Human Anatomy Lesson 3
Lesson 3 - Vertebrae, Joints, Ligaments
Welcome! This is the third lesson in a semester-long undergraduate-level course in Human Anatomy. In this lesson, you will be learning about the bones, joints, and ligaments of the vertebral column. This lesson builds on two previous lessons that focused on anatomical terminology, position, and movement and medical imaging, systems, and the names of bones in the human skeleton. This is the third lesson in a full course on human anatomy that will give you a solid background for studying human biology, health and medical sciences, and biological anthropology.
In our tour of the human body, we are starting with the spine, nervous system, and back. This is a common sequence to follow in human gross anatomy courses. It has a few things to recommend it. First, learning the muscles of the back is relatively easy, and the back dissection is simple. Students have not yet developed dissecting skills, so their first dissection will be somewhat rough. In addition, the back dissection is self-contained, so dissection mistakes on the back will have minimal effect on other dissections later in the semester. Second, the body starts in a face-down ("prone") position, and students can work on the back and then flip the body once so that dissections for a large part of the rest of the semester can proceed with the body in face-up ("supine") position.
In our discussion of the spine and back, we will begin in this lesson with information about the bones, ligaments, and joints of the spinal column and then move to the spinal cord and somatic nervous system in lesson #4, autonomic nervous system in lesson #5, and muscles of the back in lesson #6.
Learning Objectives - By the end of this module, you should be able to ...
- Identify and describe the different regions of the vertebral column
- Identify the typical and unique characteristics of vertebrae within each region
- Describe the position of the spinal cord and other structures in the vertebral column
- Identify the types of joints that hold the vertebral column together
- Describe the different routes by which spinal nerves from the spinal cord leave the vertebral canal to reach other parts of the body
- Identify the six types of ligaments that hold the vertebrae together and explain how they limit motion in the vertebral column
The skeletal framework of the back consists mainly of the vertebrae and intervening intervertebral (IV) discs, although the skull, scapulae, pelvic bones, and ribs also provide attachment sites for muscles. The normal individual has 33 vertebrae which can be broken up into five different groups, listed from superior to inferior: (1) cervical; (2) thoracic; (3) lumbar; (4) sacral; (5) coccygeal. In humans, sacral and coccygeal vertebrae are fused into two structures that are called the sacrum and the coccyx, respectively.
Characteristics of All Vertebrae
Most vertebrae share a number of features:
- Vertebral body, or weight-bearing element.
- Vertebral arch, which encircles the spinal cord and is made up of two pedicles (which attach the vertebral arch to the body) and two laminae (which form the roof of the vertebral arch - note the different terminology in the picture above, which is from the 1918 edition of Gray's Anatomy).
- Spinous process, which arises from the laminae in the sagittal plane and participates in the structure of the roof of the vertebral arch.
- Two transverse processes, which jut out laterally at the junction between the pedicles and laminae.
- Superior and inferior articular processes, which arise from the vertebral arch and lock together in adjacent vertebrae to form the shape of the spine. The inferior articular process of an overlying vertebra articulates with the superior articular process of an underlying vertebra at a facet, which forms a joint filled with synovial fluid.
- Vertebral notches - the notches for overlying and underlying vertebrae fit together to form an intervertebral foramen, which provides passage for spinal nerves and blood vessels associated with the spinal cord to exit the vertebral canal to innervate structures in the rest of the body.
