The neural control of behavior - a summary of chapter 5 of Psychology by Gray and Bjorklund (7th edition)

Psychology
Chapter 5
The neural control of behavior

Behavior is a product of the body’s machinery, especially the nervous system.

Neurons, the building blocks of the brain

The brain contains roughly 80 to 100 billion nerve cells, or neurons, and roughly 100 trillion synapses between neurons.
These are all more-or-less active, and their collective activity monitors our internal and external environments, creates all of our mental experiences, and controls all of our behavior.
The magic of this nervous system, lies in the organization of their multitudes.

Each neuron is itself a complex decision-making machine.
Each neuron receives information from multiple sources, integrates that information, and sends its response out to many other neurons or, in some cases, muscle cells or glands.

Three basic varieties of neurons, and structures common to them

The brain and spinal cord make up the central nervous system.
Extensions from the central nervous system, called nerves, make up the peripheral nervous system.

A neuron is a single cell of the nervous system
A nerve is a bundle of many neurons (or a bundle consisting of the axons of many neurons) within the peripheral nervous system.
Nerves connect the central nervous system to the body’s sensory organs, muscles and glands.

The central nervous system and peripheral nervous system are parts of an integrated whole. 

Neurons come in a wide variety of shapes and sizes and serve countless specific functions.
They can be grouped into three categories according to their functions and their locations in the overall layout of the nervous system.

• Sensory neurons
  Bundled together in nerves, carry information from sensory organs into the central nervous system.
• Motor neurons
  Bundled in nerves, carry messages out from the central nervous system to operate muscles and glands
• Interneurons
  Exist entirely within the central nervous system and carry messages from one set of neurons to another. They collect, organize, and integrate messages from various sources. They also outnumber the other two types. 
  They make sense of the input that comes from sensory neurons, generate all our mental experiences and initiate and coordinate all our behavioral actions through their connections to motor neurons.

All neurons contain the same basic parts.

• The cell body
  The widest part of the neuron. It contains the cell nucleus and other basic machinery common to all body cells.
• Dendrites
  Thin, tube like extensions that branch extensively and function to receive input for the neuron.
  In motor neurons and interneurons, the dendrites extend directly off the cell body and generally branch extensively near the cell body (forming bush-like structures). These structures increase the surface area of the cell and thereby allow for receipt of signals from many other neurons.
  In sensory neurons, dendrites extend from one end of the axon, rather than directly from the cell body. They extent into a sensory organ and respond to sensory signals, such as sound waves in the ear or touch on the skin.
• Axon
  A thin, tube-like extension from the cell body.
  It carries messages to other neurons or (in case of motor neurons) muscle cells.
  Some axons are extremely long (5 feet or more).
  Most axons form may branches some distance away from the cell body. Each branch ends with a small swelling called an axon terminal. Axon terminals are designed to release chemical transmitter molecules onto other neurons or, in case of motor neurons, onto muscle cells or glandular cells.
  The axons of some neurons are surrounded by a casing called myelin sheath. Myelin is a fatty substance produced by supportive brain cells called glial cells. This sheath helps to speed up the movement of neural impulses along the axon.

How neurons send messages down their axons

Neurons exert their influence on other neurons and muscle cells by firing off all-or-none impulses called action potentials.

In motor neurons and interneurons, action potentials are triggered at the junction between the cell body and the axon. They travel rapidly down the axon to the axon terminals.

In sensory neurons, they are triggered at the dendritic end of the axon and travel through or past the cell body to the axon terminals.

Action potentials occur or don’t occur. They don’t occur in different sizes or gradations.
Although each action potential is all-or-none a neuron can convey varying degrees of intensity in its message by varying its rate or producing action potentials.
A given neuron might fire off action potentials at a rate anywhere from zero to a 1,000 per second. By varying its rate of action potentials, a neuron varies the strength of its effect on other neurons or muscle cells.

The resting neuron has a constant electrical charge across its membrane

A cell membrane encloses each neuron. This is a porous skin that permits certain chemicals to flow into and out of the cell, while blocking others.
You can think of the neuron as a tube, the walls of which are the cell membrane.
This tube is filled with a solution of water and dissolved chemicals called intracellular fluid and is bathed on the outside by another solution of water and dissolved chemicals called extracellular fluid.

