BIO 004 · Human Anatomy
The Nerve Plexuses
Block 5 · Module 5: The Peripheral Nervous System, the Nerve Plexuses
A brief reference for the nerve plexuses. A plexus is a network where the anterior rami of spinal nerves regroup before traveling to the body. This page covers the principal plexuses at a general level: where each one forms and what region it serves, so you can predict the area of the body affected if a plexus is damaged.
How to use this sheet Toggle the toolbar above. Notes prints the full reference for review. Study prints as a fill-in-the-blank worksheet. Print it, then write each definition while you watch the video or read your book. Quiz me is on-screen typing practice: type the term, click Reveal to check yourself. The comparison grids respond to Study and Quiz too, with a Reveal button on each row.
The Foundations video gives you a complete foundational understanding of this topic, enough on its own for a foundational course. Learn it first, then move on to the Deep dive, which adds the majors-level depth for this course.
- Define a nerve plexus and explain why the anterior rami regroup.
- Name the principal nerve plexuses and the spinal nerves that form each.
- Predict the region of the body affected if a given plexus is damaged.
Your pre-work
Work through these the evening before class. None of it is turned in. It is how you learn the material and build your spaced recall.
This is more than a checklist. Ticking these boxes is the start, not the finish. Committing this material to memory and being able to apply it takes considerable time and repeated effort. You are not done when the boxes are checked. Put in the real hours, and keep coming back for frequent recall and review until the material is genuinely yours.
The four plexuses in place
Add a posterior view showing where the cervical, brachial, lumbar, and sacral plexuses sit along the spine.
Anterior rami forming a plexus
Add a diagram showing the anterior rami of several spinal nerves weaving into a plexus.
Regions served
Add a body map shading the region each principal plexus supplies.
What a Nerve Plexus Is
Most spinal nerves do not run straight to their targets. The anterior rami of neighboring spinal nerves first weave together into a network called a plexus, then sort their axons into the nerves that leave it. The thoracic region is the exception.
- Nerve plexusa network where the anterior rami of several spinal nerves join and regroup before their axons travel on to the body
- Anterior ramithe branches of the spinal nerves that form the plexuses and supply the limbs and the front of the body
- Why a plexus formsregrouping lets each body region receive axons from several spinal nerves, so damage to one spinal nerve does not erase all function in that area
- Intercostal nervesthe exception; the anterior rami of T2 to T12 do not form a plexus but run directly to the muscles and skin of the thoracic wall
- Principal plexusesthe cervical, brachial, lumbar, sacral, and coccygeal plexuses, each named for the region it serves
The Principal Nerve Plexuses
Each plexus is formed by a known set of spinal nerves and serves a known region of the body. Compare them.
| Plexus | Formed by | Region served |
|---|---|---|
| Cervical plexus | the anterior rami of C1 to C4, with a contribution from C5 | the skin and muscles of the neck and the upper shoulders; it also gives rise to the phrenic nerve, the motor supply to the diaphragm |
| Brachial plexus | the anterior rami of C5 to C8 and T1 | the shoulder and the entire upper limb |
| Lumbar plexus | the anterior rami of L1 to L4 | the anterolateral abdominal wall, the external genitals, and part of the lower limb |
| Sacral plexus | the anterior rami of L4 to L5 and S1 to S4 | the buttocks, the perineum, and most of the lower limb |
| Coccygeal plexus | the anterior rami of S4 to S5 and the coccygeal nerves | a small area of skin over the coccyx |
Predicting the Effect of Plexus Damage
Because each plexus serves one region, the location of a plexus injury predicts the region of the body that loses movement and sensation. This is the level of detail to know for this course.
| Plexus damaged | Region affected |
|---|---|
| Cervical plexus | weakness or numbness of the neck and shoulder; if the phrenic nerve is involved, the diaphragm on that side is weakened and breathing is affected |
| Brachial plexus | weakness and sensory loss across the shoulder and the whole upper limb on that side |
| Lumbar plexus | weakness and sensory loss of the lower abdominal wall and the front and inner surfaces of the thigh |
| Sacral plexus | weakness and sensory loss of the buttock, the back of the thigh, and the leg and foot |
| Coccygeal plexus | numbness of the small patch of skin over the coccyx |
See also: The Peripheral Nervous System for nerve structure and the spinal nerves, and The Spinal Cord for the roots these rami arise from.
Study questions
Work on answering these in writing, in your own words. They are the questions to bring to class, and good practice for the reasoning the exams ask for.
- Name the four major nerve plexuses and the region of the body each one supplies.
- Explain what a plexus is, and why nerves regroup into one rather than running straight from the cord.
- Predict which region would be affected if the brachial plexus were injured, and which if the lumbar plexus were.
- Compare the cervical and sacral plexuses by the body region each one serves.
Step 2 . Retrieval check
Now explain it back, in your own words.
In 60 words or more, pull together what the video just taught you. Include the key concepts. This is the point where the learning actually sticks. After you submit, your spaced-recall cards for this topic unlock.