Introduction to PsychologyThe Nervous System:

Biological Control CenterChapter 3

The brain not only thinks and calculates but also feels and controls motivation.

The brain is connected to the spinal cord, a thick bundle of long nerves running through the spine.

Individual nerves exit or enter the spinal cord and brain, linking the brain to every part of the body. Some of these nerves carry messages from the body to the brain to inform the brain to regulate the body’s function and the person’s behavior.

Without the nervous system, the body would be a mass of uncoordinated parts that couldn’t act, reason, or experience emotion.

The Nervous System

The most important unit of the nervous system is the individual nerve cell known as the neuron.

In the early 1900’s, Santiago Roman y Cajal, the scientist that discovered neurons, described them as “the mysterious butterflies of the soul”. Since then, we have learned about the building blocks of the nervous system.

Neurons

Neurons range in length from less than a millimeter to more than a meter in length. There are the same three parts in every neuron.

1) The cell body – contains a neuron’s nucleus and other parts essential for the cell’s preservation and nourishment.

2) Dendrites – braches that extend out and receive messages from other neurons.

3) Axons – are branches at the other end of the neuron that carry neural messages away from the cell body and transmit them to the next neuron. The messages usually stem from the dendrite through the axon, but can go the opposite direction.

The human nervous system is made up of 100 billion neurons. The human body contains trillions of neural connections, most of them in the brain.

Neuron is not the same as nerve. A nerve is a bundle of many long neurons outside the brain and spinal cord.

Parts of the Neuron

Neurons are their own living wires, with their own built-in supplies of electrical power. Neurons are sacs filled with one type of fluid on the inside and bathed in a different type of fluid on the outside.

These fluids are “soups” of dissolved chemicals, including ions, the particles that carry either negative or positive electrical charge.

There are more negative ions inside a cell. This means the charge is negative. These cells tend to attract positively charged ions.

The outside out of the cell becomes saturated with positive ions, particularly sodium (Na+). When neurons are in a resting state, they are 10 times more positively charged ions on the outside than on the inside.

Neural Transmission

Many ions are able to move freely through the cell membrane of the neuron, but sodium ions cannot. The membrane is said to be semipermeable in its normal resting state-only some chemicals can pass through the holes in the membrane. There is a balance as there are both positive and negative aspects.

While the neurons are in a resting state, they are said to be electrically polarized.

When a membrane is stimulated, positively charged ions, rush into the neuron. This process is called depolarization because the neuron is no longer mostly negative on the inside.

Depolarization creates a chain of events known as the action potential. When sodium ions rush through, it makes that part of the cell permeable. When the sodium comes through the cell pumps it back out, bring polarization back to the cell.

This polarization lasts one ten-thousandths of a second. Ramon y Cajal believed this was an “all-or-nothing principle. He believed that all action potential was the same. They now know that neurons transmit messages through graded electrical potentials that vary in magnitude.

Neural Transmission

Many axons are encased in a white, fatty coating called the myelin sheath. This sheath insulates the axon and greatly increases the speed at which the axon conducts neural impulses.

The myelin sheath continues to grow until late adulthood. The layers are thicker in females than in males. This may be indicative of more efficient neural processing of some kinds of information by females.

Multiple sclerosis destroys the myelin sheath. These individuals have trouble controlling their muscles; experience fatigue, dizziness, and pain, as well as suffer serious cognitive and vision problems.

Myelin Sheath and Neural Transmission

Neurons are not connected to one another, but are part of a complex chain. One neuron influences the next neuron through the synapse.

The small space between two neurons is known as the synaptic space.

The neural message is carried across the gap by chemical substances known as neurotransmitters. Not all neurotransmitters are excitable. Some axons transmit inhibitory substances across synapses, making it difficult for the next neurons to fire.

Neurotransmitters and Synaptic Transmission

Neurotransmitters are stored in tiny packets called synaptic vesicles located in the synaptic terminals, which are the knob-like ends of the axons.

When the action potential reaches the end of the axon terminal, it stimulates the vesicles to release the neurotransmitter into the gap. The neurotransmitter floats across the gap and “fits” into receptor sites on the adjacent neuron’s membrane like fitting into locks.

Hundreds of different neurotransmitter substances operate in different parts of the brain, carrying out different functions. Drugs can alter the transmission of a neuron. Psychiatric drugs can control anxiety, depression, and other psychological problems.