Among the various chemicals dissolved in the intracellular and extracellular fluid are some that have electrical charges. These include:

• Soluble protein molecules
  A-. Negative charges and exist only in the intracellular fluid. 
• Potassium ions
  K+ more concentrated in the intracellular than the extracellular fluid
• Sodium ions
  Na+
• Chloride ions
  CL-
  Those two are more concentrated in the extracellular than the intracellular fluid.

More negatively charged particles exist inside the cell than outside. This imbalance results in an electrical charge across the membrane (with inside about -70 minivolts relative to the outside). This is the resting potential and makes the action potential possible.

The action potential derives from a brief change in membrane permeability

The action potential is a wave of change in the electrical charge across the axon membrane, and it moves rapidly from one end of the axon to the other.

The action potential is initiated by a change in the structure of the cell membrane at one end of the axon. Thousands of tiny channels that permit sodium ions to pass through open up. As a result, enough sodium moves inward to cause the electrical charge across the membrane to reverse itself and become momentarily positive inside relative to outside.
This shift constitutes the depolarization phase of the action potential.

As soon as the depolarization occurs, the channels that permitted sodium to pass through close, but the channels that permit potassium to pass through remain open.
Because potassium ions are more concentrated inside the cell than outside, (and because they are repelled by the temporarily positive environment inside the cell, they are pushed outward. In this process enough positively charged  potassium ions move out of the cell to reestablish the original resting potential.
This is the re-polarization phase of the action potential.

To contain the original balance of the ions across the membrane, each portion of the membrane contains a chemical mechanism, referred to as the sodium-potassium pump. This continuously moves sodium out of the cell and potassium into it.

The action potential regenerates itself from point to point along the axon

Action potentials are triggered at one end of an axon by influences that tend to reduce the electrical charge across the cell membrane.
In sensory neurons, these influences derive from sensory stimuli acting on the dendrites.
In motor neurons and interneurons they derive from effects of other neurons that act eventually on the axon at its junction with the cell body.

The axon’s membrane is constructed in such a way that depolarization (reducing in charge across the membrane) to some critical value causes the sodium channels to open, thereby triggering an action potential.
This critical value is referred to as the cell’s threshold.
Once an action potential occurs at one location in the axon, it depolarizes the area of the axon just ahead of where it is occurring, thus triggering the sodium channels to open there.
In this way the action potential keeps renewing itself and moves continuously along the axon.
When an axon branches, the action potential follows each branch and thus reaches each of the possibly thousands of axon terminals.

The speed at which an action potential moves down an axon, is affected by the axon’s diameter.
Large diameter axons present less resistance to the spread of electric currents. So it moves faster than thin ones.
The presents of a myelin sheath speeds of the rate of conduction.
Myelin protects and insulates axons, speeding the rate at which nervous impulses can be sent and reducing interferences from other neurons.
Each action potential skips down the axon from one node to the next. This is faster than it could come as a continuous wave.

The process of developing myelin is called myelination.
It begins before a person is born, but is not completed until sometime in adulthood.
Neurons in the sensory system are the first to be myelinated.

How neurons are influences by other neurons: synaptic transmission

Neurons generate action potentials at rates that are influenced by all the information that is sent to them form other neurons.
The cell and body of a typical motor neuron or interneuron are blanketed by tens of thousands of axon terminals, which come from the branching axons of hundreds of thousands of different neurons.

Synapse: the junction between each axon terminal and the cell body or dendrite of the receiving neuron
When an action potential reaches an axon terminal, it causes the terminal to release packets of chemical substance, a neurotransmitter.
These neurotransmitters include dopamine, acetylcholine, serotonin and GABA (among others). They move across the space between the cells and alter the receiving neuron in ways that influence its production of action potentials.

The synaptic cleft; a very narrow gap that separates the axon terminal from the membrane of the cell that it influences.

The presynaptic membrane: the membrane of the axon terminal that abuts the cleft

The postsynaptic membrane: the membrane of the other side of the cleft.

Within the axon terminal are hundreds of tiny globe-like vesicles, each of which contains several thousand molecules of a chemical neurotransmitter.

When an action potential reaches an axon terminal, it causes some of the vesicles to spill their neurotransmitter molecules into the cleft.
The molecules than diffuse through the fluid in the cleft and some become attached to special receptors on the postsynaptic membrane. Each neurotransmitter molecule ca be thought of as a key, and each receptor can be thought of as a lock. A molecular key entering a receptor lock opens a gate in the channel allowing ions to pass through.
If the postsynaptic cell is a muscle cell → the flow of ions triggers a biochemical process that causes the cell to contact.
If it is a neuron, the result is a change in the polarization of that neuron. The direction of change depends on whether the synapse is excitatory or inhibitory.

