When you have read the online lecture area in Module 1 about natural protections for the brain, think about what man-made protections there are including precautions we ourselves do, technologies we can use and laws that govern possible protections. Talk about your ideas in this discussion and particularly the ones that pertain to you and your activities.
The lecture note
Chapter 2 Neuroanatomy – Cells and Stuctures
Learning Objectives/Outcomes – After studying this chapter and online notes the student should be able to understand the following:
1. Contrast the differing roles of the glia with the roles of the neurons in the nervous system2. Apply the directional terms used in studying the brain and the peripheral nervous system3. Differentiate the central nervous system from the peripheral nervous system4. Analyze the protections of the brain5. Contrast the roles of the three functional types of neurons6. Analyze the pervasiveness of the nervous system throughout the body7. Compare the brain structures in the hindbrain, midbrain and forebrain
Functionally There are 3 Types of Neurons
If we look at neurons and how they function, we can categorize them into 3 categories:
1. Sensory neurons – these are responsible for transducing the environmental energy into neural energy of the action potential.
2. Motor Neurons – these are responsible for enervating (causing excitation) in a motor fiber. Muscles of the body, whether skeletal or the muscles of the gut are made of muscle fibers and neurons are responsible for make them move.
3. Interneurons – if a neuron is not a sensory neuron or a motor neuron, it is an interneuron. Interneurons continue the messages from sensory neurons up afferently toward the brain and down efferently from the brain to a muscle.
All of these pictures were taken at UTPB in the lab with a full human brain and a half human brain. It is one thing to look at plastic models and those can be quite helpful, but very different to look at the brain itself. This brain has been preserved in Formalin so that it would not break down in the normal process of all organic tissue. This process makes the tissues shrink somewhat and so know that the living brain would be considerably less dense and contracted. It is a bit like sticking your finger into partially congealed jello – I know that sound weird, but do know that the brain itself has no pain neurons so it is not painful to touch parts. However, the scalp and other tissues certainly have pain sensors.
Bottom view The hindbrain is visible with the medulla and pons toward the bottom with the cerebellum on each side. The cranial nerves come off the hindbrain and the higher portion of the midbrain. In the middle you will see twin structures making a “V” shape. Those are the tracts for the optic nerve.
Top view of human brain. Notice the two cortex hemispheres, the sulcus and gyrus formations, and the central fissure that divides the cortex into the hemispheres.
This shows the way the cerebellum “hands” off the cortex area. The central fissure is also evident and the corpuscallosum is almost visible in the crevice of the central fissure.
This is a saggital cut through the profile. This cut allows us to see the most variety of structures. Read and study your book and then see if you can test yourself here. Remember that there has been a lot of compacting of tissue to to the Formalin fixative. Also, you will see the swellings of the pons right above the cerebellum. However, the medulla is missing. Close up of the sub-arachnoid layer You have read about and that is one of the protections of the brain. Cerebral-spinal fluid runs through it in the living brain and those dark areas are blood vessels that run through it to help nourish the brain. In the living brain it would be more organized than what we see here of course. Cross section of the cerebellum. Its fern-like appearance shows how important the functions are and gives more surface area to allow for many functions. This is similar to the gyrus and sulcus formation which allow for more surface area of the cortex for the same reason. The cerebellum is often called the “mini brain” because of its appearance and broad functions. Close-up of optic nerve tract and olfactory tract Note the two thickish white “V” structure. That is the optic nerve that the axons from the ganglion cells of the retina go through on their way through the visual pathway to the visual (striate) cortex. The intersect of the “V” is the opticchiasm where axons from each eye go contralateral andipsilateral to the visual cortex. That way, both sides of the visual cortex have information from both eyes. Just above each of these tracts you will see a sort of worm like structure running up and down in our picture. Those are the olfactory tracts for the axons of the olfactory bulb for smell. These axons synapse directly into the frontal lobe as smell is the only sense that does not go through the thalamus.
Natural Protections of the Brain
The brain is a vital organ that obviously needs protection from harm because it is a survival organ. There are 7 main natural protections of the brain:
1. Skull – This hard, bony structure helps protect the underlying soft tissue from injury. In newborn babies, the skull is not fully formed and is softer so great care must be taken to protect such developing skulls.e
2. Dura Mater – This strong tissue lies right underneath the skull and is whitish and may remind you of a swimmers latex swim cap. It further protects the soft tissue below because of it strong nature.
3. Sub-Arachnoid Layer – This layer of tissue is web-like and thus, it’s name is that of the spider. It is a web-like layer that has many blood vessels to supply oxygen and glucose to the brain and the cerebral-spinal fluid flows within it. This helps cushion the brain within the skull cavity.
4. Pia Mater – this layer of protective tissue is not visible to the human eye but is below the sub-arachnoid layer and help protect the tender tissues of the cortex from toxins and other agents that are foreign to the brain.
5. Cerebral-Spinal Fluid – This fluid flows in the skull and the spinal cord and circulates and is replaces daily by the body. It brings vitals substances to the brain as well as “washing” wastes from the brain. It’s main function is to give the brain bouncy. When any trauma comes from the outside, it allows the brain to “slosh” around so that the trauma is lessened or avoided. One interesting thing to mention is the idea of a Contrecoup injury. If the head is hit hard as in an automobile accident, the brain will slosh to the other side and hit the skull on the opposite side of the trauma. There may be injury to the side of the head where the trauma hit, but there may also be injury to the other side of the head where the brain impacted the inside of the skull. This is often what happens in football injuries and can have serious results. It is probably that Mohammad Ali had many of these during his boxing days.
6. Ventricles – there are 4 ventricles of the brain. Two are in the upper part (the lateral ventricles) and can be seen if you slice the top of the brain in lateral cuts though thesulci and gyri until the brain cavity opens up. The other two are below these one on top of the other. Cerebral-spinal fluid flows within all of them. They also act as a protection by the give that they have if the brain is compressed in trauma. They allow for some give and take and try to minimize damage. There are also important neurons that line these ventricles for such functions as hunger and thirst.
7. Blood Brain Barrier – this involves all the vessels and capillaries of the circulatory system that service the brain and spinal cord tissue. While these capillaries are much like those of the whole rest of the body, they have a unique feature. The walls of all these vessels and capillaries are non-fenestrated or continuous so that they only allow very small molecules to pass through and thus get into the brain. They allow oxygen and glucose through but keep out most viruses, bacteria, toxins and metals. This is a protection to the brain tissue. Most capillaries (fenestrated ones) in other parts of the body allow for the passage of much larger molecules that could harm the brain. As good as this protection is, it sometimes poses a problem in getting medications into the brain that may be needed.