Some of the drugs can fit the receptor sites much like the original neurotransmitters. Some drugs can block receptor sites.

Neurotransmitters and Synaptic Transmission

Neurons are greatly outnumbered by a second class of cells called glial cells.

Glial cells help the neurons carry out their function in three ways:

1) New neurons grow from glial cells throughout life.

2) Glial cells support neurons and transport nutrients from blood vessels to neurons.

3) Some glial cells produce the myelin sheath that insulates axons.

4) Glial cells also influence the transmission of messages from one neuron to another across synaptic gaps. They can increase/decrease the speed of synaptic transmission. The glial cells absorb neurotransmitters from the synaptic gap, release the neurotransmitter into the synaptic gap, or by chemically preparing the synapse for transmission.

Glial Cells

The two major divisions of the nervous system are: 1) the Central Nervous System and the 2) Peripheral Nervous System.

1) The Central Nervous System – consists of the brain and the spinal cord. The brain controls the functions of the nervous system. The spinal cord’s main job is to relay messages between the brain and body. It also processes reflexes.

Example – withdrawing quickly from a hot object. Reflexes are activated in the spinal cord which reach a certain neuron called the interneuron. This gives a message to the muscles of the limb to contract.

Divisions of the Nervous System

2) The Peripheral Nervous System – composed of the nerves that branch from the brain and the spinal cord to the body.

Messages can travel across a synapse only in one direction. Those messages coming from the body into the central nervous system are carried by one set of neurons, the afferent neurons. Messages going out from the central nervous system to the body are carried by efferent neurons.

Divisions of the Nervous System

The peripheral nervous system is divided into two sections: 1) the somatic nervous system, and 2) autonomic nervous system.

The peripheral nervous system – carries messages from the central nervous system to the skeletal muscles that control movements of the body. These include voluntary and involuntary movements. This nervous system also receives incoming messages from the sensory receptors and transmits them to the nervous system.

Divisions of the Peripheral Nervous System

The autonomic nervous system – is composed of nerves that carry messages to the glands and visceral organs (heart, stomach, and intestines). The autonomic nervous system plays a key role in:

1) Essential body functions – regulates many essential functions of the organs. E.g. heartbeat, breathing, digestion, sweating, and sexual arousal.

2) Emotion – the autonomic system is activated during times of stress. This can create discomfort and make someone feel nausea (flight or fight syndrome).

Divisions of the Peripheral Nervous System

The autonomic nervous system is composed of two parts:

1) the sympathetic nervous system.

2) the parasympathetic nervous system.

Divisions of the Autonomic Nervous System

The sympathetic nervous system prepares the body for emergencies and times of stress. The sympathetic nervous system activates organs to improve our ability to respond to stress. In other cases, it inhibits organs that are not needed during times of stress.

The sympathetic nervous system: 1) Dilates (opens) the pupils of the eyes to let

in light. 2) Decreases salivation. 3) Speeds the beating of the heart. 4) Dilates the passageways (bronchi) of the

lungs to increase air flow. 5) Inhibits the digestive tract (stomach,

pancreas, intestines) 6) Releases sugar (glycogen) from the liver. 7) Stimulates the secretion of epinephrine

from the adrenal glands. 8) Inhibits contraction of the urinary bladder. 9) Increases blood flow and muscle tension in

the large muscles.

The Sympathetic Nervous System

This system helps to maintain balance regulation of the internal organs and the large body muscles. When levels of physical and emotional stress are low, it stimulates maintenance activities and energy conservation.

1) Constricts (closes) the pupils of the eyes.

2) Increases saliva to facilitate digestion. 3) Slows the beating of the heart. 4) Constricts the bronchi of the lungs. 5) Activates the digestive tract. 6) Releases bile from the liver to aid in

digestion of fats. 7) Inhibits secretion of epinephrine from

the adrenal glands. 8) Contracts the urinary bladder. 9) Reduces blood flow and muscle

tension in the large muscles.

The Para-sympathetic Nervous System

No function of the brain is carried out solely by one part. The brain can be viewed as having three major parts: 1) the hind-brain, 2) the midbrain, and 3) the forebrain.

These parts of the brain are divided into smaller sections.

Structures and Functions of the Brain

The hindbrain is the lowest part of the brain, located near the base of the skull. It tries to keep the body working properly. There are three parts to this portion of the brain:

1) the Medulla 2) the Pons 3) the Cerebellum

The medulla is a swelling just above the top of the spinal cord, where the cord enters the brain. It controls breathing and a variety of reflexes.