• Excitatory synapse: the transmitter opens sodium channels in the postsynaptic membrane. The movement of the positively charged sodium ions into the cell causes a slight depolarization of the receiving neuron, which tends to increase the rate of action potentials triggered in a neuron.
• An inhibitory synapse → the transmitter opens either chloride CL- channels or potassium K+ channels. The movement of negatively charged chloride ions into the cell or of positively charged potassium ions out of the cell causes a slight hyper-polarization of the receiving neuron, which tends to decrease the rate of action potentials triggered in that neuron.

Postsynaptic neurons integrate their excitatory and inhibitory inputs.

At any given moment a single neuron may receive input at dozens, hundreds, or even thousands of its fast synapses. Some of these are excitatory (causes a slight depolarization) and some are inhibitory (causes a slight hyperpolarization).
These effects spread passively through the dendrites and cell body, combining the have an integrated effect on the electrical charge across the membrane of the axon at its junction with the cell body.

When the axon membrane is depolarized below the critical value, the action potential is triggered.
The greater the degree of depolarization below that value, the greater the number of action potentials per second.

The development of neurons

Newborn infants have more neurons in their brain than adults do.

Neurogenesis

Neurogenesis: the process of creating new neurons.
This occurs during the first 20 weeks after conception, peaking in the third and fourth months of gestation. During this peak, the fetal brain generates several hundred thousand neurons each minute.

Once neurons are born, they migrate to their permanent position in the brain. Beginning about 20 weeks after conception, they enter the last stage of their development, the differentiation. During this time, neurons grow in size and increase their numbers of dendrites and axon terminals as well as the number of synapses they form. (This doesn’t stop at birth).

Cell death and synaptic pruning

Beginning late in the prenatal period and continuing after birth, the primary changes are in losses of neurons and synapses.
Both the number of neurons and the number of synapses decrease over early development.
Neurons die in a process known as selective cell death (or apoptosis). This begins before birth and continues well into the teen years. This is at different rates in different parts of the brain.
Adults heave fewer, but stronger and more effective neuronal connections than they had as children.

The brain first overproduces neurons and synapses and then experience, hormones and genetic signals shape the brain.

Mirror neurons, the basis for social learning?

Mirror neurons are active both when a subject engages in a behavior and when the subject observes some else perform a similar behavior.
It serves to recognize when another is doing something that the self can do to. In humans mirror neurons code for movements forming an action, not for the action itself. They are important in imitative learning where the specific behaviors a model performs are as important as the goal the model attains.

Methods of mapping the brain’s behavioral functions

Neurons in the central nervous system are organized into nuclei and tracts.
A nucleus is a cluster of cell bodies in the central nervous system. (Don’t confuse with a cell nucleus, which exist within each cell).
A tract is a bundle of axons that course together from once nucleus to another.
Tacts are referred to collectively as white matter because the myelin sheaths around axons make tracts appear relatively white.

Nuclei are referred to collectively as gray matter.

In general, neurons whose cell bodies occupy the same nucleus and whose axons run in the same tract have identical or similar functions. Groups of nuclei located near one another have functions that are closely related to one another.

Because of this pattern of organization, we can speak of the general functions of relatively large anatomical structures within the brain.

Methods used for studying the human brain

Three general categories.

• Observing behavioral deficits that occur when a part of the brain is destroyed or is temporarily inactivated.
• Observing behavioral effects of artificially stimulating specific parts of the brain
• Recording changes in neural activity that occur in specific parts of the brain when a person or animal is engages in a particular mentor or behavioral task.

Observing effects of localized brain damage

One must be cautious when interpreting the effects of brain damage on psychological functioning. Brain damage can rarely be narrowed to one area and frequently involves complications beyond that of simple lesions, or areas of specific damage. Lesions in one area can also lead to changes in the other brain areas.

Observing effects of magnetic interference with normal brain activity

Transcranial magnetic stimulation, TMS.
A pulse of electricity is sent through a small copper coil, inducing a magnetic field around the coil. The coil is held just above a person’s scalp so that the magnetic field passes through the scalp and skull and induces an electric current in the neurons immediately below the coil.
Repetitive pules causes a temporary loss in those neurons’ abilities to fire normally. The effect is comparable to that of lesioning a small area of the brain. The effect is temporally and reversible.

