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13. 4. 2015 Kaan Yücel M.D., Ph.D. http://fhs122.org [email protected] Dr. Kaan Yücel fhs122.org Introduction to neuroanatomy & Spinal cord I do not know the question, but the brain is the answer . As they say “You are what you eat!”, but it should be ”You are what your nervous system permits”. From the moment your day starts and to the moment the next day starts, we are under the control of the magic box: brain. The humans and animals interact with the world. They also need all the systems in the body should be working in harmony. Any stimulus is received by the nervous system. It can originate from the outer world or the inner world inside your body. Some of these stimuli do not reach your consciousness level. Think of a man trying to get your attention at a street! He is shouting at you! He might be swearing at you! Not good! Or he may be actually trying to give your wallet he has just found at the restaturant you just left! That is good! As you see the stimulus (what the man is saying, his apperance, what is in his hand; a knife or your wallet?) is received and your organism plans an action. You respond to that stimulus. As you can see here, there are two ends; get the stimulus and respond. But there is also other complex processes. Your memory, moral values, emotions, prejuduice, planning, reasoning….etc… Welcome to the World of the Neurons & BraiN! There are two types cells in the nervous system. Neurons & Neuroglia Neuron: The functional unit of the nervous system The best estimate is that the human brain contains about 1011 neurons (100 billion neurons). Although nerve cells can be classified into different types, they share many common features. The structural base of the functional unit of the nervous system is simple and similar in all neurons. There is the cell body. There is the nucleus embedded within the cell body. The cell body is bounded externally by a plasma membrane. The cell bodies of the small granular cells of the cerebellar cortex measure about 5 µm in diameter, whereas those of the large anterior horn cells may measure as much as 135 µm in diameter. The fibers exiting from the neuron and coming to the neuron are called neurites. When we say neurite we understand two kinds of projections from the cell body of a neuron: A long, always single axon and a bunch of dendirites. The dendirites are fibers bringing information to the neuron from the neurons around. The axon carries the information to other neurons or structures. The bundles of axons (tract or peripheral nerve) looks white thanks the myelin sheath it is covered by. Synapse is the connection of a neuron with the neurites or the cell body of another neuron. The neuron conveying the signal is called the presynaptic neuron. The neuron receiving the signal is called the postsynaptic neuron. The space between these two structures is called the synaptic cleft. Another feature of this neuronal connection is the presynaptic terminal. Most of the time, the signal is transmitted from one neuron to another via neurotransmitters. Rarely, the connection is electrical. Popular neurotransmitters are acetylcholine (movement, memory), adrenaline (sympathetetic system), serotonin (mood), dopamine (addiction). Types of neurons Unipolar neurons are round. The cell body has one single neurite. This neurite gives branches. It gives off an axon and a series of dendrites. There are no dendirites exiting from the cell body. These neurons are found in some ganglia of the autonomic nervous system. Bipolar neurons possess an elongated cell body. From each end a single neurite starts. Examples of this type of neuron are found in the retinal bipolar cells and the cells of the sensory cochlear and vestibular ganglia, and olfactory bulb. Pseudo-unipolar neurons is a special form of bipolar neurons. One neuron has one short neurite with two branches. One goes to the periphery. The other branch goes centrally. They are sensory neurons. The peripheral branch brings the touch, pressure, pain and temperature sensations to the neuronal body. The central branch conveys this information to the spinal cord. Examples of this form of neuron are found in the dorsal root ganglion. Multipolar neurons have a number of neurites arising from the cell body. With the exception of the long process, the axon, the remainder of the neurites are dendrites. Most neurons of the brain and spinal cord are of this type. 1 Dr. Kaan Yücel fhs122.org Introduction to neuroanatomy & Spinal cord The boss sits upstairs. So is the boss of the body; the brain Nerve fiber is the name given to an axon (or a dendrite) of a nerve cell. Bundles of nerve fibers found in the central nervous system are often referred to as nerve tracts; bundles of nerve fibers found in the peripheral nervous system are called peripheral nerves. Two types of nerve fibers are present in the central and peripheral parts of