Parts Of The Brain Mcat

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1.3 Organization of the Brain
A Brief History of Neuropsychology

Throughout this section, refer to Figure 1.4, which identifies various anatomical structures inside the human brain. As we discuss different parts of the brain, it’s important to remember the functions of these brain structures. Different parts of the brain perform remarkably different functions. For instance, one part of the brain processes sensory information while an entirely different part of the brain maintains activities of the internal organs. For complex functions such as playing a musical instrument, several brain regions work together. For the MCAT, you will need to know some of the basics about how the brain integrates input from different regions.

Figure 1.4. Anatomical Structures Inside the Human Brain

The brain is covered with a thick, three-layered sheath of connective tissue collectively called the meninges. The outer layer of connective tissue is the dura mater, and is connected directly to the skull. The middle layer is a fibrous, weblike structure called arachnoid mater. And the inner layer, connected directly to the brain, is known as the pia mater. These three layers of connective tissue are shown in Figure 1.5. The meninges help protect the brain by keeping it anchored within the skull, and the meninges also resorb cerebrospinal fluid, which is the aqueous solution that nourishes the brain and spinal cord and provides a protective cushion. Cerebrospinal fluid is produced by specialized cells that line the ventricles (internal cavities) of the brain.

Figure 1.5. Layers of the Meninges

The human brain can be divided into three basic parts: the hindbrain, the midbrain, and the forebrain. Notice that brain structures associated with basic survival are located at the base of the brain and brain structures with more complex functions are located higher up. The meaningful connection between brain location and functional complexity is no accident. In evolutionary terms, the hindbrain and midbrain were brain structures that developed earlier. Together they form the brainstem, which is the most primitive region of the brain. The forebrain developed later, including the limbic system, a group of neural structures primarily associated with emotion and memory. Aggression, fear, pleasure, and pain are all related to the limbic system. The most recent evolutionary development of the human brain is the cerebral cortex, which is the outer covering of the cerebral hemispheres. In humans, the cerebral cortex is associated with everything from language processing to problem solving, and from impulse control to long-term planning. Most of the key brain regions described in the following sections are summarized in Table 1.1.

Major Divisions and Principal Structures

· Forebrain

o Cerebral cortex

o Basal ganglia

o Limbic system

o Thalamus

o Hypothalamus

o Complex perceptual, cognitive, and behavioral processes

o Emotion and memory

o Sensory relay station

o Hunger and thirst; emotion

· Midbrain

o Inferior and superior colliculi

o Sensorimotor reflexes

· Hindbrain

o Cerebellum

o Medulla oblongata

o Reticular formation

o Pons

o Refined motor movements

o Heart, vital reflexes (vomiting, coughing)

o Arousal and alertness

o Communication within the brain, breathing

Table 1.1. Anatomical Subdivisions of the Brain

In prenatal life, the brain develops from the neural tube. At first, the tube is composed of three swellings, which correspond to the hindbrain, midbrain, and forebrain. Both the hindbrain and forebrain later divide into two swellings, creating five total swellings in the mature neural tube. The embryonic brain is diagrammed in Figure 1.6, and its subdivisions are described further in the following sections. Understanding the relationship between the structures of the developing brain and the fully developed brain is important. So the following sections describe both the structures of the developing brain and what those structures develop into.

Figure 1.6. Subdivisions of the Embryonic Brain

Located where the brain meets the spinal cord, the hindbrain (rhombencephalon) controls balance, motor coordination, breathing, digestion, and general arousal processes such as sleeping and waking. In short, the hindbrain manages vital functioning necessary for survival. During embryonic development, the rhombencephalon divides to form the myelencephalon (which becomes the medulla oblongata) and the metencephalon (which becomes the pons and cerebellum). In the developed brain, the medulla oblongata is a lower brain structure that is responsible for regulating vital functions such as breathing, heart rate, and digestion. The pons lies above the medulla and contains sensory and motor pathways between the cortex and the medulla. At the top of the hindbrain, mushrooming out of the back of the pons, is the cerebellum, a structure that helps maintain posture and balance and coordinates body movements. Damage to the cerebellum causes clumsiness, slurred speech, and loss of balance. Notably, alcohol impairs the functioning of the cerebellum, and consequently affects speech and balance.

