The spinal cord extends from the foramen magnum of the occipital bone to the level of the first lumbar vertebra (L1), just below the bottom rib. What protects the spinal cord? The spinal cord is protected by bone (vertebrae), meninges, cerebrospinal fluid (CSF), and the blood-brain barrier.
The CSF sample is examined for blood, pathogens, pressure or other abnormal signs. Why is the patient positioned as shown here? This position helps spread the vertebrae so that the needle can be inserted more easily. Why is there no danger of injuring an adult’s spinal cord with the needle? The cord only extends to L1 in an adult; therefore, there is no danger of injuring an adult’s spinal cord because the needle is inserted between L3 and L4.
Gray matter is located in the center of the spinal cord and is shaped like a butterfly. It contains mostly cell bodies and interneurons. The central canal communicates with the ventricular system and contains cerebrospinal fluid. The white matter contains tracts which carry information up and down the cord. Tracts are composed primarily of groups of myelinated axons.
Why are the ascending tracts called sensory tracts and the descending tracts called motor tracts? The sensory tracts carry information from the periphery, up the spinal cord, and toward the brain—hence the name, ascending tracts. The motor tracts, or descending tracts, carry information from the brain, down the spinal cord, and toward the periphery. Ascending tracts carry information such a temperature, pressure, pain, and touch. The motor tracts carry information to the muscles for movement. Refer students to Table 11-1. The name of the tract often indicates its origin and destination. For example, the spinocerebellar tract originates in the spinal column (spino-) and its destination is the cerebellum (-cerebellar). What is another name for the pyramidal tract? It is also called the corticospinal tract. Decussation is the crossing over of the tracts from one side to the other. Most motor tracts decussate at the level of the medulla oblongata and descend contralaterally, whereas most sensory tracts decussate in the spinal cord and travel up the opposite side of the cord to the brain.
Do the neurons of the brain and spinal cord regenerate if injured or destroyed? No, they do not. What causes paraplegia and quadriplegia? Both conditions involve the spinal cord being severed. The difference is the location of the injury. If the spinal cord is severed in the lumbar region, paraplegia results. If the cord is severed at the neck region, quadriplegia results.
The dorsal (back) root and the ventral (front) root attach the spinal nerves to the spinal cord. Which types of fibers are associated with each root? Sensory nerve fibers from the periphery travel to the cord through the dorsal root. The dorsal root ganglia contain the cell bodies of the sensory fibers that are gathered together. Motor fibers comprise the ventral root and are distributed to muscles and glands. The dorsal and ventral roots are packaged together to form a spinal nerve. All spinal nerves are mixed nerves because they contain both sensory and motor fibers.
The spinal cord provides a pathway from the periphery to the brain for sensory information, and it also provides a pathway from the brain to the periphery. The spinal cord also acts as a major reflex center. It processes much reflex information without involving the brain—for example, coordinating the movement of arms and legs during walking.
The five stages of the reflex arc are the following: 1. A receptor organ (here, the patellar tendon) is stimulated. 2. Sensory information is carried to the spinal cord. 3. The integrating center allows the sensory neuron to make contact with more than one neuron. This illustration does not show an interneuron within the integrating center. 4. Motor information comes from the spinal cord to the effector organ (quadriceps femoris muscle, in this example). 5. The effector organ performs.
Reflexes help protect from injury, such as in this illustration of the withdrawal reflex. Organ reflexes, such as the pupillary reflex, the knee jerk reflex, and the baroreceptor reflex, help regulate organ function. Some reflexes, such as the Babinski reflex, are also used diagnostically to assess nerve function. Additional clinically significant reflexes are discussed in Table 11-2.
A neuron is a single nerve cell. A nerve contains many neurons bundled together with blood vessels and wrapped in connective tissue. Nerves are located outside the CNS. When found inside the CNS, a nerve is called a tract.
Somatic afferent nerves carry sensory information, such as touch, pressure and pain, to the parietal lobe of the cerebrum. Somatic efferent nerves carry motor information to the skeletal muscles. The autonomic system controls the viscera and is the subject of the next chapter.
All cranial nerves arise from the base of the brain with the exception of CN X (vagus nerve). Most cranial nerves supply the head, neck, and shoulders. Table 11-4 presents the clinical assessment of cranial nerves and specific disorders. The functions of the cranial nerves will be further discussed in Chapter 13.
