Somatic nervous systemThe somatic nervous system (SNS) is the part of the peripheral nervous system associated with the voluntary control of body movements through the action of skeletal muscles, and with reception of external stimuli, which helps keep the body in touch with its surroundings (e.g., touch, hearing, and sight).The system includes all the neurons connected with skeletal muscles, skin, and sense organs. Somatic Nervous SystemThe somatic nervous system (SNS), composed of somatic parts of the CNS and PNS, provides sensory and motor innervation to all parts of the body, except the viscera in the body cavities, smooth muscle, and glands. The somatic sensory system transmits sensations of touch, pain, temperature, and position from sensory receptors. Most of these sensations reach conscious levels (i.e., we are aware of them). The somatic motor sys-tem innervates only skeletal muscle, stimulating voluntary and reflexive movement by causing the muscle to contract, as occurs in response to touching a hot iron.

Autonomic Nervous SystemThe autonomic nervous system (ANS), classically described as the visceral nervous system or visceral motor system consists of motor fibers that stimulate smooth (involuntary) muscle, cardiac muscle, and glandular (secretory) cells. It has visceral efferent fibers and visceral afferent fibers. These nerve fibers and ganglia of the ANS are organized into two systems or divisions: the sympathetic (thoracolumbar) division and the parasympathetic (craniosacral) division. In contrast to somatic nervous system both divisions of the ANS, conduction of impulses from the CNS to the effector organ involves a series of two multipolar neurons. The nerve cell body of the first presynaptic (preganglionic) neuron is located in the gray matter of the CNS. Its fiber (axon) synapses only on the cell body of a post-synaptic (postganglionic) neuron, the second neuron in the series. The cell bodies of these second neurons are located outside the CNS in autonomic ganglia, with fibers terminating on the effector organ (smooth muscle, modified cardiac muscle, or glands).

SYMPATHETIC (THORACOLUMBAR) DIVISION OF ANSThe cell bodies of the presynaptic neurons of the sympathetic division of the ANS are found in only one location: lateral gray horn fromT1-L2. The cell bodies of postsynaptic neurons of the sympathetic nervous system occur in two locations, the paravertebral and prevertebral ganglia:

Paravertebral ganglia are linked to form right and left sympathetic trunks (chains) on each side of the vertebral column and extend essentially the length of this column. Prevertebral ganglia are in the plexuses that surround the origins of the main branches of the abdominal aorta (for which they are named), such as the two large celiac ganglia that surround the origin of the celiac trunk (a major artery arising from the aorta). The axons of presynaptic neurons leave the spinal cord through anterior roots and enter the anterior rami of spinal nerves T1–L2 or 3.Almost immediately after entering, all the presynaptic sympathetic fibers leave the anterior rami of these spinal nerves and pass to the sympathetic trunks through white rami communicantes (communicating branches).Within the sympathetic trunks, presynaptic fibers follow one of four possible courses.

1. Ascend in the sympathetic trunk to synapse with a post-synaptic neuron of a higher paravertebral ganglion.

2. Enter and synapse immediately with a postsynaptic neuron of the paravertebral ganglion at that level

3. Descend in the sympathetic trunk to synapse with a post-synaptic neuron of a lower paravertebral ganglion.

4. Pass through the sympathetic trunk without synapsing, continuing through an abdominopelvic splanchnic nerve (a branch of the trunk involved in innervating abdominopelvic viscera) to reach the prevertebral ganglia.

