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Neuroanatomy and the Brain

MattPD

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Welcome! The human brain is a marvelously complex web of structures all working together to help create what we subjectively experience as reality. This guide has been designed to help you better understand the complex structures which make up your brain and allow you to function in daily life.

The main purpose of creating this FAQ was to inform you, the reader, about how your brain does what it does. The terms contained herein can help you to understand journal articles, scientific and news media reports about drug effects, and also to understand how the drugs you choose to ingest affect the functioning of your brain as a whole.

Included in this FAQ are italicized portions. These portions pertain to specific licit and illicit drug effects which occur in or at that brain structure. These inclusions are by no means a complete account of how these drugs exert their effects, but instead give a basis from which to continue research.
 
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Table of Contents

1. Neurons
  • The Cell Membrane
  • The Cell Body
  • The Axon
  • The Myelin-Sheath
  • The Dendrite
  • The Synaptic Cleft
2. Types of Neurons
  • Motor Neurons
  • Sensory Neurons
  • Local Neurons
3. The Spinal Chord

4. The Hindbrain
  • The Medulla
  • The Pons
  • The Reticular Formation and The Raphe System
  • The Cerebellum
5. The Midbrain
  • The Tectum
  • The Tegmentum
  • The Substantia Nigra
6. The Forebrain (and Limbic System)
  • The Thalamus
  • The Hypothalamus
  • The Pituitary Gland
  • The Basal Ganglia
  • The Basal Forebrain
  • The Hippocampus
7. Lobes of the Brain
  • The Occipital Lobe
  • The Temporal Lobe
  • The Parietal Lobe
  • The Frontal Lobe
8. Ventricals and CSF
 
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1. Neurons

A neuron is, in the simplest sense, the cell which receives and transfers information within the nervous system. The human brain, once it reaches maturity may contain as many as one hundred billion neurons, however this is just an estimate as it would be impossible to count all these cells.

What are the different parts of a neuron?

The neuron is composed of a vast amount of separate parts; these follow below. Differences between sensory and motor neurons will be discussed in greater detail later.

brain1.JPG


The Cell Membrane All cells in the body are surrounded by a membrane. These membranes are composed of an interconnecting bi-lipid (2 layers of fat) layer. These layers are interconnected, but are still able to move around each other. Embedded in this bi-lipid layer are specialized protein molecules which are known as protein channels. These channels allow exchange of chemicals, wastes, and nutrients necessary for the cells survival.

Cell membranes help to protect the cell from damaging agents or chemicals, and also help to preserve the specific charges within a neuron necessary for proper functioning.

The Cell Body (or Soma) This structure contains many components necessary to the continued survival of the neuron. These include the nucleus, which contain the chromosomes of the neuron; the mitochondria, which is a structure in which the cell performs metabolic functions (discussed later), and finally ribosomes which synthesize new protein molecules for the cell. These ribosomes may be floating unattached in the cell; others are connected to the endoplasmic reticulum which is a series of tubes which transport new proteins to other locations within the cell.

The Axon This structure is long and fibrous and extends from the soma to the presynaptic terminals on nearby neurons. The axon is basically the information sender of the neuron. It may convey impulses to target muscles to invoke a movement or to other neurons to create a response.

Axons contain many branches, at the tip of each of these is a bulbous formation known as the presynaptic terminal. These terminals are the site of neurotransmitter release. Drugs such as ecstasy work at these sites to cause the pleasurable effects of the drug. These are also the site of 'pumps' which pull released neurotransmitters like dopamine back into the axon. These dopamine pumps are the pumps which are blocked when cocaine is used. This blockade of the reuptake of dopamine causes increased amounts to act upon the receptor at the next neuron, thus causing the high. Due to the great amount of energy necessary for synthesis and release of the chemicals from the terminal many mitochondria can be found in these cellular areas.

A neuron is limited to only a single axon; however it may have many branches. Axons also vary widely in length. Some are incredibly long, reaching from your spinal chord all the way down to your legs. Others are incredibly short and localized.

The Myelin Sheath This is a fatty substance which covers many axons in the human brain. This substance is created in Scwann cells in the periphery of the body. These sheaths both insulate and increase the speed at which info may be sent through the axon.

Found along myelin sheathes are breaks in the sheath known as Nodes of Ranvier. At these nodes the charge traveling down the axon is strengthened.

The Dendrite Somewhat the opposite of the axon, this structure extends from the soma into the periphery of the nervous system. Its name comes from the Greek word for tree, and dendrites do indeed resemble very small trees. Dendrites are used as the information receiving parts of the neuron, specifically at sites known as synaptic receptors. A dendrite’s ability to process larger or smaller amounts of info is based on its surface area. To take advantage of this dendrites often branch out from their main stalks and develop even smaller outgrowths known as dendritic spines. These spines and branches greatly increase the surface area of the neuron.

A neuron may have any number of dendrites.

The Synaptic Cleft Although not technically part of a neuron, this is the space between presynaptic terminals and synaptic receptors. In this area neurotransmitters are sent from axons to dendrites (where they interact with receptors for those specific neurotransmitters), destruction of neurotransmitters occurs and neurotransmitters are reabsorbed.
 
