either make it more susceptible to firing an action potential, or make it more reluctant to fire one. In the first case, we say that it has an excitatory effect; in the latter case we say that it is inhibitory. These effects are determined by the manner in which the receiving or post-synaptic membrane reacts to the neurotransmitter. The structure of a synapse is illustrated in fig. 6. Note that this is a Axon Neuron 1 Synaptic vesicles Dendrite or soma Neuron 2 Fig. 6. Synapse Synaptic cleft simple illustration it does not show the great variety of synapses. Further, this account does not describe varied chemical nature of neurotransmitters or the effects they have on the membrane of the next neuron. Such detail is beyond the scope of this unit. This elementary description is aimed at creating awareness of the nature of chemical transmission between neurons. Equally important, similar synaptic junctions are present between nerve fibres and muscle cells as well. Divisions of the nervous system. Central nervous system (CNS). The brain, enclosed in the skull, and the spinal cord in the vertebral canal constitute the CNS. The CNS is enclosed in three protective coverings, the meninges. The outermost covering is a tough sheet, the dura mater (the term literally means the tough mother!). The middle covering is a network of delicate strands like a spider s web, and hence called the arachnoid mater. The innermost covering, a thin, delicate membrane, follows all the small irregularities of the surface of the CNS, and is called the pia mater. (Pia = pious, faithful!). Between the arachnoid and the pia is a space (subarachnoid space) occupied by the cerebrospinal fluid. The fluid is produced in cavities inside the CNS (remember that the CNS develops as a tube!) and flows out into this space. At the top of the brain, the fluid is absorbed back into the blood stream. Fig. 7. Brain The brain itself (fig. 7) comprises the cerebrum (the largest part), the cerebellum and the brainstem. (These details are not core knowledge for this unit. However, it helps to have a comprehensive picture as a background.) The cerebrum and the cerebellum have a cortex (= shell) of grey matter, with white matter inside. Within the white matter are additional grey masses, not seen in this picture. Note : Fig. 7 shows the medial surface of the right half of the brain. The spinal cord is of considerable interest to us. It extends from the base of the skull to the first lumbar vertebra in the adult. In this unit, the nerves emerging from the spinal cord are of great interest to us. We therefore describe the nerves in conjunction with the spinal cord. Peripheral nervous system (PNS). Nerves attached to the brainstem and the spinal cord constitute the peripheral nervous system. There are twelve pairs of cranial nerves attached to the brain, ten of them to the brainstem. This is a matter of study in Neurobiology (2217) and in part, Human Functional Anatomy (2213). However, one cranial nerve is of special importance to us ; the 10th cranial nerve, named vagus. While other cranial nerves have their territory limited to the head and the neck, the vagus nerve travels all the way through the neck and the thorax into the abdomen. The nerve is in fact named vagus for this reason ( the wanderer ). 4

5 Thirty-one pairs of spinal nerves emerge from the spinal cord. They are named according to the regions of the vertebral column they are associated with : there are eight cervical nerves, twelve thoracic, five lumbar and five sacral nerves. The four vertebrae of the tail fuse to form the coccyx, and only one spinal nerve, the coccygeal is associated with it. Note that there are seven cervical vertebrae, but eight cervical nerves the first cervical nerve emerges between the skull and the atlas. While cranial nerves exhibit considerable functional variety, spinal nerves have a fairly constant pattern. This, and the anatomy of the spinal cord deserves some study. The Spinal Cord. C1 The spinal cord, like the vertebral column, reflects the segmental pattern and development of the body. There are no landmarks on the surface of the spinal cord to indicate its segmental nature. However, the fact that there are spinal nerves emerging from it at intervals, is due to its segmental structure. A segment of the spinal cord is the slice that sends one pair of spinal nerves to the body. Thus we speak of 8 cervical, 12 thoracic, 5 lumbar and 5 sacral segments of the spinal cord. In fig. 8, note that the first nerve in each region is labelled. Also note that each nerve arises by two sets of tiny roots ( rootlets ) which converge to form two roots, which in turn unite to form a spinal nerve. The two lines in the thoracic region indicate a slice of the spinal cord which gives rise to one pair of spinal nerves. Such a slice is a spinal segment. This concept is illustrated in the magnified picture in fig. 11, along with a description of spinal nerve roots T L1 7 8 S1 Fig. 9. Formation of the cauda equina. Fig. 10. A specimen of cauda equina. To begin with, the spinal cord has the same length as the vertebral column. The spinal nerves emerge through the corresponding intervertebral