Short Description of the Central Nervous System (CNS)

It is composed of the brain and spinal cord. The CNS is surrounded by bone-skull and vertebrae. Fluid and tissue also insulate the brain and spinal cord.



Areas of the brain. The above image is from http://www.prs.k12.nj.us/schools/PHS/Science_Dept/APBio/pic/brain.gif.


The brain is composed of three parts: the cerebrum (seat of consciousness), the cerebellum, and the medulla oblongata (these latter two are "part of the unconscious brain").
The medulla oblongata is closest to the spinal cord, and is involved with the regulation of heartbeat, breathing, vasoconstriction (blood pressure), and reflex centers for vomiting, coughing, sneezing, swallowing, and hiccuping. The hypothalamus regulates homeostasis. It has regulatory areas for thirst, hunger, body temperature, water balance, and blood pressure, and links the Nervous System to the Endocrine System. The midbrain and pons are also part of the unconscious brain. The thalamus serves as a central relay point for incoming nervous messages.

The cerebellum is the second largest part of the brain, after the cerebrum. It functions for muscle coordination and maintains normal muscle tone and posture. The cerebellum coordinates balance.

The conscious brain includes the cerebral hemispheres, which are are separated by the corpus callosum. In reptiles, birds, and mammals, the cerebrum coordinates sensory data and motor functions. The cerebrum governs intelligence and reasoning, learning and memory. While the cause of memory is not yet definitely known, studies on slugs indicate learning is accompanied by a synapse decrease. Within the cell, learning involves change in gene regulation and increased ability to secrete transmitters.

The Brain 

During embryonic development, the brain first forms as a tube, the anterior end of which enlarges into three hollow swellings that form the brain, and the posterior of which develops into the spinal cord. Some parts of the brain have changed little during vertebrate evolutionary history. Click here to view an diagram of the brain, and here for a clickable map of the brain.



Parts of the brain as seen from the middle of the brain. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.
Vertebrate evolutionary trends include

1. Increase in brain size relative to body size.
2. Subdivision and increasing specialization of the forebrain, midbrain, and hindbrain.
3. Growth in relative size of the forebrain, especially the cerebrum, which is associated with increasingly complex behavior in mammals.

The Brain Stem and Midbrain

The brain stem is the smallest and from an evolutionary viewpoint, the oldest and most primitive part of the brain. The brain stem is continuous with the spinal cord, and is composed of the parts of the hindbrain and midbrain. The medulla oblongata and pons control heart rate, constriction of blood vessels, digestion and respiration.
The midbrain consists of connections between the hindbrain and forebrain. Mammals use this part of the brain only for eye reflexes.

The Cerebellum

 

The cerebellum is the third part of the hindbrain, but it is not considered part of the brain stem. Functions of the cerebellum include fine motor coordination and body movement, posture, and balance. This region of the brain is enlarged in birds and controls muscle action needed for flight.

The Forebrain

 

The forebrain consists of the diencephalon and cerebrum. The thalamus and hypothalamus are the parts of the diencephalon. The thalamus acts as a switching center for nerve messages. The hypothalamus is a major homeostatic center having both nervous and endocrine functions.
The cerebrum, the largest part of the human brain, is divided into left and right hemispheres connected to each other by the corpus callosum. The hemispheres are covered by a thin layer of gray matter known as the cerebral cortex, the most recently evolved region of the vertebrate brain. Fish have no cerebral cortex, amphibians and reptiles have only rudiments of this area.
The cortex in each hemisphere of the cerebrum is between 1 and 4 mm thick. Folds divide the cortex into four lobes: occipital, temporal, parietal, and frontal. No region of the brain functions alone, although major functions of various parts of the lobes have been determined.



The major brain areas and lobes. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.
The occipital lobe (back of the head) receives and processes visual information. The temporal lobe receives auditory signals, processing language and the meaning of words. The parietal lobe is associated with the sensory cortex and processes information about touch, taste, pressure, pain, and heat and cold. The frontal lobe conducts three functions:

1. motor activity and integration of muscle activity
2. speech
3. thought processes

Functional areas of the brain. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.
Most people who have been studied have their language and speech areas on the left hemisphere of their brain. Language comprehension is found in Wernicke's area. Speaking ability is in Broca's area. Damage to Broca's area causes speech impairment but not impairment of language comprehension. Lesions in Wernicke's area impairs ability to comprehend written and spoken words but not speech. The remaining parts of the cortex are associated with higher thought processes, planning, memory, personality and other human activities.



