The Human Nervous System 28-9-2010 By: Raghas Khasawneh

Lecture#2: The Nervous System

The Nervous system is divided into two main parts: the CNS (Centeral Nervous System), which consists of the brain, spinal cord, and the peripheral nervous system, which consists of 12 pairs of cranial nerves and 31 pairs of spinal nerves and their associated Ganglion (A collection of cell bodies outside the nervous system).

As you can see the spinal cord contains skeletal compartments, which are the skull and the vertebral column.

The central nervous system is composed of large numbers of nerve cells and their processes, supported by specialized tissue called neuroglia (neuroglia is The supportive tissue of the nervous system) .

The CNS communicates with the body and different body regions through Nerves (a Nerve is a bundle of axons).

In this picture, you can see a multipolar neuron, and neuron is the term given to the nerve cell and all its processes.

Each neuron is made up of three main components : 1- Cell body

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The Human Nervous System 28-9-2010 By: Raghas Khasawneh

2- Axons: long processes of cell body3- Dendrites: short processes of the cell body and they are highly branched.

According to the book, The nerve cell has two types of processes, called dendrites and an axon.

In the terminals of the neuron, the cell will communicate with another cell, so the whole process begins again, with the dendrites sending information to the cell body and the cell body sending information down the axon and into the next neuron..

- Most of the time not all of the time, the cell bodies are located in the CNS.

The spinal cord is divided into 31 segments and these segments are not anatomical segments, because if you dissect it, you will not see discrete segments, so we just know these segments by relating it to adjacent vertebrae. For example: we have 8 cervical vertebrae (in the neck), 12 thoracic vertebrae ( in the thoracic region ) , and so on( 5 lumbar, 5 sacral, and 1 coccygeal ), as you see in the below picture >

Notice that the spinal cord is not the same length as the vertebral column, the vertebral column will grow out the spinal cord and reach longer length, so this is why you will see that the nerves from the lower segments are longer to reach its respective level of the vertebral column, and this lead to the formation of a bundle of nerves inside the vertebral column that we call it the cauda equina, it resembles a horse’s tail. We have the CNS and the nerves that exit the vertebral column that we call “spinal nerves”, and there are other types of nerves like the cranial nerves. On dissection, the cranial and spinal nerves are seen as grayish white cords. They are made up of bundles of nerve fibers (axons) supported by delicate areolar tissue.

REMEMBER: (peripheral nervous system = spinal nerves + cranial nerves).

o Where do you think these cranial nerves come from?

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The Human Nervous System 28-9-2010 By: Raghas Khasawneh

These cranial nerves emanate from the brain and the brain stem actually (we’ll talk about it in details next semester).

There are 12 pairs of cranial nerves that leave the brain and pass through foramina in the skull. All the nerves are distributed in the head and neck except the 10th or X th (vagus), which also supplies structures in the thorax and abdomen.

Let’s talk about the structure of the spinal nerve, so if we take a cross-section to the spinal cord, we’ll see this :

This is the spinal cord and you notice a butterfly-shaped or an H-shaped, which is the gray matter surrounded by the white matter, this is how the anterior of the CNS is organized into gray and white matter, so the gray matter where are the cell bodies ( nerve cells) are concentrated, and the white matter which is mostly consist of axons (nerve fibers ) , and they are both embedded in neuroglia.

Simple question: Why do we call it white matter?

Because axons are myelinated “myelin is found usually around axons”, and myelinated axons are white in appearance.

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The Human Nervous System 28-9-2010 By: Raghas Khasawneh

Each spinal nerve has two nerve roots. The root in the front, called the anterior or ventral root, and is called efferent fibers, Which Transmits impulses from the spinal cord to the muscles and cause them to contract are called motor fibers. The root leaving from the back part of the spinal column, known as the posterior or dorsal root and are called afferent fibers, carries sensory information (about touch, position, pain, and temperature) from the body to the spinal cord.

Again, the anterior root of each spinal nerve is made of nerve fibers which come from the anterior column of the spinal cord. It consists solely of motor nerve fibers. The posterior root springs from the posterior part of the spinal cord, and consists of fibers which lead to the posterior column of the cord. These fibers are all sensory fibers.

On the posterior there is a small bulge (a swelling). It is called the posterior root ganglion and is caused by the bodies of the nerve cells which form the sensory nerve fibers.

Ganglion: Any structure containing a collection of nerve cell bodies in the central nervous system (CNS) or peripheral nervous system (PNS) and often also numbers of synapses. Plural: ganglia.

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The Human Nervous System 28-9-2010 By: Raghas Khasawneh

At each intervertebral foramen, the anterior and posterior roots unite to form a mixed spinal nerve. As each spinal nerve leaves the intervertebral foramen it divides into two branches. The smaller branch curves sharply backwards and is called the posterior ramus. It contains both motor and sensory fibers. Soon it divides into many small branches to supply the muscles and the skin of the back.

The large branch of each spinal nerve is called the anterior ramus. It, too, contains both motor and sensory nerve fibers. The anterior ramus runs forward in the tissues, dividing into many small branches which serve the muscles and the skin of the front of the body.

In addition to the anterior and posterior rami (rami is the plural of ramus), spinal nerves give a small meningeal branch that supplies the vertebrae and the coverings of the spinal cord (the meninges). Thoracic spinal nerves also have branches, called rami communicates, that are associated with the sympathetic part of the autonomic nervous system

(meningeal : the three layers of membranes covering the brain and spinal cord )

Functionally, the nervous system can be further divided into the somatic nervous system, and the autonomic nervous system.

