Central Nervous System

The Nerve Impulse

When a nerve is stimulated the resting potential changes. Examples of such stimuli are pressure, electricity. chemicals., ect. Different neurons are sensitive to different stimuli(although most can register pain). The stimulus causes sodium ion channels to open. The rapid change in polarity that moves along the nerve fiber is called the "ACTION POTENTIAL." This moving change in polarity has several stages:

Depolarization

The upswing is caused when positively charged sodium ions(Na+) suddenly rush through open sodium gates into a nerve cell.The mambrane potential of the stimulated cell undergoes a localized change from-65 millivolts to 0 in a limited area. As additional soduum rushes in, the membrane potential actually reverses its polarity s

The central nervous system is the control center for the body. It regulates organ function, higher thought, and movement of the body. The central nervous system consists of the brain and spinal cord.

Generation & propagation of an action potential

Electrical characteristics of a neurochemical action potential.

o that the outside of the membrane is negative relative to the inside. During this change of polarity the membrane actually develops a positive value for a moment(+40 millivolts). The change in voltage stimulates the opening of additional sodium channels (the are voltage-gated). This is an example of a positive feedback loop.

Repolarization

(The downswing) is caused by the closing of sodium ion channels and the opening of potassium ion channels. Release of positively charged potassium ions (K+) from the nerve cell when potassium gates open. Again, these are opened in response to the positive voltage--they are voltage gated. This expulsion acts to restoret the localized negative membrane potential of the cell (about -65 or -70 mV is typical for nerves).

Refractory phase

is a short period of time after the depolarization stage. Shortly after the sodium gates open they close and go into an inabtive conformation. The sodium gates cannot be opened again until the membrane is repolarized to its normal resing potential. The sodium-potassium pump returns sodium ionss to the outside and potassuim ions to the inside. During the refractory phase this particular area of the nerve cell membrane cannot be depolarized. This refractory area explains why action potentials can only move forward from the point of stimulation.

Increased permeability of the sodium channel occurs when there is a deficit of calcium ions. when there is a deficit of calcium ions (Ca+2) in the interstitial fluid the sodium channels are activated (opened) by very little increase of the membrane potential above the normal resting level. The nerve fiber can therefore fire off action potentials spontaneously, resulting in tetany. Coud be caused by the lack of hormone from parathyroid glands. could be caused by hyperventilation, which leads to a higher pH, which causes calcium to bind and become unavailable. Speed of conduction. This area of depolarization/replarization/recovery moves along anerve fiber like a very fast wave. In nonmyelinated fibers, conduction is hundreds of times faster because the action potential only occurs at the nodes of Ranvier by jumping from node to node. this is called "saltatory" conduction. Damage to the myelin sheath by the disease can cause secere impairment of nerve cell function. Some posons and drugs interfere with nerve impulses by blocking sodium channels in nerves. See discussion on drug at the end of this outline.

Brain

A color-coded image of the brain, showing the main sections.

The brain is found in the cranial cavity. Within it are found the higher nerve centers responsible for coordinating the sensory and motor systems of the body (forebrain). The brain stem houses the lower nerve centers (consisting of midbrain, pons, and medulla),

Medulla

The medulla is the control center for respiratory, cardiovascular and digestive functions.

 Pons

The pons houses the control centers for respiration and inhibitory functions. Here it will interact with the cerebellum.

Cerebrum

The cerebrum, or top portion of the brain, is divided by a deep crevice, called the longitudinal sulcus. The longitudinal sulcus separates the cerebrum in to the right and left hemispheres. In the hemispheres you will find the cerebral cortex, basal ganglia and the limbic system. The two hemispheres are connected by a bundle of nerve fibers called the corpus callosum. The right hemisphere is responsible for the left side of the body while the opposite is true of the left hemisphere. Each of the two hemispheres are divided into four separated lobes: the frontal in control of specialized motor control, learning,planning and speech; parietal in control of somatic sensory functions; occipital in control of vision; and temporal lobes which consists of hearing centers and some speech. Located deep to the temporal lobe of the cerebrum is the insula.

Cerebellum

The cerebellum is the part of the brain that is located posterior to the medulla oblongata and pons. It coordinates skeletal muscles to produce smooth, graceful motions. The cerebellum receives information from our eyes, ears, muscles, and joints about what position our body is currently in. It also receives output from the cerebral cortex about where these parts should be. After processing this information, the cerebellum sends motor impulses from the brainstem to the skeletal muscles. The main function of the cerebellum is coordination. The cerebellum is also responsible for balance and posture. It also assists us when we are learning a new motor skill, such as playing a sport or musical instrument.

The Limbic System and Higher Mental Functions

Image of the brain, showing the Limbic system.