foramina. The vertebral column grows to a greater extent than the spinal cord, which appears to be shorter than the vertebral column. In the adult, the spinal cord ends, on an average, at the level of the disc Co between vertebrae L1 and L2 (at the lower border of L1, if that sounds simpler). The emerging spinal nerves remain anchored to the corresponding intervertebral foramina. Fig. 8. Spinal cord Below the first lumbar vertebra, we find a bundle of spinal nerve roots. At every succeeding level, one pair makes its exit. The bundle therefore grows smaller as we proceed lower down. The bundle of nerve roots appears like a horse s tail and is indeed called the cauda equina (cauda = tail, equus = horse). This is illustrated diagrammatically in fig. 9. The numbers given to the spinal cord segments in the diagram are arbitrary. The appearance of the cauda equina in a real specimen is shown in fig. 10. The long arrow shows the end of the spinal cord. The short arrow shows spinal nerves emerging through intervertebral foramina. Note how the cauda equina reduces in size as the nerves leave it. 5

6 The cauda equina floats in CSF; with a tube of dura mater surrounding it all. A needle passed anywhere below the end of the spinal cord can be used to draw a few drops of CSF for laboratory testing in many clinical situations. This is called lumbar puncture, and is best done between the spines of L3 and L4. Spinal nerves can also be anaesthetized by injecting local anaesthetic drugs outside the dura mater. In the vertebral canal, the dura mater is not fused with the bone there is a narrow potential space between the dura and the bone (epidural space). Due to this arrangement the entire dural tube can slide within the canal with the movements of the vertebral column and there is no damage to the spinal cord. The grey matter of the spinal cord is a continuous column which on a cross section is H -shaped, with ventral (anterior) and dorsal (posterior) horns. The white matter outside this carries bundles of nerve fibres up and down. Bundles of functionally similar fibres are called tracts. Just above the beginning of the spinal cord the major motor tracts cross. Damage to the spinal cord at its upper end (common in injuries to the joint complex between the skull, atlas and the axis) threatens disruption of motor control routes to the entire body below the head and can result in quadriplegia (paralysis of all four limbs). Road accidents and certain sports injuries can lead to such damage. A note on terminology. In the study of the nervous system and embryology we often use animal models. To avoid confusion between terms like anterior and inferior, we prefer to use the term ventral (towards the belly) and dorsal (towards the back) instead of anterior and posterior respectively. Formation of a spinal nerve. The nervous system gathers information from all over the body and its surroundings. Examples of sensations are touch, pain, sensation of joint position, vibration and many others. We also have special sensations like vision, sound and smell. This distinction is not rigid, as (for example) touch sensation can arise within the body but its cause may be outside the body, like a breeze moving hairs on the skin. All sensory information is analysed, interpreted and often stored in memory by the CNS. The CNS is also the source of commands to muscles in the body, bringing about movements of parts of whole of the body. A typical spinal nerve (fig. 11) has sensory and motor fibres (you need not know the single exception). Sensory fibres enter the spinal cord on its dorsal side as bundles called dorsal roots. Motor fibres leave the spinal cord via ventral roots. In other words, we say that a spinal nerve has two roots the dorsal root is sensory, the ventral root is motor. Each root is in fact made of a number of smaller bundles, the rootlets ( little roots ). Dorsal Root Ganglion Ramus rootlets Ramus Root Rootlets Ventral Fig. 11. A spinal nerve. Left : a spinal segment with a spinal nerve. Right : cross section, with vertebral canal (Vc) and dura mater (D). The two roots unite close to the intervertebral foramen to form the mixed spinal nerve. Just after its exit from the foramen, a spinal nerve divides into its first major branches, two in number; both mixed nerves. These are the primary rami (primary because they are the first branches, ramus = branch). As seen in fig. 11, the spinal nerve itself is rather short it divides into the two rami soon after its formation. It is extremely important to realise the difference between roots and rami : Roots of a spinal nerve are within the vertebral canal. The dorsal root is sensory, the ventral root is motor. The spinal nerve is a mixed nerve (has both sensory and motor fibres. The rami are both mixed nerves. They are located outside the vertebral canal. 6