the nervous system: myelinated and nonmyelinated. A myelinated nerve fiber is one that is surrounded by a myelin sheath. The myelin sheath is not part of the neuron but is formed by a supporting cell. The smaller axons of the central nervous system, the postganglionic axons of the autonomic part of the nervous system, and some fine sensory axons associated with the reception of pain are nonmyelinated. Neuroglia (glia) glia, Gr. glue There are between 10 and 50 times more glial cells than neurons in the CNS. Glial cells have other roles than processing information. The neuroglia are non-neural supporting cells. They are considered as specialized connective tissue cells of the nervous system. The neurons have te feature of excitability. The neuroglia do not have this feature. The neuroglia are known as macroglia, ependymal cells and microglia. The microglia are the macrophages of the nervous system. 1- They serve as supporting elements. They provide firmness and structures to the brain. They also separate and occasionally insulate groups of neurons from each other. 2- Oligodendrocyte in the CNS forms myelin. Myelin is the insulating sheath that covers most large axons. 3- Some glial cells remove debris after injury or neuronal death. 4- Some glial cells take up and remove chemical transmitters released by neurons during synaptic transmission. 5- Some glial cells have nutritive functions for nerve cells. Central Nervous System (CNS) & Peripheral Nervous System The nervous system is divided into two pars anatomically. This classification depends on the locations. The central nervous system and peripheral nervous system. The nervous system is also divided into two parts functionally. The somatic nervous system (skin & skeletal muscles; voluntary actions) and autonomic nervous system (involuntary actions). Central Nervous System (CNS) Structural (Morphological) division of the central nervous system: Brain & Spinal cord Brain in the cranium 1. Forebrain (Prosencephalon) Telencephalon Diencephalon 2.Midbrain (Mesencephalon) 3. Hindbrain (Rhombencephalon) Metencephalon (Pons & Cerebellum) Myelencephalon (Medulla oblongata-Medulla; Bulbus; Omurilik soğanı) Spinal cord (Medulla spinalis) in the vertebral column Brain stem (Truncus encephali) : Midbrain (Mesencephalon) + Pons + Medulla oblongata As you see the brain is divided into five different regions/parts. They are grouped under three bigger parts (forebrain, midbrain and hindbrain). These three major parts have embriyological references. The brain doubles its weight when the human being becomes 2 years old. In adult, it weighs between 1250-1450 gr. Nucleus Group of neurons coming together for the same function in the central nervous system Tract Group of axons coming together as bundles for the same function Fasciculus (less common) White matter (Substantia alba; beyaz cevher): where the axons are (looks brighter thanks to the myelin sheath) Grey matter (Substantia grisea; gri cevher) where the neurons are (looks darker) thanks to pigments within. The grey matter is outside in the brain. The grey matter is inside, the white matter is outside in the spinal cord. 2 Dr. Kaan Yücel fhs122.org Introduction to neuroanatomy & Spinal cord A neuron can be located in the cortex of the cerebral hemispheres forming a sheath. This sheath of grey matter is also called pallium (wall, kabuk). Under the cortex there is the white matter of the hemispheres. It is formed by the axons of the neurons in the cortex. A neuron might be located in the subcortical structures forming nuclei. The concept of “subcortical” mean under the hemispheres level. Thalamus, basal ganglia are all subcortical structures. A neuron can be also in groups in the periphery. This group of neurons outside the CNS is called ganglion. Some neurons are motor neurons. They carry information from the CNS to the parts of CNS or to the PNS. Some are sensory neurons. They carry information to the CNS. Some neurons are interneurons. They set the connections between the neurons of the CNS. They outnumber the motor and sensory neurons. Peripheral Nervous System (PNS) The peripheral nervous system is composed of 12 pairs of cranial nerves, 31 pairs of spinal nerves, their associated ganglia, and nerve fibers of the autonomic system. The PNS is in physical continuity with the CNS. The cell bodies of many of the nerve fibers (axons) of the PNS are located in the CNS. SOMATIC NERVOUS SYSTEM The somatic nervous system is under the individual’s conscious control. The somatic nervous system is involved in sensory information coming from the outer world, and from the skin, joints and muscles and motor impulses to the striated muscles. It is the autonomic system involved in regulating the involuntary actions (under the control of the brain), i.e. smooth muscles, cardiac muscles and glands. “Peripheral nerves” is a collective term for the cranial and spinal nerves. Each peripheral