(Transcribed from Dr. Glasser’s lecture, 21 Feb 2000 by Brian Buschman)
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Synapses communicate from vesicles of presynaptic terminals to post synaptic membranes via the release of chemical neurotransmitters. The nervous system is electrochemical in that neurons carry signals within themselves electrically and between neurons chemically.
The CNS is composed of the brain and the spinal cord. The brain is made of the telencephalon (cerebral cortex), the diencephalon, cerebellum and brain stem.
The cerebral cortex has an outer layer of gray matter and in inner portion of white matter. The neocortex gives the cerebral cortex where the gray matter consists of six layers of neurons that are separated by fibers. The paleocortex gives the cerebellar cortex where the gray matter is three layers thick. The white matter contains basal ganglia that are not true ganglia because they are in the CNS that makes then nuclei but the traditional name has stuck.
The cerebellum is also gray matter on the outside and like the paleocortex has three layers. In this case, they are the molecular layer, Purkinje cell layer and granular layer. The Purkinje cells have the largest somas and the granular cells have the smallest somas of any nervous cell in the body.
The diencephalon is made of four structures:
1) Thalamus that is the relay center to the cerebral cortex. You can think about it as if the cerebral cortex is the boss and to get to the boss you must go through his secretary, the thalamus.
2) Hypothalamus that is the head ganglion of the ANS.
3) Subthalamus that has relay nuclei for the thalamus and regulatory for the hypothalamus.
4) Epithalamus that has a similar function to that of the subthalamus.
The brain stem is made of nuclei and tracts that go all over the place. It contains the reticular formation that helps regulate muscular and visceral (with the help of the hypothalamus) functions.
The spinal cord is made up of a central H of gray matter that is surrounded by white matter. In the middle of the gray matter is the central canal.
The gray matter can be divided up into three general region; dorsal gray, intermediate gray and ventral gray. The gray mater can be further divided up into the ten lamina sections.
I-V makes up the dorsal gray
VI-VII makes up the intermediate gray
VIII-IX makes up the ventral gray
X is found around the central canal
Each of the sections have great details that we will discuss very well next semester.
For the most part spinal nerves are mixed nerves that have efferent fibers from the ventral gray and afferent fibers from the dorsal gray.
The CNS has no connective tissue in it so its support comes from the three meninges that surround it. From superficial to deep they are the dura mater, arachnoid and pia mater. The dura is dense connective tissue that is found in the brain in two layers. The outer layer is the periostium of the cranium. The inner layer is separated from the outer layer by the potential epidural space and between the dura and the arachnoid is the subdural space. In the spinal cord, only the inner layer exists. Dura is pain sensitive and has a vascular supply. It has folds that produce the dural venous sinuses that contain blood an cerebral spinal fluid (CSF).
The arachnoid is avascular and made of two components:
1) A layer of elastic and collagenous connective tissue.
2) Trabeculae that extend from the arachnoid into the pia forming the subarachnoid space.
In some areas, the arachnoid extends superficially to penetrate the dural sinuses to allow drainage of CSF. This happens through channels called arachnoid granulations or arachnoid villi. Arachnoid villi have valves to prevent the backflow from the sinuses.
Pia mater is composed primarily of loose connective tissue containing blood vessels that are thin and transparent. The pia follows every curve of the brain and spinal cord very tightly and is only separated from the CNS by the astrocytic end feet that make up part of the blood brain barrier. Arteries enter the CNS with a sleeve of pia attached to them. There is a perivascular space between the pia and the artery called the Virchow-Robins space. Pia disappears from the arteries as the capillaries are formed.
Due to the arachnoid trabeculae, the arachnoid and pia are bound so tightly that some people consider them as one meningeal layer called the arachnoid-pia.
The blood-brain barrier is composed of:
1) Endothelial tight junctions.
2) Fused astrocytic end feel.
3) Has no fenestrations.
4) Has few pinocytotic vesicles at the level of the capillaries.
Epidural bleeding is an arterial bleed since it results from an injury of the arteries in the dura which transverse the epidural space. A venous bleed will be the result of damage to the dural venous sinuses. An arterial bleed will be high pressure and can easily compress the brain. The cerebral cortex is the part of the brain that would be closest to the injury so it would be the first part damaged. A venous bleed on the other hand would be low pressure and would be able to drain into the dural venous sinuses. It would probably give you a headache but not kill you.
The CSF is made by the choroid plexus that are projections from the blood vessels in the ventricles of the brain. The ventricular system in the brain is the continuation of the central canal from the spinal cord. CSF fills the ventricles and surrounds the brain and spinal cord (it does not enter the central canal of the spinal cord). CSF functions as a shock absorber to protect the brain and spinal cord. It protects the spinal cord from outside and protects the brain from both within and without. CSF also assists in electrolyte balance that is very important for neurotransmission.
- About 500 ml of CSF are made per day.
- The whole system holds about 130 ml of CSF.
- The ventricular system holds about 25 ml of CSF.
- About 105 ml of CSF are in the subarachnoid space.
There is no regulatory mechanism that is able to shut off the production of CSF. Should a drainage problem occur it will result in increased subarachnoid pressure that will crush the brain. This is known as hydrocephalus.
Nerve fibers are surrounded by reticular fibers called endoneurium. The endoneurium holds the vascular supply for the given nerve. Bundles of nerve fibers are surrounded by perineurium. Entire nerves are surrounded by a layer of dense connective tissue, epineurium.
Ganglia of the PNS can be placed into one of three types:
1) Sensory ganglia that are located in the dorsal roots. Dorsal root ganglia (DRG) do not synapse in the ganglia, as they are pseudounipolar.
2) Autonomic fibers have the two remaining types:
a) Sympathetic fibers synapse in the sympathetic chain ganglia or in splanchnic ganglia.
b) Parasympathetic fibers synapse primarily in ganglia located in the organ innervated by the specific nerve.
Materials flow along the axon in either anterograde (away from the axon) or retrograde (toward the axon). Anterograde flow can be slow, 2-4 mm/day, intermediate, moves mitochondria, or fast, 20-400 mm/day and moves neurotransmitter vesicles. Retrograde flow is always fast.
If a neuron is destroyed, it is unable to regenerate but neuroglia is able to. In the PNS, it is possible for an axon to reform if the axon is injured but the soma is unharmed. It follows a set pattern of steps:
1) First, there will be degeneration and removal by macrophages of the segment distal to the site of damage.
2) Then Schwann cells will proliferate and form a column to guide the neuron back to the structure it is to innervate.
3) The some will enlarge, the nissl will degenerate (chromatolysis) and the nucleus will move to the periphery.
4) The proximal segment will degenerate a short distance with it’s myelin sheath.
5) The proximal segment will give off many processes in search of the pathway to guide it back to the original tissue.
It can happen that a motor neuron will follow the path for a sensory neuron and vise versa. In this case, the neuron will be inactive since it is the improper type for the given innervation and the muscle innervated will undergo deinnervation atrophy.
This process does not work in the CNS because the oligodendrocytes do not form good channels back to the original endpoint.
In limb amputation the nerves will try to regenerate but obviously will not be able to but instead will give a nerve bundle called a neuroma which is believed to be the source of phantom limb pain.
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