It can also be used for temporary activation.

Recording brain activity electrically through the scalp

Constant activity of the brain’s billions of neurons produces continuous electrical chatter which to some degree penetrates the overlying skull and scalp. By placing electrodes on a person’s scalp, researchers can detect and amplify these signals.
The resulting record of brain activity is an electroencephalogram or EEG.
It can be uses as an index of whether a person is highly aroused, relaxed or asleep and can be used to identify various stages of sleep. 

The brief change in the EEG record immediately following the stimulus is an event-related potential. Or ERP.
By testing the person repeatedly in such task and averaging the EEG records obtained, it is possible to produce an average ERP for each electrode location.
Comparison of the average ERPs recorded at different scalp locations reveals the pattern of activity in the brain as the person detects and responds to the stimulus.

Viewing brain activity with imaging methods sensitive to blood flow

Some methods for localizing brain activity rely on the fact that increased neural activity in any area of the brain is accompanied by increased blood flow in that area.
When a portion of the brain becomes more active, blood vessels there immediately enlarge, so more blood enters the portion.

Using technically complex methods, researchers can create three-dimensional pictures, referred to as neuroimages that depict the relative amount of blood flowing through each part of the brain.
The assumption is that increased blood flow reflects increased neural activity.

The first of these neuroimaging methods was positron emission tomography or PET.
This method involves injecting a radioactive substance into the blood and measuring the radioactivity that is emitted from each portion of the brain.

Another method is functional magnetic resonance imaging or FMRI
This method involves the creation of a magnetic field around a person’s head, which causes hemoglobin molecules that are carrying oxygen in the blood to give off radio waves of a certain frequency. Which can be detected and used to assess the amount of blood in each part of the brain.

With either method, the persons head must be surrounded by a set of sensors.
PET and FMRI can depict activity anywhere in the brain, not just on the surface near the skull. These methods also produce a more fine-grained picture of the spatial locations of activity than possible with EEG.

Methods used for studying the brains of nonhuman animals

With nonhuman animals, researchers can localize brain functions using methods that are more intrusive than those uses with humans. They can destroy, stimulate or record neural activity in small, well-localized areas anywhere in the brain in order to assess the behavioral functions of those areas.

Observing effects of deliberately placed brain lesions

Producing areas of damage:

• Stereotaxic instrument (with a thin wire electrode)
• Cannula (a small amount of chemicals).

Effects of stimulating specific areas of the brain

Also be accomplished either electrically or chemically.

Electrical recording from single neurons

Electrodes can be used to record neural activity in specific areas as the animal engages in some behavioral task.
Extremely thin microelectrodes with very fine points that can penetrate into the cell bodies of single neurons can be permanently implanted in the brain.

Functional organization of the nervous system

The nervous system is hierarchically organized.

• The sensory-perceptual hierarchy
  Involved in data processing. It receives sensory data about a person’s internal and external environment and it analyzes those data to make decisions about the person’s bodily needs and about threats and opportunities in the outside world. From sensory receptors to perceptual centers in the brain.
• The motor-control hierarchy
  Involved in the control of movement. Top: executive centers that make decisions about the activities that a person as whole should engage in. Bottom: centers that translate those decisions into specific patterns of muscle movement.

Peripheral nerves. The nervous system’s interface with the world.

The peripheral nervous system consist of the entire set of nerves which connect the central nervous system to the body’s sensory organs, muscles and glands.

Nerves are divided into two classes that correspond to the portion of the central nervous system from which they protrude.

• Cranial nerves
  Project directly from the brain
• Spinal nerves
  Project from the spinal cord

Nerves exist in pairs. There is a right and a left member in each pair.
Humans have 12 pairs of cranial nerves and 31 pairs of spinal nerves. These nerves extend to all portions of the body.

Three pairs of cranial nerves contain only sensory neurons and five pairs contain only motor neurons. The remaining four pairs of cranial nerves and all 31 pairs of spinal nerves contain both sensory and motor neurons.

Without interneurons the nervous system would have no way of receiving sensory input or controlling the person’s actions.

Sensory neurons provide data needed for governing behavior

Sensory neurons are activated at their dendritic ends by the effects of sensory stimuli.
They send their action potentials all the way into the central nervous system by way of their long axons.

