2.12: Lab Exercise 14- The Spinal Cord and Selected Spinal Nerves

Lab Summary: In a previous lab, you focused on learning the anatomy and physiology of the brain. However, the spinal cord is equally important when it comes to the transmission and processing of information throughout the body; that will be the focus of this lesson. Additionally, you will study a selected group of spinal nerves through a series of clinical application questions. In a future lab, you will consider these nerves in further detail as part of reflexes.

• In a C5-Spinal Cord Cross Section Model, you should be able to identify and list the function(s) of o Dorsal horns
  • Lateral horns
  • Ventral horns
  • Dorsal root ganglion o Dorsal root
  • Ventral root
  • Spinal nerve
  • Central canal
  • White matter tracts
• In a Longitudinal Spinal Cord Model, you should be able to identify and list the function(s) of o Conus medullaris
  • Cauda equina
  • Filum terminale
  • Spinal nerves (generally where they are located)
• Identify location, nerve roots, and function(s) of
  • Cervical plexus, and Phrenic nerve
  • Brachial plexus, and Median nerve, Ulnar nerve, Radial nerve, Axillary nerve, Musculocutaneous nerve
  • Lumbar plexus, and Femoral nerve and Obturator nerve o Sacral Plexus, and Sciatic nerve

Longitudinal Anatomy of the Spinal Cord

Think of the spinal cord as a superhighway for information traffic (action potentials) that are incoming and outgoing from the CNS. In an adult, the spinal cord is about eighteen inches long and extends from the foramen magnum of the skull to approximately the first lumbar vertebra and is divided into regions that correspond to regions of the vertebral column (Figure \(\PageIndex{1}\), Figure \(\PageIndex{2}\)). The name of each spinal cord region corresponds to the level at which spinal nerves pass through the intervertebral foramina. Immediately adjacent to the brain stem is the cervical region, followed by the thoracic, then the lumbar, and finally the sacral region. The spinal cord has two areas where the diameter of the spinal cord is enlarged because of increased neural structures associated with the appendages. The cervical enlargement is caused by nerves moving to and from the arms and is located from approximately C3 through T2. The lumbar enlargement is caused by nerves moving to and from the legs and is located from about T7 through T11 (Figure \(\PageIndex{1}\), Figure \(\PageIndex{2}\)).

Figure \(\PageIndex{1}\): Nerve plexuses of the body.

The spinal cord does not extend the full length of the vertebral column because the spinal cord does not grow significantly longer after the first or second year while the skeleton continues to grow. As the vertebral column continues to grow, spinal nerves grow with it and result in a long bundle of nerves that resembles a horse’s tail, called the cauda equina (Figure \(\PageIndex{1}\), Figure \(\PageIndex{2}\)). Some of the largest neurons of the spinal cord extend from the cauda equina including the motor neuron that causes contraction of the big toe which is located in the sacral region of the spinal cord. This motor neuron’s axon reaches all the way to the belly of that muscle which can be over a meter in length in a tall person. The neuronal cell body that maintains that long fiber is also necessarily quite large, possibly several hundred micrometers in diameter, making it one of the largest cells in the body. Immediately superior to the cauda equina, the solid part of the spinal cord terminates at the conus medullaris, which begins around vertebra L1. Beyond the conus medullaris, the meninges that cover the spinal cord (discussed below) continue as a thin, delicate strand of tissue called the filum terminale (terminal filum), which anchors the spinal cord to the coccyx.

Thirty-one pairs of spinal nerves extend from the spinal cord and each pair is named for the level of the spinal cord from which each pair emerges: C1 to C8, T1 to T12, L1 to L5, S1 to S5, Co1. The first nerve, C1, emerges between the first cervical vertebra and the occipital bone. The second nerve, C2, emerges between the first and second cervical vertebrae. The same occurs for C3 to C7, but C8 emerges between the seventh cervical vertebra and the first thoracic vertebra. For the thoracic and lumbar nerves, each one emerges between the vertebra that has the same designation and the next vertebra in the column. The sacral nerves emerge from the sacral foramina along the length of that unique vertebra.

Figure \(\PageIndex{2}\): Spinal Cord, Longitudinal View. From Mettler FA: Neuroanatomy, ed 2, St. Louis, 1948, Mosby.

