THE SPINAL CORD
— Spinal cord is the part of CNS. The spinal cord is continuous above with the medulla oblongata and extends from the upper border of the atlas to the lower border of the 1st lumbar vertebra. It is approximately 45 cm long in adult males, and is about the thickness of the little finger
Coverings of spinal cord-
— Like brain the spinal cord is also completely surrounded by three layers of tissue, known as spinal meninges . These are-
— Dura mater
— Arachnoid mater
— Pia mater
— The spinal cord, although roughly cylindrical, is flattened slightly antero-posteriorly. Like Brain it is also supported by CSF. Cerebrospinal fulid lies inside central canal and subarachnoid space around spinal cord
— Cross section of the spinal cord reveals regions of white matter that surround an inner core of gray matter. The white matter of the spinal cord consists primarily of bundles of myelinated axons of neurons. Two grooves penetrate the white matter of the spinal cord and divide it into right and left sides
— The anterior median fissure is a wide groove on the anterior (ventral) side. The posterior median sulcus is a narrow furrow on the posterior (dorsal) side. The gray matter of the spinal cord is shaped like the letter H or a butterfly.
— The arrangement of grey matter in the spinal cord resembles the shape of the letter H, having two posterior, two anterior and two lateral columns.
— Posterior columns of grey matter- These are composed of cell bodies that are stimulated by sensory impulses from the periphery of the body. The nerve fibres of these cells contribute to the formation of the white matter of the cord and transmit the sensory impulses upwards to the brain.
— Anterior columns of grey matter- These are composed of the cell bodies of the lower motor neurones that are stimulated by the upper motor neurones or the connector neurones linking the anterior and posterior columns to form reflex arcs.
— Lateral columns of grey matter- These are composed of motor neurons of autonomic nervous system. which sequentially pass throughthe lateral gray horn, anterior gray horn, and anterior root toenter the spinal nerve.
SPINAL REFLEXES-
— A reflex action is an involuntary and immediate motor response to a sensory stimulus. Many connector and motor neurons may be stimulated by afferent impulses from a small area of skin, e.g. the pain impulses initiated by touching a very hot surface with the finger are transmitted to the spinal cord by sensory fibres in mixed nerves.
— These impulses stimulate many connector (interneurons) and lower motor neurons in the spinal cord, which results in the contraction of many skeletal muscles of the hand, arm and shoulder, and the removal of the finger. Reflex action happens very quickly. This system is known as reflex arc.
SPINAL NERVS-
— There are 31 pairs of spinal nerves attached to the spinal cord within the human body, which are named and numbered according to the region and level of the vertebral column from which they emerge.
31 pairs of spinal nerves are:
— 8 Paris Cervical nerves.
— 12 Pairs Thoracic Nerves.
— 5 Pairs Lumber Nerves.
— 5 Pairs Sacral Nerves.
— 1 Pair Coccegeal Nerve.
— The spinal nerves provide the paths of communication between the Brain( through spinal cord) and specific regions of the body as they connect the central nervous system to sensory receptors, muscles and glands in all the parts of the body.
— A typical spinal nerve has two connections to the spinal cord – a posterior root and an anterior root, which unite to form a spinal nerve
— After coming out of inter-vertebral foramen, a spinal nerve divides into several branches. These branches are known as rami (branches). The posterior ramus serves the deep muscles and skin of the posterior surface of the trunk. The anterior ramus serves the muscles and structures of the upper and lower limbs and the skin of the lateral and anterior surfaces of the trunk.
PLEXUS
— Axons from the anterior rami of spinal nerves, except for thoracic nerves T2–T12, do not go directly to the body structures they supply. Instead, they form networks on both the left and right sides of the body by joining with various numbers of axons from anterior rami of adjacent nerves. Such a network of axons is called a plexus
CERVICAL PLEXUS-
— The cervical plexus is formed by the roots (anterior rami) of the first four cervical nerves (C1–C4), with contributions from C5 . There is one on each side of the neck alongside the first four cervical vertebrae.
— The cervical plexus supplies the skin and muscles of the head, neck, and superior part of the shoulders and chest. The phrenic nerves arise from the cervical plexuses and supply motor fibers to the diaphragm and help in respiration
BRACHIAL PLEXUS-
— The anterior rami of spinal nerves C5–C8 and T1 form the brachial plexus which extends inferiorly and laterally on either side of the last four cervical and first thoracic vertebrae.
