Neuroanatomy (download pdf version)
The human skull, brain, and spinal cord are very complex anatomical structures with numerous parts. The nervous system is divided into two broad categories; the peripheral nervous system and central nervous system. The central nervous system consists of the brain and the spinal cord. The peripheral nervous system consists of all the nerves in the arms, legs and torso which actually leave the spinal cord. A nerve is simple a bundle of neurons. (Most neurosurgeons treat patients with diseases which affect either the central or peripheral nervous system.)
The brain and spinal cord are made up of billions of individual neurons (see figure 1), as well as supporting cells called ‘glia,’ and blood vessels which supply nutrients to the system. A neuron is an independent cell which has the ability to process and transmit both chemical and electrical information from one neuron to another. (Thus, the one of the more common type of tumors which affects the brain is called a glioma and is specifically a tumor of the glia “supporting cell.”) Although in the brain it controls many higher-level complex processes, a large fraction of the neurons are dedicated to two functions – processing sensory information transmitted from the body (touch, pain, pressure, etc.) and planning and coordination of muscle movement. Other functions of these neurons include regulation of hormones, wakefulness, personality, thought, etc.
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| Figure 1: Neurons and glia |
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| A cartoon of neurons and some surrounding cells. The larger triangulated structures are neurons and the smaller circles are the glia. Notice the intricate connections between neurons. These limbs are how neurons communicate with each other. |
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| The signals from the neurons in the brain must be transmitted to and from the rest of the body and organs. There are a special set of 12 nerves called the cranial nerves which transmit information from the face and senses to and from the brain. (see figure 2) For example, the optic nerve is the bundle of neurons which transmit the light and vision to and from the brain. The information and interpretation of these signals is process with a different area of the brain. |
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| Figure 2: Base of brain – The following is a view of the base of the brain. Some of the cranial nerves can be seen. The Olfactory nerve detects smells and odors. The optic nerve relays information regarding light and vision. The oculomotor and trochlear nerves are two of the three nerves which helps to move the eyeball in different directions. The trigeminal nerve relays sensation from the face. As mentioned above, there are a total of twelve pairs of cranial nerves. |
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| Similarly, it is through the spinal cord and then the peripheral nerves that this circuit is completed. There is a physical connection and millions of neurons in the spinal cord which helps to relay the brains information to the rest of the body. For example, there is actually a single, extremely long group of neurons which start from the middle of the brain, travel through the spinal cord the length of the spine makes only one connection which reaches and allows movement of the feet. |
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| The central nervous system is irreplaceable. As such, it must be protected from any potential damage. The components of the central nervous system by large do not have any ability to regenerate. Thus, the skull encases the brain and the spinal vertebra and discs (see spinal anatomy) protects it from injury. Directly below the skull are three overlapping membranes that cover the brain and the spinal cord. These are called the meninges. The outermost layer very tough and is called the dura mater. If a blood vessel ruptures above this membrane (between the skull and the dura), it is called an epidural hematoma, whereas bleeding below this membrane is called a subdural hematoma. These are common problems which a neurosurgeon must attend to urgently because they have the ability to rapidly damage the underlying brain tissue. |
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| Figure 3: AP and Lateral Skull – The figure on left is a straight on view of the skull with a view of the orbit, the part which encases the eyes. The figure on the right is a side view of the skull. |
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| The arachnoid mater is below the dura and resembles a sticky membranous fresh egg white type of spider web. The pia lies directly on the brain and contains the brains superficial blood vessels. A clear, nutrient rich liquid called cerebrospinal fluid (commonly referred to as spinal fluid) bathes the brain and spinal cord and also provides shock absorption. |
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| Looking at the brain from a side view, it can be seen that there are many ridges (‘gyri’) and grooves (‘sulci’). This gives the brain a greater surface area and helps pack in more neurons into a smaller space. Each neuron has a cell body and long extensions known as dendrites (which receive information) and axons (which transmit information). The cell bodies comprise the surface part of the brain (cerebral cortex or ‘grey matter’) and the dendrites make up a large portion of the inner matter of the brain (‘white matter’). |