The rates and patterns of action potentials in sensory neurons are the data that perceptual areas of the central nervous system use to figure out the state of the external and internal environment. This is the base for its behavior-controlling decisions.

Sensory input form the specialized sensory organs of the head enters the brain by way of cranial nerves.
Sensory input from the rest of the body enters central nervous system by way of all of the spinal nerves and some of the cranial nerves.

The sensations conveyed by these inputs are referred to collectively as somatosensation.

Motor neurons are the final common path for control of behavior.

Motor neurons have their cell bodies in the central nervous system and sent their long axons out (by way of cranial or spinal nerves) to terminate on muscles or glands.

All of the behavioral decisions of the nervous system are translated into patterns of action potentials in the axons of motor neurons. Those patterns determine our behavior.
Motor neurons are the final common path of the nervous system.
Only through them can the nervous system control behavior.

The motor system includes skeletal and autonomic divisions

Motor neurons act on two broad classes of structures

• Skeletal muscles
  The muscles that are attached to bones and produce externally observable movements of the body when contracted.
• Visceral muscles and glands.
  Muscles that are not attached to bones. They form the walls of such structures as the heart, and stomach.
  Glands are structures that produce secretions (like sweat glands).

Neurons act on skeletal muscles are those that make up the skeletal portion of the peripheral motor system. → initiate activity on skeletal muscles. Completely inactive in the absence of neural input.

Those that act on visceral muscles and glands make up the autonomic portion. → modulate activity in the visceral muscles. Built-in nonneural mechanisms for generating activity.
Most visceral muscles and glands receive two sets of neurons which produce opposite effects and come from two anatomically distinct divisions of the autonomic system.

• Sympathetic division
  Responds especially to stressful stimulation and helps to prepare the body for possible ‘flight or fight’.
• Parasympathetic
  Serves regenerative, grow-promoting, and energy conserving functions through effect that include the opposites of those listed for the sympathetic division.

The spinal cord, a conduit and an organizer of simple behaviors

The spinal cord connects the spinal nerves to the brain. It also organizes some simple reflexes and rhythmic movements.

The spinal cord contains pathways to and from the brain

The spinal cord contains:

• Ascending tracts
  Carries somatosensory information brought in by the spinal nerves of the brain.
• Descending tracts.
  Carries motor control commands down from the brain to be transmitted out by spinal nerves to muscles.

The spinal cord organizes simple reflexes

Some reflexive are organized by the spinal cord alone.
Flexion reflex

The spinal cord contains pattern generators for locomotion

The spinal cord is capable of generating sustained organized movements without the involvements of the brain.
The spinal cord contains networks of neurons that stimulate one another in a cyclic manner and thereby produce bursts of action potentials that wax and wane in a regular, repeating rhythm.
These networks, called pattern generators, activate motor neurons in the spinal cord in such a way as to produce the rhythmic sequence of muscle movements that results in walking, running, flying or swimming.
Normally, pattern generators are controlled by neurons descending from the brain.

Subcortical structures of the brain.

The more primitive parts of the brain are referred to as subcortical structures.
Their position is beneath the cerebral cortex.

The brainstem organizes species-typical behavior patterns

As it enters the head, the spinal cord enlarges and becomes the brainstem.
The parts of the brainstem, beginning the closest to the spinal cord and going upward toward the top of the head, are the medulla, pons, and midbrain.

The brainstem is quite similar to the spinal cord, but is more elaborate.
The brainstem is the site of 10 of the 12 cranial nerves.
Both the brainstem and the spinal cord contain ascending (sensory) and descending (motor) tracts that communicate between nerves and higher parts of the brain.
The brainstem also has some neural centers that organize reflexes and certain species-typical behavior patterns.

The medulla and pons organize reflexes that are more complex and sustained than spinal reflexes. They include:

• Postural reflexes
  Helps an animal maintain balance.
• Vital reflexes
  Such as those that regulate heart rate and breathing rate in response to input signaling the body’s metabolic needs.

The midbrain contains neural centers that help govern most of an animal’s species-typical movement patterns (like eating, drinking, attacking or copulating).
Also in the midbrain are neurons that act on pattern generators in the spinal cord to increase or decrease the speed of locomotion.

The thalamus is a relay station for sensory, motor and arousal pathways

Direct atop of the brainstem is the thalamus. It is seated squarely in the middle of the brain.
It is most conveniely thought of as a relay station that connects various parts of the brain with one another.