The pons is concerned with balance, hearing, and some parasympathetic functions.

Hindbrain and Midbrain: Housekeeping Chores and Reflexes

The cerebellum consists of two rounded structures located to the rear of the pons. It plays a key role in coordination of complex muscle movements and plays a role in memory and learning.

The reticular formation – is a set of neurons that spans the medulla and the pons. This plays a role in maintaining muscle tone and cardiac responsiveness to changing circumstances.

Hindbrain and Midbrain: Housekeeping Chores and Reflexes

The midbrain – the small area at the top of the hindbrain that helps to control important postural systems, especially those that deal with the senses. As an example, it controls the movement of the eyes.

Hindbrain and Midbrain: Housekeeping Chores and Reflexes

This portion of the brain deals with two distinct areas.

One area contains: the thalamus, the hypothalamus, and most of the limbic system.

The other area made up of the cerebral cortex, sits over the lower parts of the brain.

Forebrain: Cognition, Motivation, Emotion, and Action

Thalamus – routes incoming stimuli from the sense organs to the appropriate parts of the brain and links the upper and lower centers of the brain.

Hypothalamus – lies underneath the thalamus, just in front of the midbrain. It is involved in our emotions and our motives. It regulates the body’s temperature, sleep, endocrine gland activity, and resistance to disease. It maintains a normal pace and rhythm of such body functions like blood pressure and heartbeat.

Thalamus, Hypothalamus, and Limbic System

The Limbic System contains: 1) Amygdala – plays a role in

emotion and aggression. It helps to form memories about emotionally charged events.

2) Hippocampus – helps in forming new memories. It “ties together” the sights, sounds, and meanings of memories stored in various parts of the cerebral cortex and is particularly involved in spatial memory. Those with Alzheimer’s disease will have damage to this area.

3) Cingulate cortex – works with the hippocampus to process cognitive information related to emotion.

Thalamus, Hypothalamus, and Limbic System

The largest structure in the forebrain is the cerebral cortex. It is involved in conscious experiences, voluntary actions, language, and intelligence. It is the primary brain structure related to the somatic nervous system.

The thin outer surface of the cerebrum is a densely packed mass of billions of cell bodies of neurons. This portion is gray and is often called the “gray matter of the brain”. The fatty myelin underneath this area gives neurons their white appearance. These gray and white areas can be seen in the MRI image.

Cerebral Cortex: Sensory, Cognitive, and Motor Functions

Frontal lobes – is located behind your forehead, extending back to the middle of your head. It plays a key role in thinking, remembering, making decisions, speaking, predicting future consequences of actions, controlling movement, and regulating emotions.

The frontal lobe also contains Broca’s area (in the left cerebral hemisphere), which is involved in our ability to speak language. This is named for French neurologist Paul Broca. He performed autopsies on non-fatal stroke patients that had damage parts of their cerebral cortex and had difficulty speaking.

Lobes of the Cerebral Cortex

Persons with expressive aphasia – understand what is said to them, but have difficulty speaking. This means damage has occurred to Broca’s area.

The frontal lobe also controls some voluntary movement of the body. Near the middle of the top of the head, a strip called the motor area runs across the back portion of the frontal lobes. Damage to this area can result in paralysis or loss of motor control.

The Frontal Lobe

The frontal lobe also deals with decisions and appropriate social behaviors. This was revealed in the case of Phineas Gage. (See case study.)

150 years later, psychologist Christina Myers saw a similar case. A 33 year-old man by the name of J.Z. had a tumor removed from the same area that Phineas Gage was struck. After the surgery his personality changed dramatically. Whereas he was once honest, stable, and a reliable husband…he became dishonest, irritable, irresponsible, and grandiose in his beliefs.

The Frontal Lobe

The parietal lobe is located right behind the frontal lobe on the top of your skull. There is a strip that runs right next to the motor strip known as the sensorimotor strip.

This strip tells us what our hands and feet are doing and gives us a sensation when someone touches us. Different parts of the strip serve us in different areas of the body.

The Parietal Lobe

The temporal lobes stem backwards from the area of your temples, occupying the middle area at the base of the brain beneath the frontal and parietal lobes.

The temporal lobe contains the auditory areas. These are just inside the skull by the ears. They are involved in the sense of hearing.

Wernicke’s area – is located just behind the auditory area in the left hemisphere. This portion helps to understand spoken language.

