Neuroscience
I. Introduction
The brain is an essential part of the nervous system, a complex, highly coordinated network of tissues that communicate via electrochemical signals. We use our brains in virtually everything we do, from keeping our heart beating to deducing the existence of black holes. Within our brains lie our deepest secrets, our earliest memories, our most amazing capabilities, and the keys to the mystery of consciousness itself.
Hippocrates (460–377 B.C.), the most famous physician of the ancient world, first theorized that our thoughts, feelings, and ideas came from the brain, while others at the time thought the heart and stomach were the seats of emotion. Today, researchers are paying more attention to the roles played by the brain and the hormones that affect it in experiences such as mother-infant bonding, religious ecstasy and prayer, extreme stress, and meditation. Researchers now realize that though our minds and brains may not be exactly the same thing, they are intimately connected.
The brain is an essential part of the nervous system, a complex, highly coordinated network of tissues that communicate via electrochemical signals. We use our brains in virtually everything we do, from keeping our heart beating to deducing the existence of black holes. Within our brains lie our deepest secrets, our earliest memories, our most amazing capabilities, and the keys to the mystery of consciousness itself.
Hippocrates (460–377 B.C.), the most famous physician of the ancient world, first theorized that our thoughts, feelings, and ideas came from the brain, while others at the time thought the heart and stomach were the seats of emotion. Today, researchers are paying more attention to the roles played by the brain and the hormones that affect it in experiences such as mother-infant bonding, religious ecstasy and prayer, extreme stress, and meditation. Researchers now realize that though our minds and brains may not be exactly the same thing, they are intimately connected.
II. The Nervous System
The nervous system is a complex, highly coordinated network of tissues that communicate via electro chemical signals. It is responsible for receiving and processing information in the body and is divided into two main branches: the central nervous system and the peripheral nervous system.
The nervous system is a complex, highly coordinated network of tissues that communicate via electro chemical signals. It is responsible for receiving and processing information in the body and is divided into two main branches: the central nervous system and the peripheral nervous system.
The Central Nervous System
The central nervous system receives and processes information from the senses. The brain and the spinal cord make up the central nervous system. Both organs lie in a fluid called the cerebrospinal fluid, which cushions and nourishes the brain. The blood-brain barrier protects the cerebrospinal fluid by blocking many drugs and toxins. This barrier is a membrane that lets some substances from the blood into the brain but keeps out others.
The spinal cord connects the brain to the rest of the body. It runs from the brain down to the small of the back and is responsible for spinal reflexes, which are automatic behaviors that require no input from the brain. The spinal cord also sends messages from the brain to the other parts of the body and from those parts back to the brain.
The brain is the main organ in the nervous system. It integrates information from the senses and coordinates the body’s activities. It allows people to remember their childhoods, plan the future, create term papers and works of art, talk to friends, and have bizarre dreams. Different parts of the brain do different things.
Damage to the Spinal Cord
The spinal cord is what connects the brain and body, and it is protected by the bones in the spinal column. Injuries to the spinal cord can cause serious problems, such as paralysis. Even relatively minor damage to the spinal cord can cause loss of feeling in parts of the body, impaired organ function, and loss of muscular control. Though spinal cord injuries are usually permanent, current research into regenerated axons and stem cells offers hope that one day these injuries may be treated successfully.
The Peripheral Nervous System
All the parts of the nervous system except the brain and the spinal cord belong to theperipheral nervous system. The peripheral nervous system has two parts: the somatic nervous system and the autonomic nervous system.
The Somatic Nervous SystemThe somatic nervous system consists of nerves that connect the central nervous system to voluntary skeletal muscles and sense organs. Voluntary skeletal muscles are muscles that help us to move around. There are two types of nerves in the somatic nervous system:
The sympathetic nervous system’s activation may manifest as a rapidly thumping heart, sweaty palms, pale skin, or panting breath—the kinds of things we experience during a crisis. We may experience these kinds of symptoms during a panic attack, for example.
The central nervous system receives and processes information from the senses. The brain and the spinal cord make up the central nervous system. Both organs lie in a fluid called the cerebrospinal fluid, which cushions and nourishes the brain. The blood-brain barrier protects the cerebrospinal fluid by blocking many drugs and toxins. This barrier is a membrane that lets some substances from the blood into the brain but keeps out others.
