Deep Back and Spinal Cord

Learning Objectives

Moore's Clinically Oriented Anatomy, 7th ed.; pages 46-65, 439-482, 496-507

Upon completion of this session, the student will be able to:

1. Describe the spine and its movement

The spine extends from the base of the skull (C1 = atlas vertebra) to the coccyx and forms the main part of the axial skeleton. It consists of 33 vertebrae arranged in five regions: 7 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 4 coccygeal. Motion occurs among only 24 vertebra (cervical, thoracic, and lumbar) since the sacral and coccygeal vertebrae are typically fused. Movement of the spine includes:

- - Flexion (touching your toes)
  - Extension (bending backwards)
  - Lateral bending
  - Rotation

Based on their structure, different vertebrae or regions of the spine are more or less responsible for different movements; ie, lumbar vertebrae are involved more in flexion while thoracic vertebrae are involved more in rotation.

2. Identify the osteologic features of a typical vertebra, of the special vertebrae, and of the sacrum as listed in the List of Structures. See BlueLink Images

3. Describe and differentiate between the Central Nervous System (CNS) and the Peripheral Nervous System (PNS).

The nervous system allows the body to react to the constantly changing external and internal environments of the body. Structurally, the nervous system is divided into the CNS (brain and spinal cord) whose main function is to integrate and coordinate incoming and outgoing signals, and the PNS (the nerves connecting the rest of the body to the CNS, known as spinal nerves and cranial nerves). The primary function of the PNS is to conduct nerve impulses from the body (or periphery) to the CNS and from the CNS to the rest of the body (periphery).

A nerve is:

- - a bundle of nerve fibers or fascicles
  - the connective tissue surrounding and binding these fibers together
  - blood vessels that nourish the fibers and connective tissue

Within a nerve, there are often fibers conducting impulses in both directions – so afferent fibers conveying an impulse from a sensory receptor to the CNS and efferent fibers conveying an impulse from the CNS to affect a muscle or gland. This kind of nerve (with both afferent and efferent fibers) is called a mixed nerve.

Spinal nerves arise in bilateral pairs from specific segments of the spinal cord. There are 31 pairs of spinal nerves. The nerves arise from the spinal cord as rootlets. The rootlets converge to form two nerve roots – the ventral root and the dorsal root. The ventral root carries efferent (motor) fibers and the dorsal root carries afferent (sensory) fibers. At the intervertebral foramen, the ventral and dorsal roots unite to form a mixed spinal nerve. This spinal nerve immediately begins dividing into branches to reach different parts of the body.

4. Define a dermatome.

The area of skin that is supplied or innervated by sensory fibers of a single spinal nerve is called a dermatome. These distal nerve branches (that innervate the skin) are called cutaneous nerves. On the torso, the dermatome containing the nipple of the breast is typically innervated by cutaneous branches of spinal nerve T4, and the dermatome containing the umbilicus is innervated by cutaneous branches of T10. You should begin learning the dermatome map of the human body; it will be useful in your clinical studies of potential nerve lesions, particularly of the upper and lower limbs.

5. Describe and identify the coverings, spaces, and supporting structures of the spinal cord.

Three layers of tissue collectively make up the meninges – the pia mater, arachnoid mater, and dura mater. The meninges and the cerebral spinal fluid (CSF) surround and protect the CNS. The innermost meningeal layer, the pia mater, directly covers the brain and spinal cord. It is a delicate, transparent covering. The next layer is the arachnoid mater. The CSF is located between the pia mater and the arachnoid mater in the subarachnoid space. External to the arachnoid mater is the thick, tough dura mater. In the spinal cord region, the space external to the dura mater is filled with fat and is called the epidural space.

The denticulate ligament is a toothed thickening of the pia mater running longitudinally on the sides of the spinal cord. The ligament separates the dorsal and ventral roots of the spinal nerves. Its denticulations (or “teeth”) extend laterally and anchor the spinal cord within the vertebral canal.

6. Differentiate between the terminal end of the spinal cord vs. the dural sac. Why is this important clinically?

In adults, the spinal cord ends at about the vertebral level of L2; the caudal end of the cord is called the conus medullaris. However, the dura mater (with the arachnoid mater adhering to the inner side of the dura) continues to vertebral level of about S2. This means that the subarachnoid space and its CSF continue to bathe the nerve roots that hang down from the spinal cord. Note that the nerve roots arise from rootlets coming from the spinal cord (which ends at L2). However, the roots don’t fuse into the spinal nerves until they are exiting through the intervertebral foramen at the “appropriate” vertebral level; eg, the L5 rootlets exit the spinal cord superiorly to where the L5 spinal nerve exits the vertebral column. The nerve roots, then, hang down from the caudal end of the spinal cord, and are bathed in CSF since the dural sac continues to S2. This collection of long nerve roots is called the cauda equina. It is of clinical importance because CSF can be withdrawn from this space without danger of hitting the spinal cord with a needle or anesthetic can be inserted into the subarachnoid space without danger to the spinal cord. The needle is inserted at L4/L5 and is well below the caudal end of the spinal cord. This spine level may be found by following the plane between iliac crests.

In order to anchor the caudal end of the spinal cord, the pia mater forms a filament or ligament from the conus medullaris at L2 to the inner side of the dural sac at S2. This pia mater structure is called the filum terminale. The dural sac then anchors to the coccyx with a ligament from its distal end at S2 to the coccyx. This is called the coccygeal ligament.

7. Prepare for lab by reading through the condensed, step-by-step dissection summary found here.

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