In general, biological psychologists are reductionist. They believe that behavior (and experience) can be reduced to biology and explained in its terms.
The success of chemical treatments for depression and anxiety may support this view.
The Death (and Rebirth) of the Homunculus
In cartoons or our imagination (even in Woody Allen movies) we see the brain and mind as a vast complex, like a machine, a factory, or a city.
In charge of this city is a little person (a "homunculus" in Latin) who sits somewhere behind our eyes.
This little person receives all our sensory input, looks up things in our memory banks, makes a decision and issues a command.
Suppose that I am at a buffet, deciding whether to have pizza with double cheese (dripping with grease) or salad with fat-free vinaigrette.
In the homunculus model, my inner person takes note of the smell from the pizza, sees the rising steam, etc. That is combined with memories of previous delicious indulgences. On the other hand, he recalls stories of heart disease and perhaps memories of heartburn. Maybe he takes a reading on some sort of guilt meter. He weighs the alternatives and then announces in my mind "I think I'll have the pizza." Shortly thereafter, he sends a signal to the hand to go for the cholesterol pie.
This idea is similar to the idea of the "ghost in the machine," an immaterial entity that inhabits our body and makes our decisions. According to Descartes and to many other thinkers, this ghost is who we actually are.
Obviously there is no little man, but we might hypothesize that there is some sort of "central processing unit" or "executive circuits" in the brain.
Cognitive scientists believe that the mind (and the brain) have a modular organization. One or more of the modules may have "executive" functions. Think about the role of attention in perception (we will discuss in a couple of weeks).
Normal functioning of the mind involves many processes dependent on many structures in the mind. The frontal lobes seem to be especially important in planning and decision making.
Still, brain scans would show that the motor impulses driving my hand toward baked heart attack initiate in certain parts of my brain before I was even aware of my decision to have pizza.
Behaviorists such as BF Skinner used the problem of infinite regress to dismiss the idea of a central consciousness in the mind. If consciousness is a little man in the head, what is conscious in the little man's head?
There is one sense in which there is a little person in the brain. The motor cortex stretches left and right from the center line (the longitudinal fissure) of the brain along the ridge at the back of the frontal lobe. (This ridge runs in front of the central fissure, so it is called the precentral gyrus). In this ridge are the cell bodies of neurons that initiate voluntary movements. Each point on this ridge corresponds to a muscle somewhere in the body. Adjacent areas of the motor cortex control adjacent areas of the body, though the relative size of the cortical areas is distorted relative to the size of the body part (see figure 4.10 on page 67 in Myers). This mapping of body to brain is called the "motor homunculus."
Likewise, in back of the central fissure, at the front of the parietal lobe is a ridge (the postcentral gyrus) with the cell bodies of sensory neurons receiving info from different areas of the body. This is called the "sensory homunculus."
This sort of topographic organization is found in many areas of the brain, for example in the visual cortex.
Neurobiology
There is a thorough set of online tutorials on neurons and the brain, with pictures, from Athabasca University at http://psych.athabascau.ca/html/Psych402/Biotutorials/
The Neuron
The nervous system depends on specialized cells called neurons. The brain contains about 100 billion neurons.
Structure
Like all cells, neurons are surrounded by a cell membrane.
The membrane consists of a lipid bilayer.
Neurons are mostly fat by dry weight.
A number of biochemical structures are imbedded in the lipids, including:
ion channels - allow ions (like sodium or chloride) to pass through the membrane
receptors - respond to the chemical environment outside the cell
Neurons have three parts:
cell body
nucleus - contains the genetic material of the cell
metabolic functions
The gray matter of the brain and spinal cord has a high density of cell bodies.
one axon
carries signals away from the cell body
branched at the end, each branch tipped with a terminal button
presynaptic membrane releases messages to other cells
The white matter of the brain and spinal cord is composed largely of myelinated axons (nerve fibers).
Note: A "nerve" is a bundle of axons. There are axons in the body several feet long (for example, from your spine to your toes).
one or more dendrites
carry signals toward the cell body
post synaptic membrane receives messages from other cells
There is a high density of dendrites in gray matter as well; there is much communication between neurons here.
Function
Action Potentials
Every living cell maintains chemical differences between the extracellular environment and the cytoplasm inside the cell.
Sodium ions are actively pumped out of the cell, creating a negative charge inside the cell (about 70 mV in neurons at rest).
When properly stimulated, neurons open ion channels in the cell membrane, allowing sodium ions to enter the cell, reducing then reversing the membrane potential to about +40 mV.
When one point on the membrane is discharged, it can induce discharge in an adjacent point. Thus the discharge is conducted along the membrane (down the dendrite or axon).
