Health Psychology. Michael Murray

      Figure 2.5 The brain, brainstem, medulla, pons and other important brain structures

      Source: © 2010 Terese Winslow, U.S. Govt.

      Another cortical input consists of fibres from the cortex itself, from either the same hemisphere or the opposite one. Once sensory signals arrive in their primary cortical area, they diverge into various local circuits responsible for information processing. These cortical microcircuits comprise the same types of cell distributed in the same six layers of the cortex. The results of ‘computations’ performed by these microcircuits ultimately converge at pyramidal neurons whose axons are the only output pathways from the cortex. A high proportion of axons that leave the cortex return to it, on the same or the opposite side. Other axons emerging from the cortex terminate in subcortical centres such as the thalamic nuclei, where they come into contact with the sensory fibres that send their axons to the cortex.

      ‘Feedback looping’ is a fundamental characteristic of information processing by the brain. At every stage, some of the fibres and connections loop back to the preceding stage to provide feedback that helps to control it. For instance, feedback loops enable the brain’s motor control centres to correct and adjust their signals to the muscles, right up to the moment these signals are sent. Feedback loops like these let us keep our balance while walking against sudden gusts of wind. Feedback loops are also found in bodily reflexes, such as the leg withdrawal reflex. A complex task, such as playing a piano, involves highly complex connections because it requires the pianist to contract and relax many different muscles simultaneously, which is controlled by the cerebellum.

      Neuromodulation

      Neuromodulation occurs when a neurone uses a chemical to regulate diverse populations of neurones. Neuromodulators are secreted by a small group of neurons and diffused through large areas of the NS, instead of into a synaptic gap, affecting multiple neurones at the same time. Just one of these neurones can influence over 100,000 others through the neuromodulators that it secretes into the brain’s extracellular space.

      Neuromodulators spend significant amounts of time in the cerebrospinal fluid (CSF), ‘modulating’ the activity of several other neurones. Some of the same chemicals that act as neurotransmitters are also neuromodulators, specifically serotonin, acetylcholine, dopamine and norepiniphrene. The neurons of the ‘hormonal brain’ differ from those of the ‘wired brain’ in several ways. The hormonal neurones are concentrated mainly in the brainstem and the central region of the brain. They form small masses of thousands of cells, but these cells project their axons into large areas of the forebrain and the midbrain. Many drugs and medications, including those prescribed for affective disorders and schizophrenia, act on the neuromodulators of the diffuse projection neurons in the brainstem. For this reason, the function and distribution of the projections of these neurons have been the subject of much research using tracing techniques, because the axons of these neurons are not myelinated and do not form readily identifiable bundles. The results have confirmed how widely diffused these projections are. For example, a single axon from one of these neurons may subdivide and innervate both the cortex and the cerebellum.

      The four main neuromodulators are norepinephrine (diffused by the locus coereleus), serotinin (diffused by the Raphe nuclei), acetylcholine (diffused by the basal nucleus of Meynert, pedunculopontine and pontine nuclei) and dopamine (diffused by the substantia nigra and ventral tegmental area). Each of these groups of neurones projects axons into large areas of the CNS and thus modulates numerous behaviours. The diffusion of the four main neuromodulators is illustrated in Figure 2.6.

      The Somatic and Autonomic Nervous Systems

      The somatic nervous system is the organism’s apparatus for responding to the external environment. It sends information to the brain from the body’s various sensory detectors. The somatic nerves enable us to respond to these stimuli by moving through our environment, taking voluntary action, reacting and speaking. One of the principal roles of the somatic NS is maintaining homeostasis in the external environment, as discussed later in this chapter.

      The autonomic nervous system (ANS) maintains homeostasis in the internal environment by regulating vital organs, such as those involved in digestion, respiration, blood circulation, excretion, and the secretion of hormones. The ANS is divided into two subsystems, the sympathetic and parasympathetic systems. The two branches of the ANS generally work in opposite directions, enabling a continuous upward or downward control of the internal organs to maintain homeostasis in the internal environment.