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| As a whole, the brain is divided into 4 main lobes; frontal, parietal, temporal and occipital. There are two large fissures or gaps or sulci (grooves) which can be seen on the side of the brain, these serve as very important anatomic landmarks in the brain. The fissure traveling from front to back is called the Sylvian fissure and the one that runs upwards towards the middle of the brain is called the central sulcus. The fissures define different lobes of the brain. The purpose of defining different lobes of the brain is job differentiation. For example, the chief purpose of the occipital lobe is to process vision. |
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| Figure 4: Side view of the brain – the illustration below represents a side view of the brain showing the four main lobes of the brain, frontal, parietal, temporal and occipital lobes. Also, the two large grooves called the central sulcus and the sylvian fissure can be seen. The central sulcus separates the movement part of the brain from the sensory part and is an important landmark for the surgeon. |
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| The ridge directly in front of the central sulcus is called the precentral gyrus. This is an important region because it houses the primary motor cortex of the brain. This diagram shows how the arrangement of neurons corresponds to the level of motor control required. Areas that need fine motor control, like the fingers and face, have a higher concentration of neurons than other areas. When muscle movement is required, these neurons send signals down long extensions called dendrites that travel down the spinal cord and relay the signal to other neurons that eventually send the signal to the muscle and cause the contraction. |
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| The area of the brain where sensations (touch, vibration, etc.) are perceived is located directly behind the precentral region in an area of the brain known as the postcentral gyrus. It is similar to the precentral gyrus because it is also arranged so areas receiving higher amounts of sensory information (fingers, face, etc.) have more neurons dedicated to them. |
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| At the base of the brain, there is a critical structure called the brainstem. This region forms the eventual connection between the brain and the spinal cord. It subconsciously controls many functions important for life such as heart rate, blood pressure, and breathing so we don’t have to think about doing them. This area is also important for sleep and wakefulness. Damage to the brainstem area can have severe and devastating effects or death. |
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| The structure at the back of the brain is called the cerebellum and is very important in combining sensory information from eyes, ears, and muscles to coordinate movements. Damage to the cerebellum can cause ‘intention tremors’ – a trembling that occur only when conscious movement is carried out. |
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| The hippocampus is yet another important structure of the brain that is difficult to see from any surface view. It is important in learning and short-term memory and damage to this area can lead to amnesia and learning defects. There is also a close connection of this area with the nerves involved in smell, which is may be why certain smells remind us of specific places or events from years ago. |
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| Epilepsy is a brain disorder in which clusters of neurons, and sometimes the whole brain, transmit signals abnormally. Depending on where this abnormal activation occurs in the brain, strange sensations, emotions, muscle spasms, and even a loss of consciousness might occur. The exact reasoning for why these neurons fire abnormally is unknown, but only after two seizures is the diagnosis of epilepsy made. When seizures can be localized to one specific area, surgery sometimes can be performed to remove the area and stop further seizures. Some seizures start on one side and travel across the corpus callosum (the main connection between the two hemispheres) into the other hemisphere. In this case, surgery is sometimes performed to cut the corpus callosum and disconnect the two hemispheres. |
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| In Parkinson’s disease, cells in the substantia nigra (a specific area of the brainstem) selectively degenerate or wear out. These cells produce dopamine which is an important neurotransmitter involved in movement. There are many symptoms associated with this disease including muscle stiffness, problems with walking, a resting tremor, and changes in speech. There are many treatments currently being looked into trying to increase the dopamine levels in the brain. |
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| Clearly, the brain anatomy is critical in understanding the functioning of the human body and in the treatment of neurological disorders. Neurosurgeons have a very detailed understanding of this anatomy and that education is used to correlate your symptoms to a specific anatomical or functional problem. If these correlate, then a diagnosis can be typically made and a treatment planned. Sometimes when these symptoms do not correlate, then other investigations or diagnostic test may have to be performed to figure out the problem. |
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