Nervous+System+Organization

toc =Organization of the Nervous System=

Dr. Bennett-Clarke, PhD 2 Nov 2007 @ 10 AM

Objectives
> The nervous system can be differentiated into the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS consists of the brain and spinal cord, and the PNS consists of cranial and spinal nerves. > > > The spinal cord is cylindrical and about 16-18 inches in length. It is larger superiorly and tapers caudally at the conus medullaris. It has two enlargements: a cervical enlargement between C5-T1 (upper limbs) and a lumbar enlargement between L3/4-S3 (lower limbs). The cauda equina is the dorsal and ventral roots of the lower spinal nerves located inferior to the L2 vertebral level. > > There are three menigeal layers that surround the spinal cord. > The outermost layer is the dura mater "tough mother" which is a tough dense connective tissue and forms tubular sheath from C1 to S2 held inferiorly by a thin extension to the coccyx. > The next layer internally is the arachnoid mater "spidery mother" which consists of two layers itself. The membranous layer is a thin transparent membrane in contact with the internal surface of the dura, and the arachnoid layer which consists of web-like extensions extending to the pia mater. > The innermost layer is the pia mater "delicate mother" which is a very thin layer of connective tissue in direct contact with the spinal cord. The inferior extension of the pia mater that extends to the dura mater and anchors to the coccyx is known as the filum terminale. Lateral extensions of the pia mater to the dura mater are known as the denticulate ligaments. > The spaces between the meningies are known as the epidural space (above the dura), subdural space (below the dura) and the subarachnoid space (below the arachnoid. > > > The spinal nerve is formed by the combination of the dorsal roots (sensory information) and ventral roots (motor information), and contains both sensory and motor information. Therefore spinal nerves are mixed nerves (but dorsal and ventral roots are not). The spinal nerve then branches to become a dorsal rami (back muscles and skin) and ventral rami (all other muscles and the rest of the skin). > > There are 31 pairs of spinal nerves, 8 cervical pairs, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal. The C1 nerve exits spinal cord between the skull and C1, and the C8 nerve exits between C7 and T1. Therefore subsequent nerves exit between the same level vertebrae above and the one below (ie. the L3 nerve exits between L3 and L4) > > A spinal cord segment is the portion of the spinal cord that contributes to the formation of a single spinal nerve, which is not the same as the vertebral level which is simply an anatomical landmark. > > A dermatome is an area of the skin that is known to be served by a specific spinal nerve. It can be used to determine which spinal nerves are damaged depending on whether sensation is felt in specific areas of the body. > > Since the spinal cord terminates (conus medullaris) around the L2 level, inserting a needle between L3 and L4 means that the needle will be entering the cauda equina. Since the cauda equina consist of nerve roots, it is like poking a needle into spaghetti, which means there is a low likelihood of nerve damage
 * 1) **Describe the basic anatomical organization of the nervous system.**
 * 1) **Disgram a typical neuron, list it's parts. Identify distinguishing characteristics of motor and sensory neurons.**
 * 1) **Describe the gross anatomical features of the spinal cord.**
 * 1) **Describe the location, organization, and structure of the spinal meninges.**
 * 1) **Diagram a cross section through the spinal cord. Be able to label all of the parts as described in class.**
 * 1) **Describe the components of a typical spinal nerve and explain the normal branching pattern.**
 * 1) **Explain the numbering of spinal nerve pairs.**
 * 1) **Give the definition of a spinal cord segment. Explain the difference between the spinal cord segment and the vertebral level.**
 * 1) **Define the term dermatome and state it's clinical significance.**
 * 1) **Give the anatomical basis for administering a successful lumbar puncture.**

I. Organization of the nervous system
The nervous system can be organized either funcionally or anatomically. The anatomical organization is most frequently used.
 * Anatomical Organization
 * Central Nervous System (CNS)
 * Brain
 * Spinal Cord
 * Peripheral Nervous System (PNS)
 * Cranial Nerves - attached to brain, 12 pair, supplies structures of the head and neck
 * Spinal Nerves - attached to spinal cord, 31 pair, supplies structures of the trunk and limbs

II. Basic unit of the nervous system
The neuron is the basic unit of both the CNS and the PNS.
 * Parts of the neuron
 * Cell Body (soma)
 * Cell Processes
 * Dendrites
 * Axons
 * Synapse
 * Types of neurons
 * multipolar (motor neurons)
 * pseudounipolar (primary sensory neurons)

III. List of Definitions
This is a list of important definitions that we will be using throughout the course.
 * Nucleus - collection of neuronal cell bodies in the CNS
 * Ganglion - collection of neuronal cell bodies in the PNS
 * Tract - collected neuronal processes in the CNS
 * Nerve - collected neuronal processes in the PNS
 * Gray Matter - unmylenated areas of the CNS (usually cell bodies, dendrites)
 * White Matter - mylenated areas of the CNS (usually neuronal processes incl. axons)