The spinal cord connects the brain to the rest of the body. It runs from the brain down to the small of the back and is responsible for spinal reflexes, which are automatic behaviors that require no input from the brain. The spinal cord also sends messages from the brain to the other parts of the body and from those parts back to the brain.
The brain is the main organ in the nervous system. It integrates information from the senses and coordinates the body’s activities. It allows people to remember their childhoods, plan the future, create term papers and works of art, talk to friends, and have bizarre dreams. Different parts of the brain do different things.
Damage to the Spinal Cord
The spinal cord is what connects the brain and body, and it is protected by the bones in the spinal column. Injuries to the spinal cord can cause serious problems, such as paralysis. Even relatively minor damage to the spinal cord can cause loss of feeling in parts of the body, impaired organ function, and loss of muscular control. Though spinal cord injuries are usually permanent, current research into regenerated axons and stem cells offers hope that one day these injuries may be treated successfully.
The Peripheral Nervous System
All the parts of the nervous system except the brain and the spinal cord belong to theperipheral nervous system. The peripheral nervous system has two parts: the somatic nervous system and the autonomic nervous system.
The Somatic Nervous SystemThe somatic nervous system consists of nerves that connect the central nervous system to voluntary skeletal muscles and sense organs. Voluntary skeletal muscles are muscles that help us to move around. There are two types of nerves in the somatic nervous system:
- Afferent nerves carry information from the muscles and sense organs to the central nervous system.
- Efferent nerves carry information from the central nervous system to the muscles and sense organs.
- The sympathetic nervous system gets the body ready for emergency action. It is involved in the fight-or-flight response, which is the sudden reaction to stressful or threatening situations. The sympathetic nervous system prepares the body to meet a challenge. It slows down digestive processes, draws blood away from the skin to the skeletal muscles, and activates the release of hormones so the body can act quickly.
- The parasympathetic nervous system becomes active during states of relaxation. It helps the body to conserve and store energy. It slows heartbeat, decreases blood pressure, and promotes the digestive process.
The sympathetic nervous system’s activation may manifest as a rapidly thumping heart, sweaty palms, pale skin, or panting breath—the kinds of things we experience during a crisis. We may experience these kinds of symptoms during a panic attack, for example.
III. Neurons: Cells of the Nervous System
There are two kinds of cells in the nervous system: glial cells and neurons. Glial cells, which make up the support structure of the nervous system, perform four functions:
The other cells, neurons, act as the communicators of the nervous system. Neurons receive information, integrate it, and pass it along. They communicate with one another, with cells in the sensory organs, and with muscles and glands.
Each neuron has the same structure:
There are two kinds of cells in the nervous system: glial cells and neurons. Glial cells, which make up the support structure of the nervous system, perform four functions:
- Provide structural support to the neurons
- Insulate neurons
- Nourish neurons
- Remove waste products
The other cells, neurons, act as the communicators of the nervous system. Neurons receive information, integrate it, and pass it along. They communicate with one another, with cells in the sensory organs, and with muscles and glands.
Each neuron has the same structure:
- Each neuron has a soma, or cell body, which is the central area of the neuron. It contains the nucleus and other structures common to all cells in the body, such as mitochondria.
- The highly branched fibers that reach out from the neuron are called dendritic trees. Each branch is called a dendrite. Dendrites receive information from other neurons or from sense organs.
- The single long fiber that extends from the neuron is called an axon. Axons send information to other neurons, to muscle cells, or to gland cells. What we callnerves are bundles of axons coming from many neurons.
- Some of these axons have a coating called the myelin sheath. Glial cells produce myelin, which is a fatty substance that protects the nerves. When an axon has a myelin sheath, nerve impulses travel faster down the axon. Nerve transmission can be impaired when myelin sheaths disintegrate.
- At the end of each axon lie bumps called terminal buttons. Terminal buttonsrelease neurotransmitters, which are chemicals that can cross over to neighboring neurons and activate them. The junction between an axon of one neuron and the cell body or dendrite of a neighboring neuron is called a synapse.
Role of Myelin
People with multiple sclerosis have difficulty with muscle control because the myelin around their axons has disintegrated. Another disease, poliomyelitis, commonly called “polio,” also damages myelin and can lead to paralysis.
Communication Between Neurons
In 1952, physiologists Alan Hodgkin and Andrew Huxley made some important discoveries about how neurons transmit information. They studied giant squid, whose neurons have giant axons. By putting tiny electrodes inside these axons, Hodgkin and Huxley found that nerve impulses are really electrochemical reactions.
