A&P Chap 16
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The spinal cord is protected by the vertebral column, the meninges, cerebrospinal fluid, and denticulate ligaments.
The three meninges are covering that run continuously around the spinal cord and brain. They are the dura mater, arachnoid mater, and pia mater.
The spinal cord begins as a continuation of the medulla oblongata and ends at about the second lumbar vertebra (L2) in an adult.
The spinal cord contains cervical and lumbar enlargements that serve as points of origin for nerves to the limbs.
The tapered inferior portion of the spinal cord is the conus medullaris, from which arise the filum terminale and cauda equina.
Spinal nerves connect to each segment of the spinal cord by two roots. The posterior or dorsal root contains sensory axons, and the anterior or ventral root contains motor neuron axons.
The anterior median fissure and the posterior median sulcus partially divide the spinal cord into right and left sides.
The gray matter in the spinal cord is divided into horns, and the white matter into columns. In the center of the spinal cord is the central canal, which run the length of the spinal cord.
Parts of the spinal cord observed in transverse section are the gray commissure; central canal; anterior posterior, and lateral gray horns; and anterior, posterior, and lateral white columns, which contain ascending and descending tracts. Each part has specific functions.
The spinal cord conveys sensory and motor information by way of ascending and descending tracts, respectively.
The 31 pairs of spinal nerves are named and numbered according to the region and level of the spinal cord from which they emerge.
There are 8 pairs of cervical, 12 pairs of thoracic, 5 pairs of sacral, and 1 pair of coccygeal nerves.
Spinal nerves typically are connected with the spinal cord by a posterior root and an anterior root. All spinal nerves contain both sensory and motor axons (are mixed nerves).
Three connective tissue coverings associated the spinal nerves are the endoneurium, perineurium, and epineurium.
Branches of a spinal nerve include the posterior ramus, anterior ramus, meningeal branch, and rami communicantes.
The anterior rami of spinal nerves, except for T2-T12, form networks of nerves called plexi.
Emerging from the plexi are nerves bearing the names that typically describe the general regions they supply or the route they follow.
Nerves of the cervical plexus supply the skin and muscles of the head, neck, and upper part of the shoulders; they run parallel with some cranial nerves and innervate the diaphragm.
Nerves of the brachial plexus plexus supply the upper limbs and several neck and shoulder muscles.
Nerves off the lumbar plexus supply the anterolateral abdominal wall, external genitals, and part of the lower limbs.
Nerves of the sacral plexus supply the buttocks, perineum, and part of the lower limbs.
nerves of the coccygeal plexus supply the skin of the coccygeal region.
Anterior rami of nerves T2-T12 do not form plexi and are called intercostal (thoracic) nerves. They are distributed directly to the structures they supply in the intercostal spaces.
Sensory neruons within spinal nerves and the trigeminal (V) cranial nerve serve specific constant segments of the skin called dermatomes.
Knowledge of dermatomes helps a physician determine which segment of the spinal cord or which spinal nerve is damaged.
The white matter tracts in the spinal cord are highways for nerve impulse propagation. Along these tracts, sensory input travels toward the brain, and motor output travels from the brain toward skeletal muscles and other effector tissues.
Sensory input travels along two main pathways in the white matter of the spinal cord: the posterior column-medical lemniscus pathway and the anterolateral (spinothalamic) pathways.
Motor output travels along two main routes in the white matter of the spinal cord: direct pathways and indirect pathways.
A second major function of the spinal cord is to serve as an integrating center for spinal reflexes. This integration occurs in the gray matter.
Muscle spindles monitor changes in the length of skeletal muscles and participate in stretch reflexes.
Tendon organs monitor changes in the tension of skeletal muscles.
A reflex is a fast, predictable sequence of involuntary actions, such as muscle contractions or glandular secretions, which occurs in response to certain changes in the environment.
Reflexes may be spinal or cranial and somatic anr autonomic (visceral).
The components of a reflex arc are sensory receptor, sensory neuron, integrating center motor neuron, and effector.
Somatic spinal reflexes include the stretch reflex, the tendon reflex, the flexor (withdrawal) reflex, and the crossed extensor reflex; all exhibit reciprocal inhibition.
A two-neuron or monosynaptic reflex arc consists of one sensory neuron and one motor neuron A stretch reflex, such as the patellar reflex is an example.
The stretch reflex is ipsilateral and is important in maintaining muscle tone.
A polysynaptic reflex arc contains sensory neurons, interneurons, and motor neurons. The tendon reflex, flexor (withdrawal) reflex, and crossed extensor reflex are examples.
The tendon reflex is ipsilateral and prevents damage to muscles and tendons when muscle force becomes too extreme. The flexor reflex is ipsilateral and moves a limb away from the source of a painful stimulus. The crossed extensor reflex extends the limb contralateral to a painfully stimulated limb, allowing the weight of the body to shift when a supporting limb is withdrawn.
Several important somatic reflexes are used to diagnose various disorders These include the patellar reflex, Achilles reflex, Babinski sign and abdominal reflex.
Types of paralysis are dependent on the location and extent of spinal nerve damage.
Complete transection of the spinal cord results in a loss of sensation and voluntary movements below the level of transection.
Spinal shock is an immediate response to spinal cord injury characterized by temporary areflexia, a loss of reflex function.
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