CN I, the olfactory nerve, carries information from the nose to the brain. Damage to the olfactory nerve may cause loss of the sense of smell and loss of taste because the sense of smell is involved with the appeal of food. CN II, the optic nerve, is a sensory nerve that carries visual information from the eye to the brain. CN III, the oculomotor nerve, is primarily a motor nerve that causes contraction of the extrinsic eye muscles. Compression of the nerve by a brain tumor or increased intracranial pressure interferes with pupillary response to light and the raising of the eyelid. More severe compression might cause the pupils to become dilated and fixed. CN IV, the trochlear nerve, innervates one of the extrinsic muscles of the eyeball. Damage might cause an inability to rotate the eye properly or double vision. Damage causes blindness or diminished vision in the affected eye.
CN V, the trigeminal nerve, has three branches supplying the facial region: two sensory branches and one motor branch that innervates the chewing muscles. Nerve damage causes a loss of sensation and impaired movement of the lower jaw. Trigeminal neuralgia, an inflammation of the trigeminal nerve, causes bouts of severe facial pain. CN VI, the abducens nerve, innervates only one of the extrinsic eye muscles. Nerve damage prevents a lateral rotation of the eye. CN VII, the facial nerve, is called the nerve of facial expression. Damage to the facial nerve is indicated by the absence of facial expression on the affected side of the face, which is called Bell’s palsy. Because this nerve is the motor arm of the corneal reflex, stimulation such as dust in the eye will result in tears and blinking.
CN VIII, the vestibulocochlear nerve, carries information for hearing and balance from the inner ear to the brain. A common name for eighth cranial nerve damage is ototoxicity. It can cause loss of hearing or balance, or both. Why does a middle ear infection sometimes cause dizziness? Middle ear infection can cause pressure within the ear and stimulate the vestibular branch, causing dizziness (vertigo). CN IX, the glossopharyngeal nerve, carries taste sensation from the posterior tongue to the brain. Damage to this nerve can cause a loss of the gag reflex, which increases the risk for choking. Why would you not give an unconscious person anything by mouth? Because the gag reflex is not working, the patient may aspirate contents into the respiratory tract.
CN X, the vagus nerve, innervates the tongue, pharynx, larynx, and many organs in the thoracic and abdominal cavities. Hoarseness or loss of voice, impaired swallowing, and diminished motility of the digestive tract are results of damage to the vagus nerve. Cardiac effects of vagal innervation will be discussed in later chapters. CN XI, the accessory nerve, supplies the sternocleidomastoid and the trapezius muscles. What does nerve damage to the accessory nerve cause? It causes the inability to shrug the shoulders. CN XII, the hypoglossal nerve controls movement of the tongue. Nerve damage causes the tongue to deviate toward the injured side.
Thirty-one pairs of spinal nerves emerge from the spinal cord. Each pair of spinal nerves is numbered according to the level of the spinal cord from which it arises. The lumbar and sacral nerves extend to the end of the spinal cavity before exiting from the vertebral column. They are called the cauda equina because they look like a horse’s tail. The tiny holes in the vertebrae through which nerves exit from the bony vertebral column are called foramina.
Spinal nerves exit the spinal column. These fibers converge into nerve plexuses, or networks, at various points. Each plexus sorts out the many fibers and sends them to a specific part of the body. Fibers from the cervical plexus supply the muscles and skin of the neck. Motor fibers from this plexus also pass into the phrenic nerve, which stimulates the contraction of the diaphragm. The nerves from the brachial plexus supply the muscles and skin of the shoulder, arm, forearm, wrist, and hand. The axillary nerve travels through the shoulder into the arm. The lumbosacral plexus gives rise to nerves that supply the muscles and skin of the lower abdominal wall, external genitalia, buttocks, and lower extremities. The sciatic nerve supplies the muscles of the leg and foot. Table 11-5 lists the nerve plexuses. Table 11-6 describes the peripheral nerves that emerge from the plexuses and the result from damage.
The phrenic nerve innervates the diaphragm, the major breathing muscle. High spinal cord injuries are much more serious if they damage the phrenic nerve because the patient may become ventilator-dependent. Improper use of crutches (crutch palsy) can damage the axillary nerve. Damage to the radial, ulnar, and median nerves can damage the function of the wrist. Table 11-6 summarizes the major peripheral nerves and results of damage to them.
The figure gives several examples of damage to various nerves. When repetitive motion damages the median nerve, carpal tunnel syndrome can result. A common cause of sciatica is a ruptured or herniated disc. This condition is also common in pregnant women because pressure of the enlarging abdomen can compress the sciatic nerve and produce the symptoms of sciatica.
Each spinal nerve innervates a particular area of skin. This distribution of nerves is called a dermatome. Each dermatome is named for the nerve that serves it. Dermatomes are useful clinically. For example, if the skin of the shoulder is stimulated with a pin and the person cannot feel it, it is likely that the fourth cranial nerve is impaired.
The Human Body in Health and
Illness, 4th edition
Nervous System: Spinal Cord and