Presynaptic sympathetic fibers that provide autonomic innervation within the head, neck, body wall, limbs, and thoracic cavity follow one of the first three courses, synapsing within the paravertebral ganglia. The presynaptic sympathetic fibers innervating viscera within the abdominopelvic cavity follow the fourth course.Those postsynaptic sym-pathetic fibers, destined for distribution within the neck, body wall, and limbs, pass from the paravertebral ganglia of the sympathetic trunks to adjacent anterior rami of spinal nerves through gray rami communicantes . By this means, they enter all branches of all 31 pairs of spinal nerves, including the posterior rami. The postsynaptic sympathetic fibers stimulate contraction of the blood vessels (vasomotion) and arrector muscles associated with hairs (pilomotion, resulting in “goose bumps”), and to cause sweating (sudo motion). Postsynaptic sympathetic fibers destined for the viscera of the thoracic cavity (e.g., the heart, lungs, and esophagus) pass through cardiopulmonary splanchnic nerves to enter the cardiac, pulmonary, and esophageal plexuses. The presynaptic sympathetic fibers involved in the innervation of viscera of the abdominopelvic

cavity (e.g., the stomach and intestines) pass to the prevertebral ganglia through abdominopelvic splanchnic nerves. All presynaptic sympathetic fibers of the Abdominopelvic splanchnic nerves, except those involved in innervating the suprarenal (adrenal) glands, synapse in prevertebral ganglia. The postsynaptic fibers from the prevertebral ganglia form periarterial plexuses, which follow branches of the abdominal aorta to reach their destination. Some presynaptic sympathetic fibers pass through the celiac pre vertebral ganglia without synapsing, continuing to terminate directly on cells of the medulla of the suprarenal gland. The suprarenal medullary cells function as a special type of postsynaptic neuron that, instead of releasing their neurotransmitter substance onto the cells of a specific effector organ, release it into the bloodstream to circulate throughout the body, producing a widespread sympathetic response. Thus the sympathetic innervation of this gland is exceptional. As described earlier, postsynaptic sympathetic fibers are components of virtually all branches of all spinal nerves. By this means and via peri arterial plexuses, they extend to and innervate all the body’s blood vessels (the sympathetic system’s primary function) as well as sweat glands, arrector muscles of hairs, and visceral structures. Thus the sympathetic nervous system reaches virtually all parts of the body, with the rare exception of such avascular tissues as cartilage and nails. Because the two sets of sympathetic ganglia (para-and prevertebral) are centrally placed in the body and are close to the midline (hence relatively close to the spinal cord), in this division the presynaptic fibers are relatively short, where as the postsynaptic fibers are relatively long, having to extend to all parts of the body.

PARASYMPATHETIC (CRANIOSACRAL) DIVISION OF ANS

Presynaptic parasympathetic nerve cell bodies are located in two sites within the CNS, and their fibers exit by two routes. This arrangement accounts for the alternate name “craniosacral” for the parasympathetic division of the ANS: In the gray matter of the brainstem, the fibers exit the CNS within cranial nerves III, VII, IX, and X; these fibers constitute the cranial parasympathetic out flow.In the gray matter of the sacral segments of the spinal cord (S2–4), the fibers exit the CNS through the anterior roots of sacral spinal nerves S2–4 and the pelvic splanchnic nerves that arise from their anterior rami; these fibers constitute the sacral parasympathetic out flow.Not surprisingly, the cranial out flow provides parasympathetic innervation of the head, and the sacral out flow provides the parasympathetic innervation of the pelvic viscera. However, in terms of the innervation of thoracic and abdominal viscera, the cranial outflow through the vagus nerve (CN X) is dominant. It provides innervation to all thoracic viscera and most of the gastrointestinal (GI)

tract from the esophagus through most of the large intestine (to its left colic flexure).The sacral outflow to the GI tract supplies only the descending and sigmoid colon and rectum. Regardless of the extensive influence of its cranial outflow, the parasympathetic system is much more restricted than the sympathetic system in its distribution. The parasympathetic system distributes only to the head, visceral cavities of the trunk, and erectile tissues of the external genitalia. With the exception of the latter, it does not reach the body wall or limbs, and except for the initial parts of the anterior rami of spinal nerves S2–4, its fibers are not components of spinal nerves or their branches. Four discrete pairs of parasympathetic ganglia occur in the head. Elsewhere, presynaptic parasympathetic fibers synapse with postsynaptic cell bodies, which occur singly in or on the wall of the target organ (intrinsic or enteric ganglia). Consequently, in this division, most presynaptic fibers are very long, extending from the CNS to the effector organ, whereas the postsynaptic fibers are very short, running from a ganglion located near or embedded in the effector organ.