The Resting Potential
Nerves are specially built to transmit electrochemical signals. Fluids exist both inside and outside neurons. These fluids contain positively and negatively charged atoms and molecules called ions. Positively charged sodium and potassium ions and negatively charged chloride ions constantly cross into and out of neurons, across cell membranes. An inactive neuron is in the resting state. In the resting state, the inside of a neuron has a slightly higher concentration of negatively charged ions than the outside does. This situation creates a slight negative charge inside the neuron, which acts as a store of potential energy called the resting potential. The resting potential of a neuron is about –70 millivolts.
The Action Potential
When something stimulates a neuron, gates, or channels, in the cell membrane open up, letting in positively charged sodium ions. For a limited time, there are more positively charged ions inside than in the resting state. This creates an action potential, which is a short-lived change in electric charge inside the neuron. The action potential zooms quickly down an axon. Channels in the membrane close, and no more sodium ions can enter. After they open and close, the channels remain closed for a while. During the period when the channels remain closed, the neuron can’t send impulses. This short period of time is called the absolute refractory period, and it lasts about 1–2 milliseconds. The absolute refractory period is the period during which a neuron lies dormant after an action potential has been completed.
The All-or-None Law
Neural impulses conform to the all-or-none law, which means that a neuron either fires and generates an action potential, or it doesn’t. Neural impulses are always the same strength—weak stimuli don’t produce weak impulses. If stimulation reaches a certain threshold, or minimum level, the neuron fires and sends an impulse. If stimulation doesn’t reach that threshold, the neuron simply doesn’t fire. Stronger stimuli do not send stronger impulses, but they do send impulses at a faster rate.
The Synapse
The gap between two cells at a synapse is called the synaptic cleft. The signal-sending cell is called the presynaptic neuron, and the signal-receiving cell is called the postsynaptic neuron.
Neurotransmitters are the chemicals that allow neurons to communicate with each other. These chemicals are kept in synaptic vesicles, which are small sacs inside the terminal buttons. When an action potential reaches the terminal buttons, which are at the ends of axons, neurotransmitter-filled synaptic vesicles fuse with the presynaptic cell membrane. As a result, neurotransmitter molecules pour into the synaptic cleft. When they reach the postsynaptic cell, neurotransmitter molecules attach to matching receptor sites. Neurotransmitters work in much the same way as keys. They attach only to specific receptors, just as certain keys fit only certain locks.
When a neurotransmitter molecule links up with a receptor molecule, there’s a voltage change, called a postsynaptic potential (PSP), at the receptor site. Receptor sites on the postsynaptic cell can be excitatory or inhibitory:
Neurotransmitter effects at a synapse do not last long. Neurotransmitter molecules soon detach from receptors and are usually returned to the presynaptic cell for reuse in a process called reuptake.
People with multiple sclerosis have difficulty with muscle control because the myelin around their axons has disintegrated. Another disease, poliomyelitis, commonly called “polio,” also damages myelin and can lead to paralysis.
Communication Between Neurons
In 1952, physiologists Alan Hodgkin and Andrew Huxley made some important discoveries about how neurons transmit information. They studied giant squid, whose neurons have giant axons. By putting tiny electrodes inside these axons, Hodgkin and Huxley found that nerve impulses are really electrochemical reactions.
The Resting Potential
Nerves are specially built to transmit electrochemical signals. Fluids exist both inside and outside neurons. These fluids contain positively and negatively charged atoms and molecules called ions. Positively charged sodium and potassium ions and negatively charged chloride ions constantly cross into and out of neurons, across cell membranes. An inactive neuron is in the resting state. In the resting state, the inside of a neuron has a slightly higher concentration of negatively charged ions than the outside does. This situation creates a slight negative charge inside the neuron, which acts as a store of potential energy called the resting potential. The resting potential of a neuron is about –70 millivolts.
The Action Potential
When something stimulates a neuron, gates, or channels, in the cell membrane open up, letting in positively charged sodium ions. For a limited time, there are more positively charged ions inside than in the resting state. This creates an action potential, which is a short-lived change in electric charge inside the neuron. The action potential zooms quickly down an axon. Channels in the membrane close, and no more sodium ions can enter. After they open and close, the channels remain closed for a while. During the period when the channels remain closed, the neuron can’t send impulses. This short period of time is called the absolute refractory period, and it lasts about 1–2 milliseconds. The absolute refractory period is the period during which a neuron lies dormant after an action potential has been completed.
