Spinal cord

Rabia Mustafa

King Edward Medical University

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Spinal Cord Overview:

The Spinal Cord is connected to the brain and is about the diameter of a human finger. From the brain the spinal cord descends down the middle of the back and is surrounded and protected by the bony vertebral column. The spinal cord is surrounded by a clear fluid called Cerebral Spinal Fluid (CSF), that acts as a cushion to protect the delicate nerve tissues against damage from banging against the inside of the vertebrae.

The anatomy of the spinal cord itself consists of millions of nerve fibres which transmit electrical information to and from the limbs, trunk and organs of the body, back to and from the back. The nerves which exit the spinal cord in the mid and lower section of the back control the trunk and legs, as well as bladder, bowel and sexual function.

The nerves which carry information from the brain to muscles are called Motor Neurones. The nerves which carry information from the body back to the brain are called Sensory Neurones. Sensory Neurones carry information to the brain about skin temperature, touch, pain and joint position.

The brain and spinal cord are referred to as the Central Nervous System, whilst the nerves connecting the spinal cord to the body are referred to as the Peripheral Nervous System.

Embryology:

The spinal cord is made from part of the neural tube during development. As the neural tube begins to develop, the notochord begins to secrete a factor known as Sonic hedgehog or SHH. As a result, the floor plate then also begins to secrete SHH, and this will induce the basal plate to develop motor neurons. Meanwhile, the overlying ectoderm secretes bone morphogenetic protein (BMP). This induces the roof plate to begin to secrete BMP, which will induce the alar plate to develop sensory neurons. The alar plate and the basal plate are separated by the sulcus limitans

Medulla spinals of 8-week-old human embryo

Additionally, the floor plate also secretes netrins. The netrins act as chemoattractants to decussation of pain and temperature sensory neurons in the alar plate across the anterior white commissure, where they then ascend towards the thalamus.

Lastly, it is important to note that the past studies of Viktor Hamburger and Rita Levi-Montalcini in the chick embryo have been further proven by more recent studies which demonstrated that the elimination of neuronal cells by programmed cell death (PCD) is necessary for the correct assembly of the nervous system.

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The Spinal Cord

Provides a vital link between the brain and the rest of the body. Exhibits some functional independence from the brain. Spinal cord and its attached spinal nerves serve two important functions .pathway for sensory and motor impulses responsible for reflexes

Structure of the Spinal Cord

A typical adult spinal cord ranges between 42 and 45 Centimeters (cm) (16 to 18 inches) in length. viewed in cross section, it is roughly cylindrical, but Slightly flattened both posteriorly and anteriorly. the posterior (or dorsal) median sulcus,

Arrangement and Functions ofthe Spinal cord

Structures that encircle the spinal cord, listed from outermost to innermost are:

vertebra epidural space dura mater subdural space arachnoid subarachnoid space pia mater

the posterior surface the anterior (or ventral) median fissure, is observed on its anterior surface

Regions of the Spinal Cord

The cervical region is the superior-most region of the spinal cord continuous with the medulla oblongata contains neurons whose axons form the cervical spinal nerve The thoracic region lies inferior to the cervical region. attached to this region are the thoracic spinal nerves The lumbar region is a shorter segment of the spinal cord that contains the neurons for the lumbar spinal nerves The sacral region lies inferior to the lumbar region and contains the neurons for the sacral spinal nerves The coccygeal region is the most inferior “tip” of the spinal cord. One pair of coccygeal spinal nerves arises from this region.

Location and Distribution ofWhite Matter

The white matter of the spinal cord is external to the gray matter White matter on each side of the three regions. A posterior funiculus lies between the posterior gray horns on cord is also

partitioned into The posterior side of the cord and the posterior median sulcus. The white matter region on each lateral side of the spinal cord is the lateral

funiculus.