The anatomical distinction between the sympathetic and parasympathetic divisions of the ANS is based primarily on: 1.The location of the presynaptic cell bodies, and 2.Which nerves conduct the presynaptic fibers from the CNS

A functional distinction of pharmacological importance for medical practice is that the postsynaptic neurons of the two divisions generally liberate different neurotransmitter substances: norepinephrine by the sympathetic division (except in the case of sweat glands) and acetylcholine by the parasympathetic division.

FUNCTIONS OF DIVISIONS OF ANSAlthough both sympathetic and parasympathetic systems innervate involuntary (and often affect the same) structures, they have different, usually contrasting yet coordinated, effects. In general, the sympathetic system is a catabolic (energy-expending) system that enables the body to deal with stresses, such as when preparing the body for the fight-or-flight response. The parasympathetic systemIs primarily a homeostatic or anabolic (energy-conserving) system, promoting the quiet and orderly processes of the body, such as those that allow the body to feed and assimilate. The primary function of the sympathetic system is to regulate blood vessels. This is accomplished by several means having different effects. Blood vessels throughout the body are tonically innervated by sympathetic nerves, maintaining a resting state of moderate vasoconstriction. In most vascular beds, an increase in sympathetic signals causes increased vasoconstriction, and a decrease in the rate of sympathetic signals allows vasodilation. However, in certain regions

of the body, sympathetic signals are vasodilatory (i.e., sympathetic transmitter substances inhibit active vasoconstriction, allowing the blood vessels to be passively dilated by the blood pressure). In the coronary vessels, the vessels of skeletal muscles, and the external genitals, sympathetic stimulation results in vasodilation.

VISCERAL SENSATIONVisceral afferent fibers have important relationships to the ANS, both anatomically and functionally. We are usually un-aware of the sensory input of these fibers, which provides information about the condition of the body’s internal environment. This information is integrated in the CNS, often triggering visceral or somatic reflexes or both. Visceral reflexes regulate blood pressure and chemistry by altering such functions as heart and respiratory rates and vascular resistance. Visceral sensation that reaches a conscious level is generally perceived as pain that is either poorly localized or felt as cramps or that may convey a feeling of hunger, fullness, or nausea. Surgeons operating on patients who are under local anesthesia may handle, cut, clamp, or even burn (cauterize) visceral organs without evoking conscious sensation. How-ever, adequate stimulation, such as the following, may elicit pain: •Sudden distension. • Spasms or strong contractions. • Chemical irritants. • Mechanical stimulation, especially when the organ is active. • Pathological conditions (especially ischemia) that lower the normal thresholds of stimulation. Normal activity usually produces no sensation but may do so when the blood supply is inadequate (ischemia). Most visceral reflex (unconscious) sensation and some pain travel in visceral afferent fibers that accompany the parasympathetic fibers retrograde (backward). Most visceral pain impulses (from the heart and most organs of the peritoneal cavity) travel centrally along visceral afferent fibers accompanying sympathetic fibers.

SKELETAL SYSTEMThe skeletal system may be divided into two functional parts:

The axial skeleton consists of the bones of the head (cranium or skull), neck (hyoid bone and cervical vertebrae), and trunk (ribs, sternum, vertebrae, and sacrum).

The appendicular skeleton consists of the bones of the limbs, including those forming the pectoral (shoulder) and pelvic girdles.

