9.9 Secrets of Spinal Cord or Why Christopher Reeve?

Physician's Notebooks 9 - http://physiciansnotebook.blogspot.com - See Homepage


9. Spinal Cord - Update 10 Septr 2021
(Special Note: This is packed with information; so it should be read slowly, without trying to remember, i.e., for pure reading pleasure, in small segments with reference to internet as needed and re-read when better understanding is needed.)

The following descending column presents headings in order of appearance and may be used to search & find or to scroll down, plus it gives an idea of what is in the chapter

Spinal cord injuries

The spinal cord is an information superhighway

outer membranes

Spinal Cord Segments

The Dermatomes

Spinal Reflex

Danger from Aerobics

Vascular malformations

  Spinal lumbar puncture (LP)
LP in 5% makes severe headache

Spinal cord injuries are up. Why? More tourism to swimming resorts where tourist, often dulled by alcohol, dives into shallow water and hits head, compressing the neck spinal bones to cause high cervical spinal injury of the worst type - pentaplegia, paralysis of all 4 extremities (quadri- or tetra-plegia) plus of the diaphragm breathing muscle and of the neck. Also riding a horse (The late movie Superman Christopher Reeve) where the horse suddenly stopped and threw the rider forward on rider`s head.

   The spinal cord is an information superhighway of nerve fibers that pass sensory signals up the cord to brain. Oppositely, it transmits motor signals down from brain. It is also a center of reflex muscle tone to keep our bodies erect and to control the bladder and rectum.

   The spinal cord is inside the bony vertebrae and its covering outer membranes that contain the cerebrospinal fluid is a space where bacteria or virus infection-causing meningitis may enter the CNS and spread up to the brain. (Note recent meningitis from back pain injections.)

Spinal Cord Segments: Highest, the cervical (C, neck) 1 to 8. (There are only 7 cervical bones compared to 8 cervical spinal cord segments; C1 and C2 spinal cord segments share the C1 bone space). Next lower, the thoracic (T, rib) 1 to 12; and below it the lumbar (L, low back) 1 to 5; and, lowest, the 5 fused sacral (S) with coccyx, or tail bone. 

                  Use magnifier for inspection.

Thoracic-level spinal segment. Note the spinal cord slice (labeled “Cord segment”), its central outline in the shape of fancy letter H. The H area is colorless here, but in a real cord it would be gray matter because packed with neurons. The part of cord outside the H is the white matter of fibers. Most ascending and descending fiber tracts cross from one side to the other side at some level in the spinal cord explaining why one side of brain causes opposite side of body weakness (hemiplegia) in brain stroke. There are also fibers that run horizontally, connecting each side's neurons at same level. 

   Note, on each side of spinal cord, left and right, dorsal (back side) and ventral (belly side) nerve roots, each root made of rootlets on each side of a cord segment. Signals in the ventral rootlets pass outwardly from the cord in the motor nerve fibers to a spinal nerve. Each smallest fiber comes from a final spinal motor neuron in the gray matter of same-side cord segment, the end of a long relay that runs from opposite-side brain's cerebral cortex rear frontal lobe motor neurons and passes the motor action signal down through brain stem and across to opposite side of spinal cord to relay the signal to final spinal neurons in the gray matter at the spinal segment from where the muscle target of the nerve fiber is located.

   Now give attention to left and right dorsal root and ganglion. Rootlets combine into the dorsal root and it immediately becomes a dorsal root ganglion.”(Labeled only on your right) Each left and right dorsal root ganglion, one of 31 pairs in the spinal cord, is the location of  the most distal (from CNS) neurons that grow the sensory nerve fibers to the body's periphery. The sensory neuron receives signals of the senses - touch, heat, etc. - from the periphery and the signals continue upwards into the spinal cord gray matter where the fiber synapses (connects) with 2^nd relay sensory neuron and continues the signal towards the brain’s thalamus and cerebral cortex. The central fibers of these dorsal root-ganglion neurons transmit centrally directed sensory signals from the periphery as each ascends and crosses left-right, right-left in the white-matter columns of the spinal cord.