Spinal Nerves

31 pairs of spinal nerves connect the central nervous system to muscles, glands, and receptors Each spinal nerve is formed from the union of Thousands of motor and sensory axons. Motor axons originate from the spinal cord. Each anterior root and its corresponding posterior root unite within the

intervertebral foramen to Become a spinal nerve. Contain both motor axons and sensory axons. Each spinal nerve is associated with the vertebra of The same number. All of the 31 pairs of spinal nerves, except the first pair and those in the sacrum, exit the vertebral column through intervertebral Foramena located between adjacent vertebrae. The first pair of spinal nerves exits between the skull and the first cervical Vertebra. The nerves of the sacrum exit from the single bone of

The sacrum through the sacral foramina (see chapter 7). Eight spinal nerve pairs exit the vertebral column in the cervical region, 12 in the thoracic region, 5 in the lumbar region, 5 in the Sacral region and 1 in the coccygeal region (figure 12.13). For Convenience, each of the spinal nerves is designated by a number. The letter

indicates the region of the vertebral, each ofcolumn from which the nerve emerges: C, cervical; T, thoracic; L,

Lumbar; and S, sacral. The single coccygeal nerve is often not Designated, but when it is, the symbol often used is Co. The number indicates the location in each region where the nerve emerges from the vertebral column, with the smallest number Always representing the most superior origin. For example, the most superior nerve exiting from the thoracic region of the vertebral Column is designated T1. The cervical nerves are designated C1–C8, the thoracic nerves T1–T12, the lumbar nerves L1–L5 and the sacral nerves S1–S5.

Dermatomes

A specific segment of skin supplied by a single spinal nerve. All spinal nerves except for C1 innervate a segment of skin, and so each of these

nerves is associated with a dermatome. The skin of the body may be divided into sensory segments that collectively make

up a dermatome map

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Intercostal Nerves

Each spinal nerve has a dorsal and a ventral ramus

Anterior rami of spinal nerves T1–T11. Travel in the intercostal space sandwiched between Two adjacent ribs. (Rami branch). Additional rami (rami)called communicating rami, from the thoracic and upper lumbar spinal cord regions carry axons associated

with the sympathetic division The dorsal rami (rami) innervate most of the deep muscles of the Dorsal trunk responsible for movement of the vertebral column. . They also innervate the connective tissue and skin near the midline Of the back. The ventral rami are distributed in two ways. In the thoracic region, the ventral rami form intercostal (between ribs) nerves, which extend along the inferior margin of each rib and Innervate the intercostal muscles and the skin over the thorax.

Nerve plexuses:

The term plexus means braid and describes the organization produced by the intermingling of the nerves. The ventral rami of different spinal nerves, called the roots of the plexus, join with each other to form a plexus. These roots should not be confused with the dorsal and ventral roots from the spinal cord, which are more medial. Nerves that arrive from plexuses usually have axons from more than one spinal

Nerve and thus more than one level of the spinal cord.

The ventral rami of spinal nerves C1-C4 form the cervical plexus, C5-T1 form the brachial plexus, L1-L4 form the lumbar plexus, L4-S4 Form the sacral plexus, and S4, S5, and the coccygeal nerve form the coccygeal

plexus. Several smaller somatic plexuses, such as the pudenda Plexus in the pelvis, are

derived from more distal branches of them pineal nerves.

Autonomic plexuses also exist in the thorax and abdomen. Net work of interweaving anterior rami of spinal nerves.

Anterior rami of most spinal nerves form nerve

Plexuses on both the right and left sides of the body. nerve plexuses then split into multiple “named” Nerves that innervate various body structures. Principal plexuses are the cervical plexuses, brachial plexuses, lumbar plexuses, and sacral plexuses

Cervical Plexus

The cervical plexus is a relatively small plexus originating fromSpinal nerves C1–C4 (figure 12.16). Branches derived from this Plexus innervate superficial neck structures, including several of the muscles attached to the hyoid bone.

Brachial Plexus

The brachial plexus originates from spinal nerves C5–T1 A connection is also present from C4 of the cervical plexus To the brachial plexus. The five ventral rami that constitute the Brachial plexus join to form three trunks, which separate into six Divisions and then join again to create three cords (posterior, lateral, and medial) from which five branches, or nerves of the upper limb, emerge. The five major nerves emerging from the brachial plexus to Supply the upper limb are the axillary, radial, musculocutaneous, Ulnar and median nerves. The axillary nerve innervates part of the Shoulder; the radial nerve innervates the posterior arm, forearm, And hand; the musculocutaneous nerve innervates the anterior Arm; and the ulnar and median nerves innervate the anterior forearm and hand. Smaller nerves from the brachial plexus

innervate the shoulder and pectoral muscles.