The All-or-None Law
Neural impulses conform to the all-or-none law, which means that a neuron either fires and generates an action potential, or it doesn’t. Neural impulses are always the same strength—weak stimuli don’t produce weak impulses. If stimulation reaches a certain threshold, or minimum level, the neuron fires and sends an impulse. If stimulation doesn’t reach that threshold, the neuron simply doesn’t fire. Stronger stimuli do not send stronger impulses, but they do send impulses at a faster rate.
The Synapse
The gap between two cells at a synapse is called the synaptic cleft. The signal-sending cell is called the presynaptic neuron, and the signal-receiving cell is called the postsynaptic neuron.
Neurotransmitters are the chemicals that allow neurons to communicate with each other. These chemicals are kept in synaptic vesicles, which are small sacs inside the terminal buttons. When an action potential reaches the terminal buttons, which are at the ends of axons, neurotransmitter-filled synaptic vesicles fuse with the presynaptic cell membrane. As a result, neurotransmitter molecules pour into the synaptic cleft. When they reach the postsynaptic cell, neurotransmitter molecules attach to matching receptor sites. Neurotransmitters work in much the same way as keys. They attach only to specific receptors, just as certain keys fit only certain locks.
When a neurotransmitter molecule links up with a receptor molecule, there’s a voltage change, called a postsynaptic potential (PSP), at the receptor site. Receptor sites on the postsynaptic cell can be excitatory or inhibitory:
- The binding of a neurotransmitter to an excitatory receptor site results in a positive change in voltage, called an excitatory postsynaptic potential orexcitatory PSP. This increases the chances that an action potential will be generated in the postsynaptic cell.
- Conversely, the binding of a neurotransmitter to an inhibitory receptor site results in an inhibitory PSP, or a negative change in voltage. In this case, it’s less likely that an action potential will be generated in the postsynaptic cell.
Neurotransmitter effects at a synapse do not last long. Neurotransmitter molecules soon detach from receptors and are usually returned to the presynaptic cell for reuse in a process called reuptake.
IV. Neurotransmitters
So far, researchers have discovered about 15–20 different neurotransmitters, and new ones are still being identified. The nervous system communicates accurately because there are so many neurotransmitters and because neurotransmitters work only at matching receptor sites. Different neurotransmitters do different things.
So far, researchers have discovered about 15–20 different neurotransmitters, and new ones are still being identified. The nervous system communicates accurately because there are so many neurotransmitters and because neurotransmitters work only at matching receptor sites. Different neurotransmitters do different things.
Neurotransmitter Acetylcholine Dopamine Serotonin Endorphins Norepinephrine GABA Glutamate |
Major Functions Muscle movement, attention, arousal, memory, emotion Voluntary movement, learning, memory, emotion Sleep, wakefulness, appetite, mood, aggression, impulsivity, sensory perception, temperature regulation, pain suppression Pain relief, pleasure Learning, memory, dreaming, awakening, emotion, stress-related increase in heart rate, stress-related slowing of digestive processes Main inhibitory neurotransmitter in the brain Main excitatory neurotransmitter in the brain |
Excess is associated with...
Schizophrenia Multiple Sclerosis |
Deficiency is associated with...
Alzheimer’s disease Parkinsonism Depression Depression |
Agonists and Antagonists
Agonists are chemicals that mimic the action of a particular neurotransmitter. They bind to receptors and generate postsynaptic potentials.
For Example: Nicotine and Receptors
Nicotine is an acetylcholine agonist, which means that it mimics acetylcholine closely enough to compete for acetylcholine receptors. When both nicotine and acetylcholine attach to a receptor site, the nerve fibers become highly stimulated, producing a feeling of alertness and elation.
Antagonists are chemicals that block the action of a particular neurotransmitter. They bind to receptors but can’t produce postsynaptic potentials. Because they occupy the receptor site, they prevent neurotransmitters from acting.
For Example: Paralysis and Poison Arrows
Curare is a drug that causes paralysis. As an acetylcholine antagonist, it binds to acetylcholine receptors at nerve-muscle junctions, preventing communication between nerves and muscles. Doctors sometimes use curare to immobilize patients during extremely delicate surgery. South American tribes have long used curare as an arrow poison.
