The Handy Psychology Answer Book. Lisa J. Cohen
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FunctionsMemory
What are the basal ganglia?
The basal ganglia are centrally involved with action and motor behavior. The basal ganglia are actually a group of brain regions, including the putamen, globus pallidus, and caudate nucleus. This part of the brain is relatively old, phylogenetically, and handles the more automatic aspects of behavior. When we learn a new behavioral sequence, like riding a bicycle, we initially depend on the frontal lobes while we are concentrating on what we are doing. After the behavior is learned and becomes more automatic, however, it is handled by the basal ganglia. Damage to the basal ganglia can severely disrupt motor behavior, as is found in neurological diseases like Parkinson’s or Huntington’s disease.
What does the thalamus do?
The thalamus is a large bulb in the middle of the brain, which most of the limbic system wraps around. Like most of the brain, the thalamus is bilateral; in this case that means that the thalamus is actually comprised of two symmetrical bulbs lying side by side. The thalamus is centrally involved with sensation and perception. It serves as a relay station for sensory information, filtering input from the sense organs on the way to the cortex. The thalamus processes visual, auditory, tactile, and gustatory (taste) information but it does not process olfactory (sense of smell) input.
Paul MacLean felt that the human brain can be separated into three regions based on how it evolved over time.
What is the brain stem?
The brain stem is the oldest and most primitive part of the brain. It regulates basic physiological processes necessary for life, such as breathing, temperature regulation, sleep-wake cycle, and cardiac function. The brain stem has been relatively conserved across evolution and therefore does not dramatically differ across animal species.
What is the triune model?
In 1964, Paul D. MacLean (1913–2007) divided the brain into three general regions, the reptilian, paleomammalian, and neomammalian regions, which he believed to correspond with different periods of evolution. The neomammalian region of the brain refers to the neocortex, which includes the frontal lobe and most cortical regions. These parts of the brain are most developed in more complex and evolutionarily younger mammals, such as primates. The paleomammalian region incorporates the limbic system, which is found in all mammals. The reptilian area of the brain refers to the brain stem and the cerebellum, phylogenetically ancient regions that are found in some of the most ancient and primitive species (e.g., reptiles). Although MacLean’s triune model has been criticized as overly simplistic, it does provide a useful way for non-specialists to picture the brain.
THE NEURON
What is a neuron?
A neuron is a brain cell, the basic building block of the brain. The entire brain is actually a huge network of interlocking, interacting neurons. There are about one hundred billion neurons in the human brain and several times that amount of glial and other smaller cells that support neural function. A neuron is composed of a cell body, an axon, and a profusion of branching dendrites.
What are the input and output sections of the cell?
Brain cells have both input and output sections. Dendrites are the tree-like extensions that reach out from the cell body. They are the input section of the cell and carry electrical information into the cell body from the axons of other neurons. Axons are the output section of the cell and carry electrical information from the cell out to other neurons. Axons can be extremely long, reaching from the brain all the way down to the base of the spine. While some axons branch into two sections, by and large there is only one axon per neuron. At the end of the axon, the cell branches into numerous axon terminals. One axon can have thousands of axon terminals. The majority of axon terminals connect with the dendrites of other cells, resulting in trillions of neuronal connections in the typical brain.
The structure of the neuron.
What is a synapse?
The contact point between the dendrite of one cell and the axon terminal of another is called the synapse. Neurons communicate across synapses with chemical messengers known as neurotransmitters. The neuron sending the message is called the pre-synaptic neuron while the neuron receiving the message is the post-synaptic neuron. The gap between the neurons is called the synaptic cleft.
What are neurotransmitters and what do they do?
Neurotransmitters are the chemical messengers that neurons use to communicate with each other. When an axon meets up with the dendrite of another cell at the synapse, neurotransmitters are released into the synaptic cleft. Some of these neurotransmitters are excitatory and others are inhibitory. Both types of neurotransmitters change the electrical charge of the postsynaptic neuron.
How do neurons fire?
When a neuron fires, it sends an electrical impulse down the length of the axon to the axon terminals. Excitatory neurotransmitters make it easier for the post-synaptic neurons to fire; inhibitory neurotransmitters make it more difficult. Each cell receives excitatory and inhibitory inputs from many synapses. When the sum total of these inputs reaches a certain threshold, the neuron fires, sending an electrical impulse down the axon. This firing is known as an action potential. When the action potential reaches the bottom of the axon, the axon terminals release their own neurotransmitters, stimulating (or inhibiting) action potentials in the next group of neurons. In this way, wide networks of neurons communicate with each other in very little time. Action potentials commonly travel at speeds of fifty meters per second and can occur every five hundredths of a millisecond.
Neurons communicate information to each other by sending chemicals called neurotransmitters across synaptic clefts.
What is the difference between white matter and gray matter?
Axons are coated by a fatty sheath known as myelin that speeds up the rate that the action potential travels down the axon. Because myelinated axons are white in appearance, brain tissue made up of these fibers is called white matter. In contrast, gray matter refers to brain tissue made up of dendrites and cell bodies (as well as glial cells and capillaries). The surface of the cortex is composed of gray matter.
BRAIN DEVELOPMENT
THE HUMAN BRAIN ACROSS EVOLUTION
How has the human brain changed across evolution?
Because we see the brain through the lens of evolutionary theory, it is important to consider how the human brain has changed across evolution. We make inferences about how our brain has evolved based on comparisons between human and animal brains. For one, our brain has become much larger relative to our body size. Evolutionarily older and more primitive animals have much smaller brains relative to their body size. Think of the dinosaurs with their tiny brains and huge bodies. In fact, our brains are about three times the size of the brains of our closest relative,