Cartilage and BonesThe skeleton is composed of cartilages and bones. CARTILAGE Cartilage is a specialized dense connective tissue. It is hard but not rigid like bone. It can be bent and also brought back into its original form when bending force is withdrawn.This cartilage forms the skeletal basis of some parts of the body (auricle of the ear, external nose). No nerves or blood vessels occur in cartilage. At the time of birth, many parts of the skeletal frame work of the newborn are made up of cartilage. Later this cartilage will be converted into bones by a process called ossification.Cartilage consists of cells called chondrocytes, fibers & ground substance.Based on the type of fibers present in the matrix, the cartilages are classified into three types—1. Hyaline cartilage EX: trachea and ends of bone where they form joints. 2. Elastic cartilage EX: Lobe of the ear, the epiglottis 3. Fibrocartilage EX: discs between the vertebrae BONE

Bone, a living tissue, is a highly specialized, hard form of connective tissue that makes up most of the skeleton. Bones of the adult skeleton provide:

Support for the body and its vital cavities; it is the chief supporting tissue of the body.Protection for vital structures (e.g., the heart).The mechanical basis for movement (leverage).Storage for salts (e.g., calcium, phosphorus, etc).A continuous supply of new blood cells (produced by the marrow in the medullary cavity of many bones).

Osseous tissue, or bone tissue, is the major structural and supportive connective tissue of the body. Osseous tissue forms the rigid part of the bone organs that make up the skeletal system.Formation of osseous tissue or bone tissueBone tissue is a mineralized connective tissue. It is formed by cells, called osteoblasts, that deposit a matrix of Type-I collagen and also release calcium, magnesium, and phosphate ions that ultimately combine chemically within the Collagenous matrix into a crystalline mineral, known as bone mineral,

in the form of hydroxyapatite. The combination of hard mineral and flexible collagen makes bone harder and stronger than cartilage without being brittle.

Classification of bones, based on theA. Types of osseous tissue or bone tissue that make up the bone :Cortical bone, synonymous with compact bone, is one of the two types of osseous tissue that form bones. Cortical bone facilitates bone's main functions: to support the body, protect organs, provide levers for movement, and store and release chemical elements, mainly calcium. As its name implies, cortical bone forms the cortex, or outer shell, of most bones, which is made up of bony plates called lamella. These bony plates are arranged very compactly. Again, as its name implies, compact bone is much denser than cancellous bone, which is the other type of osseous tissue. Furthermore, it is harder, stronger and stiffer than cancellous bone. Cortical bone contributes about 80% of the weight of a human skeleton. The primary anatomical and functional unit of cortical bone is the osteon.Cancellous bone, synonymous with trabecular bone or spongy bone, is one of the two types of osseous tissue that form bones. Compared to compact bone, which is the other type of osseous tissue, it has a higher surface area but is less dense, softer, weaker, and less stiff because bony plates are arranged irregularly leaving spaces in between them that gives a spongy appearance. It typically occurs at the ends of long bones, proximal to joints and within the interior of vertebrae. Cancellous bone is highly vascular and frequently contains red bone marrow where hematopoiesis, the production of blood cells, occurs. The primary anatomical and functional unit of cancellous bone is the trabeculae.

B. Bones are classified according to their shape. Long bones are tubular (e.g., the humerus in the arm).

Short bones are cuboidal and are found only in the tarsus (ankle) and carpus (wrist).Flat bones usually serve protective functions (e.g., the flat bones of the cranium protect the brain).Irregular bones have various shapes other than long, short, or flat (e.g., bones of the face).

C. Based on development All the bones are developed from the mesoderm

1. Membranous bone: The mesenchymal tissue (developed from mesoderm) is directly transformed in to a bone EX: CLAVICLE

2. Cartilaginous bones: The mesenchymal tissue is first transformed into a cartilage. Later cartilage undergoes ossification to form bones ex: limb bones.

D. SPECIAL TYPE OF BONES 1. Pneumatic bone These are flat or irregular bones with hollow spaces in their body. These spaces contain air. Ex: ethmoid, maxilla.2. Sesamoid bone (sesamoid seed-like)

Sesamoid bones (e.g., the patella or knee cap) develop in certain tendons and are found where tendons cross the ends of long bones in the limbs; they protect the tendons from excessive wear and often change the angle of the tendons as they pass to their attachments.Macroscopic structure of a bone:

