Ganglion – a collection of cell bodies located outside the Central Nervous System. The spinal ganglia or dorsal root ganglia contain the cell bodies of sensory neurons entering the cord at that region.
Nerve – a group of fibers (axons) outside the CNS. The spinal nerves contain the fibers of the sensory and motor neurons. A nerve does not contain cell bodies. They are located in the ganglion (sensory) or in the gray matter (motor).
Tract – a group of fibers inside the CNS. The spinal tracts carry information up or down the spinal cord, to or from the brain. Tracts within the brain carry information from one place to another within the brain. Tracts are always part of white matter.
Gray matter – an area of unmyelinated neurons where cell bodies and synapses occur. In the spinal cord the synapses between sensory and motor and interneurons occurs in the gray matter. The cell bodies of the interneurons and motor neurons also are found in the gray matter.
White matter – an area of myelinated fiber tracts. Myelination in the CNS differs from that in nerves. dermatomes are somatic or musculocutaneous areas served by fibers from specific spinal nerves. Spinal nerves join together in plexuses.
A plexus is an interconnection of fibers which form new combinations as the “named” or peripheral nerves. There are four voluntary plexuses (there are some autonomic plexuses which will be mentioned later): they are the cervical plexus, the brachial plexus, the lumbar plexus, and the sacral plexus. Each plexus gives rise to new combinations of fibers as the peripheral nerves. The nerves and plexuses you need to know are:
Cervical Plexus – the phrenic nerve travels through the thorax to innervate the diaphragm.
Brachial Plexus Axillary nerve – innervates the deltoid muscle and shoulder, along with the posterior aspect of the upper arm.
Musculocutaneous nerve – innervates anterior skin of upper arm and elbow flexors.
Radial nerve – innervates dorsal aspect of the arm and extensors of the elbow, wrist, and fingers, abduction of thumb.
Median nerve – innervates the middle elbow, wrist and finger flexors, adducts the thumb.
Ulnar nerve – innervates the medial aspect wrist and finger flexors.
Lumbar Plexus genitofemoral – to the external genitalia obturator – to the adductor muscles femoral – innervates the skin and muscles of upper thigh, including the quadriceps.
Sacral Plexus gluteal nerves (superior and inferior) – superior innervates the gluteus medius and minimus, inferior innervates the gluteus maximus. sciatic nerve – the body’s largest nerve, consisting of two major branches, the tibial and common peroneal. Together they innervate most all of leg including the flexors of the knee, part of adductor magnus, muscles for plantar flexion, dorsiflexion, and other movements of the foot and toes. Structure of a nerve: A peripheral nerve is arranged much like a muscle in terms of its connective tissue. It has an outer covering which forms a sheath around the nerve, called the epineurium. Often a nerve will run together with an artery and vein and their connective coverings will merge. Nerve fibers, which are axons, organize into bundles known as fascicles with each fascicle surrounded by the perineurium. Between individual nerve fibers is an inner layer of endoneurium. The myelin sheath in peripheral nerves consists of Schwann cells wrapped in many layers around the axon fibers. Not all fibers in a nerve will be myelinated, but most of the voluntary fibers are. The Schwann cells are portrayed as arranged along the axon like sausages on a string. (A more apt analogy would be like jelly rolls!) Gaps between the Schwann cells are called nodes of Ranvier. These nodes permit an impulse to travel faster because it doesn’t need to depolarize each area of a membrane, just the nodes. This type of conduction is called saltatory conduction and means that impulses will travel faster in myelinated fibers than in unmyelinated ones. The myelin sheath does several things: 1) It provides insulation to help prevent short circuiting between fibers. Diseases which destroy the myelin sheath lead to inability to control muscles, perceive stimuli etc. One such disease is multiple sclerosis, an autoimmune disorder in which your own lymphocytes attack the myelin proteins. 2) The myelin sheath provides for faster conduction. 3) The myelin sheath provides for the possibility of repair of peripheral nerve fibers. Schwann cells help to maintain the micro-environments of the axons and their tunnel (the neurilemma tunnel) permits re-connection with an effector or receptor CNS fibers, not having the same type of myelination accumulate scar tissue after damage, which prevents regeneration What are spinal tracts? These are communication pathways within the CNS… they are like cables of axons that run up and down the spinal cord in the white matter. Some run up the spinal cord (cephalad), while others descend the spinal cord from the brain. What tracts exist? There are many tracts. Each tract carries specific types of information from one location to another. So, if you want to carry painful information from the spinal cord to the brain, you use one particular tract (the spinothalamic tract), while if you want to carry voluntary movement commands from the brain to the spinal cord, you use a different tract (the lateral corticospinal tract). Motor tracts: Corticospinal tracts soundicon. *Lateral corticospinal tract This tract carries information about movement from the brain to the spinal cord. That means that if your brain decides that it wants your legs to move, it has to send this voluntary movement command down to the region of the spinal cord that is responsible for leg movements. (The anterior corticospinal tract is much more minor, but it crosses at the spinal level of the motor neuron it synapses on… so it runs contralaterally.) Sensory tracts: Dorsal Columns: this specific term is not in your book, but is a common term to describe the tracts that carry touch and proprioceptive information to your brain. Therefore, light touch, deep pressure, and proprioceptive senses (carried out by muscle spindles and other sensory receptors) are carried through these pathways. Fasciculus cuneatus This spinal tract carries this sensory information from your thoracic and cervical body areas up to the brain. These areas include your arms. Fasciculus gracilis This spinal tract carries this sensory information from your sacral and lumbar body areas up to the brain. These areas include your legs. Spinothalamic tracts .Pain and temperature (the free nerve endings in skin) sensory information is carried by these tracts to your brain. lateral spinothalamic tract anterior spinothalamic tract