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The Human Nervous System 28-9-2010 By: Raghas Khasawneh

- What's the difference between Somatic Nervous System and Autonomic Nervous System?

Somatic nervous system “It is a single-neuron pathway” includes all of the neural pathways (neurons) that result in voluntary actions, basically anything that you can tell your brain to do like move your arm or stick out your tongue. The autonomic nervous system “It is a two-neuron pathway.” includes all of the neural pathways that result in involuntary actions, like your brain telling your eyes to blink or your intestines to contract, basically anything that you cannot control.

- The autonomic nervous system has two divisions: (1) the sympathetic and (2) the parasympathetic, and both parts have afferent and efferent nerve fibers.

What’s the difference between sympathetic and parasympathetic ?

sympathetic nervous system, which is responsible for stimulating activities associated with the fight-or-flight response, so it takes control of your body when you’re afraid and running and it’ll push the blood from your GI system and there will be no saliva in your mouth ( your mouth will be dry), because it’s not time for ingestion now!

The parasympathetic system specifically is responsible for stimulation of activities that occur when the body is at rest, including salivation, lacrimation, urination, digestion and defecation, so the blood will go to your GI system and your mouth will be full of salvia.

The nervous system, together with the endocrine system, controls and integrates the activities of the different parts of the body.

The sympathetic part of the autonomic system inhibits peristalsis of the intestinal tract and closes the sphincters. And the parasympathetic part of the autonomic system slow the heart rate, increase peristalsis of the intestine and glandular activity, and open the sphincters.

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The Human Nervous System 28-9-2010 By: Raghas Khasawneh

The hypothalamus of the brain controls the autonomic nervous system and integrates the activities of the autonomic and neuroendocrine systems, thus preserving homeostasis in the body.

Sympathetic and parasympathetic system :

Now the motor or the efferent neuron, we have two neuron system, First one neuron reaches from the CNS to the anterior root then to the spinal nerve, and it doesn’t complete its trip to the spinal nerve. No, it escapes through s structure called a gray ramus to enter a chain of ganglion , That’s located a long either side of vertebral column and it may synapsing with another neuron ( this is the first option), so this the first neuron we call it “Preganglionic” and pre means before. And the neuron that comes later, it synapse to another neuron and this neuron take the massage to the organ, we call it “postganglionic” neuron, this is the second neuron, So the Preganglionic neuron escape from the spinal nerve through the gray ramus, once it does that it either synapse immediately with another neuron, Or it may go up or down through the chain “second option” or it may leave the chain without synapse and synapse outside “the third option” .

Question from one of the student: If the somatic motor for example makes a synapse through a ganglion!!?

No, ganglion it has to have a cell body outside the CNS, but the cell body of a somatic motor neuron lies in the CNS.

Since we have here postganglionic neuron which means we’ll have the cell body outside the CNS, so we have a ganglion chain, So what the somatic motor will do here? It just leave through the anterior root and merge with spinal nerve then either leave with posterior or anterior ramus, it’ll not do anything with gray ramus or the chain of ganglion here.

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The Human Nervous System 28-9-2010 By: Raghas Khasawneh

This picture just present the parasympathetic division of the autonomic nervous system and as you notice, the parasympathetic fibers doesn’t leave at all segments of the CNS, we have fibers coming out from the brain and fiber coming out at the sacral of the spinal cord, This why we call the parasympathetic, it has a craniosacral outflow, because it comes from cranial and sacral.

The cranial part gives fibers to the ganglionic system.

For the parasympathetic, we don’t have a chain of ganglion, why?? Because the para sympathetic division, the neuron leaves the CNS and it’ll not synapse in a middle way, it will synapse just within the organ innervates .

Whenever we have pregonglionic autonomic fiber, we call it splanchnic nerve, and it means preganglionic fiber for sympathetic and parasympathetic. In case of the pelvic splanchnic nerve, its parasympathetic =)

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The Human Nervous System 28-9-2010 By: Raghas Khasawneh

The sympathetic division comes out the thoracic part of the spinal cord and some of the lumbaric part, as you see the fiber exit and merge in the sympathetic trunk ( it’s a chain of ganglion ), so you can see that by your naked eye on the cadaver, if we have corpse in dissecting room, and after it enter the chain, it either will make a synapse at the same level like T4 for example, the neuron which left the CNS it will go to ganglion and it either synapse here and the postganglion will leave or the same fiber will go up or down to make a synapse at different levels or it’ll leave the chain without synapsing and it’ll synapse in different sympathetic ganglion.

So, what type of ganglion I use for the synapse between the postganglionic and the postganglionic sympathetic fiber?

First, we have the sympathetic trunk which is the chain of ganglion collecting to each other at either side of the vertebral column, we also have paravertebral ganglion and also another type of ganglion we call it prevertebral, it lies anterior to the vertebral column.

As I just said, some fibers do not synapse inside the ganglion chain and they just leave and make the synapse somewhere else, this somewhere else is the prevertebral ganglion.

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The Human Nervous System 28-9-2010 By: Raghas Khasawneh

- The thoracic gray matter have an extra horn, we usually have either posteror horns or anterior horns, in the thoracic segment we have something called the lateral horn where the sympthatic neurons resides.

- Gray and white ramus together we call it rami communicates. - Why it’s white ramus? Because the preganglionic fiber are myelinated .

“ I tried to do my best and forgive me guys for any mistakes “ Good luck,,

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And btw I think u’re the youngest in our dof3a, Happy birthday mn hl2 ;) *_^

Done by:

Raghas khasawneh =)

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