IV. Spinal Cord
Structure and function.
 * Functions of the spinal cord - connects the brain to the body
 * Sensory (afferent) - carries sensory information from the periphery towards the brain
 * Motor (efferent) - carries motor information down the spinal cord towards the periphery
 * Features of the spinal cord
 * Size and shape - 16 to 18 in. in length, cylindrical
 * Various diameters - larger superiorly
 * Cervical enlargement - between C5-T1 due to nerve info to/from upper limbs
 * Lumber enlargement - between L3 or L4 to S3 due to info to/from lower limbs
 * Conus medullaris - tapering caudally of the spinal cord
 * Cauda equina - dorsal and ventral roots of the spinal nerves, located inferior to the L2 vertebral level
 * Protection for the spinal cord
 * Vertebral column - bony protection, vertebral foramina form the vertebral column which contains the spinal cord
 * Meninges - three connective tissues coverings that encircle the brain and spinal cord. They are listed below as they are located from the outside in
 * Dura mater "tough mother" (dural sac) - tough dense connective tissue, forms tubular sheath from C1 to S2 held inferiorly by a thin extension of the dura to the coccyx.
 * Arachanoid mater "spidery mother"
 * membranous layer - thin transparent membrane in contact with the internal dura mater
 * arachnoid layer - web-like extensions which extend to the pia mater
 * Pia mater "delicate mother" - very thin later of connective tissue in direct contact with the spinal cord
 * Related structures to the meninges
 * Filum terminale - inferior extension of the pia mater that extends to the dura mater
 * Denticulate ligaments - lateral extensions of the pia mater to the dura mater
 * Spaces and potential spaces associated with the meninges
 * Epidural space (above the dura) - site of many common anesthesias
 * Subdural space (below the dura) - between the dura and the arachnoid mater
 * Subarachnoid space (below the arachnoid) - contains CSF
 * CSF - provides buoyancy, shock absorption
 * Internal structure of the spinal cord - the spinal cord in cross-section
 * Locate the following structures
 * Gray and white matter portions of the spinal cord
 * Dorsal horn (sensory receptive area)
 * Ventral horn (motor effector area)
 * Lateral horn (autonomic nervous system)
 * Formation of the spinal nerve
 * Sensory components
 * Dorsal root - carries the central process from the dorsal root ganglion into the spinal cord
 * Dorsal root ganglion - all the bodies of the sensory neurons
 * Motor components
 * Ventral root - leaves ventral horn to spinal nerve, carries motor information
 * Spinal Nerves - formed where the dorsal root and ventral root meet
 * Mixed nerves - nerves that carry both sensory and motor components (eg. spinal nerves) Dorsal roots and ventral roots are not mixed nerves
 * Intervertebral foramen - where the spinal nerves exit the spinal cord
 * Branches of a typical spinal nerve
 * Dorsal primary ramus - to the back muscles and skin
 * Ventral primary ramus - to all other muscles and skin, much bigger
 * Notes: spinal nerves do not supply internal organs, dorsal/ventral rami are mixed nerves
 * Number of spinal nerves - 31 pair
 * 8 Cervical pair
 * C1 uses the space between the C1 vertebrae and the skull
 * C2-C8 named after lower vertebrae that form the intervertebral foramen the nerve exits
 * 12 Thoracic pair
 * Named after the upper vertebrae that form the intervertebral foramen the nerve exits
 * 5 Lumbar pair
 * 5 Sacral pair
 * 1 Coccygeal pair

V. Spinal cord segment (level) vs. vertebral level

 * Spinal cord segment - the portion of the spinal cord that contributes to the formation of a single spinal nerve
 * Vertebral level - the anatomical landmark associated with a specific numbered vertebrae
 * Why aren't they the same? They are the same during the first trimester of development, but afterwards the vertebral column grows while the spinal cord remains the same length (remember 16 to 18 inches), thus resulting in the mismatch between spinal cord segments and vertebral levels.

VI. Dermatomes
A dermatome is an area of the skin that is known to be served by a specific spinal nerve. It can be used to determine which spinal nerves are damaged depending on whether sensation is felt in specific areas of the body.

VII. Lumbar Puncture
Since the spinal cord terminates (conus medullaris) around the L2 level, inserting a needle between L3 and L4 means that the needle will be entering the cauda equina. Since the cauda equina consist of nerve roots, it is like poking a needle into spaghetti, which means there is a low likelihood of nerve damage. A lumbar puncture pierces the dural sac (through the pia mater as well) to get CSF from the lumber cistern.

Resources
Last Year's Objectives