Most of the tracts that ascend through the brainstem terminate in special nuclei in the thalamus. Those nuclei send their output to specific areas in the cerebral cortex.
The thalamus also has nuclei that relay messages from higher parts of the brain to movement-control centers in the brainstem.

The thalamus also plays a role in the arousal of the brain as a whole.
Arousal pathways in the midbrain converge in the center of the thalamus and then project diffusely to all areas of the cerebral cortex.

The cerebellum and the basal ganglia help to coordinate skilled movements

Both specialized to use sensory information to guide movement, but in different ways.

Cerebellum means little brain in Latin.
Is riding piggyback on the rear of the brainstem.
Rapid, well-timed sequences of muscle movement.
Uses information in a feed-forward manner. Crucial for movements that occur too rapidly to be modified once they are in progress.

The basal ganglia

Are a set of interconnected structures lying on each side of the thalamus.
Coordinate slower, deliberate movements.
Uses sensory information in a feedback manner.

Portions of the cerebellum, basal ganglia, and certain motor-planning areas of the cerebral cortex become active not just when people are producing movements, but also when they are imagining themselves producing movements.

The limbic system and the hypothalamus play essential roles in motivation and emotion

The limbic system can be thought of as the border dividing the evolutionarily older parts of the brain below it from the newest part (the cerebral cortex) above it.
The limbic system consist of several distinct structures that interconnect with one another in a circuit wrapped around the thalamus and basal ganglia.
Some of this structures, including especially the amygdala, are involved in the regulation of basic drives and emotions.
The hippocampus is crucial for keeping track of spatial location and for encoding certain kind of memories.

The limbic systems connections with the nose are strong.
The limbic system also receives input from all the other sensory organs. It is intimately connected to the basal ganglia that connection is believed to help translate emotions and drives into actions.

The hypothalamus is a small, but vitally important structure.
It is positioned directly underneath the thalamus (hypo in this case means beneath).
It is not technically part of the limbic system, but is intimately connected to all the structures of that system.
Its primary task is to help regulate the internal environment of the body. It does this by:

• Influencing the activity of the autonomic nervous system
• Controlling the release of certain hormones
• Affecting certain drive states (like hunger)

Through its connection with the limbic system, it helps regulate emotional states.

The cerebral cortex

The evolutionarily newest part of the bran.
All the parts of the brain other than the brainstem and cerebellum.
The cerebral cortex is the outside layer of the major portion of the brain.
It is the largest part of the human brain (80 %!)
It surface area is greater because it folds inward in many places.
The entire folded cerebral cortex is divided into left and right hemispheres.
Each hemisphere is divided into four lobes, or segments, demarcated at least partly by rather prominent inwardly folding creases, referred to as fissures.
The lobes are (from back to front):

• The occipital lobe
• The temporal lobe
• The parietal lobe
• The frontal lobe

The cortex includes sensory, motor and association areas

The functions of the cortex are divided into three categories of functional regions, or areas.

• The primary sensory areas
  Receive signals from sensory nerves and tracts by way of relay nuclei in the thalamus.
• Primary motor area
  Sends axons down to motor neurons in the brainstem and spinal cord.
• Association areas
  Receive input from the sensory areas and lower parts of the brain and are involved in the complex processes that we call perception, thought and decision making.
  (Biggest part by far!)

The primary sensory and motor areas are topographically organized

Principle of topographic organization: adjacent neurons receive signals from or send signals to adjacent portions of the sensory or muscular tissue to which they are ultimately connected.

In other animals, other body parts have greater representation, depending on the range and delicacy of their movements.

The primary cortex is part of the chain of command in controlling movements, but is not at the top of that chain and not involved in all types of movement.
It receives input form the basal ganglia and cerebellum and is specialized to fine-tune the signals going to small muscles (like fingers) which must operate in a finely graded way.

Premotor areas help organize specific patterns of movement

Premotor areas: in from of the primary motor area lies a set of cortical areas devoted to motor control.
Set up neural programs for producing organized movements or patterns of movements.
To choose what program to set up, they use information sent to them anterior (forward) portions of the frontal lobe that are involved in overall behavioral planning.
To execute an action program they send information out the cerebellum, basal ganglia, and motor cortex, which refine the program further before sending messages down to the muscles.

Premotor areas become active during the mental rehearsal of coordinated movements as well as during the actual production of such movements.