Cross-sectional Anatomy of the Spinal Cord

Each section of the spinal cord has its associated spinal nerves forming two nerve routes that include a combination of incoming sensory (afferent) axons and outgoing motor (efferent) axons. For example, the radial nerve contains fibers of cutaneous sensation in the arm, as well as motor fibers that move muscles in the arm. The sensory axons that form a part of the radial nerve enter the spinal cord as the dorsal (posterior) nerve root, whereas the motor fibers emerge as the ventral (anterior) nerve root (Figure \(\PageIndex{3}\) and Figure \(\PageIndex{4}\)). The cell bodies of sensory neurons synapse at the dorsal root ganglion, causing an enlargement of that portion of the spinal nerve. Note that you may sometimes see the terms dorsal and ventral used interchangeably with posterior and anterior, particularly in reference to nerves and the structures of the spinal cord.

In cross-section (Figure \(\PageIndex{3}\) and Figure \(\PageIndex{4}\)), the distribution of gray matter of the spinal cord is often compared to an inkblot test or butterfly, with the spread of the gray matter subdivided into regions referred to as horns, replicated on both left and right halves of the spinal cord. The dorsal horn receives information from the dorsal root and is, therefore, responsible for sensory processing. The ventral horn sends out motor signals to the ventral root to cause movement of skeletal muscles. The lateral horn, which is only found in the thoracic and upper lumbar regions, is a key component of the autonomic nervous system. It sends motor information to organs such as the heart, lungs, pharynx, esophagus, and intestines. The ventral/anterior median fissure marks the anterior midline; the posterior median sulcus marks the posterior midline. Each side of the gray matter is connected by the gray commissure and located in the center of the gray commissure is the central canal, which runs the length of the spinal cord. The central canal is continuous with the ventricles of the brain and transports CSF and nutrients to the spinal cord.

Figure \(\PageIndex{3}\): Cross-section of spinal cord

Figure \(\PageIndex{4}\): Cross-section of spinal cord and associated structures

Comparable to the gray matter being separated into horns, the white matter of the spinal cord is separated into columns (Figure \(\PageIndex{4}\) and Figure \(\PageIndex{5}\)). Ascending tracts of nervous system fibers in these columns carry sensory information from the periphery to the brain, whereas descending tracts, such as the corticospinal tracts, carry motor commands from the brain to the periphery. Looking at the spinal cord longitudinally, the columns extend along its length as continuous bands of white matter. In cross-section, the dorsal/posterior columns can be seen between the two dorsal horns of gray matter, whereas the ventral/anterior columns are bounded by the ventral horns. The white matter on either side of the spinal cord, between the dorsal horn and the ventral horn, are the lateral columns. The dorsal/posterior columns are composed of axons of ascending tracts, whereas the ventral/anterior and lateral columns are composed of many different groups of axons of both ascending and descending tracts.

Figure \(\PageIndex{5}\): Cross-section of the spinal cord showing white matter tracts

The Meninges of the Spinal Cord

The spinal cord and brain are covered by the meninges which are a continuous, layered unit of tissues that provide support and protection to the delicate structures of the nervous system. As you will recall from a previous lab, the meninges, from superficial to deep, are the dura mater, arachnoid mater, and pia mater (Figure \(\PageIndex{6}\)). There are two noticeable differences for the spinal cord. The dura mater is a single layer (as opposed to the dual layered dura mater around the brain). Additionally, the pia mater is thicker and less vascular in the spinal cord than in the brain. As with the brain, the subarachnoid space is filled with cerebrospinal fluid (CSF) which protects the CNS by providing cushioning. Because the CSF around the spinal cord is more easily accessible, CSF can be removed by performing a spinal tap or lumbar puncture. This procedure is typically done at the end of the spinal cord, where the terminal filum extends from the inferior end of CNS at the upper lumbar region to the sacral end of the vertebral column. Because the spinal cord does not extend through the lower lumbar region of the vertebral column, a needle can be inserted in this region through the dura and arachnoid layers to withdraw CSF with minimal risk of damaging the nervous tissue of the spinal cord. One example of a disease commonly diagnosed via lumbar puncture is meningitis, which is an inflammation of the meninges caused by either a viral or bacterial infection. Symptoms include fever, chills, nausea, vomiting, sensitivity to light, soreness of the neck, and severe headache. More serious are the possible neurological symptoms, such as changes in mental state including confusion, memory deficits, other dementia-type symptoms, hearing loss, and even death due to the close proximity of the infection to nervous system structures, such as the brain stem (which is has centers for control of basic life functions including breathing, pulse rate, and blood pressure).

Figure \(\PageIndex{6}\): Spinal Cord. (From Mettler FA: Neuroanatomy, ed 2, St. Louis, 1948, Mosby)

Activity 14.1: Identifying Selected Structures in C5-Spinal Cord Cross Section Model Procedure for Activity 14.1: For this activity, you will use spinal cord models to study selected

structures and learn their functions.