   Note the contrast between the sensory and motor parts of the nervous system at this level: The sensory system (dorsal roots) has its most distal (farthest from the CNS) neurons outside the CNS in a dorsal root spinal ganglion. In contrast, the motor ventral root contains no neurons but is only a route for its motor nerve fibers to leave the spinal cord; each ventral root fiber's neuron of origin is in the anterior gray matter of the spinal cord at same level. And these motor neurons transmit thru synapse (relay connection to final motor neuron in cord) to relay the signaling from the brain to the muscle target. And note, too, the dorsal route does not use synapses but instead relies on pseudo-polar connectivity (See Peripheral nerve chapter).

   Note that on each side, beyond the dorsal and ventral roots,  the distal fibers combine together to form the left and right mixed motor and sensory spinal peripheral nerves for the body segment. The resulting spinal nerve on each side divides into two main branches, the “primary dorsal ramus” which curves back and is the peripheral nerve for the rear of that body segment (the back), and the “primary ventral ramus” which branches to form the peripheral nerves for the front body segment and whose main trunk runs just under each rib with an artery and vein to form a subcostal nerve on each side in the thoracic segments.

   Finally note on each side and in front of the cord segment, the labeled “Sympathetic chain ganglion”, left and right, a group of neurons that receive their fibers from the spinal nerve, and send signals to the vital organs (heart, lungs, glands). These ganglia are part of the sympathetic outflow of the autonomic nervous system. A particular infectious disease of the dorsal nerve neurons, Herpes zoster, aka Shingles, is caused by the chickenpox virus lying dormant in the dorsal root neuron and then in older age due to lower immunity or trauma or toxin, spreading down the spinal nerve of a particular segment of one side or the other.  Its diagnostic characteristic is an infectious, itchy chickenpox rash that shows only along the strip of skin on either left or right side (only one side or the other) starting at mid back and extending laterally around side of body or down extremity or side of face.

(Main Text Continues) Sensory signals are transmitted into the spinal cord from the skin and other organ or tissue. The sensory receptors are for touch, position, pressure, pain, temperature, stretch. When stimulated, a sensory receptor generates an electric potential that starts a signal that passes into the cord to make first connection by synapse in gray matter of the cord at that level. Then as the signal ascends in the spinal cord it crosses over and passes into the opposite side and, after another neuron relay by synapse, it ends in the opposite-side thalamus, where it synapses with a thalamic neuron and then is flashed to the parietal lobe cerebral cortex (and other parts of Brain) and as a result each of us becomes aware of the stimulus as pain or heat or pressure or pleasure and also is able to recognize objects by touch-feel.

   The main trunk of a spinal nerve carries both motor, sensory and autonomic fibers that have joined from peripheral sites, run together and then separate for each one's specific function. 

The Dermatomes are horizontal or semi horizontal surface strips of the body that delineate each spinal nerve root control area.

Four years ago I started getting a new kind of headache in back of head on awakening from sleep. A pain pill relieved it but the headache caused anxiety. Did I have a brain tumor? I looked in the neurology book and saw the dermatomes, front and rear.  I noted my new headache site coincided with dermatomes whose sensory nerves come from spinal segments cervical #2 and #3 of back of neck and I realized that the headache was coming from my neck which contains the C2 and C3 jointed vertebrae and, when overextended because of sleeping on small hard Japanese pillow, it could irritate the C2-C3 spinal nerves. I tested that idea by changing to a soft pillow and wearing a soft cervical collar to relieve tension and pressure on my neck and – Lo! – immediately my “pillow” headaches went away. 

  It’s one of many instances where knowledge of the dermatomes is an anxiety reliever.

  Where are the dermatomes?  Take a look!

The numbered dermatomes. Note shoulders are C4 and bare numbers below it are Thoracic (chest) segments and should be labeled T2, T3, T4, T5 … T12, and in inguinal zone, L1 is Lumbar 1. The dermatomes seen on the torso start in the mid line front of chest (Left and Right), could be traced around to rear and on both sides would meet at backbone in the mid line. Each dermatome consists of separate left and right. The dermatome zones represent areas innervated by the same numbered spinal nerve rami (branches) with the anterior transmitting sensory stimuli from front chest, and posterior from rear. Also note on frontal view that numbered zones on your right (the figure’s left) are all even and at slightly higher level than the odd number zones on your left. This is to indicate the normal overlapping of dermatomes. So T2 and T3 share their lower and upper dermatome-shown parts of zones respectively on both sides of chest. 