Characteristics of Particular Vertebrae
In addition to the characteristics typical to most vertebrae, each type of vertebra has its own unique characteristics. Next, let's break down the vertebral column by vertebra type, and note the typical and unique components of each type:
- Cervical vertebrae: There are 7 cervical vertebrae. This number is conserved across vertebrates. Even giraffes with their long necks only have seven (extremely tall!) cervical vertebrae. In humans, five of these cervical vertebrae are variations on a theme and are considered to be typical. Vertebrae 3-7 have short, small, square-shaped vertebral bodies when viewed from the superior surface, with a concave superior surface and a convex inferior surface; small transverse processes pierced by a transverse foramen (Gray's Anatomy uses the term foramen transversarium); a short, bifid (split) spinous process; triangular vertebral foramen; and horizontally- to mildly-inferiorly-projecting spinous processes when viewed from the lateral side. The tips of the spinous processes point more inferiorly the further down the cervical spine you go, and the spinous process of C7 is, along with T1, the most projecting spinous process of the whole spinal series, and can be easily palpated at the base of the neck. The unique features of cervical vertebrae C3-C7, the transverse foramina and bifid spinous process, are due to the passage of vertebral vessels (including the vertebral artery and vein and sympathetic nerves) in the neck and the presence of the nuchal ligament, respectively - see below.The first two cervical vertebrae, the atlas and axis, are unique. C1, the atlas, is "all" vertebral canal, laminae, and pedicles, with a small vestigial vertebral body called the anterior tubercle and a small vestigial spinous process called the posterior tubercle. It is shaped like a ring. C2, the axis, is the opposite - it has more body than it should, with a somewhat substantial spinous process but diminished transverse elements. The dens of the axis projects superiorly into the space left by the "missing" body of the atlas, and the two are tightly bound together by the transverse ligament of atlas and the alar ligaments. The lateral masses of atlas hold facets for the occipital condyles on the inferior side of the cranium. The atlas and head, anchored at the occipital condyles, rotate medially and laterally (as in the gesture "no") on the axis, and the head rocks anteriorly and posteriorly (as in "yes") on the atlas. The position of the dens of the axis relative to the atlas, occipital condyles, and foramen magnum is of great clinical interest. In a car or motorcycle accident, the head and neck experience traumatic "whiplash" forces when the head is thrown forward and quickly back. In such cases, the dens can fracture. Since the dens projects superiorly through the arch of the atlas, it is in close approximation to the non-cortical brain in the foramen magnum at the base of the cranium. Entrance into the non-cortical brain through the foramen magnum results in a quick, painless death, since the heart and lungs immediately cease to function.
- Thoracic vertebrae: There are 12 thoracic vertebrae, although this number can vary, with a small proportion of individuals having one fewer or one more thoracic vertebra. The superior thoracic vertebrae share some characteristics of the overlying cervical vertebrae (but not a bifid spinous process or transverse foramina), and the inferior thoracic vertebrae resemble the upper-most lumbar vertebrae to some degree. In general, thoracic vertebrae have long spinous and transverse processes; a heart-shaped vertebral body and circular vertebral foramen in superior view; four partial facets (called demifacets or superior and inferior costal processes) for articulation with the head of the rib associated with that vertebra and the head of the rib below that; and transverse costal processes on the transverse processes for articulation with the neck of a rib. Spinous processes of thoracic vertebrae overlap one another and also the next vertebra in line down the spine, which seriously limits flexion and extension of the spine in the thoracic region as compared to the cervical and lumbar regions (although a fair amount of lateral flexion and rotation are still possible).
- Lumbar vertebrae: There are 5 lumbar vertebrae although there is a fair amount of variation in numbers in human populations, with a small percentage of individuals having 6 lumbar vertebrae and a small percentage having 4. In general, lumbar vertebrae are large, with thin, long transverse processes (except for L5, in which the iliolumbar ligaments anchor the transverse processes to the ilium of the pelvis). Note that the picture to the right below is of L5, so the transverse processes are more substantial. Lumbar vertebrae have wide, heavily-built vertebral bodies, a triangular vertebral foramen, and tall, horizontally-oriented rectangular spinous processes that do not overlap one another. The superior and inferior articular processes of lumbar vertebrae face medially and laterally, respectively, and are oriented largely in the sagittal plane. They fit together like two gloves, which limits rotation, but not flexion and extension, of the lumbar spine.
- Sacrum: the sacrum is one structure that is the amalgamation of 5 fused sacral vertebrae. In anterior view, the sacrum is triangular in shape, with the widest superior portion made up of two alae that articulate with the pelvic bones at the sacroiliac joints. The narrowest part is inferior, where the last sacral segment articulates with the coccyx. The sacrum has a concave anterior surface and convex posterior surface, and the inferior part of the sacrum and the coccyx point forward in anatomical position, much like a scorpion's tail. The elements of the five fused vertebrae are readily discernible on the sacrum: the spines of sacrum or median sacral crest are remnants of spinous processes, the sacral bodies are remnants of vertebral bodies, the alae are remnants of transverse processes, and the sacral canal is equivalent to the vertebral canal. There are four pairs of foramina (the plural of foramen) on the posterior and anterior surfaces of the sacrum that pass the posterior and anterior rami of S1 to S4 spinal nerves. In an adult, the spinal cord only extends to the level of the intervertebral disc between L1 and L2, with some variation. Below this level, spinal nerves still pass inferiorly through the vertebral canal into the sacrum at the sacral canal. The sacral canal is often incompletely closed at the inferior end of the sacrum. This used to be a popular site to administer an epidural, but it is important to be aware that it is a site of significant natural anatomical variation.
- Coccyx: this is a small, triangular bone that is the remnant of 3-4 coccygeal (tail) vertebrae. The coccyx does not have vertebral arches or canal, and it is, in fact, the fused remnant of tail vertebrae.