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2. Types of Neurons

There are several types of neurons which help to send info and messages throughout our entire bodies.

Motor Neuron This type is usually located within the spinal chord, extending out to target muscles. This type of neuron receives chemical and electric messages from other neurons in a domino style fashion originating within areas of the brain. When excited it conducts impulses from the soma (cell body) down the axon and to the target muscle. This either contracts or relaxes said muscle.

All axons of this type of cell are known as efferent axons. This means, essentially, that it carries info away from a structure. Basically it receives messages and carries the impulses into muscle cells.

Sensory Neuron This type is highly specialized at its dendrite end to be ultra sensitive to a certain type of stimulation. For example a neuron which sends info about touch from the skin may have dendrites which merge directly from the skin to the axon located in the spinal chord.

Sensory neurons are known as afferent neurons. This is the opposite of the above, efferent neurons. These cells carry information into a structure, i.e. it receives sensory info from outside stimulus, and carries it to a place where it may be processed into one of the five senses.

Local neurons These neurons have either an incredibly short or completely non-existent axons. These types are only capable of communicating with cells directly adjacent to it. Many cells directly within the brain are of this type.
 
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3. The Spinal Chord

The spinal chord communicates with sense organs and muscles below the level of the head.

brain3.JPG


Sensory neurons are located in clusters outside of the spinal chord. These enter the spinal chord towards the back and carry info up to the brain. Motor neurons are located within the spinal chord, and exit it towards the stomach to carry impulses to muscles and glands.

Muscle relaxants work on GABA neurons located in the spinal chord and in the brain stem. They are complete agonists, which means that they bind to GABA receptors and stimulate the neuron. This causes the muscle relaxant effect.
 
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4. The Hindbrain

The hindbrain is the part of your brain closest to the back of your head. It contains many structures important to continuing life.

brain4.JPG


The Medulla This structure is an enlarged, elaborate extension of the spinal chord. It sits atop it and controls a number of vital functions. These include breathing, heart rate, vomiting, salivation, sneezing, etc. Damage to the hindbrain is frequently fatal due to the fact that many of the activities it is involved in controlling are quite vital.

Most drugs which can cause respiratory depression and inhibit gag reflex work their negative actions at structures in the hindbrain. This effect may lead to death. Opiates which cause vomiting are working at the chemoreceptor trigger zone also located on the medulla.

Cranial nerves are located on the medulla. There are 12 pairs of these nerves, of which numbers 5 through twelve are located on the medulla and pons. These nerves are both sensory and motor, and control the skin and muscles in the head. Each nerve originates in a nucleus which integrates the different (sensory and motor) messages to regulate feelings and motions.

The Pons The pons is located towards the front and bottom of the medulla. Besides containing the nuclei for several cranial nerves, the pons acts as a crossover station for many axons to go from one side of the brain to the other.

The Reticular Formation and The Raphe System Contained within the medulla and pons is the reticular formation. This structure has two portions, the ascending and descending areas. The ascending area sends output to much of the cerebral cortex. This output will selectively increase attention or arousal in one area of the brain. The descending portion is one of several brain areas which send messages regarding motor control to the spinal chord.

Also included in the reticular formation is the raphe system. This system sends axons to many parts of the forebrain and serves to increase or decrease the brain’s readiness to respond to stimuli.

The Cerebellum This structure is large and contains many folds and grooves. The cerebellum contributes to control of movement, as well as balance and coordination. The cerebellum can be impaired in it’s functioning by alcohol and other depressant drugs. Damage to this area of the brain may cause trouble shifting attention from auditory to visual stimulus and impairment in timing and coordinating movements.

Loss of coordination and balance and staggering brought on by alcohol appears to be related to depression of cerebral function. Benzodiazepines and anti-convulsant drugs can also have this effect.
 
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5. The Midbrain

The midbrain is located in the middle part of the brain.

The Tectum The top of the midbrain is known as the tectum. To the sides of the tectum are two swellings known as the superior and inferior colliculus.

The Tegmentum Under the tectum is the tegmentum. This is the intermediate level of the midbrain and contains several structures. These include the nuclei for the third and fourth cranial nerves, parts of the reticular formation system (mentioned above) and extensive pathways between the forebrain and hindbrain/spinal chord.

Substania Nigra Also located in the midbrain is the substania nigra. This dopamine rich path way has been implicated in reinforcing effects of drugs of abuse as well as being the site of rapid dopamine loss in patients presenting with Parkinson’s disease.
 
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6. The Forebrain (and Limbic System)

The forebrain is the part of your brain closest to the front of your head. The first structure one encounters in the forebrain is the cerebral cortex. This is the dense, grooved gray matter surface most people envision when they think of the brain.

Under the cerebral cortex is the limbic system. This system contains many structures, forms a border around the brain stem, and is especially important for motivated action.