 The Limbic System

The Limbic System is a complex set of structures found just beneath the cerebrum and on both sides of the thalamus. It combines higher mental functions, and primitive emotion, into one system. It is often referred to as the emotional nervous system. It is not only responsible for our emotional lives, but also our higher mental functions, such as learning and formation of memories. The Limbic system explains why some things seem so pleasurable to us, such as eating and why some medical conditions are caused by mental stress, such as high blood pressure. There are two significant structures within the limbic system and several smaller structures that are important as well. They are:

1. The Hippocampus
2. The Amygdala
3. The Thalamus
4. The Hypothalamus
5. The Fornix and Parahippocampus
6. The Cingulate Gyrus

Structures of the Limbic System

Hippocampus

The Hippocampus is found deep in the temporal lobe, shaped like a seahorse. It consists of two horns that curve back from the amygdala. It is situated in the brain so as to make the prefrontal area aware of our past experiences stored in that area. The prefrontal area of the brain consults this structure to use memories to modify our behavior. The hippocampus is responsible for memory.

Amygdala

The Amygdala is a little almond shaped structure, deep inside the anteroinferior region of the temporal lobe, connects with the hippocampus, the septi nuclei, the prefrontal area and the medial dorsal nucleus of the thalamus. These connections make it possible for the amygdala to play its important role on the mediation and control of such activities and feelings as love, friendship, affection, and expression of mood. The amygdala is the center for identification of danger and is fundamental for self preservation. The amygdala is the nucleus responsible for fear.

Thalamus

Lesions or stimulation of the medial, dorsal, and anterior nuclei of the thalamus are associated with changes in emotional reactivity. However, the importance of these nuclei on the regulation of emotional behavior is not due to the thalamus itself, but to the connections of these nuclei with other limbic system structures. The medial dorsal nucleus makes connections with cortical zones of the prefrontal area and with the hypothalamus. The anterior nuclei connect with the mamillary bodies and through them, via fornix, with the hippocampus and the cingulated gyrus, thus taking part in what is known as the Papez's circuit.

 Case Study

Central Pain Syndrome

I was 42 years old when my life changed forever. I had a stroke. As an avid viewer of medical programs on television I assumed that I would have physical therapy for my paralyzed left side and get on with my life. No one ever mentioned pain or the possibility of pain, as a result of the stroke. I did experience unusual sensitivity to touch while still in the hospital, but nothing to prepare me for what was to come.

The part of my brain that is damaged is the Thalamus. This turns out to be the pain center and what I have now is an out of control Thalamus, resulting in Thalamic Pain syndrome, also called Central Pain Syndrome. This means that 24 hours a day, seven days a week, my brain sends messages of pain and it never goes away. I am under the care of physicians, who not only understand chronic pain, but are also willing to treat it with whatever medications offer some help. None of the medications, not even narcotic medications, take the pain away. They just allow me to manage it so I can function.

Image of the brain showing the location of the hypothalamus.

Hypothalamus

The Hypothalamus is a small part of the brain located just below the thalamus on both sides of the third ventricle. Lesions of the hypothalamus interfere with several vegetative functions and some so called motivated behaviors like sexuality, combativeness, and hunger. The hypothalamus also plays a role in emotion. Specifically, the lateral parts seem to be involved with pleasure and rage, while the medial part is linked to aversion, displeasure, and a tendency to uncontrollable and loud laughing. However, in general the hypothalamus has more to do with the expression of emotions. When the physical symptoms of emotion appear, the threat they pose returns, via the hypothalamus, to the limbic centers and then the prefrontal nuclei, increasing anxiety.

The Fornix and Parahippocampal

These small structures are important connecting pathways for the limbic system.

The Cingulate Gyrus

The Cingulate Gyrus is located in the medial side of the brain between the cingulated sulcus and the corpus callosum. There is still much to be learned about this gyrus, but it is already known that its frontal part coordinates smells and sights, with pleasant memories of previous emotions. The region participates in the emotional reaction to pain and in the regulation of aggressive behavior.

 Memory and Learning

Memory is defined as : The mental faculty of retaining and recalling past experiences, the act or instance of remembering recollection. Learning takes place when we retain and utilize past memories.

There are three basic types of memory:

1. Sensory Memory
2. Short Term Memory
3. Long Term Memory

Sensory Memory

The sensory memories act as a buffer for stimuli through senses. A sensory memory retains an exact copy of what is seen or heard: iconic memory for visual, echoic memory for aural and haptic memory for touch. Information is passed from sensory memory into short term memory. Some believe it lasts only 300 milliseconds, it has unlimited capacity. Selective attention determines what information moves from sensory memory to short term memory.

Short Term Memory

Short Term Memory acts as a scratch pad for temporary recall of the information under process. For instance, in order to understand this sentence you need to hold in your mind the beginning of the sentence as you read the rest. Short term memory decays rapidly and also has a limited capacity. Chunking of information can lead to an increase in the short term memory capacity, this is the reason why a hyphenated phone number is easier to remember than a single long number. The successful formation of a chunk is known as closure. Interference often causes disturbance in short term memory retention. This accounts for the desire to complete a task held in short term memory as soon as possible.