Parts of the cerebral cortex and the relative areas that are devoted to controlling various body regions. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.

Types of Aphasia

 
Types of Aphasia 


Retrieved From: American Stroke Association 

Excerpted from "The Language of Aphasia," Stroke Connection Magazine, May/June 2003 (Science Update August 2009)


Aphasia | Comprehension | Wernicke's | Broca's 

| Global | Remember

Language is much more than words. It involves our ability to recognize and use words and sentences. Much of this capability resides in the left hemisphere of the brain. When a person has a stroke or other injury that affects the left side of the brain, it typically disrupts their ability to use language.

Through language, we:

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  Communicate our inner thoughts, desires, intentions and motivations.
  
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  Understand what others say to us.
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  Ask questions.
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  Give commands.
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  Comment and interchange.
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  Listen.
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  Speak.
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  Read.
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  Write.

A stroke that affects the left side of the brain may lead to aphasia, a language impairment that makes it difficult to use language in those ways. Aphasia can have tragic consequences.


People with aphasia:

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  May be disrupted in their ability to use language in ordinary circumstances.
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  May have difficulty communicating in daily activities.
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  May have difficulty communicating at home, in social situations, or at work.
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  May feel isolated.

Scientists and clinicians who study how language is stored in the brain have learned that different aspects of language are located in different parts of the left hemisphere. For example, areas in the back portions allow us to understand words. When a stroke affects this posterior part of the left hemisphere, people can have great difficulty understanding what they hear or read.
Imagine going to a foreign country and hearing people speaking all around you. You would know they were using words and sentences. You might even have an elemental knowledge of that language, allowing you to recognize words here and there, but you would not have command of the language and couldn’t follow most conversation. This is what life is like for people with comprehension problems.

People with comprehension problems:

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  Know that people are speaking to them.
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  Can follow some of the melody of sentences — realizing if someone is asking a question or expressing anger.
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  May have great difficulty understanding specific words.
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  May have great difficulty understanding how words go together to convey a complete thought.
  

Wernicke's Aphasia

People with serious comprehension difficulties have what is called Wernicke’s aphasia and:

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  Often say many words that don’t make sense.
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  May fail to realize they are saying the wrong words; for instance, they might call a fork a “gleeble.”
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  May string together a series of meaningless words that sound like a sentence but don’t make sense.
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  Have challenges because our dictionary of words is shelved in a similar region of the left hemisphere, near the area used for understanding words.

Broca's Aphasia
 
When a stroke injures the frontal regions of the left hemisphere, different kinds of language problems can occur. This part of the brain is important for putting words together to form complete sentences. Injury to the left frontal area can lead to what is called Broca’s aphasia. Survivors with Broca's aphasia:

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  Can have great difficulty forming complete sentences.
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  May get out some basic words to get their message across, but leave out words like “is” or “the.”
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  Often say something that doesn’t resemble a sentence.
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  Can have trouble understanding sentences.
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  Can make mistakes in following directions like “left, right, under, and after.”

“Car…bump…boom!” This is not a complete sentence, but it certainly expresses an important idea. Sometimes these individuals will say a word that is close to what they intend, but not the exact word; for example they may say “car” when they mean “truck.”
A speech pathologist friend mentioned to a patient that she was having a bad day. She said, “I was bitten by a dog.” The stroke survivor asked, “Why did you do that?” In this conversation, the patient understood the basic words spoken, but failed to realize that the words of the sentence and the order of the words were critical to interpreting the correct meaning of the sentence, that the dog bit the woman and not vice verse.






Global Aphasia


When a stroke affects an extensive portion of the front and back regions of the left hemisphere, the result may be global aphasia. Survivors with global aphasia:

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  May have great difficulty in understanding words and sentences.
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  May have great difficulty in forming words and sentences.
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  May understand some words.
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  Get out a few words at a time.
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  Have severe difficulties that prevent them from effectively communicating.

Remember, when someone has aphasia:

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  It is important to make the distinction between language and intelligence.
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  Many people mistakenly think they are not as smart as they used to be.
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  Their problem is that they cannot use language to communicate what they know.
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  They can think, they just can’t say what they think.
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  They can remember familiar faces.
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  They can get from place to place.
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  They still have political opinions, for example.
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  They may still be able to play chess, for instance.

The challenge for all caregivers and health professionals is to provide people with aphasia a means to express what they know. Through intensive work in rehabilitation, gains can be made to avoid the frustration and isolation that aphasia can create.