7 We now put together the anatomy of a vertebra, the development of the body wall and the anatomy of a segmental spinal nerve together. Spinal nerve and its rami : correlation with the body wall. Each spinal nerve belongs to a segment of the body. Each segment has its own skeletal muscle, developed from the myotome. The segment also has its share of the dermis, formed by the dermatome. Further: A spinal nerve supplies motor nerve fibres to all the skeletal muscle of the segment and sensory fibres to the dermis (skin) of the segment. The dorsal ramus of the nerve supplies the segmental epaxial muscle and the skin covering it. The ventral ramus supplies the segmental hypaxial muscle and the skin covering it. The dorsal (sensory) root of a spinal nerve has a small swelling called the dorsal root ganglion. This ganglion has the neuron bodies of sensory neurons. The nerve fibre that comes towards this neuron body is like a dendrite (bringing information towards the neuron body). However, it is long, single and can be myelinated. It is therefore preferable to call this a peripheral process rather than a dendrite. The true axon goes towards the spinal cord (CNS) through the dorsal root; and is often called the central process. These concepts are illustrated in fig. 12. The asterisk indicates the dorsal root ganglion, with a representative neuron body shown within. A is the peripheral process bringing in sensation. It is long, Fig. 12. Dorsal root ganglion, peripheral and central processes. and may be myelinated like an axon. B is the central process (axon) of the same neuron. It is important to realise that the two processes are parts of the same neuron there is no synapse in the ganglion. Autonomic Nervous System. Since we are going to deal with internal organs in the thorax and the abdomen, it is necessary for us to have an overall idea of the organisation and function of the autonomic nervous system. The autonomic nervous system controls all involuntary functions in the body. We refer here to control by the nervous system. Certain chemical messengers also play a role in the control of internal organs, but this is beyond the scope of the current discussion. The autonomic nervous system (ANS) is anatomically not a distinct entity. Parts of the ANS are to be found in both CNS and PNS. We are largely concerned with its organisation as a part of the PNS. Clarification on some of the terms used. The ANS is often called the visceral nervous system as it deals with viscera (internal organs, singular : viscus). In contrast the nerves that supply skeletal muscle, skin or body wall structures are called somatic. Motor fibres in both are called efferent fibres (efferent = going away from). Sensory fibres in both are called afferent (afferent = coming towards). The ANS has both afferent and efferent components. Some textbooks describe only the efferent component as the ANS. This is unfortunate, as both these are essential and integrated for the normal functioning of internal organs. For our purpose in this unit, the meaning of the terms afferent and efferent as described here is satisfactory. However, in the study of neurobiology these meanings have finer distinctions. Bear this in mind if you are studying neurobiology! 7

8 We begin by describing the efferent motor component first. Even while thinking of the motor component, we come across two organisational divisions of the ANS. These are : sympathetic division and parasympathetic division. On any given organ, these divisions have opposite effects. Normal functioning of an organ depends on a balance between these two. Thus, the sympathetic division accelerates the heart and makes it beat more powerfully; while the parasympathetic slows the heart down and reduces the force of contraction. On the other hand, the parasympathetic increases the movements of the muscle in the stomach and increases the secretion of digestive juices by the stomach. Both these divisions have common anatomical features, which we describe first. Refer to fig. 13. The salient common features for both sympathetic and parasympathetic divisions are : Fig. 13. CNS (Spinal cord or brainstem) Organ Diagrammatic common plan of neurons in the efferent ANS routes. There is always a ganglion (G) between the CNS and the organ to be controlled. There are two neurons on this control route. The first neuron has its body in the brainstem or the spinal cord that is, in the CNS. The second neuron has its body in the ganglion. In the ganglion, the axon of the first neuron hands over some information to the second neuron. Thus, in an autonomic ganglion, there is a synapse. The axon of neuron 1 is before the ganglion : it is called the preganglionic fibre (X). The axon of neuron 2 is after the ganglion : it is called the postganglionic fibre (Y). Next, we consider the special features of these divisions. Sympathetic division. (Fig. 14). First neurons on the sympathetic route are located in the thoracic and upper lumbar segments of the spinal cord. Since there are five lumber segments, by upper lumbar segments we mean L1 and L2, sometimes L3. The axons of these neurons are preganglionic sympathetic nerve fibres. Since these are efferent (motor) fibres, they emerge with the corresponding ventral roots and the spinal nerves. The sympathetic division is therefore called thoracolumbar outflow. Sympathetic ganglia are located close to the vertebral column and form a chain (sympathetic chain). Many postganglionic fibres join the spinal nerve again and travel