Lumbar and Sacral Plexuses

The lumbar plexus originates from the ventral rami of spinal Nerves L1-L4 and the sacral plexus from L4-S4. Because of Their close, overlapping relationship and their similar Distribution, however, the two plexuses often are considered together as a single lumbosacral plexus. Four major nerves exit the lumbosacral plexus and enter the422 Part 3 Integration and Control Systems Lower limb: the obturator, femoral, tibial, and common fibular (peroneal). The obturator nerve innervates the medial thigh; the femoral nerve innervates the anterior thigh; the tibial nerve innervates the posterior thigh, the leg, and foot; and the common fibular nerve innervates the posterior thigh, the anterior and lateral Leg and the foot. Other lumbosacral nerves supply the lower Back, the hip, and the lower abdomen.

Function of spinal cord:

The spinal cord has two major functions: (a) carrying information, and (b) coordinating reflexes.

First, it receives sensory information through the afferent nerves from the sensory receptors throughout the body, and sends them to the brain. It also carries information from the brain through efferent fibers to the muscles and glands. Second, it coordinates reflexes without the involvement of the brain, thus, the spinal cord has both communicative and integrative functions.

Reflexes

Rapid, automatic, involuntary reactions of muscles or glands to A stimulus. All reflexes have similar properties. a stimulus is required to initiate a response to sensory input a rapid response requires that few neurons be involved and synaptic delay be minimal an automatic response occurs the same way every time An involuntary response requires no intent or pre-awareness of the reflex

activity.

Reflexes are usually not suppressed. Awareness of the stimulus occurs after the reflex action has been completed, in time to correct or avoid a potentially dangerous situation.

Components of a Reflex Arc:

Communicates with the CNS. Ends at a peripheral effector (muscle orgland) cell.

Ipsilateral and ContralateralReflex Arcs:

Ipsilateral is when both the receptor and effector organs of the reflexes are on the same side of the spinal cord.

for example, an ipsilateral effect occurs when the muscles in your left arm contract to pull your left hand away from a hot object Contralateral is when the sensory impulses from a receptor organ cross over

through the spinal cord to activate effector organs in the opposite limb. for example, contralateral effect occurs when you step on a sharp object with your left foot and then contract the muscles in your right leg to maintain balance as you

Monosynaptic Reflexes

The simplest of all reflexes. Interneurons are not involved in processing this reflex. The patellar (knee-jerk) reflex is a monosynaptic reflex that physicians use to

assess the functioning of the spinal cord. By tapping the patellar ligament with a reflex hammer, the Muscle spindles in the quadriceps muscles are stretched. Produces a noticeable kick of the leg.

Polysynaptic Reflexes

Have more complex neural pathways that exhibit a number of synapses involving interneurons within the reflex arc. Because this reflex arc has more components, there

withdraw your left leg from the damaging is a more prolonged delay between stimulus and response object

Monosynaptic reflex That monitors and regulates skeletal muscle length. When a stimulus results in the stretching of a muscle, That muscle reflexively contracts. The patellar (knee-jerk) reflex is an example of a stretch reflex. The stimulus (the tap on the patellar tendon) initiates

Contraction of the quadriceps femoris muscle and extension of the knee joint.

Golgi Tendon Reflex

Prevents skeletal muscles from tensing excessively. Golgi tendon organs are nerve endings located within Tendons near a muscle–tendon junction. activation of the Golgi tendon organ signal interneurons in the spinal cord, which in turn inhibit the actions of the motor neurons The associated muscle is allowed to relax, thus protecting the muscle and tendon from excessive Tension damage.

Golgi tendon organs detect tension applied to a tendon.4. Inhibition of the alpha motor neurons causes muscle relaxation, relieving the tension applied to the tendon.2. Sensory neurons conduct action potentials to the spinal cord.3. Sensory neurons synapse with inhibitory interneurons that synapse with alpha motor neurons.

Reflex Testing in a Clinical

Setting

Reflexes can be used to test specific muscle groups and specific spinal nerves or segments of the spinal cord. Consistently abnormal reflex response may indicates Damage to the nervous system or muscles. A reflex response may be normal, hypoactive, or hyperactive.

