Spinal Cord and Spinal Nerves
Directions
- Print this PDF worksheet for a hardcopy guide as you work through this lesson.
- Within the lesson click the red linked headings to bring up the desired starting point within the cadaver for your work.
- Use the provided images on the worksheet to annotate and identify specific anatomical structures.
Three protective membranes, called meninges, surround and protect the brain and spinal cord. They are the dura mater, arachnoid mater and pia mater.
- Dura Mater - In the dissection area, locate and highlight the dura mater surrounding the spinal cord. Note its location in the cross sectional view, too. The dura mater ("tough mother") forms a sac from the foramen magnum where it is continuous with the brain dura mater. The dura is the most superficial of the three meninges that surround and protect both the brain and the spinal cord. The space superficial to the dura mater between it and the vertebral canal wall is the epidural space. A protective cushion of fat and connective tissue is located here. An "epidural" is a form of anesthesia frequently used during childbirth in which a needle is placed between the vertebrae into the epidural space. A small catheter is then threaded through the needle into the space and the needle withdrawn. The catheter is left in place and taped so that it does not move. Small amounts of pain medication can then be given continuously or as a single bolus. An epidural relaxes the pelvic muscles and the nerves are bathed in the local anesthetic medication which causes an insensitivity to pain. Pressure and the urge to push is still felt. After the baby is born, the catheter is removed.
- Arachnoid Mater - Deep to the dura mater is a very delicate network of collagen and elastic fibers that form the next meninx, the arachnoid mater. Due to its delicate nature, it is not visible in our specimen. Use another source to locate it in an appropriate diagram. You'll see that it is the middle of the three protective meninges. The subarachnoid space located between the arachnoid mater and the deeper pia mater is where a needle is inserted to remove cerebrospinal fluid during a lumbar puncture procedure.
- Subdural Space - Between the dura mater and the arachnoid mater is the subdural space that contains interstitial fluid.
- Cerebrospinal Fluid Space - Deep to the arachnoid mater, locate the cerebrospinal fluid space.
- Pia Mater - The innermost meninx is the pia mater. It is a thin transparent connective tissue layer that adheres directly to the spinal cord surface. Again, due to its nature, it is not visible in your specimen.
- Conus Medullaris - Dissect away the spinal dura mater and the lumbar cerebrospinal fluid space. Locate and highlight the spinal cord emerging from beneath the thoracic cerebrospinal fluid space. Notice how it tapers to a rather sharp conical point particularly noticeable in the sagittal cross section. This is called the conus medullaris which ends between the first and second lumbar vertebrae in adults. Confirm this by locating the L1 and L2 roots of the lumbar plexuses.
- Cauda Equina - Nerves that arise inferior to the conus medullaris leave the vertebral column inferior to where they exit the cord. The roots of these nerves angle inferiorly in the vertebral canal from the end of the spinal cord like wisps of hair. These roots are collectively named the cauda equina meaning "horse's tail." Again, this is not labeled in your specimen but it is clearly visible.
- Filum Terminale - Inferior to the conus medullaris and emerging from the cauda equina, the filum terminale is an extension of the pia mater that anchors the spinal cord to the coccyx. This is not specifically labeled in your specimen but it is the central strand from the tip of the conus medullaris through the lumbar and sacral canal and attaching to the coccyx.
First, let's define what we're looking at. This is a network of nerve axons called a plexus. As you can see from the external anatomy, this is the brachial plexus innervating the upper limbs and shoulder. Individual nerves emerge from a plexus. The primary plexuses are the cervical, brachial, lumbar, and sacral. There is also a smaller coccygeal plexus.
- Nerve Roots - T1 Spinal nerve roots have been already selected for easy identification. Locate both the posterior and anterior roots in the cross section (anterior is "up"). These two roots unite to form a spinal nerve at the intervertebral foramen. The posterior root contains sensory axons and the anterior root contains motor axons, all spinal nerves are "mixed."
- Rami - Shortly after passing through the intervertebral foramen, spinal nerves divide into several branches known as rami. The posterior (dorsal) ramus serves the deep muscles and skin of the dorsal surface of the trunk. The anterior (ventral) ramus serves the muscles and structures of the upper and lower limbs and skin of the lateral and ventral trunk surfaces. Due to their delicate nature, these structures are difficult to define in your specimen. None the less, if you look VERY carefully in this view you see posterior rami at L2.
- Phrenic nerve - The phrenic nerve innervates the diaphragm. Severing the spinal cord above the origin of the phrenic nerves (C3, 4, and 5) causes respiratory arrest. Breathing stops because the phrenic nerves no longer send impulses to the diaphragm.
We come back to the brachial plexus to examine a few nerves that generate from here. Mouse over the nerves of this plexus to identify each of the following:
- Musculocutaneous Nerve - The musculocutaneous nerve innervates the coracobrachialis, biceps brachii, and brachialis muscles.
- Median Nerve - The median nerve innervates most forearm flexors and most of the lateral palm and finger skin.
- Radial Nerve - The radial nerve innervates the triceps brachii and other arm and forearm extensor muscles as well as posterior arm and forearm skin, and most of the dorsal/lateral hand and fingers.
- Ulnar Nerve - The ulnar nerve innervates most of the hand muscles and much of the hand skin, too. This is the nerve that causes the hands to go numb when riding a bicycle for long distances. Weight placed on the hands puts pressure directly on this nerve resulting in the loss of feeling in the hands.
- Axillary Nerve - As you should know, the axillary region is the armpit. To fully reveal the axillary nerve, you need to dissect away the right musculocutaneous nerve and the right radial nerve. This will reveal a small length of the right axillary nerve near the right armpit. Over on the left side, the axillary nerve is visible just superior to the armpit. The axillary nerve innervates the deltoid and teres minor muscles as well as the overlying skin.
On we go to the lumbar plexus formed by spinal nerves L1 - L4. We'll examine two nerves that emerge from the lumbar plexus. Locate the following in your specimen:
- Femoral Nerve - The femoral nerve innervates thigh and leg flexors and extensors. Fibers also innervate some leg and thigh skin.
- Obturator Nerve - The obturator nerve innervates leg adductor muscles and some thigh skin.
Our last plexuses are often referred to collectively as the sacrococcygeal plexus. As you'll see, they are practically continuous with each other. Identify the combined sacral and coccygeal plexuses and identify the following nerves. Rotate your specimen laterally to gain the best perspective.
- Sciatic Nerve - The sciatic nerve is actually two nerves, the tibial and common fibular, bound together in a common sheath of connective tissue. These two nerves separate from each other near the knee.
- Tibial Nerve - The tibial nerve innervates the gastrocnemius, plantaris, soleus, popliteus, tibialis posterior, and some foot muscles.
- Common Fibular Nerve - The common fibular nerve divides to form the superficial and deep fibular nerves that innervate muscles of the leg and foot.
- Pudendal Nerve - The pudendal nerve innervates muscles of the perineum, skin of the penis and scrotum in males and clitoris labia majora/minora and vagina in females.
Self-test Labeling Exercises