with its branches to supply blood vessels and sweat glands in the skin. However, spinal nerve above and below the thoracolumbar limits also need to carry postganglionic fibres. Therefore : The chain extends into the neck above and the lower lumbar and sacral regions below. You can see some preganglionic fibres running up into the neck and some running down in the sympathetic chain, where they synapse. Some postganglionic fibres go from the chain to the organs of the thorax and the abdomen. However : Many preganglionic fibres do not synapse in the chain ganglia they pass through the ganglia and form nerves to the organs. (These nerves are called splanchnic nerves. Splanchnic is a term of Greek origin meaning viscera). They synapse in ganglia very close to these organs. From these ganglia, postganglionic fibres follow a short course to the organs. Fig. 14. General plan of the sympathetic division. 8

9 There are other minor features which would make this description more detailed. However, these are not core material for this unit. A couple of more points will be added when we study the thorax and the abdomen. Parasympathetic division. (Fig. 15). Preganglionic fibres emerge with cranial nerves (III, VII, IX and X) and with sacral spinal nerves S2, 3 and 4. The parasympathetic division is also called craniosacral outflow. The ganglia are located very close to the organs or in the walls of the organs. We are not concerned with the details of parasympathetic fibres in cranial nerves III, VII and IX. These nerves are responsible for parasympathetic supply to structures in the head and neck. These are included in ANHB We shall discuss the vagus (X). The vagus nerve (vagus = wanderer) descends through the neck into the thorax, supplies the organs in the thorax and then passes into the abdomen. Preganglionic parasympathetic fibres travel with this nerve to the organs in the thorax and most of the organs in the abdomen. These end in ganglia close to the organs. The lower abdominal organs (the distal part of the large intestine is shown here, but also including pelvic organs) are supplied by parasympathetic fibres from the sacral spinal cord (segments S2, 3 and 4). Postganglionic fibres reach the organs through ganglia located nearby. We shall understand a few more relevant features as we study the thoracic and abdominal organs and their development. Fig. 15. Plan of the parasympathetic division. Visceral sensations and sensory nerves. The brain perceives sensations arising in different parts of the body differently. Sensations arising in the body wall structures are called somatic sensation. Body wall structures include the skin and underlying connective tissue, skeletal muscle, tendons, connective tissue associated with joints, bone and parietal layers of serous membranes. Such sensations can be described qualitatively for example, we speak of pain arising from a pinprick, a cut, or a burn differently. Touch can also be described differently if it is caused by a breeze against the hair or skin, or a hand resting on one s shoulder etc. Further, these sensations can be localised one can point to an area of touch or pain even with eyes closed. Sensations arising in internal organs including the visceral layers of serous membranes and mesenteries are called visceral sensations. Visceral sensations are rather vague ( discomfort, pain or a gripe ), cannot be easily localised to the organ in which they arise (even if you know anatomy!). Moreover, the viscera are sensitive to three main types of stimuli stretching, pressure and lack of blood supply. A number of visceral sensations arising in normal organs in fact do not even reach consciousness. They are used by the nervous system for reflex regulation of visceral function. We thus speak of two kinds of visceral sensations. Those which are a part of normal functioning and do not reach conscious level ( physiological sensations), are carried by sensory nerve fibres which accompany parasympathetic nerves. The vagus nerve that was mentioned earlier in fact carries a huge number of such 9

10 sensory nerve fibres from all the organs it supplies. Sensations which arise in abnormal situations and are perceived as pain, are carried by nerve fibres which travel with sympathetic nerves. Since sympathetic nerves are associated with specific spinal nerves, such visceral sensations are interpreted by the brain as arising from the body wall structures of the corresponding segments of the body. This is called referred pain. This is a very important concept in functional anatomy and in medical practice. For example, the heart receives its sympathetic supply largely (though not entirely) from the upper few thoracic segments. Sensory fibres from the heart therefore enter the spinal cord at these segments. Pain arising from a heart attack (which is loss of blood supply to a part of the heart), is therefore referred to the parts of the thoracic wall or the inner side of the arm, which are areas of the body wall supplied by the same segmental spinal nerves. We shall elaborate upon this concept as we study the thoracic and abdominal organs. *********************************************** 10