Within short term memory there are three basic operations:

1. Iconic memory - the ability to hold visual images
2. Acoustic memory - the ability to hold sounds. Can be held longer than iconic.
3. Working memory - an active process to keep it until it is put to use. Note that the goal is not really to move the information from short term memory to long term memory, but merely to put it to immediate use.

The process of transferring information from short term to long term memory involves the encoding or consolidation of information. This is not a function of time, that is, the longer the memory stays in the short term the more likely it is to be placed in the long term memory. On organizing complex information in short term before it can be encoded into the long term memory, in this process the meaningfulness or emotional content of an item may play a greater role in its retention in the long term memory. The limbic system sets up local reverberating circuits such as the Papaz's Circuit.

Long Term Memory

Long Term Memory is used for storage of information over a long time. Information from short to long term memory is transferred after a short period. Unlike short term memory, long term memory has little decay. Long term potential is an enhanced response at the synapse within the hippocampus. It is essential to memory storage. The limbic system isn't directly involved in long term memory necessarily but it selects them from short term memory, consolidates these memories by playing them like a continuous tape, and involves the hippocampus and amygdala.

There are two types of long term memory:

1. Episodic Memory
2. Semantic Memory

Episodic memory represents our memory of events and experiences in a serial form. It is from this memory that we can reconstruct the actual events that took place at a given point in our lives. Semantic memory, on the other hand, is a structured record of facts, concepts, and skills that we have acquired. The information in the semantic memory is derived from our own episode memory, such as that we can learn new facts or concepts from experiences.

There are three main activities that are related to long term memory:

1. Storage
2. Deletion
3. Retrieval

Information for short term memory is stored in long term memory by rehearsal. The repeated exposure to a stimulus or the rehearsal of a piece of information transfers it into long term memory. Experiments also suggest that learning is most effective if it is distributed over time. Deletion is mainly caused by decay and interference. Emotional factors also affect long term memory. However, it is debatable whether we actually ever forget anything or whether it just sometimes becomes increasingly difficult to retrieve it. Information may not be recalled sometimes but may be recognized, or may be recalled only with prompting. This leads us to the third operation of memory, information retrieval.

There are two types of information retrieval:

1. Recall
2. Recognition

In recall, the information is reproduced from memory. In recognition the presentation of the information provides the knowledge that the information has been seen before. Recognition is of lesser complexity, as the information is provided as a cue. However, the recall may be assisted by the provision of retrieval cues which enable the subject to quickly access the information in memory.

Language and Speech

Language depends on semantic memory so some of the same areas in the brain are involved in both memory and language. Articulation, the forming of speech, is represented bilaterally in the motor areas. However, language analysis and speech formation take place in most individuals in regions of the left hemisphere only. The two regions involved are:

1. Broca's Area
2. Wernicke's Area

Broca's area is located just in front of the voice control area of the left motor cortex. This region assembles the motor of speech and writing. For example, patients with lesions in this area:

1. Understand language perfectly
2. May be able to write perfectly
3. Seldom speak spontaneously

Wernicke's area is part of the auditory and visual associations cortex. This region is responsible for the analysis and formation of language content. For example, patients with lesions in this area:

1. Are unable to name objects
2. Are unable to understand the meaning of words
3. Articulate speech readily but usually nonsensically

Diseases of the Limbic System

There are several well known diseases that are disorders of the limbic system. A few are:

1. Psychosis
2. Schizophrenia
3. Depression

An increased DA response in the limbic system results in schizophrenia. DA may be synthesized or secreted in excess, DA receptors may be supersensitive, and DA regulatory mechanism may be defective. Symptoms are decreased by drugs which block DA receptors. Symptoms of schizophrenia are:

1. Loss of touch with reality
2. Decreased ability to think and reason
3. Decreased ability to concentrate
4. Decreased memory
5. Regress in child-like behavior
6. Altered mood and impulsive behavior
7. Auditory hallucinations

Symptoms may be so severe that the individual cannot function.

Depression is caused by decreased levels of NE and/or serotonin in the limbic system. Drugs which increase NE and/or serotonin decrease the symptoms of depression. Depression is the most common major mental illness and is characterized by both emotional and physical symptoms. Symptoms of depression are:

1. Intense sadness and despair
2. Anxiety
3. Loss of ability to concentrate
4. Pessimism
5. Feelings of low self esteem
6. Insomnia or hypersomnia
7. Increased or decreased appetite
8. Changes in body temperature and endocrine gland function

10 to 15% of depressed individuals display suicidal behavior during their lifetime.

Another common form of depression is manic depression. Manic is an acute state characterized by:

1. Excessive elation and impaired judgment
2. Insomnia and irritability
3. Hyperactivity
4. Uncontrolled speech

Manic depression, also known as bipolar disorder, displays mood swings between manic and depression. The limbic system receptors are unregulated. Drugs used are unique mood stabilizers.

The hippocampus is particularly vulnerable to several disease processes, including ischemia, which is any obstruction of blood flow or oxygen deprivation, Alzheimer’s disease, and epilepsy. These diseases selectively attack CA1, which effectively cuts through the hippocampal circuit.