Agonists are chemicals that mimic the action of a particular neurotransmitter. They bind to receptors and generate postsynaptic potentials.
For Example: Nicotine and Receptors
Nicotine is an acetylcholine agonist, which means that it mimics acetylcholine closely enough to compete for acetylcholine receptors. When both nicotine and acetylcholine attach to a receptor site, the nerve fibers become highly stimulated, producing a feeling of alertness and elation.
Antagonists are chemicals that block the action of a particular neurotransmitter. They bind to receptors but can’t produce postsynaptic potentials. Because they occupy the receptor site, they prevent neurotransmitters from acting.
For Example: Paralysis and Poison Arrows
Curare is a drug that causes paralysis. As an acetylcholine antagonist, it binds to acetylcholine receptors at nerve-muscle junctions, preventing communication between nerves and muscles. Doctors sometimes use curare to immobilize patients during extremely delicate surgery. South American tribes have long used curare as an arrow poison.
V. Studying the Brain
To examine the brain’s functions, researchers have to study a working brain, which means they can’t use cadavers. Invasive studies, in which researchers actually put instruments into the brain, can’t be done in humans, though they can be done occasionally during medically necessary brain surgery. Researchers usually use invasive techniques in animal studies. There are two main types of invasive animal studies:
Because they cannot use such invasive techniques on humans, researchers study human brains in two ways:
To examine the brain’s functions, researchers have to study a working brain, which means they can’t use cadavers. Invasive studies, in which researchers actually put instruments into the brain, can’t be done in humans, though they can be done occasionally during medically necessary brain surgery. Researchers usually use invasive techniques in animal studies. There are two main types of invasive animal studies:
- Lesioning studies: Researchers use an electrode and an electric current to burn a specific, small area of the brain.
- Electric stimulation of the brain: Researchers activate a particular brain structure by using a weak electric current sent along an implanted electrode.
Because they cannot use such invasive techniques on humans, researchers study human brains in two ways:
- They examine people with brain injuries or diseases and see what they can and can’t do.
- They use electroencephalographs (EEGs), which can record the overall electrical activity in the brain via electrodes placed on the scalp.
- Computerized tomography (CT): In CT, a number of x-rays are taken of the brain from different angles. A computer then combines the x-rays to produce a picture of a horizontal slice through the brain.
- Magnetic resonance imaging (MRI): Both brain structure and function can be visualized through MRI scans, which are computer-enhanced pictures produced by magnetic fields and radio waves.
- Positron emission tomography (PET): For PET scans, researchers inject people with a harmless radioactive chemical, which collects in active brain areas. The researchers then look at the pattern of radioactivity in the brain, using a scanner and a computer, and figure out which parts of the brain activate during specific tasks, such as lifting an arm or feeling a particular emotion.
VI. Structure and Functions of the Brain
The brain is divided into three main parts: the hindbrain, the midbrain, and the forebrain.
The Hindbrain
The hindbrain is composed of the medulla, the pons, and the cerebellum. The medulla lies next to the spinal cord and controls functions outside conscious control, such as breathing and blood flow. In other words, the medulla controls essential functions. The pons affects activities such as sleeping, waking, and dreaming. The cerebellum controls balance and coordination of movement. Damage to the cerebellum impairs fine motor skills, so a person with an injury in this area would have trouble playing the guitar or typing a term paper.
The Midbrain
The midbrain is the part of the brain that lies between the hindbrain and the forebrain. The midbrain helps us to locate events in space. It also contains a system of neurons that releases the neurotransmitter dopamine. The reticular formation runs through the hindbrain and the midbrain and is involved in sleep and wakefulness, pain perception, breathing, and muscle reflexes.
The Forebrain
The biggest and most complex part of the brain is the forebrain, which includes the thalamus, the hypothalamus, the limbic system, and the cerebrum.
Thalamus
The thalamus is a sensory way station. All sensory information except smell-related data must go through the thalamus on the way to the cerebrum.
Hypothalamus
The hypothalamus lies under the thalamus and helps to control the pituitary gland and the autonomic nervous system. The hypothalamus plays an important role in regulating body temperature and biological drives such as hunger, thirst, sex, and aggression.