About a millisecond after depolarization, the membrane begins pumping ions again until the resting potential is restored.
The impulse travels along the neuron as a transient point of depolarization.
Most neurons in the brain and many elsewhere are myelinated, ie covered with an insulated sheet (actually formed by another cell called a Schwann cell). The myelin sheath has regular gaps in it and the discharge jumps from gap to gap, travelling more quickly and reliably than down an unmyelinated membrane.
In multiple sclerosis, the myelin sheath breaks down.
The entire sequence is called an action potential or nerve impulse.
Synaptic Transmission
Nerve cells communicate with each other.
The cells do not touch each other but rather are separated by a very thin gap called a synapse.
The action potential cannot cross this gap, the signal is instead carried by chemical messengers called neurotransmitters (NT).
The action potential reaches the terminal button.
Within the terminal button synaptic vesicles (tiny sacs of neurotransmitters) moved toward and merge with the presynaptic membrane.
The synaptic vesicles release their contents into the synaptic cleft.
The neurotransmitters cross the synapse and reach receptors on the postsynaptic membrane.
Receptors are specialized biochemical structures (mostly protein) that match a particular neurotransmitter.
The NT is the "key" that fits the receptors "lock."
Receptors are on dendrites, or sometimes cell bodies, of other neurons.
The receptor responds in one of two ways:
Excitatory transmitters increase the likelihood of an action potential in the postsynaptic cell.
Inhibitory transmitters decrease the likelihood of an action potential.
Either can carry a message--in the absence of synaptic events, a neuron fires at a baseline rate. The signal can be in the form of an increase or decrease in firing rate.
Following release and receptor activation, the NT is deactivated by degradation (being broken down by enzymes) or reuptake (being put back into vesicles inside the neuron).
Putting it together
The function of the nervous system depends on the combined activity of many neurons.
Each neuron receives synaptic transmissions from many other neurons. The cell combines the multiple excitatory and inhibitory messages and either fires or not.
A cell membrane can be partially depolarized, but an action potential is all-or-none.
Structure of the Nervous System(s)
The central nervous system consists of the brain and spinal cord.
Neurons in the CNS divide and grow up to about age 6 and then stop.
Damage to the CNS (such a spinal injury) is not usually repaired by regeneration of the damaged tissue.
Brain damage is often overcome because other cells will take over functions previously carried out by the damaged tissue.
There are also non-neuronal cells in the CNS that serve structural roles. These continue to divide (and in fact can become cancerous).
The peripheral nervous system consists of the nerves that run to the body from the spinal cord, as well as the cranial and facial nerves (that run directly out of the brain).
PNS nerves continue to grow and divide throughout your life.
Damage to the PNS is usually repaired.
The Brain
The Three Brains
Paul McClean was working on a theory of the "triune brain" as early as the 1950's
There are a number of schemes for dividing the brain into three parts, the details do not always agree.
The basic idea is that the higher most human functions (such as reason and language) are dependent on the newest, outermost and topmost layer (the cerebral cortex).
More primitive functions such as emotions, are localized in brain structures within or below the neocortex.
The most primitive functions are localized in the deepest parts of the brain.
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Cerebral Cortex (neocortex) |
frontal lobe |
sensation |
New Brain (forebrain) |
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Limbic System (subcortex) |
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sensory preprocessing |
Mammalian Brain Reptilian Brain |
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Brainstem (and company)(old brain) |
locus coeruleus reticular formation pons medulla oblongata |
visual and auditory processing basic bodily processes arousal |
midbrain
hindbrain |
The Brainstem
This part of the brain is shared by a wide group of organisms (virtually all vertebrates). It is the "oldest" part in evolutionary terms. Damage to the brainstem is usually fatal.
The medulla is a swelling on the top of the spinal cord. It regulates basic processes such as heart rate, breathing, blood pressure, sneezing, coughing, etc.
The pons connects the neocortex and the cerebellum.
Spread through the brainstem (and into the limbic system) is the reticular system. This controls sleep and arousal.
The midbrain contains the locus coeruleus, part of the reticular system. It is also involved in vision and hearing. It regulates visual tracking.
The Limbic System
We have the limbic system in common with some lower vertebrates. It includes the "mammalian brain" and perhaps parts of the "reptilian brain."
The limbic system is involved in the four Fs:
Feeding
Fleeing
Fighting
Family Matters J
Thalamus is a sensory relay station, receiving sensory information (except olfactory) and sending it to other parts of the brain. Sensory information (eg, visual stimuli) can influence feeling and behavior here even before or without conscious awareness (ie, cortical involvement)
Victims of cortical blindness can still respond reflexively or emotionally to visual cues.