This system contains many structures important to the pharmacology and abuse potential of many drugs. It includes dopamine rich reward centers. Drugs which affect these neurons include those that act on GABA (gamma-amino butyric acid); such as GHB, alcohol, and benzodiazepines; as well as those that work on opiod receptors and dopamine receptors; including heroin, codeine, and morphine for opiates, and cocaine and nicotine on dopamine. Also in the cortex are a large amount of NMDA receptors. These receptors are blocked when one uses the dissaciative drugs such as PCP and ketamine.

What makes up the limbic system?

brain6.JPG


Thalamus This structure is located in the center of the forebrain, and can best be described as two avocados side by side. The thalamus functions as a relay station for you brain. Most sensory stimulus is first sent to the thalamus. The thalamus processes this info and sends it to the cerebral cortex and into our consciousness. Many nuclei within the thalamus receive primary input from one sense system (such as taste, or touch, but not olfaction) and then send them to specific areas within the cortex (more on this further down.)

The thalamus is implicated in the control of pain processing (i.e. determining level of pain experienced) and opiate analgesics work by altering this process and decreasing associated pain.

Hypothalamus Located under, and near the back of the thalamus is the hypothalamus. The hypothalamus is sent messages from the thalamus and in turn sends messages to the pituitary gland to regulate hormone production. This structure contains many distinct nuclei, including the medial forebrain bundle, which is often referred to as the brains’ "pleasure center."

This structure also controls some motivated behaviors such as feeding, drinking, and temperature regulation. Regulation of homeostasis is a large part of the hypothalamus’s job. Damage to this region of the brain can cause abnormal eating (gorging or starving) inability/desire to maintain osmotic (fluid) balance, and so on.

Pituitary Gland Attached to the base of the hypothalamus is the pituitary gland. This gland synthesizes hormones as directed by the hypothalamus and releases them into the blood stream or at the target organ to create the necessary change.

Basal Ganglia The basal ganglia are a group of 3 major structures to the sides of the thalamus. These include the caudate nucleus, putamen, and globus pallidus. These sub-divisions each exchange information with different parts of the cerebral cortex. This info usually pertains to emotional expression, planning action and memory. This area is high in dopamine neurons and Parkinson’s and Huntington’s disease cause a great amount of deterioration to these areas.

Stereotypic, repetitive activities associated with the use of amphetamines and other stimulants are believed to be caused by stimulation of the dopaminergic neurons within the putamen and the caudate nucleus.

Basal Forebrain On the top surface of the forebrain is the basal forebrain. This structure contains the nucleus basalis which receive information from the hypothalamus and sends axons to widespread areas of the cerebral cortex. The basal forebrain acts as an intermediary between the emotional nature of the hypothalamus and the strictly informational areas of the cortex. The acetylcholine containing axons extending from the nucleus basilis play a large part in the brains wakefulness system.

Hippocampus This is located between the thalamus and the cortex. It is closer to the rear of the forebrain than the aforementioned structures. The hippocampus is critical to storing certain kinds of memory. The fornix links the hippocampus to the hypothalamus and other structures.

The hippocampus is full of GABA sites. It is here that drugs such as alcohol and benzodiazepines exert their amnestic affect by agonizing (binding to the receptors and causing them to be stimulated) these GABAa receptors.
 
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7. Lobes of the Brain

Lobes are the organization of the cerebral cortex. The cerebral cortex consists of two hemispheres. Neurons in each hemisphere communicate with their counterparts through two bundles of axons, the larger corpus callosum, and the smaller anterior commissure.

Both hemispheres are divided into rows and columns of cells. The layers are known as laminae and there are six present. These layers are parallel to the surface of the cortex and are organized into columns with similar properties. The lobes of the brain are named for the skull bones in their proximity.

brain7.JPG


Occipital Lobe This lobe is near to back of the cortex. It is the main target of axons extending from the thalamus that carry info from the optic nerve to create vision. The furthest pole is known as the primary visual cortex. Destruction of any part of this causes cortical blindness.

Parietal Lobe Between the occipital lobe and the central sulcus lies the Parietal lobe. This lobe is the primary target for touch sensations and info from muscle and joint receptors. This lobe contains four bands of cells, each of which receive info from different body areas. This lobe also helps integrate visual and auditory info.

Temporal Lobe Located near the temples is the, fittingly enough, Temporal lobe. This is the primary cortical target for auditory info. These lobes are essential for understanding spoken language, as well as more complex aspects of vision, such as face recognition and movement. This lobe also plays a role in modulating motivational behaviors such as fears and anxieties.

Frontal Lobe This structure contains two important components, the motor cortex and the prefrontal cortex. The rear of the motor cortex is known as the precentral gyrus. The precentral gyrus is specialized to control fine motor responses.

The prefrontal cortex is the structure closest to the front of the lobe. Although this structure is not a primary target of any single sensory system it is quite important in the storage of working memories (i.e. memories about how to do things.) It also helps to control shifting of attention, monitoring events, and calculating orders of actions.
 
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8. Ventricals and CSF

Ventricals are four cavities within the brain (two in each hemisphere) filled with cerebral spinal fluid (CSF). This fluid flows around the brain through canals and small openings which surround the brain and spinal chord. CSF acts as a shock absorber for the brain, provides a reservoir for hormones and nutritive fluids and provides buoyancy.
 
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