Just above the hindbrain is the midbrain (mesencephalon), which receives sensory and motor information from the rest of the body. The midbrain is associated with involuntary reflex responses triggered by visual or auditory stimuli. There are several prominent nuclei in the midbrain, two of which are collectively called colliculi. The superior colliculus receives visual sensory input, and the inferior colliculus receives sensory information from the auditory system. The inferior colliculus has a role in reflexive reactions to sudden loud noises.

Above the midbrain is the forebrain (prosencephalon), which is associated with complex perceptual, cognitive, and behavioral processes. Among its other functions, the forebrain is associated with emotion and memory; it is the forebrain that has the greatest influence on human behavior. Its functions are not absolutely necessary for survival, but are associated instead with the intellectual and emotional capacities most characteristic of humans. During prenatal development, the prosencephalon divides to form the telencephalon (which forms the cerebral cortex, basal ganglia, and limbic system) and the diencephalon (which forms the thalamus, hypothalamus, posterior pituitary gland, and pineal gland).

METHODS OF MAPPING THE BRAIN

Neuropsychology refers to the study of functions and behaviors associated with specific regions of the brain. It is most often applied in research settings, where researchers attempt to associate very specific areas in the brain to behavior. Neuropsychology is also applied in clinical settings with evaluations of patient cognitive and behavioral functioning, as well as the diagnosis and treatment of brain disorders. Neuropsychology has its own experimental methodology and technology.

Studying human patients with brain lesions is one way that researchers have determined the functions of the brain. In order to conclude that a specific structure of the brain is responsible for a specific function, researchers look for patients that exhibit damage to that structure coupled with a loss of the function. One problem in studying human brain lesions, however, is that such lesions are rarely isolated to specific brain structures. When several brain structures are damaged, the impairment could be attributed to any of the damaged structures, and pinpointing a specific link between brain structure and function becomes difficult.

One method for studying the relationship of brain regions and behaviors is to study brain lesions in lab animals. The advantage of this approach is that precisely defined brain lesions can be created in animals by extirpation. Researchers can also produce lesions by inserting tiny electrodes inside the brain and then selectively applying intense heat, cold, or electricity to specific brain regions. Such electrodes can be placed with great precision by using stereotactic instruments, which provide high-resolution, three-coordinate images of the brain. Ethical or cruelty concerns notwithstanding, such studies have greatly increased our understanding of comparable neural structures in humans.

Another neuropsychology method involves electrically stimulating the brain and recording consequent brain activity. While operating on the brain, a surgeon can stimulate a patient’s cortex with a small electrode. This stimulation causes groups of neurons to fire, thereby activating the behavioral or perceptual processes associated with those neurons. For instance, if the electrode stimulates neurons in the motor cortex, the stimulation can lead to specific muscle movements. If the electrode stimulates the visual cortex, the patient may “see” flashes of light that are not really there. By using electrical stimulation, neurosurgeons can thus create cortical maps. This method relies on the assistance of the patient, who is awake and alert. Because there are no pain receptors in the brain, only local anesthesia is required. Electrodes have also been used in lab animals to study deeper regions of the brain. Depending on where these electrodes are implanted, they can elicit sleep, sexual arousal, rage, or terror. Once the electrode is turned off, these behaviors cease.

Electrodes can also be used to record electrical activity produced by the brain itself. In some studies, individual neurons are recorded by inserting ultrasensitive microelectrodes into individual brain cells and recording their electrical activity. Electrical activity generated by larger groups of neurons can be studied using an electroencephalogram (EEG), which involves placing several electrodes on the scalp. Broad patterns of electrical activity can thus be detected and recorded. Because this procedure is noninvasive (it does not cause any damage), electroencephalograms are commonly used with human subjects. In fact, research on sleep, seizures, and brain lesions relies heavily on EEGs, like the one shown in Figure 1.7.