Damage from strokes or injuries to this area of the cortex results in Wernicke’s aphasia. These people cannot make sense out of language that is spoken to them by others. They can make their own speech sounds, but say it makes little sense.

Temporal Lobes

These lobes are at the base of the back of the head. It is the part of the brain located farthest from the eyes. The visual area is the most important part of this lobe.

This part of the brain plays an essential role in processing sensory information due to the eyes. Damage to this area can result in partial or complete blindness.

The unlabeled parts of the brain not included with the lobes are known as the association areas. They work closely with other nearby areas.

Occipital Lobes

In the last 40 years, technology has increased in order to be able to view the living brain. We can see electrical activity, magnetic waves, and other forms of radiation.

Electroencephalogram (EEG) – the technician places electrodes on the person’s scalp and electrical activity is recorded. This is generally used to study a person’s sleep cycle and to diagnose medical conditions, such as seizure disorder. Multiple EEG’s can be used to create computer generated maps in color.

Images in the Brain at Work

PET scanning – positron emission tomography. They can watch what happens to the brain as different chemicals enter the body.

The best technology for the brain is the MRI (magnetic resonance imaging). This technique detects magnetic activity from the nuclei of atoms in living cells and creates visual images of the anatomy of the brain. Functional MRI – is capable of measuring the changes in oxygen because of the use by neurons that reflect their activity level. There is no exposure to radiation as there is with PET.

Images in the Brain at Work

The cerebral cortex has 4 lobes, which are in charge of different functions.

The cerebral cortex is also made up of two separate halves called the cerebral hemispheres. These two hemispheres are linked by the corpus callosum, allowing communication between the two sides of the brain. The left hemisphere controls most of the circumstances on the right side of the body and vice versa.

Functions of the Hemispheres of the Cerebral Cortex

The areas that control language are generally in the left hemisphere given 90% of human beings. The left side of the brain tends to deal with logic.

The right hemisphere seems to be in control of understanding space and the locations of items.

The left side tends to handle logic, reasoning, and analysis.

The right side tends to handle musical abilities, space, and emotion.

Functions of the Left and Right Central Hemispheres

The largest and most important bridge between the two hemispheres of the brain is the corpus callosum. Sometimes, when a person has bad seizures they will sometimes cut the corpus callosum so the seizure can’t move from one side of the brain to the other.

When the portions of the brain have been disconnected, scientists have learned much more about what each half does.

Scientists have done experiments where they show only one eye a word to see how the person will react. Because opposite sides of the brain are responsible for opposite sides of the body, the right eye would see and understand words (the left hemisphere of the brain). The left eye (the right hemisphere) would not understand the word.

Split Brains

The cerebral cortex also deals with emotional information.

The right hemisphere – plays a role in expression and perception of negative emotions (fear, sadness, and anger).

The left hemisphere – plays a role in the perception of positive emotions.

In 1861, physician Paul Broca noticed that patients that suffered a stroke in the left cerebral hemisphere were more likely to become depressed. This is because this hemisphere tends to deal with positive emotions. This is not true all the time, though.

Hemispheres of the Cerebral Cortex and Emotion

Severe damage to the cerebral cortex often results in the loss of important psychological functions. These can be recovered if the damage occurs early in life. This is called plasticity in that other parts of the brain can take over this lost portion.

If a child loses a portion of the left hemisphere that is related to language, portions of the right hemisphere will take over.

Plasticity of the Cortex

Our brain continues to change in structure throughout our lives.

The total weight of the brain does not change much after early childhood.

White matter (myelinated neural fibers) increase in the cerebral cortex from childhood through middle age. There is a continued growth of myelin through adolescence and early adulthood.

Myelin increases the rate of neurons and speeds the transmission of neural impulses.

The gray matter (neural cell bodies) decrease in the cortex at about the same rate from childhood through the middle ages. This happens because of neural pruning – eliminating unneeded neural cells. This is supposed to increase the efficiency of the brain.

After the 50’s, both gray and white matter begin to decrease which results in memory loss and cognitive speed loss.

The Brain is a Developing System

There are many neurons that keep growing in areas of the brain even during adulthood. This is known as neurogenesis.

These new neurons come from glial cells in the brain that can be transformed into neurons.

These new neurons help with learning and storing new memories.

Learning a new skill can cause an average increase of 3% in areas of the cortex.