nerve consists of parallel bundles of nerve fibers. There are 31 pairs of spinal nerves exiting from the spinal cord. There are 12 pairs of cranial nerves exiting from the brain and brainstem. The cranial nerves pass through openings (foramine or fissures) in the skull. Some of these nerves are composed entirely of afferent nerve fibers bringing sensations to the brain (CNI; olfactory,CN II optic, and CN VIII; vestibulocochlear), others are composed entirely of efferent fibers (CN III; oculomotor, CN IV; trochlear, CN VI; abducent, CN XI; accessory, and CN XII; hypoglossal), while the remainder possess both afferent and efferent fibers (CN V; trigeminal,CN VII; facial, CN IX; glossopharyngeal, and CN X; vagus). All cranial nerves except vagus (CN X) supply the head and neck. The vagus nerve also supplies organs in the thorax and abdomen. The peripheral nerves are classified as afferent and efferent fibers. The afferent fibers carry the sensory information coming from the receptors to the central nervous system. The efferent fibers carry the motor responses to the effector organs from the central nervous system. EFFERENT FIBERS Somatic efferent fibers Visceral (autonomic) efferent fibers Somatic efferent fibers (Somatic motor- Somatomotor) fibers go to the striated muscles. They end in the motor end plate. The motor end plate is in the nerve-muscle juction. The cell bodies of the somatomotor fibers either originate from the neurons in the ventral horn of the spinal cord or somatic neurons in the brain stem. They are called as general somatic efferent (GSE) fibers. Special visceral efferent nerves (fibers): Muscles of the pharynx, larynx, soft palate, facial muscles, muscles of mastication and muscles in the middle ear Visceral (autonomic) efferent fibers go to the heart muscle (cardiomotor), smooth muscles in the organs (visceromotor), smooth muscles in the walls of vessels (vasomotor), smooth muscles in the roots of the hair follicles (pilomotor), glands in the digestive system (secretomotor). They are called as general visceral efferent (GVE) fibers. AFFERENT FIBERS Somatic afferent fibers Visceral afferent fibers Somatic afferent fibers are sensory fibers. They bring information from the skin, muscle, tendon and joints. The sensory information they bring: touch, pressure, pain, temperature and proprioception. This sensory 3 Dr. Kaan Yücel fhs122.org Introduction to neuroanatomy & Spinal cord information is perceived by the receptors in the body and is carried into the spinal cord. The spinal cord sends this information to the upper levels in the central nervous system. Somatic afferent fibers are called as general somatic afferent (GSA) fibers. General visceral afferent (GVA) fibers carry information perceived in the viscera. This sensory information originates from the organs, glands and membranes. The information is carried to the spinal cord. The information can be on pain due to stretching, spasm. Special visceral afferent (SVA) fibers carry special sensory information unique to the organs (such as nose and tongue). Examples are gustatory (taste) and olfactory (smell) sensations. SENSORY INFORMATION The superficial sensory information is divided into two parts (Source # 4). One is the protopathic sensation. These are simple sensations. The four: 1) Touch 2) Pressure 3) Temperature 4) Pain. In protopathic sensations, you can define the degree of the sensation roughly. The other one is the epicritic sensation. The epicritic sensation is a discriminative sensation. You can define small differences between heat for example. You can define the difference in the heat between 10 0 and 200. You can tell the differences between the pain stimuli. One epicritic discrimation is “two-point discrimination". Think of a compass (pergel) touching your tongue. As the receptors are dense there, you can feel the two points of touching separetely until 1.4 mm. Less than 1.4 mm, you feel the compass at one point of contact. In the back of the neck this distance is 36.2 mm. The superficial sensations are carried from the exteroreceptors located in the skin or just under the skin. The discriminative (fine) touch defines the type, intensity, localization of the touched site. The crude touch is a light touch on the skin. An example is the shirt touching the skin lightly. There is no discriminative feature in crude (nondiscriminative) touch. The deep sensory information (sensations) is proprioceptive, vibration and deep muscle pain. Their receptors are located in the muscles, tendons, joint capsule and ligaments. The proprioceptive receptors are also located in the inner ear. They give information about our location in the space. They are very important for maintaining our balance. Not all the proprioceptive sensations reach our concious (cortex). Vibration sensation is the repetitive sensations of pressure. The visceroreceptors (interoceptors) carry the visceral sensory