Prefrontal association areas create general plans for action

Expanded the most in human beings compared to other animals, consisting of the entire frontal lobe anterior to the premotor areas.

Executive function: the processes involved in regulating attention and in determining what to do with information just gathered or retrieved from long-term memory.
A central role in planning and behaving flexibility, particularly when dealing with novel information.

Association areas in the rear parts of the cortex analyze information that comes to them from sensory areas. These areas send output to prefrontal association areas, which also receive information about the internal environment through strong connections with the limbic system.
Combining all this information, the prefrontal areas set op general plans for action that can be put into effect through connections to the premotor cortex and through downward links to the basal ganglia and cerebellum.

How hormones interact with the nervous system

Hormones are chemical messengers that are secreted into the blood. They are carried by the blood to all parts of the body, where they act on specific target tissues.

Classic hormones (not all the hormones) are secreted by special hormone-producing glands called endocrine glands.

How hormones affect behavior

Hormones influence behavior in many ways.

• The growth of peripheral bodily structures, so influence behavioral capacity
• Affect metabolic processes throughout the body and thereby influence the amount of energy that is available for action
• Act in the brain in ways that influence drives and moods.

Some effects of hormones are long term of irreversible and some of these occur before birth.
Short-term effects of hormone range in duration from a few minutes to many days. These hormones are also taken up by neurons in certain parts of the brain and apparently act there to help the animal adapt behaviorally to the stressful situation.

How hormones are controlled by the brain

The pituitary, which sits at the base of the brain, produces hormones that stimulate the production of other hormones in other glands, including the adrenal cortex and gonads (ovaries in the female and tested in the male).

The rear part of the pituitary, the posterior lobe, is in fact a part of the brain.
It consist mainly of modified neurons (neurosecretory cells), which extend down from the hypothalamus.
When these neurosectretory cells are activated, by brain neurons that lie above them, they release their hormones into a bed of capillaries.
Once these hormones enter the capillaries, they are transported into the rest of the circulatory system to affect various parts of the body.

The remainder of the pituitary, the anterior lobe, is not part of the brain (no neurons descend into it) but is intimately connected to the brain by a specialized set of capillaries.

Neuronsecretory cells in the brain’s hypothalamus produce releasing factors, hormones that are secreted into the special capillary system and are carried to the anterior pituitary, where they stimulate the anterior pituitary cells to synthesize and release hormones into capillaries that carry hormones into the general bloodstream.
Different releasing factors act selectively to stimulate the production of different anterior pituitary hormones.

Asymmetry of higher functions of the cerebral cortex

Nearly every part of the brain exists in duplicate.

The part of the brain in which the right-left division is most evident, is the cerebral cortex.

Each half of the cortex folds inward where it would about the other half, forming a deep, fore -to-aft midline fissure that divides the cortex in two hemispheres.
The two hemispheres are connected by a massive bundle of axons called the corpus callosum.
The hemispheres are quite symmetrical in their primary sensory and motor functions. Each does the same job, but for a different half of the body.
Most paths are contralateral. (Crossed).
Sensory neurons that arise from the skin on the right side of the body send their signals to the somatosensory area of the left hemisphere, and vice versa. Such symmetry breaks down in the association areas.

• Large areas in the left hemisphere are specialized for language
• Large areas in the right hemisphere are specialized for nonverbal, visuospatial analysis for information.

Effects of surgical separation of the hemispheres: split brain, split mind.

The two hemispheres are separated by an operation in which the corpus callosum was cut.

How each hemisphere can be tested separately after the split-brain operation

It is possible to send visual information to just one hemisphere by presenting the stimulus in only the opposite half of the visual field.
Send tactile (touch) information to just one hemisphere by having the subject feel an object with the opposite hand.
Test the knowledge of just one hemisphere by have the subject respond with the hand opposite to that hemisphere.

How patients with split brain cope as well as they do

The patient’s ability to coordinate the two hemispheres probably involves several mechanisms.

• Only the cerebral cortex and some parts of the limbic system are divided when the corpus callosum is cut. Motor centers that control movement of the larger muscles lie in the lower, undivided parts. Some sensory information also passes from one hemisphere to the other by way of those lower routes.
• The intact connections allow each hemisphere to inhibit the motor output of the other, so the more competent hemisphere can take control of any given task.
• Each hemisphere learns to communicate indirectly with the other by observing and responding to the behavior that the other produces. This is called cross-cueing.