Figure 1.7. Electroencephalogram (EEG) during REM Sleep

Another noninvasive mapping procedure is regional cerebral blood flow (rCBF), which detects broad patterns of neural activity based on increased blood flow to different parts of the brain. rCBF relies on the assumption that blood flow increases to regions of the brain that are engaged in cognitive function. For example, listening to music may increase blood flow to the right auditory cortex because music is processed in that region in most individuals’ brains. To measure blood flow, the patient inhales a harmless radioactive gas; a special device that can detect radioactivity in the bloodstream can then correlate radioactivity levels with regional cerebral blood flow. This research method uses noninvasive computerized scanning devices.

Some of the other common scanning devices and methods of visualization used for brain imaging include:

· CT (computed tomography), also known as CAT (computed axial tomography) scan, in which multiple X-rays are taken at different angles and processed by a computer to produce cross-sectional images of the tissue.

· PET (positron emission tomography) scan, in which a radioactive sugar is injected and absorbed into the body, and its dispersion and uptake throughout the target tissue is imaged.

· MRI (magnetic resonance imaging), in which a magnetic field that interacts with hydrogen atoms is used to map out hydrogen dense regions of the body.

· fMRI (functional magnetic resonance imaging), which uses the same base technique as MRI, but specifically measures changes associated with blood flow. fMRI is especially useful for monitoring neural activity, since increased blood flow to a region of the brain is typically coupled with its neuronal activation.

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MRI techniques are dependent on the reaction of hydrogen to a magnetic field, and the scientific principles behind MRI scans are also applied in NMR techniques, which can be found in Chapter 11 of MCAT Organic Chemistry Review.

MCAT Concept Check 1.3:

Before you move on, assess your understanding of the material with these questions.

1. What are the main functions of the hindbrain? Midbrain? Forebrain?

Hindbrain

Behavior and Biology for the MCAT: Everything You Need to Know

Learn key MCAT concepts about behavior and biology, plus practice questions and answers

(Note: This guide is part of our MCAT Psychology and Sociology series .)

Part 1: Introduction to biology and behavior

Part 2: Anatomy of the nervous system

a) Divisions of the nervous system

b) Neurotransmitters

c) Regions of the brain

Part 3: Reflexes and innate behavior

Part 4: Methods used to study the brain

Part 5: High-yield terms

Part 6: Passage-based questions and answers

Part 7: Standalone questions and answers

Part 1: Introduction to biology and behavior

Behavior refers to any and all of the actions our bodies perform: whether they are intentional, such as solving a mathematical equation, or unintentional, such as a knee-jerk reflex. Behavior can also include personality, cognition, and decision-making.

Our behavior is influenced by a complex interplay between our environment, our genes, and a variety of biological systems. Of these biological systems, perhaps the most important is the nervous system. In this guide, we’ll begin to introduce the role of the nervous and endocrine systems in shaping our behavior and interactions with the environment. What are the specific structures and pathways that are important to our behavior? And what happens to our behavior when these biological systems are disrupted or damaged?

The information presented in this guide will describe key aspects of the nervous system that are relevant to behavior. To better understand the nervous system in its entirety, be sure to refer to our guide on the nervous system.

Throughout this guide, several important keywords will be given in bold. You are encouraged to try creating your own definitions and examples to better aid your understanding. Similarly, we encourage you to sketch diagrams and annotate them with your own knowledge to deepen your understanding of different concepts. At the end of this guide, there are also MCAT-style practice questions that will test your knowledge of this material.

Part 2: Anatomy of the nervous system

a) Divisions of the nervous system

The nervous system can be divided into the central nervous system (the brain and spinal cord) and the peripheral nervous system. The names of these divisions of the nervous system are intuitive; they match their location and function. After all, the brain and spinal cord are responsible for processing information and coordinating tasks with the peripheral nervous system, which includes all sensory neurons and nerves that move the muscles.

