- The brain is the bedrock of consciousness, perception, memory, emotion, and self-awareness.
The Nervous System: A Basic Blueprint
- The function of the nervous system is to gather and process information, produce responses to stimuli, and coordinate the workings of different cells. Scientists divide it into the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS, which includes the brain and spinal cord, receives, processes, interprets, and stores information and sends messages destined for muscles, glands, and organs. The PNS transmits information to and from the CNS by way of sensory and motor nerves.
The Central and Peripheral Nervous Systems
How the Nervous System is Organized
- The peripheral nervous system consists of the somatic nervous system, which permits sensation and voluntary actions, and the autonomic nervous system, which regulates blood vessels, glands, and internal (visceral) organs. The autonomic system usually functions without conscious control.
- The autonomic nervous system is divided into the sympathetic nervous system, which mobilizes the body for action, and the parasympathetic nervous system, which conserves energy.
The Autonomic Nervous System
Communication in the Nervous System
- Neurons are the basic units of the nervous system. They are held in place by glial cells, which nourish, insulate, and protect them, and in ways still not clearly understood, help them to function properly.
Different Kinds of Neurons
- Each neuron consists of dendrites , a cell body, and an axon. In the peripheral nervous system, axons (and sometimes dendrites) are collected together in bundles called nerves. Many axons are insulated by a myelin sheath that speeds up the conduction of neural impulses and prevents signals in adjacent cells from interfering with one another.
Structure of a Neuron
- Recent research has challenged two old assumptions: that neurons in the human central nervous system cannot be induced to regenerate and that no new neurons form after very early in life. In the laboratory, neurons have been induced to regenerate. And scientists have learned that stem cells in brain areas associated with learning and memory continue to divide and mature throughout adulthood. A stimulating environment seems to enhance this process of neurogenesis.
- Communication between two neurons occurs at the synapse . Many synapses have not yet formed at birth. During development, axons and dendrites continue to grow as a result of both physical maturation and experience with the world, and throughout life, new learning results in new synaptic connections in the brain. Thus, the brain’s circuits are not fixed and immutable but are continually changing in response to information, challenges, and changes in the environment, a phenomenon known as plasticity.
- When a wave of electrical voltage (action potential) reaches the end of a transmitting axon, neurotransmitter molecules are released into the synaptic cleft. When these molecules bind to receptor sites on the receiving neuron, that neuron becomes either more or less likely to fire. The message that reaches a final destination depends on how frequently particular neurons are firing, how many are firing, what types are firing, their degree of synchrony, and where they are located.
Neurotransmitter Crossing a Synapse
- Through their effects on neural circuits, neurotransmitters play a critical role in mood, memory, and psychological well-being. Abnormal levels of neurotransmitters have been implicated in several disorders, including depression, Alzheimer’s disease, and Parkinson’s disease.
- Endorphins, which act primarily as neuromodulators that affect the action of neurotransmitters, reduce pain and promote pleasure. Endorphin levels seem to shoot up when an animal or person is afraid or is under stress. Endorphins may also be linked to the pleasures of social contact.
- Hormones, produced mainly by the endocrine glands, affect and are affected by the nervous system. Psychologists are especially interested in melatonin, which promotes sleep and regulates a "biological clock" that coordinates bodily rhythms; adrenal hormones such as epinephrine and norepinephrine, which are involved in emotions, and stress; and the sex hormones, which are involved in the physical changes of puberty, the menstrual cycle (estrogens and progesterone), sexual arousal (testosterone), and some nonreproductive functions—including, many researchers believe, mental functioning.
Mapping the Brain
- Researchers study the brain by observing patients with brain damage, by using the lesion method with animals, and by using such techniques as electroencephalograms (EEGs), transcranial magnetic stimulation (TMS), positron-emission tomography (PET scans), and magnetic resonance imaging (MRI).
- Brain scans reveal which parts of the brain are active during different tasks, but they do not tell us precisely what is happening, either physically or mentally, during the task. Thus, they do not automatically reveal "centers" for a particular function, and must be interpreted cautiously.
A Tour Through the Brain
The Human Brain
The Limbic System
- Much of the brain’s circuitry is packed into the cerebrum, which is divided into two cerebral hemispheres and is covered by thin layers of cells known collectively as the cerebral cortex.
Lobes of the Cerebrum
- The occipital, parietal, temporal, and frontal lobes of the cortex have specialized (but partially overlapping) functions. The association cortex appears to be responsible for higher mental processes. The frontal lobes, particularly areas in the prefrontal cortex, are involved in social judgment, the making and carrying out of plans, and decision making.
The Two Hemispheres of the Brain
Visual Pathways
- Studies of split-brain patients, who have had the corpus callosum cut, show that the two cerebral hemispheres have somewhat different talents, a phenomenon known as lateralization. In most people, language is processed mainly in the left hemisphere, which generally is specialized for logical, symbolic, and sequential tasks. The right hemisphere is associated with spatial–visual tasks, facial recognition, and the creation and appreciation of art and music. In most mental activities, however, the two hemispheres cooperate as partners, with each making a valuable contribution.
Divided Brain, Divided View
Two Stubborn Issues in Brain Research
- One of the oldest questions in the study of the brain is where the "self" resides. Many brain researchers and cognitive scientists believe that a unified self may be something of an illusion. Some argue that the brain operates as a collection of independent modules or mental systems, perhaps with one of them functioning as an "interpreter." But much remains to be learned about the relationship between the brain and the mind.
Gender and the Brain
- Scholars in the new field of neuroethics are raising questions about the implications of “cognitive enhancement” and the potential misuse of brain scanning techniques. In evaluating research on the brain and behavior, it is important to remember that findings about the brain are most illuminating when they are integrated with psychological and cultural ones.