Hypothalamus, located below the thalamus, has several vital roles in keeping the organism alive. Damage to the hypothalamus can easily be fatal, or cause severe abnormalities (gluttony or starvation).
Controls the autonomic nervous system
Homeostasis
eating
drinking
mating
aggression
Amygdala appears to be crucial in anxiety, fear, violence and aggression, as well as learning and memory relative to intense emotional experience.
Hippocampus has been proposed as a resonant circuit active in short term memory. It may play a crucial role in motion sickness.
The Cortex
The cortex is evolutionarily the "newest" part of the brain; it is also called the "neocortex." Damage to the cortex is not usually fatal. It may even be unobservable.
The human cerebral cortex is larger and more complex than that of any other animal. The cortex is like a large sheet of "neuron paper" crumpled over the limbic system. (Cortex means "bark").
Many areas of the cortex can be mapped onto sensory or motor functions of parts of the body or may be related to specific memories, sensations or experiences.
This mapping and specificity is taken as evidence that the mind is at least tied to the brain.
Cortical function:
localized yet diffuse
plastic (one area can take over the functions of another, temporarily or permanently)
The cortex is divided into two grossly symmetrical hemispheres. Each hemisphere is divided by large fissures (grooves) into four lobes.
Function is divided between the hemispheres (lateralization)
Speech is in the left hemisphere for right handed males.
For lefties the functions may be reversed.
For females the functions may be more shared.
Frontal lobe is forward of the central fissure, behind your forehead.
primary motor cortex (aka frontal motor area)
located along the central fissure
involved in voluntary movements
damage causes loss of specific fine motor skills (especially of the fingers)
the degree of coordination in a body region (eg, the hands) corresponds to the size of the connected area in the motor cortex
Broca's area
only in the left hemisphere
crucial for speech
other frontal areas (association areas)
involved in emotion and integration of experience
no obvious sensory or motor function
surgical destruction of this area (called a frontal lobotomy) reduces emotional expression, and was previously attempted to control extreme manic symptoms
Parietal lobe is behind the central fissure and above the lateral fissure, below the back of the top of your head.
sensory cortex (aka primary somatic projection area)
just behind the central fissure (across from the motor cortex)
receives tactile information from the body
mapped similarly to the motor cortex
Temporal lobe is below the lateral fissure, beneath your temples.
hearing
memory
Wernicke's area
left hemisphere
speech recognition
Occipital lobe is in the back of the head.
vision
damage to this area can impair visual function
cortical blindness
visual agnosia
I also include the cerebellum and corpus callosum in the cortex (for lack of somewhere else to put them)
Cerebellum (Latin for "little brain")
tucked below the temporal and occipital lobes (sticking out of the bottom of the brain)
active in the control of complex movements
appears to be a set of tuned circuits that can create "movement subroutines" launched in whole by the cortex
also hemispherically divided
connected to motor cortex and sensory nervous system via the brainstem
Corpus callosum (Latin for "tough stuff")
a dense band of nerves connecting the left and right hemisphere
located underneath the cortical lobes and above the limbic system
allows communication between the hemispheres
if this is severed, a the two hemispheres seem to function independently
The Peripheral Nervous System
Autonomic Nervous System
The autonomic nervous system controls "involuntary" functions of the body such as digestion, circulation, breathing, and so forth. It connects the CNS to glands, smooth muscles (eg, in the intestine), and internal organs. It has two main circuits:
The sympathetic division is associated with arousal, agitation and utilization of energy.
The parasympathetic division is associated with calmness, relaxation, and conservation of energy.
Somatic Nervous System
The somatic nervous system conveys "conscious" sensations and "voluntary" actions. It connects the CNS to sense receptors and striated (skeletal) muscles.
Sensory (afferent) neurons carry sense impulses to the CNS.
Motor (efferent) neurons carry motor impulses from the CNS.
Neurochemistry
Neurotransmitters
These chemicals are released by neurons to effect a change in another neuron or in some other structure (like a muscle cell).
There are now thought to be as many as 100 different chemicals involved in nervous transmission.