The peripheral nervous system and central nervous system communicate with each other through the peripheral nervous system’s efferent and afferent nerves. Afferent nerves are sensory nerves that ascend the spinal cord and carry sensory information from the environment into the brain. Efferent nerves are motor nerves that exit the brain and descend the spinal cord, relaying commands from the brain to our muscles. (It may be helpful to use this mnemonic: afferent nerves arrive at the brain, while efferent nerves exit.)

Afferent and efferent fibers are organized in the spinal cord and generally occupy different regions. The dorsal roots (back-facing side) of the spinal cord contain bundles of afferent (sensory) fibers. The ventral roots (belly-facing side) of the spinal cord contain bundles of efferent (motor) fibers.

The peripheral nervous system can be further divided into the somatic and autonomic nervous systems. The somatic nervous system governs voluntary movements and thus innervates the skeletal muscle. In contrast, the autonomic nervous system governs automatic or involuntary movements. This includes nervous system control of the digestive system, various glands, and smooth muscle.

The autonomic nervous system is further divided into the sympathetic (“fight-or-flight”) and parasympathetic (“rest-and-digest”) nervous systems. The sympathetic nervous system is designed to respond to stressors in the environment, and the parasympathetic nervous system is designed to help us rest. The sympathetic nervous system expends energy, while the parasympathetic nervous system conserves energy.

In response to a stressor, the sympathetic nervous system will cause:

• inhibition of saliva production
• dilation of the pupils: to take in as much light as possible to better assess the dangerous situation
• accelerated heart rate: to transport oxygen to the muscles
• slowed digestion: so blood can be diverted to the muscles
• airway dilation: to provide more oxygen with each inhalation
• increased glycogenolysis in the liver and increased blood glucose levels: to provide energy

Activation of the parasympathetic nervous system would cause the opposite effects. This includes the diversion of blood to the digestive tract so nutrients may be absorbed and glycogenesis in the liver is increased. (For more information on glycogenolysis and glycogenesis, refer to our guide on carbohydrate metabolism.)

When presented with a stressor or dangerous situation, the sympathetic nervous system is activated. The adrenal cortex releases cortisol while the adrenal medulla releases epinephrine: an important hormone and neurotransmitter, respectively, implicated in the stress response. Additionally, a cognitive appraisal of the situation is performed. During this appraisal, the ability to resolve the stressor or situation is assessed and likely outcomes are determined.

b) Neurotransmitters

Neurotransmitters are chemical messengers that transmit messages from a neuron cell to a target cell. They can be classified based on function (e.g., mood regulation), effect on the cell membrane potential (e.g., inhibitory, excitatory, or both), or locus of action (e.g., central or peripheral nervous system). For example, acetylcholine is the key excitatory neurotransmitter of the skeletal system and regulates movement.

Name	Function	Main Functions	Location	Disorders associated with deficit
Acetylcholine	Usually excitatory (always excitatory in the musculoskeletal system)	Motor movement, cognitive function	CNS; PNS	Alzheimer’s disease
Norepinephrine	Excitatory	“Fight-or-Flight” response	CNS; PNS	Depressed mood
Dopamine	Excitatory or inhibitory	Happiness, motor movement, alertness, learning, emotion	CNS; PNS	Parkinson’s disease
Serotonin	Inhibitory	Sleep, appetite, mood regulation, arousal	CNS	Mood disorders (e.g. depressive disorders)
Glutamate	Always excitatory (key excitatory neurotransmitter of CNS)	Stimulates the brain	CNS	Lack of focus or motivation
GABA	Always inhibitory (key inhibitory neurotransmitter of CNS)	Calms the brain	CNS	Anxiety, seizures
Endorphins	Inhibitory; provides pain relief	Pain relief	CNS	Pain