Neuroanatomy Introduction

This post will be an neuroanatomy and nervous system introduction because it is necessary to understand some basic aspects about the nervous system before trying to understand nerves in the context of anatomy. I will also assume a basic level of understanding about nerves and conduction here, so please post questions if I skip anything.


Some Basics
- A perikaryon is a term for the cell body of a neuron.
-The nervous system is generally broken into two parts, the Central Nervous System (CNS) which is essentially the brain and spinal cord, and the Peripheral Nervous System (PNS), everything else.
-A ganglion is a collection of cell bodies in the PNS.
-A nucleus is a collection of cell bodies in the CNS. BE CAREFUL it is easy to confuse this with a regular cell nucleus. Same word, it just means a different thing in neuro.
-A neuron is the structural and functional unit of the nervous system.  It exhibits two chief properties which we will over simply to keep it brief, irritability and conductivity.
-Nerves travel out of the CNS and into the PNS and go all over the body.  Most nerves traveling to the skin have the word cutaneous in their name (cutis means skin in Latin).

A basic neuron

The CNS (leaving out the brain for now so mostly just,) The Spinal Cord

As I mentioned last post, the spine is made up of individual bones called vertebrae. There are 7 cervical vertebrae, appropriately named C1 through C7, 12 thoracic, 5 lumbar, 5 sacral, and the coccyx, a fused mass of bone.

-Remember the vocab rostral and caudal because they are especially common in neuro.  Rostral means the same superior, or closer to the top of the head, while caudal means the same thing and inferior.

- The C1 nerve actually exits above the C1 vertebrae.  This is very important because there is a C8 nerve BUT NO VERTEBRAE.  The C8 nerve exits just caudal to the C7 vertebrae.  So above C7 all the nerves exit the spinal cord and enter the body ABOVE their vertebrae, and below C7 all the nerves exit and leave BELOW their vertebrae.  Or, another way, Cs above, and Ts and Ls below.

-The spinal cord begins at the medulla (in the brain) and continues until it terminates at the vertebrae levels of L1-L2.  Beyond that, nerves hang down and still exit, but the 'middle' is simply a space filled with cerebrospinal fluid.  Clinical correlation:  Ever heard of a lumbar puncture?  Ever watched the show House or any other doctor show?  To get a sample of the cerebrospinal fluid the most common method is the lumbar puncture, because with no spinal cord the chance of injury is minimal.

-There are 31 pairs of spinal nerves. C-8  T-12 L-5 S-5 C-1

-Nerves C5 C6 C7 C8 and T1 make up what is called the Brachial Plexus (more later).

A single vertebral body looks like this.  The in depth anatomy of the spine I will cover at a later point.

The Peripheral Nervous System
The PNS is everything outside the CNS, so everything except the brain and spinal cord.  It is composed of 12 pairs of Cranial nerves and 31 pairs of spinal nerves.  The cranial nerves are,

Each one has a series of functions, associated sympathetic and parasympathetics, certain pathways and relations, and is the topic of a head and neck anatomy course (maybe I'll get there eventually).

Of more immediate concern are the spinal nerves;
1. They are formed by:
         a. Dorsal Root (Sensory)
         b. Ventral Rool (Motor)
2. Divide Into:
          a. Dorsal Root (Mixed)
          b. Ventral Root (Mixed)
What does this mean?  Let me explain,

Lets say, for example, this is cervical vertebrae 5, C5.  The dorsal root is where all the sensory information it receives, in this case some skin on the arm, enters the central nervous system and is then passed up to the brain.  In contrast the ventral root is where all the motor signals sent down from the brain pass through, for example helping to contract the serratus anterior muscle.  So, think of the dorsal root as inbound traffic and the ventral root as outbound traffic.  Simple.  By the way if you didn't pick up on it, dorsal and ventral are named due to their location, I think of it as a dorsal fin on a fish, the fin being on the dorsal side, and the ventral side being a humans chest.

Looking a little further away, these two pathways mix.  They are simply called the dorsal and ventral ramus, or rami for plural (ramus means branch in Latin).  Each of them contains both sensory and motor fibers, so they are called mixed nerves.  Simple.  Got it. 

Dermatomes
A dermatome is the area of skin supplied by a single nerve.  There are very specific delineations in this pattern,

This pattern can, for example, help a doctor figure out which exact nerve is dysfunctional if a patient cannot feel anything in the specific area. I don't feel like listing out exactly what they all do so hit of Wikipedia of you need to memorize it.