— The brachial plexus passes above the first rib posterior to the clavicle and then enters the axilla. The brachial plexus provides almost the entire nerve supply of the shoulders and upper limbs
— The main nerves of the brachial plexus are:
(1) The axillary nerve supplies the deltoid and teres minor muscles.
(2) The musculocutaneous nerve supplies the anterior muscles of the arm
(3) The radial nerve supplies the muscles on the posterior aspect of the arm and forearm.
(4) The median nerve supplies most of the muscles of the anterior forearm and some of the muscles of the hand.
(5) The ulnar nerve supplies the anteromedial muscles of the forearm and most of the muscles of the hand.
THORASIC NERVES
— The anterior rami of spinal nerves T2–T12 do not enter into the formation of plexuses and are known as intercostal or thoracic nerves. These nerves supply the skin and muscles in and around the inter-costal spaces, front and back of the thorax.
LUMBER PLEXUS-
— The lumbar plexus is formed by the anterior rami of the first three and part of the 4th lumbar nerves. The plexus is situated in front of the transverse processes of the lumbar vertebrae. The lumbar plexus supplies the anterolateral abdominal wall, external genitals, and part of the lower limbs. The largest nerve arising from the lumbar plexus is the femoral nerve.
— Other nerves of lumber plexus are iliohypogastric, ilioinguinal, genitofemoral lateral cutaneous and obturator nerves.
SACRAL PLEXUS-
— The roots (anterior rami) of spinal nerves L4–L5 and S1–S4 form the sacral plexus This plexus is situated anterior to the sacrum. The sacral plexus supplies the buttocks, perineum, and lower limbs. The largest nerve in the body known as the sciatic nerve—arises from the sacral plexus. The sciatic nerve is the largest nerve in the body. It is about 2 cm wide at its origin. It passes through the greater sciatic foramen into the buttock then descends through the posterior aspect of the thigh supplying the hamstring muscles.
— At the level of the middle of the femur sciatic nerve divides to form the tibial and the common peroneal nerves which supply the lower leg.
COCCEGEAL PLEXUS-
— The coccygeal plexus is a very small plexus formed by part of the 4th and 5th sacral and the coccygeal nerves. The nerves from this plexus supply the skin around the coccyx and anal area.
ANS-
— The autonomic nervous system contributes to homeostasis by responding to subconscious visceral sensations and exciting or inhibiting smooth muscle, cardiac muscle, and glands. the ANS includes autonomic sensory neurons, integrating centers in the CNS, and autonomic motor neurons. A continual flow of nerve impulses from (1) autonomic sensory neurons in visceral organs and blood vessels propagate into (2) integrating centers in the central nervous system (CNS). Then, impulses in (3) autonomic motor neurons reach to various effector tissues,
The autonomic nervous system is separated into two divisions:
— Sympathetic nervous system (thoracolumbar outflow)
— Parasympathetic nervous system (craniosacral outflow).
The two divisions have both structural and functional differences. They normally work in an opposing manner, thereby maintaining balance of involuntary functions
— Each division has two efferent neurones between the central nervous system and effector organs. These are:
— The preganglionic neurone
— The postganglionic neurone.
The cell body of the preganglionic neurone is in the brain or spinal cord. Its axon terminals synapse with the cell body of the postganglionic neurone in an autonomic ganglion outside the CNS. The postganglionic neurone conducts impulses to the effector organ.
FUNCTIONS OF ANS-
— Sympathetic stimulation prepares the body to deal with exciting and stressful situations, e.g. strengthening its defences in times of danger and in extremes of environmental temperature. A range of emotional states, e.g. fear, embarrassment and anger, also cause sympathetic stimulation. The adrenal glands are stimulated to secrete the hormones adrenaline (epinephrine) and oradrenaline (norepinephrine) into the bloodstream.
— sympathetic stimulation prepares the body for ‘fight or flight’. The effects of stimulation on the heart, blood vessels and lungs (see below) enable the body to respond by preparing it for exercise. Additional effects are an increase in the metabolic rate and increased conversion of glycogen to glucose.
— Parasympathetic stimulation has a tendency to slow down body processes except digestion and absorption of food and the functions of the genitourinary systems. For example it decreases the rate and force of the heartbeat. Constricts the coronary arteries, reducing the blood supply to cardiac muscle.
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