Entrances, Exits and Joints
To this point, we have discussed the vertebra individually, noting the typical and unique characteristics that each type of vertebra possesses. It is important to emphasize how the vertebrae fit together and how they function as a whole. The vertebrae in the spinal column are stacked one on top of the other and are separated by intervertebral (IV) discs. When the vertebrae are stacked on top of each other, the vertebral foramina line up and form the vertebral canal. The spinal cord runs in the vertebral canal. The spinal nerves that originate from the spinal cord exit the vertebral canal through the intervertebral foramina, which are the holes made up by the superior and inferior vertebral notches of inferior and superior vertebrae, respectively. In addition, there are gaps in the posterior part of the lumbar vertebral column between the vertebral arches and spinous processes of successive non-overlapping vertebrae. As noted above, the spinal cord terminates around L2. Therefore, the gap between spinous processes below the L2 level (L3-L4, L4-L5) are popular spots to administer an epidural or spinal tap.
The junction between a vertebra and IV disc is a symphyseal joint which is formed by a layer of hyaline cartilage. In the vertebral column, the IV disc is made up of a tough outer layer of cartilage called the annulus fibrosus which surrounds a gelatinous inner layer called the nucleus pulposus. Each vertebra is associated with two symphyseal joints, one above and one below. The other type of joint associated with the vertebrae is a zygapophyseal joint, which is a synovial joint between the superior and inferior articular processes of adjacent vertebrae. Each vertebra is associated with four zygapophyseal joints, two above and two below. The facets of the articular processes that make up the zygapophyseal joints are oriented fairly horizontally in the cervical region, but are more vertically-oriented in the thoracic and lumbar regions, which limits rotation to a greater degree. The articular processes in the lumbar region are interlocking, like two mitts.
Ligaments of Spine
There are six spinal ligaments which help support the spine and keep it tightly packed together. Five of these ligaments (posterior longitudinal, ligamenta flava, supraspinous, nuchal, and interspinous) act to prevent excessive flexion of the vertebral column, but only one (anterior longitudinal ligament) acts to prevent excessive extension.
- Anterior longitudinal ligament, which runs superiorly and inferiorly along the anterior aspects of the vertebral bodies and IV discs from the cervical vertebrae down to the sacrum. The anterior longitudinal ligament is the only ligament that prevents excessive extension of the vertebral column. This ligament is often strained during whiplash of the neck. Neck collars used to treat whiplash are designed to be higher in the back than in the front, which keeps the head and neck flexed and prevents further stress to the anterior longitudinal ligament.
- Posterior longitudinal ligament, which runs superiorly and inferiorly along the posterior aspects of the vertebral bodies from the cervical vertebrae down to the sacrum, inside the vertebral canal. The posterior longitudinal ligament helps prevent excessive flexion of the vertebral body and reinforces the posterior aspect of the annulus fibrosus.
- Ligamentum flavum (pl. ligamenta flava), which passes between the lamina of adjacent vertebrae, from posteroinferior to anterosuperior, and forms part of the posterior surface of the vertebral canal. Ligamenta flava help prevent excessive flexion of the vertebral column.
- Supraspinous ligament, which runs along the tips of the spinous processes from C7 to the sacrum. The supraspinous ligament helps prevent excessive flexion of the vertebral column.
- Nuchal ligament (also called ligamentum nuchum), a tough, large continuation of the supraspinous ligament that attaches to the external occipital protuberance and foramen magnum of the cranium, the posterior tubercle of C1, and the spinous processes of the cervical vertebrae. The nuchal ligament acts to resist flexion of the head and neck and to keep the head from pitching forward during running.
- Interspinous ligaments, which run from the base to the apex of adjacent spinous processes. These ligaments blend with the supraspinous ligament and ligamenta flava and act to prevent excessive flexion of the vertebral column.
There is an extraordinary amount of tension placed on the 5 ligaments that prevent excessive flexion of the vertebral column when you attempt to lift something heavy off the ground without bending your knees. None of your back muscles are active at the initiation of the lift, so there is a great danger that you will injure these posteriorly-positioned ligaments if you are not careful. Take-home point: your mom was right, you should always bend your knees when you attempt to lift something heavy off the ground!
Preview of Upcoming Lessons
Three more lessons to go in the back region, then we move on to the thorax!
- Human Anatomy Lesson 4
This lesson covers the anatomy of the spinal cord, spinal nerves, and somatic nervous system.
- Human Anatomy Lesson 5
This lesson covers the autonomic nervous system, focusing on how sympathetic nerve fibers get to different parts of the body.
- Human Anatomy Lesson 6
This lesson covers the muscles and movements of the back.
© 2015 Robert McCarthy