information. They are located in the blood vessels, walls of the organs, and in glands as free nerve endings. They carry sensory information such as pain, pressure, chemical changes, thirst, stetching, etc. The sensations are carried by somatic afferent fibers in the spinal and cranial nerves from the abundant receptors in the skin. These receptors are simply the dendirites of the afferent neurons (Source # 4). RECEPTORS An individual receives impressions from the outside world and from within the body by special sensory nerve endings or receptors. Sensory receptors can be classified into six types according to the sensation they receive (Source #1): Mechanoreceptors. respond to mechanical deformation. Thermoreceptors respond to changes in temperature; some receptors respond to cold and others to heat. Nociceptors respond to any stimuli that bring about damage to the tissue. Photoreceptors The rods and cones of the eyes are sensitive to changes in light intensity and wavelength. Chemoreceptors respond to chemical changes associated with taste and smell and oxygen and carbon dioxide concentrations in the blood. Osmoreceptors are sensitive to differences in the osmotic pressure. AUTONOMIC NERVOUS SYSTEM The autonomic nervous system is distributed throughout the central and peripheral nervous systems. It is divided into two parts, the sympathetic and the parasympathetic and, as emphasized earlier, consists of both afferent and efferent fibers. This division between sympathetic and parasympathetic is made on the basis of anatomical differences, differences in the neurotransmitters, and differences in the physiologic effects. Both the sympathetic and parasympathetic divisions produce opposite effects in most organs and are thus considered as physiologic antagonists. However, it must be stated that both divisions operate in conjunction with one another, and it is the balance in the activities that maintains a stable internal environment. 4 Dr. Kaan Yücel fhs122.org Introduction to neuroanatomy & Spinal cord The sympathetic system is the larger of the two parts of the autonomic system and is widely distributed throughout the body, innervating the heart and lungs, the muscle in the walls of many blood vessels, the hair follicles and the sweat glands, and many abdominopelvic viscera. The function of the sympathetic system is to prepare the body for an emergency. The heart rate is increased, arterioles of the skin and intestine are constricted, arterioles of skeletal muscle are dilated, and the blood pressure is raised. There is a redistribution of blood; thus, it leaves the skin and gastrointestinal tract and passes to the brain, heart, and skeletal muscle. In addition, the sympathetic nerves dilate the pupils; inhibit smooth muscle of the bronchi, intestine, and bladder wall; and close the sphincters. The hair is made to stand on end, and sweating occurs. The activities of the parasympathetic part of the autonomic system are directed toward conserving and restoring energy. The heart rate is slowed, pupils are constricted, peristalsis and glandular activity is increased, sphincters are opened, and the bladder wall is contracted. The connector nerve cells of the parasympathetic part of the autonomic nervous system are located in the brainstem and the sacral segments of the spinal cord. Those nerve cells located in the brainstem form nuclei in the following cranial nerves: the oculomotor (parasympathetic or Edinger-Westphal nucleus), the facial (superior salivatory nucleus and lacrimatory nucleus), the glossopharyngeal (inferior salivatory nucleus), and the vagus nerves (dorsal nucleus of the vagus). The axons of these neurons are myelinated and emerge from the brain within the cranial nerves. The sacral part of the parasympathic systems’s nerve cells are found in the gray matter of the second, third, and fourth sacral segments of the spinal cord. The enteric system has beenconsidered as the third part of the autonomic nervous system. PERIPHERAL NERVE PLEXUSES Peripheral nerves are composed of bundles of nerve fibers. In their course, peripheral nerves sometimes divide into branches that join neighboring peripheral nerves. A network of nerves, called a nerve plexus, is formed. The formation of a nerve plexus allows individual nerve fibers to pass from one peripheral nerve to another. A plexus thus permits a redistribution of the nerve fibers within the different peripheral nerves. At the root of the limbs, the anterior rami of the spinal nerves form complicated plexuses. The cervical and brachial plexuses are at the root of the upper limbs, and the lumbar and sacral plexuses are at the root of the lower limbs. This allows the nerve fibers derived from different segments of the spinal cord to be arranged and distributed efficiently in different nerve trunks to the various parts of the upper and lower limbs. Cutaneous nerves, as they approach their final destination, commonly