The long bone consists of two ends (epiphysis) and a shaft (diaphysis)The shaft consists of a cylindrical cavity inside called medullary cavity, which is filled with bone marrow. The outer (cortical) part of the shaft is made up of compact bone.The two ends of the long bone are filled with tiny plates of bone containing numerous spaces called as spongy bone where the bone marrow does not extend.The outer surface of the bone is covered by a highly vascular connective tissue membrane called periosteum except at the articular surfaces. It is capable of laying down more bone (particularly during fracture healing) and provide the interface for attachment of tendons and ligamentsThe articular surfaces are covered by a cartilage called articular cartilage. The medullary connective tissue is lined by another connective tissue membrane is called endosteum. Bone marrowIt is the vascular connective tissue present in the cavity (medullary cavity) of the bone. The bone marrow differs in composition in different bones and at

different ages. It occurs in two forms, yellow marrow and red marrow. The red marrow is actively engaged in the production of blood cells. The yellow marrow derives its color form the large quantity of fat cells it contains. At birth the red marrow is present throughout the skeleton. After about fifth year of postnatal life, the red marrow is gradually replaced in the long bones by yellow marrow.

Microscopic structureAn adult long bone consists of following components:

A. Bone cellsB. Matrix

compact bone:The compact bone is made up of lamellae.Lamellae are thin plates of bone consisting of collagen fibers embedded in ground substance.Lamellae are placed one over the other.The spaces between the lamellae are called lacunaeLacunae are occupied by osteocytes and the adjacent lacunae are connected through canaliculi, which are occupied by cytoplasmic processes of osteocytes.Most of the lamellae are arranged in the form of concentric rings that surround a haversian canal, which is present at the center of each ring. Haversian canals are placed parallel to medullary and they are occupied by blood vessels and nerve fibers.Adjacent haversian canals are connected by volkman’s canal.One Haversian canal and lamella around it constitute a haversian system or an osteon.

Bone cellsOsteoblastsThese are bone forming cells, more numerous in periosteum and these are responsible for laying down organic matrix of bone including the collagen fibers. They are also responsible for calcification of the matrix.OsteocytesThese are mature bone cells, derived form osteoblasts after they have laid down the matrix. There are present in the lacunae of the bone between lamellae and

osteocytes have many cytoplasmic processes, which establish connections with other osteocytes. Osteocytes maintain the integrity of the lacunae.OsteoclastsThese are bone removing cells (demineralization) and found in relation to the surfaces of the bone. Osteoclasts are stimulated by parathyroid hormone.MATRIX (GROUND SUBSTANCE)The matrix of the bone consists of both organic and inorganic substances.a. Organic constituent (25% of the matrix)

It is mainly made up of collagen fibers and these are embedded in proteins, carbohydrates and water.The collagen fibers are responsible for toughness and resilience of bone and these fibers are synthesized by osteoblasts.Chondroitin sulphate is another important organic constituent of the bone.

a. Inorganic constituent (75%)b. Calcium, phosphate and hydroxyl ions are in the form of crystals called

hydroxyapatite crystals which lie parallel to collagen fibers.OssificationThe process of bone formation is called ossification. All the bones are developed form the mesenchymal tissue of the embryo. There are two types of ossification.Membranous ossification:The embryonic mesenchymal tissue will directly form the bone. EX: clavicle, bones of cranial vault.Cartilaginous ossification:Mesenchymal tissue is first transformed into a cartilage. Later cartilage is ossified to form boneOssification o f a long boneThe ossification of begins in one or more areas of future bone model. These areas are called centers of ossification.PRIMARY CENTER OF OSSIFICATIONThe ossification that starts in the central part of the cartilaginous model (i.e. at the future shaft) is called primary center of ossification and part of bone that develops from it is called diaphysis. The primary center ossification normally appears before birth.