Some NTs:
acetylcholine (ACh)
the first identified NT
active in the parasympathetic nervous system
triggers muscle contractions when released by motor neurons onto muscle cells
nerve gas and pesticides work by preventing the breakdown of ACh
active in cognitive functions
Alzheimer's Disease is associated with a loss of certain cholinergic neurons in the brain
nicotine is an ACh agonist (ie, it simulates the effects of choline)
dopamine (DA), norepinephrine (NE)(aka noradrenaline), and epinephrine (E)(aka adrenaline)
chemically similar
active in the sympathetic nervous system
implicated in depression (a lack of DA)
drugs that block the breakdown (MAOIs) of DA are used to treat depression
implicated in anxiety (an overabundance of NE)
serotonin
implicated in arousal by the reticular system
g-Aminobutyric Acid (GABA)
a critical inhibitory NT
slows down firing rates throughout the nervous system
alcohol, barbiturates, and BDZs (eg, Valium) act at the GABA receptor complex
alcohol and barbiturates act at different active sites in the receptor and are sometimes fatally synergistic
Glutamic Acid (Glu)
excitatory in CNS
MSG (in Chinese food) is monosodium glutamate, the same compound
some people report headache and sensory abnormalities after eating food containing MSG
Hormones
The endocrine system consists of a set of glands that release chemicals, called hormones, into the bloodstream.
Hormones are chemicals (secreted by endocrine glands or even the brain itself) which affect the long term (minutes, days, years) functioning of the body.
Some hormones are chemically identical to neurotransmitters.
The function of the endocrine system is slower than the nervous system, but is complementary to it.
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Gland |
Hormone |
Function |
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Pineal |
melatonin |
circadian rhythm |
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Pituitary |
growth hormone |
growth |
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Thyroid |
thyroxin |
metabolism |
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Parathyroid |
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calcium and phosphorus metabolism |
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Pancreas |
insulin, glucagon |
carbohydrate metabolism |
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Adrenal Medulla |
epinephrine, norepinephrine |
stress response (short term) |
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Adrenal Cortex |
|
carbohydrate metabolism |
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Gonads: Ovaries/Testes |
sex hormones |
secondary sex characteristics |
The pituitary gland, located on the edge of the brain, is the "master gland" that mediates between the hypothalamus and the rest of the endocrine system.
The pituitary can directly influence the nervous system; the hypothalamus can relese hormones to the bloodstream.
Neuropeptides are short amino acid sequences that function as both neurotransmitter and hormone.
Studying the Brain
Electrical Stimulation of the Brain
In the 1950s, Delgado et al and Olds & Milner discovered that electrical stimulation of the limbic system could produce pain or pleasure in experimental animals (actually, it was observed to be aversive or rewarding). The stimulated areas have been called the pain and pleasure centers of the brain.
Before his death in 1976, a neurosurgeon named Penfield pioneered the treatment of epilepsy by electrically destroying a small volume of neurons that had acted as a seizure focus.
Patients were awake during surgery (the brain itself is insensitive to pain). To orient himself in the cortex, Penfield would stimulate a small area and ask the patients to tell what they were experiencing. A range of sensory and motor responses were recorded. In some cases elaborate memories could be vividly evoked by electrical stimulation.
Split Brain Phenomena
In the 1960's Bogen and Sperry studied patients in whom Bogen had cut the corpus callosum to control severe epilepsy.
The epilepsy was reduced (for reasons that were not entirely understood).
The patients seemed largely normal, except:
Stimuli presented to one hemisphere was immeidately available only to that hemisphere.
An object was placed in the patients left hand. Because the sensory information crosses to the opposite cortex, the identity of the object is known to the left hemisphere. Speech is localized in Broca's area in the right hemisphere, and in fact the patient's could not readily say what the object was. They would recognize it if placed in the left hand again.
Similar effects could be demonstrated when visual stimuli were presented to the left visual field (the right hemisphere).
Sperry was awarded the Nobel Prize in 1981.
Brain Scans
electroencephalogram (EEG)
a measurement of electrical potentials at points around the brain, usually outside the skull
not very sensitive (generally reflects the sum of all electrical activity in a region)
also, the brain is well-insulated electrically
certain "brain waves" associated with mental states (eg, alpha waves with relaxation)
magnetoencephalogram (MEG)
the brain is not well shielded magnetically
using superconducting magnetic detectors (superconducting quantum interference devices--SQUIDs), high resolution may be possible
computerized axial tomography (CAT) scan
uses a large number of conventional xrays taken from different angles, then combined in a computer
reveals structural abnormalities
positron emission tomography (PET) scan
detects emission of positrons from radiolabeled glucose (usually 15O)
lights up areas that are actively taking up glucose, the brain's energy source
magnetic resonance imaging (MRI)
looks at water density or other chemical characteristics
reveals structural abnormalities
functional magnetic resonance imaging (fMRI)
can show biochemical activity in space and time
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Anthony G Benoit
abenoit@trcc.commnet.edu
(860) 885-2386
Revised