form fine plexuses. They also permit a rearrangement of nerve fibers before they reach their terminal sensory endings. The autonomic nervous system also possesses numerous nerve plexuses that consist of preganglionic and postganglionic nerve fibers and ganglia. Large collections of sympathetic and parasympathetic efferent nerve fibers and their associated ganglia, together with visceral afferent fibers, form autonomic nerve plexuses in the thorax, abdomen, and pelvis. GANGLIA Neurons are found in the brain and spinal cord and in ganglia. The sensory ganglia of the posterior spinal nerve roots and of the trunks of the trigeminal, facial, glossopharyngeal, and vagal cranial nerves have the same structure. The autonomic ganglia (sympathetic and parasympathetic ganglia) are situated at a distance from the brain and spinal cord. They are found in the sympathetic trunks, in prevertebral autonomic plexuses (e.g., in the cardiac, celiac, and mesenteric plexuses), and as ganglia in or close to viscera. MENINGES, VASCULATURE & CSF The brain in the skull and the spinal cord in the vertebral column are surrounded by three protective membranes, or meninges: the dura mater, the arachnoid mater, and the pia mater. The space between the skull and dura mater is epidural space. The space between the dura mater and arachnoid mater is subarachnoid space. The cerebrospinal fluid is here. 5 Dr. Kaan Yücel fhs122.org Introduction to neuroanatomy & Spinal cord The dura mater of the brain is conventionally described as two layers: the endosteal layer and the meningeal layer. These are closely united except along certain lines, where they separate to form venous sinuses. The venous sinuses of the cranial cavity are situated between the layers of the dura mater. The main function of the dural sinuses is to receive blood from cerebral veins and the cerebrospinal fluid from the subarachnoid space. The blood in the dural sinuses ultimately drains into the internal jugular veins in the neck. The brain gets 15 % of the cardiac output. It uses 20% of the oxygen (Source # 5). The brain is supplied by the two internal carotid and the two vertebral arteries. The four arteries lie within the subarachnoid space, and their branches anastomose on the inferior surface of the brain to form the circle of Willis. The ventricles are four fluid-filled cavities located within the brain. These are the two lateral ventricles, the third ventricle, and the fourth ventricle. The ventricles are lined throughout with ependyma and are filled with cerebrospinal fluid (CSF). As you see there is no first and second ventricle. They are the right and left lateral ventricles. The lateral ventricles are located in the telencephalon. Lobes, basal ganglia and lateral ventricles are parts of the telencephalon. The third ventricle is between the right and left halves of the diencephalon. The fourth ventricle is between the pons and medulla anteriorly, and cerebellum posteriorly. The CSF is finally drained into the dural sinuses and reaches the internal jugular vein. The CSF can be considered as a pillow of liquid around the brain and the spinal cord. Thanks to CSF, the skull feels less of the brain it is carrying. SPINAL CORD part of the CNS in the superior 2/3 of the vertebral canal It is roughly cylindrical in shape, and is circular to oval in cross-section. Internally, the cord has a small central canal surrounded by gray and white matter. The spinal cord extends from foramen magnum to lower border of L1 (first lumbar vertebra). It gives rise to 31 pairs of spinal nerves (cervical, thoracic, lumbar, sacral and coccygeal). 8 cervical spinal nerves, 12 thoracic spinal nerves, 5 lumbar spinal nerves, 5 sacral spinal nerves,1 coccygeal spinal nerve All are mixed nerves. (Motor fibers, sensory fibers, sympathetic fibers (T1-L2)/ parasympathetic fibers (S2-S4 spinal segments) Spinal nerves arise as rootlets then combine to form dorsal and ventral roots. Dorsal and ventral roots merge laterally and form the spinal nerve. Dorsal root is related to the sensory information. Anterior (ventral) root is related to the motor information. @ cervical region gives origin to the brachial plexus, and in the lower thoracic and lumbar regions, where it gives origin to the lumbosacral plexus, the spinal cord is fusiformly enlarged; the enlargements are referred to as the cervical and lumbar enlargements. Inferiorly, the spinal cord tapers off into the conus medullaris, from the apex of which a prolongation of the pia mater, the filum terminale, descends to be attached to the posterior surface of the coccyx. The cord possesses a deep longitudinal fissure called the anterior median fissure in the midline anteriorly and a shallow furrow called the posterior median sulcus on the posterior surface. Because of the shortness of the spinal cord relative to the length of the vertebral column, the nerve roots of the lumbar and sacral segments have to take an oblique course downward to reach their respective intervertebral foramina; the resulting leash of nerve roots forms the cauda equine. Along the entire length of the spinal cord are attached 31 pairs of spinal nerves by the anterior or motor roots and the posterior or sensory roots. Each root is attached to the cord by a series of rootlets, which extend the whole length of the corresponding segment of the cord. Each posterior nerve root possesses a posterior root ganglion, the cells of which give rise to peripheral and central nerve fibers. The spinal cord is composed of an inner core of gray matter, which is surrounded by an outer covering of white matter; there is no indication that the cord is segmented. 