1. Use the figures and information above to locate the required anatomical features of the spinal cord in the C5-Spinal Cord Cross Section Model: Dorsal horns, Lateral horns, Ventral horns, Dorsal root ganglion, Dorsal root, Ventral root, Spinal nerve, Central canal, white matter tracts
2. On Figure \(\PageIndex{7}\), label the anterior and posterior sides of the model.
3. Then, label the required structures listed in Step 1. As you work, you should review and make notes about the functions of each part you study.

Figure \(\PageIndex{7}\): Spinal Cord Cross-Section (Photo Credit: Julie Robinson)

Activity 14.2: Identifying Selected Structures in Longitudinal Spinal Cord Model
Procedure for Activity 14.2: For this activity, you will use a different spinal cord model to study selected

structures and learn their functions.

1. Use the figures and information above to locate the required anatomical features of the spinal cord in the Longitudinal Spinal Cord Model: Conus medullaris, Cauda equina, Filum terminale, Spinal nerves
2. On Figure \(\PageIndex{8}\), label the superior and inferior sides of the model.
3. Then, label the required structures listed in Step 1. As you work, you should review and make notes about the functions of each part you study.

Figure \(\PageIndex{8}\): Longitudinal Section of the Spinal Cord

Activity 14.3: Identifying Anatomy & Function of Selected Spinal Nerves

Spinal nerves extend outward from the vertebral column to innervate the periphery. The nerves in the periphery are not straight continuations of the spinal nerves, but rather the reorganization of the axons in those nerves to follow different courses. Axons from different spinal nerves will come together to form a peripheral nerve. This occurs at four places along the length of the vertebral column, each identified as a nerve plexus, a collection of spinal nerve roots. The nerve roots for the four major plexuses are listed in Table \(\PageIndex{1}\) and Figure \(\PageIndex{1}\). Spinal nerves of the thoracic region, T2 through T11, are not part of the plexuses but rather emerge and give rise to the intercostal nerves, which innervate the intercostal muscles found in between ribs, which are significant contributors to breathing. Figure \(\PageIndex{9}\) and Figure \(\PageIndex{10}\) show the selected nerves in situ along the skeletal system.

Table \(\PageIndex{1}\): Nerve Plexuses & Selected Spinal Nerves

Plexus	Spinal Nerve & Nerve Roots	Key Function(s)
Cervical plexus (C1-5)	Phrenic nerve (C1-C4)	Provides sole motor innervation to the majority of the diaphragm muscle Receives sensory input from the diaphragm, medial pleura of the lungs, and the pericardium
Brachial plexus (C5-T1)	Median nerve (C5-T1)	Provides motor innervation to the flexor muscles of the forearm and muscles of digits 1, 2, and the lateral half of digit 3 Receives sensory input from the anterior portions of digits 1, 2, 3, and the lateral half of digit 4
	Radial nerve (C5-T1)	Provides motor innervation to triceps brachii and some extensor muscles of the forearm
	Axillary nerve (C5-C6)	Provides motor innervation to deltoid and teres minor Receives sensory input from skin over the deltoid
	Musculocutaneous nerve (C5-C7)	Provides motor innervation to the biceps brachii, brachialis, and coracobrachialis muscles Receives sensory information from the lateral forearm
	Ulnar (C8-T1), nerve associated with the “funny bone” reaction	Provides motor innervation to some flexor muscles of the forearm and anterior hand Receives sensory input from elbow, wrist, carpal joints, phalangeal joints, and digits 4 & 5
Lumbar plexus (L1-4)	Femoral (L2-L4)	Provides motor innervation to the anterior compartment of the thigh (including the quadriceps and sartorius muscles) Receives sensory input from the hip, anterior and medial thigh, knee, and medial leg
	Obturator nerve (L2-L4)	Provides motor innervation to the medial compartment of the thigh (including the adductor muscles) Receives sensory input from the medial thigh
Sacral plexus (L4-S4)	Sciatic (L4-S2) Tibial branch (L4-S3) Fibular branch (L4-S2)	Provides motor innervation to the hamstrings, posterior muscles of the lower leg (including gastrocnemius and soleus via the tibial branch), anterior muscles of the lower leg (including tibialis anterior and peroneus muscles via the fibular branch), and many flexor and extensor muscles of the foot and ankle  Receives sensory input (via its branches) from the entire leg except for portions of the medial lower leg and medial foot)

Figure \(\PageIndex{9}\): Selected Nerves of the Cervical and Brachial Plexuses (Artist Credit: Kim-Laura Boyle)

Figure \(\PageIndex{10}\): Selected Nerves of the Lumbar and Sacral Plexuses (Artist Credit: Kim-Laura Boyle)

Procedure for Activity 14.3: Use the information above in Activity 14.3 to answer Questions 1—5 below.