Neurogenesis

The endocrine system – plays an important role in communication and the regulation of bodily processes. This system consists of a number of glands that secrete two kinds of chemical messengers.

Neuropeptides – many endocrine glands secrete neuropeptides into the bloodstream. These neuropeptides can influence their functions. Neuropeptides allow the endocrine glands to communicate with one another. These neuropeptides play important roles in stress regulation, social bonding, emotion, and memory.

Hormones – chemical substances, produced by endocrine glands that influence internal organs, including the brain.

Endocrine System: Chemical Messengers of the Body

The release of neuropeptides and hormones by the endocrine glands is regulated by several systems of the brain through the hypothalamus.

The endocrine gland gives the brain additional ways to control the body’s organs. During times of stress or emotional arousal, neuropeptides and hormones influence: metabolism, blood pressure, blood-sugar level, and sexual functioning.

Endocrine System: Chemical Messengers of the Body

The pituitary gland is located by the hypothalamus.

The pituitary is thought to be the body’s master gland because its secretions help regulate the body’s reactions to stress and resistance to disease.

This gland secretes hormones that control blood pressure, thirst, and body growth. Too much or too little of the hormone can make someone a “little person” or a “giant”.

The Pituitary Gland

The pair of adrenal glands sits atop the two kidneys. They play a role in emotional arousal and secrete hormones important in metabolism.

When the adrenal glands are stimulated by a hormone from the pituitary gland or by the sympathetic division of the autonomic nervous system, the adrenal glands secrete three hormones that are important in our reactions to stress:

1) Epinephrine 2) Norepinephrine

The Adrenal Glands

Epinephrine and norepinephrine – stimulate changes in the body to deal with physical demands that require intense body activity, including psychological threats or danger.

These two adrenal hormones operate differently. During times of stress, epinephrine increases blood pressure by increasing heart rate and blood flow.

Norepinephrine increases blood pressure by increasing heart rate and blood flow by constricting the diameter of blood vessels in the body’s muscles and by reducing the activity of the digestive system.

The adrenal glands also secrete the hormone cortisol, which also activates the body’s response to stress.

The autonomic nervous system has two ways of activating the internal organs: 1) by directly affecting the organs and 2) by stimulating the adrenals and other endocrine glands that then influence the organs with their hormones.

The Adrenal Glands

The islets of Langerhans are embedded in the pancreas – regulate the level of sugar in the blood by secreting two hormones that have opposing action.

Glucagon – causes the liver to convert its stored sugar into blood sugar and to dump it into the bloodstream.

Insulin – reduces the amount of blood sugar by helping the body’s cells absorb sugar in the form of fat. Blood sugar level is important given hunger motives and it also determines how energetic a person feels.

Islets of Langerhans

There are two sex glands: 1) ovaries in females and 2) testes in males. These glands secrete hormones important in sexual arousal and are important to the development of so-called secondary sex characteristics.

Estrogen – is the sex hormone in females.

Testosterone – is the sex hormone in males.

Each sex has a certain amount of each hormone, both estrogen and testosterone.

Testosterone influences the tendency to be socially dominant in both sexes. Sex hormones play a very important factor in adolescence.

Gonads

The thyroid gland is located just below the larynx, or voice box, which plays a role in the regulation of metabolism.

The thyroid gland secretes a hormone called thyroxin.

In children, the proper thyroid level is important for mental development.

A serious thyroid deficiency in childhood produces sluggishness, poor muscle tone, and a rare type of mental retardation known as cretinism.

In adults, people with low thyroxin levels tend to be inactive and overweight.

Thyroid Gland

The four small glands embedded in the thyroid gland are the parathyroid glands.

They secrete parathormone, which helps the nervous system to function.

Parathormone controls the excitability of the nervous system by regulating ion levels in the neurons.

Too much parathormone inhibits nervous activity and leads to lethargy; to little of it may lead to excessive nervous activity and tension.

Parathyroid

The pineal gland is located between the cerebral hemispheres, attached to the top of the thalamus. Its primary secretion is melatonin.

Melatonin is important in the regulation of biological rhythms, including menstrual cycles in females and the daily regulation of sleep and wakefulness.

Melatonin levels tend to be affected by the amount of exposure to sunlight and clock the time of day.

Melatonin also seems to play a role in regulating mood. Seasonal affective disorder, a type of depression that occurs most frequently in the winter months, is thought to occur because of the lack of light.

Pineal Gland