Blood supply

The spinal cord is supplied with blood by three arteries that run along its length starting in the brain, and many arteries that approach it through the sides of the spinal

column. The three longitudinal arteries are called the anterior spinal artery, and the right and left posterior spinal arteries.[2] These travel in the subarachnoid space and send branches into the spinal cord. They form anastamoses (connections) via the anterior and posterior segmental medullary arteries, which enter the spinal cord at various points along its length.[2] The actual blood flow caudally through these arteries, derived from the posterior cerebral circulation, is inadequate to maintain the spinal cord beyond the cervical segments.

The major contribution to the arterial blood supply of the spinal cord below the cervical region comes from the radially arranged posterior and anterior radicular arteries, which run into the spinal cord alongside the dorsal and ventral nerve roots, but with one exception do not connect directly with any of the three longitudinal arteries.[2] These intercostal and lumbar radicular arteries arise from the aorta, provide major anastomoses and supplement the blood flow to the spinal cord. In humans the largest of the anterior radicular arteries is known as the artery of Adamkiewicz, or anterior radicularis magna (ARM) artery, which usually arises between L1 and L2, but can arise anywhere from T9 to L5. [3] Impaired blood flow through these critical radicular arteries, especially during surgical procedures that involve abrupt disruption of blood flow through the aorta for example during aortic aneursym repair, can result in spinal cord infarction and paraplegia

Spinal Cord Injury

A spinal cord injury (SCI) refers to any injury to the spinal cord that is caused by trauma instead of disease.[1] Depending on where the spinal cord and nerve roots are damaged, the symptoms can vary widely, from pain to paralysis to incontinence.[2][3] Spinal cord injuries are described at various levels of "incomplete", which can vary from having no effect on the patient to a "complete" injury which means a total loss of function.

Treatment of spinal cord injuries starts with restraining the spine and controlling inflammation to prevent further damage. The actual treatment can vary widely depending on the location and extent of the injury. In many cases, spinal cord injuries require substantial physical therapy and rehabilitation, especially if the patient's injury interferes with activities of daily life.

Spinal cord injuries have many causes, but are typically associated with major trauma from motor vehicle accidents, falls, sports injuries, and violence. Research into treatments for spinal cord injuries includes controlled hypothermia and stem cells, though many treatments have not been studied thoroughly and very little new research has been implemented in standard care

Damage to the spinal cord can disrupt ascending tracts from the spinal cord to the brain, resulting in the loss of sensation, and/or descending tracts from the brain to motor neurons in the spinal cord, resulting in the loss of motor functions. About 10,000 new cases of spinal cord injury occur Each year in the United States. Automobile and motorcycle accidents are leading causes, followed by gunshot wounds, falls, And swimming accidents. Spinal cord injury is classified according to the vertebral level at which the injury occurred, whether the entire cord is damaged at that level or Only a portion of the cord, and the mechanism of injury. Most spinal cord

injuries occur In the cervical region or at the thoracolumbar junction and are incomplete. At the time of spinal cord injury, two types of tissue damage occur: (1) primary, mechanical damage and (2) secondary, tissue damage

Refrences

1. Reflexes Maton, Anthea; Jean Hopkins, Charles William McLaughlin, Susan Johnson, Maryanna Quon Warner, David LaHart, Jill D. Wright (1993). Human Biology and Health. Englewood Cliffs, New Jersey, USA: Prentice Hall. pp. 132–144. ISBN 0-13-981176-1.

2. a b c Moore, Keith; Anne Agur (2007). Essential Clinical Anatomy, Third Edition. Lippincott Williams & Wilkins. pp. 298. ISBN 0-7817-6274-X.

3. Biglioli, Paolo; ET alia (April 2004). "Upper and lower spinal cord blood supply: the continuity of the anterior spinal artery and the relevance of the lumbar arteries".

Journal of Thoracic and Cardiovascular Surgery 127 (4): 1188–1192. doi:10.1016/j.jtcvs.2003.11.038.

4. Saladin. Anatomy and Physiology, 5th Ed. 5. Saladin. Anatomy and Physiology, 5th Ed.