Limbic System
The limbic system includes thehippocampus, the amygdala, and the septum. Parts of the limbic system also lie in the thalamus and the hypothalamus. The limbic system processes emotional experience. The amygdala plays a role in aggression and fear, while the hippocampus plays a role in memory.
Cerebrum
The cerebrum, the biggest part of the brain, controls complex processes such as abstract thought and learning. The wrinkled, highly folded outer layer of the cerebrum is called the cerebral cortex. The corpus callosum is a band of fibers that runs along the cerebrum from the front of the skull to the back. It divides the cerebrum into two halves, or hemispheres. Each hemisphere is divided into four lobes or segments: the occipital lobe, the parietal lobe, the temporal lobe, and the frontal lobe:
Lateralization refers to the fact that the right and left hemispheres of the brain regulate different functions. The left hemisphere specializes in verbal processing tasks such as writing, reading, and talking. The right hemisphere specializes in nonverbal processing tasks such as playing music, drawing, and recognizing childhood friends.
Roger Sperry, Michael Gazzaniga, and their colleagues conducted some of the early research in lateralization. They examined people who had gone through split-brain surgery, an operation done to cut the corpus callosum and separate the two brain hemispheres. Doctors sometimes use split-brain surgery as a treatment for epileptic seizures.
Control of the Body
Because of the organization of the nervous system, the left hemisphere of the brain controls the functioning of the right side of the body. Likewise, the right hemisphere controls the functioning of the left side of the body.
Vision and hearing operate a bit differently. What the left eye and right eye see goes to the entire brain. However, images in the left visual field stimulate receptors on the right side of each eye, and in-formation goes from those points to the right hemisphere. Information perceived by the right visual field ends up in the left hemisphere.
In the case of auditory information, both hemispheres receive input about what each ear hears. However, information first goes to the opposite hemisphere. If the left ear hears a sound, the right hemisphere registers the sound first.
The fact that the brain’s hemispheres communicate with opposite sides of the body does not affect most people’s day-to-day functioning because the two hemispheres constantly share information via the corpus callosum. However, severing the corpus callosum and separating the hemispheres causes impaired perception.
Split-Brain Studies
If a researcher presented a picture of a Frisbee to a split-brain patient’s right visual field, information about the Frisbee would go to his left hemisphere. Because language functions reside in the left hemisphere, he’d be able to say that he saw a Frisbee and describe it. However, if the researcher presented the Frisbee to the patient’s left visual field, information about it would go to his right hemisphere. Because his right hemisphere can’t communicate with his left hemisphere when the corpus callosum is cut, the patient would not be able to name or describe the Frisbee.
The same phenomenon occurs if the Frisbee is hidden from sight and placed in the patient’s left hand, which communicates with the right hemisphere. When the Frisbee is in the patient’s left visual field or in his left hand, the patient may not be able to say what it is, although he would be able to point to a picture of what he saw. Picture recognition requires no verbal language and is also a visual-spatial task, which the right hemisphere controls.
The brain is divided into three main parts: the hindbrain, the midbrain, and the forebrain.
The Hindbrain
The hindbrain is composed of the medulla, the pons, and the cerebellum. The medulla lies next to the spinal cord and controls functions outside conscious control, such as breathing and blood flow. In other words, the medulla controls essential functions. The pons affects activities such as sleeping, waking, and dreaming. The cerebellum controls balance and coordination of movement. Damage to the cerebellum impairs fine motor skills, so a person with an injury in this area would have trouble playing the guitar or typing a term paper.
The Midbrain
The midbrain is the part of the brain that lies between the hindbrain and the forebrain. The midbrain helps us to locate events in space. It also contains a system of neurons that releases the neurotransmitter dopamine. The reticular formation runs through the hindbrain and the midbrain and is involved in sleep and wakefulness, pain perception, breathing, and muscle reflexes.
The Forebrain
The biggest and most complex part of the brain is the forebrain, which includes the thalamus, the hypothalamus, the limbic system, and the cerebrum.
Thalamus
The thalamus is a sensory way station. All sensory information except smell-related data must go through the thalamus on the way to the cerebrum.
Hypothalamus
The hypothalamus lies under the thalamus and helps to control the pituitary gland and the autonomic nervous system. The hypothalamus plays an important role in regulating body temperature and biological drives such as hunger, thirst, sex, and aggression.