6 Dr. Kaan Yücel fhs122.org Introduction to neuroanatomy & Spinal cord Gray Matter On cross section, the gray matter is seen as an H-shaped pillar with anterior and posterior gray columns, or horns, united by a thin gray commissure containing the small central canal A small lateral gray column or horn is present in the thoracic and upper lumbar segments of the cord. The amount of gray matter present at any given level of the spinal cord is related to the amount of muscle innervated at that level. Thus, its size is greatest within the cervical and lumbosacral enlargements of the cord, which innervate the muscles of the upper and lower limbs, respectively. As in other regions of the central nervous system, the gray matter of the spinal cord consists of a mixture of nerve cells and their processes, neuroglia, and blood vessels. The nerve cells are multipolar, and the neuroglia forms an intricate network around the nerve cell bodies and their neurites. Anterior column (horn) motor neurons Posterior column (horn) sensory neurons Intermediate column Interneurons, in T1-L2 becomes lateral column = sympathetetic neurons, S2-S4 parasympathetetic neurons White Matter The white matter, for purposes of description, may be divided into anterior, lateral, and posterior white columns or funiculi. The anterior column on each side lies between the midline and the point of emergence of the anterior nerve roots; the lateral column lies between the emergence of the anterior nerve roots and the entry of the posterior nerve roots; the posterior column lies between the entry of the posterior nerve roots and the midline. As in other regions of the central nervous system, the white matter of the spinal cord consists of a mixture of nerve fibers, neuroglia, and blood vessels. It surrounds the gray matter, and its white color is due to the high proportion of myelinated nerve fibers. Although some nerve tracts are concentrated in certain areas of the white matter, it is now generally accepted that considerable overlap is present. For purposes of description, the spinal tracts are divided into ascending, descending, and intersegmental tracts. On entering the spinal cord, the sensory nerve fibers of different sizes and functions are sorted out and segregated into nerve bundles or tracts in the white matter. Some of the nerve fibers serve to link different segments of the spinal cord, while others ascend from the spinal cord to higher centers and thus connect the spinal cord with the brain. It is the bundles of the ascending fibers that are referred to as the ascending tracts. The ascending tracts conduct afferent information, which may or may not reach consciousness. The information may be divided into two main groups: (1) exteroceptive information, which originates from outside the body, such as pain, temperature, and touch (2) proprioceptive information, which originates from inside the body, for example, from muscles and joints. General information from the peripheral sensory endings is conducted through the nervous system by a series of neurons. In its simplest form, the ascending pathway to consciousness consists of three neurons. The first neuron, the first-order neuron, has its cell body in the posterior root ganglion of the spinal nerve. A peripheral process connects with a sensory receptor ending, whereas a central process enters the spinal cord through the posterior root to synapse on the second-order neuron. The second-order neuron gives rise to an axon that decussates (crosses to the opposite side) and ascends to a higher level of the central nervous system, where it synapses with the third-order neuron. The third-order neuron is usually in the thalamus and gives rise to a projection fiber that passes to a sensory region of the cerebral cortex. This three-neuron chain is the most common arrangement, but some afferent pathways use more or fewer neurons. Many of the neurons in the ascending pathways branch and give a major input into the reticular formation, which, in turn, activates the cerebral cortex, maintaining wakefulness. Other branches pass to motor neurons and participate in reflex muscular activity. Imagination is more important than knowledge. Albert Einstein 7