Split-brain insight into the consciousness: the left-hemisphere interpreter

One of the natural functions of the left hemisphere is to interpret, or try to make logical sense of, everything that the person does. This is intimately connected with the brain areas that generate speech.

Language areas of the left hemisphere

Much of the left hemisphere of the human cortex is devoted in one way or another to language.
Any loss of language ability resulting from brain damage is aphasia.

Effects of damage to Broca’s area

People how have brain damage in Broca’s area suffer from a type of aphasia in which speech becomes labored and telegraphic.
This is called Broca’s aphasia or nonfluend aphasia.

Neurons in Broca’s area are crucial for:

• Articulating words and sentences in a fluent manner
• Transforming grammatically complex sentences that are heard into simpler ones in order to extract the meaning.

Effects of damage to Wernicke’s area

Difficulty:

• Understanding the meaning of words that they heard
• Finding the appropriate words to express the meanings that they wanted to convey.

A patient’s speech:
Rich in the little words that serve primarily to for the grammatical structure of a sentence, but is markedly deficient in the nouns, verbs and adjectives that give a sentence meaning.
The speech retains its fluency and grammatical structure but loses its meaning.

This disorder is called Wernicke’s aphasia or fluent aphasia.

Neurons in and near Wernicke’s area are crucially involved in translating the sounds of words into their meaning and in locating, through connections to other cortical association areas, the words needed to express intended meanings.

Identifying language areas through neuroimaging

• Viewing or hearing words, without having to act on them in any way, resulted in high activity in the relevant sensory areas. Visual areas of the occipital lobe for viewing and auditory areas of the temporal lobe for hearing.
• Repeating aloud words that were seen or heard resulted in high activity in areas of the primary motor cortex that are involved in control of the vocal apparatus.
• Generating appropriate verbs in response to seen or heard nouns resulted in high activity in an area of the frontal lobe that encompassed Broca’s area and in a portion of the temporal lobe somewhat behind Wernicke’s area.

Changes in the brain over time

Neurons are soft, pliable living cells. They can change their sizes, shapes, excitabilities and patterns of connections in ways that help adapt their possessors to life’s ever-changing circumstances.

If you use it, it will grow

Like muscles, regions of the brain tend to grow when used and to atrophy when not used.

Effects of deprived and enriched environments on the brain.

Generation of new neurons is most apparent in the hippocampus, a structure known to be involved in learning and memory.

An enriched environment tends to let the brain grow.

Restructuring of the cortex during skill development

As an animal or person develops skill at a task, ever more neurons in the brain are recruited into the performance of that skill.

Spatial learning and growth of the hippocampus

Strengthening of synapses as a foundation for learning

The strengthening of synaptic connections between already existing neurons is linked to leaning.

The Hebbian synapse: Neurons that fire together wire together.

Some synapses in the brain have the property of growing stronger whenever the postsynaptic neuron fires immediately after the presynaptic neuron fires.
Neurons could acquire the capacity to respond to input that they previously didn’t respond to.
This could provide a basis for classical conditioning and other forms of learning.

Long-term potentiation LTP (by Timothy Bliss and Terge Lomo)
Strongly supports Hebb’s theory.

The potentiation is long-term and works in the following manner:

1. There is a weak synapse between neuron A and neuron C
2. When neuron A becomes active, some of the neurotransmitter molecules it releases become bound to conventional, fast acting receptors on the postsynaptic membrane, where they produce a depolarization that is too slight to play a significant role in triggering action potentials.
3. Other molecules at the same synapse become bound temporarily to special LTP-inducing receptors on the postsynaptic membrane.
4. If neuron C then fires an action potential (due input from other neurons), the combination of that firing and the presence of transmitter molecules in the LTP-inducing receptor triggers a series of biochemical events that strengthen the synapse.

The presynaptic terminal becomes larger, able to release more transmitter substance than it could before and the postsynaptic membrane develops more conventional receptor sites than it had before.

As a result of such changes, firing in neuron a produces more depolarization in neuron C than it did before and therefore plays a greater role in triggering action potentials in that cell than it did before.

Evidence that long-term potentiation is a basis for learning

Evidence that LTP is actually involved in learning comes from many experiments showing that interference with the brain’s normal capacity for such potentiation interferes with the animal’s ability to learn.

The evolution of the human brain

The most prominent feature in the human brain is size.
The encephalization quotient or EQ is a formula for evaluating the expected ratio between brain weight and body weight for animals.