Note on your right upper part, the figure's left side of face and head, the dermatome map shows the shoulder and high front chest locations of right and left C4 dermatomes. Then note the oblique zone of C2 and C3. Actually C3 is mostly a zone around neck while C2 goes from top of head, ears and angles of jaw and below it. But there is so much overlap that it is best to consider them together as in the map on face and head.

Note that face, forehead and front of scalp are innervated by cranial nerve V (the Trigeminal nerve), with its 3 branches.

Note on your left upper view, the dermatome map of arms and hands, with palms facing you, and to right of it, with back of hand. You can see that the serial progression of dermatomes goes from thumb and outer shoulder aspect of arms to fifth finger and underarm aspect. So upper inner arm is T2 dermatome and side of shoulder is C5. Note the thumb is C6 and pinkie, C8. It is the reason that pain from coronary artery heart attack often goes down inner left arm to pinkie finger, because it is within the heart organ sensory range of C8, T1 and to T4. Also peripheral neuropathies can be usefully identified (e.g., radial nerve PN from misplaced arm injection gives numb pinky finger).

Finally note the lowest spinal segment dermatome is over the inner rear buttock surrounding anus, and note that, in the feet, the lowest spinal dermatome, S1, is over the heels. It is the reason that shooting pains of sciatica, which emanate from irritation of sacral dorsal root ganglia neurons, go down back of leg to as far as the heel. 

(Main text) Dermatomes indicate zones on surface of body innervated by anterior and posterior rami (front and rear main branch) of sensory spinal nerves of each spinal cord segment. (Only exception from spinal nerves is forehead and face by Cranial Nerve V.) Locating dermatomes is useful because diseases of spinal nerve are limited to its dermatome and to each side, e.g., herpes zoster, aka "shingles" of left T10 shows its skin rash strictly limited to the left T10 dermatome, which the body map shows to involve the left side horizontal zone of skin from navel level around left flank to same level zone on back to mid line of the vertebral column; the 1-sided rash of shingles stops abruptly at the mid line of body - an important diagnostic point.

   Keep in mind that left and right side of body are separate dermatomes at each spinal level served by right and left spinal nerve each separately.

   Some memorable dermatome surface markers: thumb C6, middle finger C7, pinky finger C8, nipple T4, bellybutton (navel) T10.

   The sensory nerves to the heart connect to left side dermatomes C8, T1, T2, T3 and T4. Thus pain from heart may radiate into left underarm and down inner side of arm to 5^th (pinky) finger. And it may occur as isolated localized pain disconnected from the chest. Or it may be felt as isolated soreness in left upper back over the wing bone (scapula). 

Knowledge of the dermatomes and related pain syndromes could save your life by allowing early diagnosis.

   The muscle equivalent of dermatomes is skeletal muscle groups innervated by spinal motor neurons from particular spinal segments. Using that knowledge we can deduce a level of spinal cord damage (by tumor, trauma) from its muscle paralyzing effect. For example spinal cord injury with quadriplegia/pentaplegia (both arms and both legs paralyzed) where the diaphragm muscle of abdominal breathing is paralyzed means damaged cord at or above C3/C4 level and in absence of immediate respiratory rescue, it leads to death by asphyxiation. An inability to lift right or left arm sideways (abduct) while retaining ability to breathe means left or right spinal cord damage at C5. These are important spinal cord damage signs based on muscle-related dermatome knowledge and can be life saving.

Spinal Reflex: Have you seen a doctor use a reflex hammer to elicit a knee jerk? The reflex knee jerk is one of several muscle responses of skeletal muscle spinal reflex to sudden stretch.

