Organization Of Neurons Into A Nervous System And Basic Neuroanatomy
For meaningful behavior to occur, information from the body and environment must get into the brain, and instructions from the brain must be delivered to the muscles and glands so that the body can perform. The portion of the nervous system that provides this interface is called the peripheral nervous system. It has two basic subsystems: the somatic system and the autonomic system. The somatic system brings information into the brain via axons originating in the sensory organs and sends messages outward through axons to skeletal muscles causing them to contract. The autonomic system is linked to our emotions and arousal states, and activates glands (e.g., sweat glands), smooth muscles (e.g., controlling the pupils of the eye, blood vessels, etc.), and heart. The autonomic system contains two subsystems: the sympathetic system readies us for action (increases heart rate, dilates pupils), whereas the parasympathetic system has the opposite effect, restoring us toward a resting level when warranted. Some age-related changes often occur in the peripheral systems, such as loss or thinning of nerve fibers or changes in the "target organs" (muscles and glands), and these may result in poorer performance.
In the central nervous system (CNS), the spinal cord extends below the brain, encased by the vertebral bones of the neck and back. The spinal cord relays incoming sensory messages from the periphery to the brain and outgoing instructions from the brain to spinal cord neurons whose axons leave the CNS to activate muscles. However, in addition to being a relay to and from the brain, the spinal cord itself contains impressive assemblies of neuronal circuitry that perform a number of sensory-motor behaviors, such as withdrawal from a painful stimulus or rhythmic movements associated with locomotion. Changes in the spinal cord can occur in older people that make it less efficient at relaying the information up and down.
Merging with the spinal cord is the brainstem, the lowest region of the brain. Its basic subdivisions (moving upwards) are the medulla, pons, and midbrain. At the top of the brainstem is the thalamus. The neural traffic between the brain and spinal cord travels along axons that traverse the brainstem. In addition, sensory information (coded in trains of action potentials) enters the brainstem from the head and the special sense organs (ears, eyes, etc.), while other messages leave the brainstem to control the face, mouth, eyes, and so on. The brainstem controls a number of activities without a necessary contribution from higher levels of the brain. For example, comparison of acoustic input from the two ears to compute the location of sounds in space is a function of brainstem circuits. Some regions of the brainstem are especially vulnerable to agerelated changes that can affect a variety of behaviors.
Behind the brainstem, near the base of the skull, is the cerebellum. It plays key roles in coordinating movements, balance, and even some types of learning. The cerebellum has numerous axons communicating with the brainstem that, in turn, communicate with the higher regions of the brain and the spinal cord below. The cerebellum is required for balance, posture, gait, and the adjustment and coordination of movements. Age-related changes observed in neurons of the cerebellum include loss of dendrites and spines and changes in neurotransmitter systems, and these are likely to affect movements.
The bulk of the brain lies above and around the thalamus. The basal ganglia are prominent parts of the interior brain adjacent to the thalamus, and deal with movement (they play important roles in cognition, as well). To the side, but folded over so as to sit deep in the middle of the brain, is the hippocampus, a structure that is essential for certain types of learning and memory as well as spatial behaviors. The amygdala, along with the hypothalamus and other parts of the limbic system, comprise the "emotional brain." At the base of the brain is the hypothalamus, a collection of small subdivisions that regulate a number of essential functions such as eating and drinking, body temperature, biological rhythms, and reproductive behavior. The hypothalamus produces hormones that influence the release of other hormones by the neighboring pituitary gland. At least some of these hormone systems become less responsive with age because of reduced production by hypothalamic neurons, changes in receptor sensitivity for the hormones, and/or changes in the endocrine glands. The fact that relatively small hypothalamic subdivisions control important biological functions has suggested that subtle changes might contribute to aging in a fundamental way.
If one looks at a human brain, it is dominated by two large, folded cerebral hemispheres. On the surface of the hemispheres (and folded into the creases or sulci) are several layers of neurons that comprise the cerebral cortex. The cerebral cortex, working in concert with the rest of the brain, is capable of incredible feats, most notably in humans, in which its size and complexity far exceeds that of other species. Language, complex thoughts, logic, and many other "higher" functions are beholden to a highly advanced cerebral cortex. The left and right hemispheres communicate with one another with millions of axons, most of which are contained in a huge band of axons called the corpus callosum. Studies have found evidence that the transfer of information between cerebral hemispheres across the corpus callosum can be slowed or diminished with age.
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