Limbic System
The limbic system includes thehippocampus, the amygdala, and the septum. Parts of the limbic system also lie in the thalamus and the hypothalamus. The limbic system processes emotional experience. The amygdala plays a role in aggression and fear, while the hippocampus plays a role in memory.
Cerebrum
The cerebrum, the biggest part of the brain, controls complex processes such as abstract thought and learning. The wrinkled, highly folded outer layer of the cerebrum is called the cerebral cortex. The corpus callosum is a band of fibers that runs along the cerebrum from the front of the skull to the back. It divides the cerebrum into two halves, or hemispheres. Each hemisphere is divided into four lobes or segments: the occipital lobe, the parietal lobe, the temporal lobe, and the frontal lobe:
- The occipital lobe contains the primary visual cortex, which handles visual information.
- The parietal lobe contains the primary somatosensory cortex, which handles information related to the sense of touch. The parietal lobe also plays a part in sensing body position and integrating visual information.
- The temporal lobe contains the primary auditory cortex, which is involved in processing auditory information. The left temporal lobe also contains Wernicke’s area, a part of the brain involved in language comprehension.
- The frontal lobe contains the primary motor cortex, which controls muscle movement. The left frontal lobe contains Broca’s area, which influences speech production. The frontal lobe also processes memory, planning, goal-setting, creativity, rational decision making, and social judgment.
Lateralization refers to the fact that the right and left hemispheres of the brain regulate different functions. The left hemisphere specializes in verbal processing tasks such as writing, reading, and talking. The right hemisphere specializes in nonverbal processing tasks such as playing music, drawing, and recognizing childhood friends.
Roger Sperry, Michael Gazzaniga, and their colleagues conducted some of the early research in lateralization. They examined people who had gone through split-brain surgery, an operation done to cut the corpus callosum and separate the two brain hemispheres. Doctors sometimes use split-brain surgery as a treatment for epileptic seizures.
Control of the Body
Because of the organization of the nervous system, the left hemisphere of the brain controls the functioning of the right side of the body. Likewise, the right hemisphere controls the functioning of the left side of the body.
Vision and hearing operate a bit differently. What the left eye and right eye see goes to the entire brain. However, images in the left visual field stimulate receptors on the right side of each eye, and in-formation goes from those points to the right hemisphere. Information perceived by the right visual field ends up in the left hemisphere.
In the case of auditory information, both hemispheres receive input about what each ear hears. However, information first goes to the opposite hemisphere. If the left ear hears a sound, the right hemisphere registers the sound first.
The fact that the brain’s hemispheres communicate with opposite sides of the body does not affect most people’s day-to-day functioning because the two hemispheres constantly share information via the corpus callosum. However, severing the corpus callosum and separating the hemispheres causes impaired perception.
Split-Brain Studies
If a researcher presented a picture of a Frisbee to a split-brain patient’s right visual field, information about the Frisbee would go to his left hemisphere. Because language functions reside in the left hemisphere, he’d be able to say that he saw a Frisbee and describe it. However, if the researcher presented the Frisbee to the patient’s left visual field, information about it would go to his right hemisphere. Because his right hemisphere can’t communicate with his left hemisphere when the corpus callosum is cut, the patient would not be able to name or describe the Frisbee.
The same phenomenon occurs if the Frisbee is hidden from sight and placed in the patient’s left hand, which communicates with the right hemisphere. When the Frisbee is in the patient’s left visual field or in his left hand, the patient may not be able to say what it is, although he would be able to point to a picture of what he saw. Picture recognition requires no verbal language and is also a visual-spatial task, which the right hemisphere controls.
The Endocrine System
The endocrine system, made up of hormone-secreting glands, also affects communication inside the body. Hormones are chemicals that help to regulate bodily functions. The glands produce hormones and dump them into the bloodstream, through which the hormones travel to various parts of the body. Hormones act more slowly than neurotransmitters, but their effects tend to be longer lasting.
The pituitary gland, which lies close to the hypothalamus of the brain, is often called the master gland of the endocrine system. When stimulated by the hypothalamus, the pituitary gland releases various hormones that control other glands in the body. The chart below summarizes the better known hormones along with some of their functions.
The pituitary gland, which lies close to the hypothalamus of the brain, is often called the master gland of the endocrine system. When stimulated by the hypothalamus, the pituitary gland releases various hormones that control other glands in the body. The chart below summarizes the better known hormones along with some of their functions.