1. Tui Anakelea is recovering from an aortic valve replacement that was performed through a major artery in the inguinal region.  Tui is encouraged to walk with assistance from Etienne, the nursing assistant.  Etienne notices that Tui seems a bit frightened, and hesitant to walk.  As they talk, Tui mentions that he feels like the muscles on “the front of my thigh aren’t working right, like my knee or hip might give out at any time.”  Etienne asks Tui to bend (flex) his hip as if he is about to take a step up.  Tui is not able to flex the hip more than 10 degrees.  Given this information, what nerve (of those listed in Table 14.9) is mostly likely compressed?  ___________________________________________________________________________

2. A patient walks into an emergency department after shoveling snow and feeling something “pop” in her neck.  She also notes increasing breathing difficulties, weakness in her arm as she tries to lift the shovel to remove more snow, and numbness and tingling in her thumb and first finger.  She is sent to the imaging center for an MRI of the cervical spine.  The radiologist finds herniation of intervertebral discs (IVDs) of C2 and C5.  Given these findings, determine two (2) nerves (of those listed in Table 14.9) that are likely affected and note your rationale for each.

a. Nerve affected: ________________________ 

Rationale: _______________________________________________________________________________________________________________________________________________________________________________________________________________________________________

 

b. Nerve affected: ________________________ 

Rationale: _______________________________________________________________________________________________________________________________________________________________________________________________________________________________________

3. Watch this short video:  https://youtu.be/XtbrVDUqV2I   If the coach had actually connected the punch leading to short-lived, significant breathing problems for the boxer, what nerve (of those listed in Table 14.9) and muscle would have been involved? __________________________________________________

4. Jesenia Alhgrin was recently promoted from her job as a precision machine operator to a project manager position.  Jesenia’s previous position required her to be on her feet walking or moving around for most of her shift.  Now, she notices that she’s sitting much more and has developed an increasing numbness and tingling down the back of her right leg.  Sometimes, it feels like the back of her calf is burning, and there’s a muscle cramp “in my butt”.   Given this information, what nerve (of those listed in Table 14.9) is mostly likely compressed?  ___________________________________________________________________________

5. A week ago, Dr. Preston Burke was shot outside the hospital at which he is the left-handed Chief of Surgery.  Dr. Burke survived, but an x-ray showed that the bullet fragmented in the left axillary region (as shown below).  Although the bullet fragments were immediately and successfully removed by Dr. Shephard, inflammation will remain in the region for several weeks.  During a rehabilitation session two-weeks post-op, the PTA notices that Dr. Burke appears to have trouble flexing the left shoulder as he moves his hand to the instrument table, and his hand shows tremors as he reaches to grasp the surgical instruments during a rehabilitation session. Given this information, determine two (2) nerves (from those listed in Table 14.9) that are likely affected and note your rationale for each.

 

a. Nerve affected: ________________________ 

Rationale: _____________________________________________________________________________ __________________________________________________________________________________________________________________________________________________________

 

b. Nerve affected: ________________________ 

Rationale: _____________________________________________________________________________ __________________________________________________________________________________________________________________________________________________________

 

Additional Learning Resources:

• Watch this video for the spinal cord slide:  http://youtu.be/F1gDA7U2JX4?hd=1
• Watch this video to learn about spinal cord structures in the C5 Cross Section Model:  https://www.youtube.com/watch?v=RteG4O-OCig&feature=youtu.be
• Watch this video to learn about spinal cord structures in the Longitudinal Spinal Cord Model: https://www.youtube.com/watch?v=uOL_o6NBqBo
• Review and Practice with these pictures of spinal cord models to learn about the anatomy of selected spinal nerves:   Practice for C5 Cross Section Model and Longitudinal Spinal Cord Model.pdf  (found at the end of your lab manual)
• Practice identifying the cells and their parts here:  (use questions 2, 3, 8, and 9):  http://academic.pgcc.edu/~aimholtz/AandP/PracPrac/2050_Lab17/neuron.html
• Practice identifying the parts of the spinal cord in models here (focus on structure listed in the objective above):  https://pierceanatomy.weebly.com/nervous-212080-862029-814150.html
• Practice identifying the parts of the C5 cross section model (focus on structure listed in the objective above):  https://pierceanatomy.weebly.com/nervous-212080-862029-988511.html
• Watch this video to learn about the anatomy, functions, and important clinical tips for selected spinal nerves:  https://www.youtube.com/watch?v=WjeCshmbVmA&feature=youtu.be&hd=1