The doctor’s hammer strikes tendon, which increases its tension. This increased tension sends signal through sensory fibers that go from tendon and muscle fiber up the local sensory nerve and into spinal cord dorsal nerve root of Lumbar 3 (L3) segment that has first-relay sensory neurons. The signal passes through sensory nerve fibers and ends in the gray matter of L3 spinal cord where the incoming sensory signals make synapses to alpha motor neurons and excite them to initiate a sudden contraction signal that causes an obvious knee jerk from the tendon hammer hit. (Review on below diagram)

Beyond common diagnostic use, the spinal motor reflexes in the legs are important in preventing sudden falling down. Without the spinal reflex, our trunk and leg muscles would be too flaccid and relaxed and we could not support selves in erect position. The reflex, especially in legs, produces a constant state of muscle strip tension (called “tone”) without which we might collapse from standing erect; and the inhibitory part of the reflex prevents this tone from being too much, in which case we would walk around like the Tin Man in The Wizard of Oz.  Spinal motor reflexes are one element producing smooth, effective, safe motor movement. By the way, the spinal reflex is an example of feedback where a signal from the periphery bounces back and affects behavior, here, automatically and note it is independent of brain. The opposite of feedback is feed forward which is the normal direction of nerve impulses from the source of the stimulus to a receiving neuron higher up in the system.

Danger from Aerobics: 1) An epidural hematoma (collection of blood outside spinal cord in and on its dura from trauma to one's back caused by pull-ups, barbell body bench pressers  and other aerobics. 2) Another aerobics accident just got reported of a 20-year-old woman who lost power and feeling in both legs and lost urine bladder control after her first aerobics. An MRI showed a hemorrhage into the spinal cord at T-4 level from a ruptured arteriovenous malformation (AVM) she had not previously been aware of and that was torn by the aerobics. She will be in a wheelchair for life. The advice seems clear: Say No to aerobics. If you must start it, get a preliminary MRI of the spine and skull to check for AVM or other abnormalities that put you at risk for traumatic hemorrhage from various activities from aerobics to riding a roller coaster. 

  Endnote: Vascular malformations (Similar to the above-mentioned AVM) affect the spinal cord by creating a pressing tumor or due to reduced blood supply. They present symptoms that suggest cancer and may be disastrously mistaken for it. My sister-in-law had one involving the thoracic cord that caused her to start falling down a lot. Everyone worried it was cancer but an MRI showed it was a malformation and surgical removal cured it, and, even before the surgery, her getting the MRI relieved much anxiety and restored happy life. 

Spinal lumbar puncture (LP) is based on the cord being surrounded and bathed in spinal fluid in its subarachnoid space. (Between the arachnoid The membrane and spinal cord pia membrane surface.) The fluid is connected with the cerebrospinal fluid circulation in the brain. The puncture (spinal tap, or lumbar puncture, or LP, when done in lumbar spine) is also used for regional anesthesia which numbs and paralyzes without loss of consciousness. It is also a test for infections on and in the brain, such as meningitis or encephalitis, and to find out about hemorrhage or tumor. Today in 2021, the LP is limited mostly to test for CNS infection since MRI is a better, safer test for hemorrhage or tumor. When intracranial pressure is high – as it is with hemorrhage or tumor –, an LP may provoke a deadly herniation of the brain substance. 

LP in 5% makes severe headache on standing up because of loss of CSF and low fluid pressure. If you are a patient who may get an LP, mention post-LP headache to the doctor doing it so that he will use the smallest bore needle possible and also do not stand up (or limit it as much as possible) for the 6 hours immediately after the LP.

Treadmill training improves locomotor function in patients with partial spinal cord injury.

A. A patient is partially supported on a moving treadmill by a harness, and stepping movements are assisted by therapists.

B. Locomotor function improved in 44 patients with chronic spinal cord injury after they received daily training lasting from 3 to 20 weeks. The functional rating ranges from 0 (unable to stand or walk) to 5 (walking without devices for more than five steps).

END OF CHAPTER. To read next click

9.10 Secrets of Nerve Signals to the Brain

For an exciting, world-spanning, sexy reading and lots of useful, educative info. at your leisure, see Slim Novels 1 to 18  by clicking http://adventuresofkimi.blogspot.com