BIO 004 · Human Anatomy
The Peripheral Nervous System
Block 5 · Module 5: The Peripheral Nervous System
A reference for the peripheral nervous system video and lab. This page covers the structure of a nerve, the spinal nerves and their roots, the rami, the nerve plexuses, and the classification of sensory receptors. The cranial nerves are covered on their own sheet. The focus is on the structures and the job each one does.
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.
- Describe the structure of a nerve and name its connective tissue wrappings.
- Identify the spinal nerves, their roots, and their rami.
- Classify the sensory receptors by stimulus, location, and structure.
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 wrappings of a nerve
Add a labeled cross-section showing the endoneurium, perineurium, epineurium, and fascicles.
A spinal nerve and its rami
Add a labeled view of a spinal nerve dividing into the posterior and anterior rami.
The sensory receptors of the skin
Add a labeled section of skin showing the named tactile receptors.
The Peripheral Nervous System, an Overview
The peripheral nervous system is everything outside the brain and spinal cord. It carries input from the body's receptors in to the CNS, and carries the response back out to the effectors.
- Peripheral nervous systemall neural structures outside the brain and spinal cord
- Functionto carry sensory input from receptors to the CNS, and to carry motor output from the CNS to the effectors
- Sensory (afferent) receptorsstructures that detect changes in the internal and external environment
- Peripheral nervesbundles of axons that connect the CNS to the body
- Gangliaclusters of neuron cell bodies located outside the CNS
- Motor (efferent) endingsthe terminals where motor neurons meet the muscles and glands
- Cranial nervesthe 12 pairs of nerves that arise from the brain, covered on their own sheet
- Spinal nervesthe 31 pairs of nerves that arise from the spinal cord
The Structure of a Nerve
A nerve is a cordlike organ: many axons bundled together and wrapped in three layers of connective tissue.
- Nervea cordlike organ of the PNS; a bundle of peripheral axons, some myelinated and some not, enclosed by connective tissue
- Axona single nerve fiber; many axons make up a nerve
- Endoneuriumthe innermost wrapping, around each individual axon
- Fasciclea bundle of axons grouped together
- Perineuriumthe middle wrapping, the thicker layer around each fascicle
- Epineuriumthe outermost wrapping, around the entire nerve
The Spinal Nerves
Thirty-one pairs of spinal nerves connect the cord to the body. Every one is a mixed nerve, carrying both sensory and motor axons.
- Spinal nervesthe 31 pairs of nerves that arise from the spinal cord
- Mixed nervesevery spinal nerve carries both sensory and motor axons
- Posterior (dorsal) rootthe sensory root that brings axons into the cord
- Anterior (ventral) rootthe motor root that carries axons out of the cord
- Intervertebral foramenthe opening between adjacent vertebrae where a spinal nerve exits the vertebral canal
The 31 pairs are grouped by the region of the cord they arise from. Compare the count in each region.
| Region | Number of pairs |
|---|---|
| Cervical | 8 pairs |
| Thoracic | 12 pairs |
| Lumbar | 5 pairs |
| Sacral | 5 pairs |
| Coccygeal | 1 pair |
The Rami of a Spinal Nerve
Just past the intervertebral foramen, each spinal nerve splits into branches called rami. Each ramus serves a different territory. Compare them.
| Ramus or branch | What it supplies |
|---|---|
| Posterior (dorsal) ramus | the deep muscles and the skin of the posterior surface of the trunk |
| Anterior (ventral) ramus | the muscles and skin of the limbs and the lateral and anterior trunk; these rami form the plexuses |
| Meningeal branch | re-enters the vertebral canal to supply the vertebrae, the ligaments, the blood vessels of the cord, and the meninges |
| Rami communicantes | small branches off the anterior ramus that carry autonomic fibers to and from the sympathetic trunk |
The Nerve Plexuses
The anterior rami of most spinal nerves do not run straight to their targets. They first join into networks called plexuses. The thoracic rami are the exception.
- Nerve plexusa network formed by the anterior rami of several spinal nerves
- Cervical plexusformed from C1 to C4; serves the neck and shoulders, and gives off the phrenic nerve to the diaphragm
- Brachial plexusformed from C5 to T1; serves the shoulder and the upper limb
- Lumbar plexusformed from L1 to L4; serves the lower abdominal wall and part of the lower limb
- Sacral plexusformed from L4 to S4; serves the buttock, the perineum, and most of the lower limb
- Intercostal nervesthe anterior rami of T2 to T12, which do not form a plexus but run straight to the thoracic wall
The plexuses are covered in more detail, including the region affected by damage, on the Nerve Plexuses sheet.
Classifying Sensory Receptors
Sensory receptors are classified in three ways: by the type of stimulus they detect, by where they are located, and by their structure.
By stimulus type
- Mechanoreceptorsrespond to mechanical force such as touch, pressure, vibration, and stretch
- Thermoreceptorsrespond to changes in temperature
- Photoreceptorsrespond to light; found in the retina of the eye
- Chemoreceptorsrespond to chemicals in solution, such as smell and taste molecules
- Nociceptorsrespond to potentially damaging stimuli and produce the sensation of pain
By location
- Exteroceptorsdetect stimuli at or near the body surface, such as touch, temperature, and the special senses
- Interoceptorsalso called visceroceptors; detect stimuli inside the body, in the organs and blood vessels
- Proprioceptorsdetect the position and movement of the body, in muscles, tendons, and joints
By structure
- Free nerve endingsnonencapsulated receptors; bare dendrite tips found in most tissues
- Encapsulated receptorsnerve endings wrapped in a connective tissue capsule
The Sensory Receptors
The named sensory receptors, with where each one is found and what it detects. Compare them.
| Receptor | Location | What it detects |
|---|---|---|
| Free nerve endings | most body tissues, dense in connective tissue and epithelium | pain, temperature, and crude touch |
| Tactile (Merkel) discs | the deepest layer of the epidermis | light, discriminative touch |
| Hair follicle receptors | wrapped around hair follicles | movement of the hair |
| Tactile (Meissner) corpuscles | the dermal papillae of hairless skin | fine, discriminative touch |
| Lamellar (Pacinian) corpuscles | the deep dermis and the hypodermis | deep pressure and vibration |
| Bulbous (Ruffini) corpuscles | the deep dermis and joint capsules | continuous, steady pressure |
| Muscle spindles | within skeletal muscles | the stretch of a muscle |
| Tendon organs | within tendons | the tension a muscle places on its tendon |
| Joint kinesthetic receptors | the capsules of synovial joints | joint position and movement |
See also: The Cranial Nerves and The Nerve Plexuses, and The Autonomic Nervous System.
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 components of the peripheral nervous system and what each one connects.
- Compare a spinal nerve's dorsal and ventral rami by the region each one supplies.
- Explain the structure of a peripheral nerve using its connective tissue wrappings.
- Classify sensory receptors by the kind of stimulus each one detects.
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.