Secondary center of ossificationThese centers appear at the two ends (epiphysis) of the long bone usually after birth. The portion of long developed from secondary center of ossification is called epiphysis. The portion of the bone which is in between the epiphysis and diaphysis are actively involved in growth called as metaphysis. The metaphysis is the wider portion of a long bone adjacent to the plate. It is this part of the bone that grows during childhood; as it grows, it ossifies near the diaphysis and the epiphyses. At roughly 18 to 25 years of age, the metaphysis stops growing altogether and completely ossifies into solid bone.Epiphyseal plates ("growth plates") are located in the metaphysis and are responsible for growth in the length of the bone.The epiphyseal plate (or epiphyseal plate, physis, or growth plate) is a hyaline cartilage plate in the metaphysis at each end of a long bone. The plate is found in children and adolescents; in adults, who have stopped growing, the plate is replaced by an epiphyseal line.Laws of ossificationEpiphysis which ossifies first unites (fuses) with the diaphysis last and the epiphysis which ossifies last fuses first. Blood supply to the long bones1. Nutrient artery: enters shaft through a nutrient foramen and on reaching the

marrow cavity they divide into ascending and descending branches.2. Epiphyseal arteries: they are several in numbers and enter the bone near the

ends.3. Metaphyseal arteries4. Periosteal arteries: they are numerous and enter the bone along the muscular

attachment.All the above the arteries forms an extensive anastomoses inside the marrow cavity. An arteriovenous anastomosis is a connection between two blood vessels, resulting in multitude of arteries and veins serving the same volume of tissue.Applied anatomy:Because the metaphysis receives rich blood supply, metaphysis of long bones are prone to hematogenous spread of Osteomyelitis in children.

Defects in the development and continued division of epiphyseal plates can lead to growth disorders. The most common defect is achondroplasia, where there is a defect in cartilage formation. Achondroplasia is the most common cause of dwarfism.

JOINTSA joint is the location at which two or more bones make contact. They are constructed to allow movement and provide mechanical support, and are classified structurally and functionally. Classification Structural classificationStructural classification names and divides joints according to how the bones are connected to each other.There are three structural classifications of joints: fibrous joint  - joined by fibrous connective tissue cartilaginous joint  - joined by cartilage synovial joint  - not directly joined

Functional classificationJoints can also be classified functionally, by the degree of mobility they allow:[4]

synarthrosis  - permits little or no mobility. Most synarthrosis joints are fibrous joints (e.g., skull sutures).

amphiarthrosis  - permits slight mobility. Most amphiarthrosis joints are cartilaginous joints (e.g., vertebrae).

Diarthroses  - permits a variety of movements. All diarthrosis joints are synovial joints (e.g., shoulder, hip, elbow, knee, etc.), and the terms "diarthrosis" and "synovial joint"

SYNOVIAL JOINTA Synovial joint, also known as a diarthrosis, is the most common and most movable type of joint in the body of a mammal. As with most other joints, synovial joints achieve movement at the point of contact of the articulating bones.Structural and functional differences distinguish synovial joints from cartilaginous joints and fibrous joints ). The main structural differences between synovial and fibrous joints is the existence of capsules surrounding the articulating surfaces of a

synovial joint and the presence of lubricating synovial fluid within that capsule (synovial cavity). Structure: articular capsule: The fibrous capsule is continuous with the periosteum of bone. It is also highly innervated but avascular (lacking blood and lymph vessels) articular cartilage : lines the epiphyses of joint end of bone. Provides the loading

and unloading mechanism to resist load and shock synovial membrane : the inner layer of the fibrous articular capsule.

The synovial membrane covers the lining of the synovial cavity where articular cartilage is absent.

Synovial fluid : it is a viscous fluid present in the joint cavity and it provides nutrition to the articular cartilage and lubrication to joint.

Nerve Supply of Synovial JointIt is derived from the nerve supply of muscles acting on the joint.Blood Supply of Synovial JointFrom the arteries sharing in the anastomosis around the joint.Movements possibleThe movements possible with synovial joints are:

Abduction : movement away from the mid-line of the body. Adduction : movement towards the mid-line of the body. Extension : straightening limbs at a joint. Flexion : bending the limbs at a joint. Rotation : a circular movement around a fixed point.

There are seven types of synovial joints. Some are relatively immobile, but are more stable. Others have multiple degrees of freedom, but at the expense of greater risk of injury.