Color Atlas of Physiology 2003 thieme

Central Nervous System

The brain and spinal cord make up the central nervous system (CNS) (!A). The spinal cord is divided into similar segments, but is 30% shorter than the spinal column. The spinal nerves exit the spinal canal at the level of their respective vertebrae and contains the afferent somatic and visceral fibers of the dorsal root, which project to the spinal cord, and the efferent somatic (and partly autonomic) fibers of the anterior root, which project to the periphery. Thus, a nerve is a bundle of nerve fibers that has different functions and conducts impulses in different directions (!p. 42).

Spinal cord (!A). Viewed in cross-section, the spinal cord has a dark, butterfly-shaped inner area (gray matter) surrounded by a lighter outer area (white matter). The four wings of the gray matter are called horns (cross-section) or columns (longitudinal section). The anterior horn contains motoneurons (projecting to the muscles), the posterior horn contains interneurons. The cell bodies of most afferent fibers lie within the spinal ganglion outside the spinal cord. The white matter contains the axons of ascending and descending tracts.

Brain (!D). The main parts of the brain are the medulla oblongata (!D7) pons (!D6), mesencephalon (!D5), cerebellum (!E), diencephalon and telencephalon (!E). The medulla, pons and mesencephalon are collectively called the brain stem. It is structurally similar to the spinal cord but also contains cell bodies (nuclei) of cranial nerves, neurons controlling respiration and circulation (!pp. 132 and 212ff.) etc. The cerebellum is an important control center for motor function (!p. 326ff.).

Diencephalon. The thalamus (!C6) of the diencephalon functions as a relay station for most afferents, e.g., from the eyes, ears and skin as well as from other parts of the brain. The hypothalamus (!C9) is a higher autonomic center (!p. 330), but it also plays a dominant role in endocrine function (!p. 266ff.) as it controls the release of hormones from the adjacent hypophysis (!D4).

basal ganglia, i.e. caudate nucleus (!C5), putamen (!C7), globus pallidus (!C8), and parts of the amygdala (!C10). The amygdaloid nucleus and cingulate gyrus (!D2) belong to the limbic system (!p. 330). The cerebral cortex consists of four lobes divided by fissures (sulci), e.g., the central sulcus (!D1, E) and lateral sulcus (!C3, E). According to Brodmann’s map, the cerebral cortex is divided into histologically distinct regions (!E, italic letters) that generally have different functions (!E). The hemispheres of the brain are closely connected by nerve fibers of the corpus callosum (!C1, D3).

Cerebrospinal Fluid

The brain is surrounded by external and internal cerebrospinal fluid (CSF) spaces (!B). The internal CSF spaces are called ventricles. The two lateral ventricles, I and II, (!B, C2) are connected to the IIIrd and IVth ventricle and to the central canal of the spinal cord (!B). Approximately 650 mL of CSF forms in the choroid plexus (!B, C4) and drains through the arachnoid villi each day (!B). Lesions that obstruct the drainage of CSF (e.g., brain tumors) result in cerebral compression; in children, they lead to fluid accumulation (hydrocephalus). The blood–brain barrier and the blood–CSF barrier prevents the passage of most substances except CO2, O2, water and lipophilic substances. (As an exception, the circumventricular organs of the brain such as the organum vasculosum laminae terminalis (OVLT;

!p. 280) and the area postrema (!p. 238) have a less tight blood–brain barrier.) Certain substances like glucose and amino acids can cross the blood–brain barrier with the aid of carriers, whereas proteins cannot. The ability or inability of a drug to cross the blood–brain barrier is an important factor in pharmacotherapeutics.

Plate 12.1 Central Nervous System

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Stimulus Reception and Processing

With our senses, we receive huge quantities of information from the surroundings (109 bits/ s). Only a small portion of it is consciously perceived (101–102 bits/s); the rest is either subconsciously processed or not at all. Conversely, we transmit ca. 107 bits/s of information to the environment through speech and motor activity, especially facial expression (!A).

A bit (binary digit) is a single unit of information (1 byte = 8 bits). The average page of a book contains roughly 1000 bits, and TV images convey more than 106 bits/s.

Stimuli reach the body in different forms of energy, e.g., electromagnetic (visual stimuli) or mechanical energy (e.g., tactile stimuli). Various sensory receptors or sensors for these stimuli are located in the five “classic” sense organs (eye, ear, skin, tongue, nose) at the body surface as well as inside the body (e.g., propriosensors, vestibular organ). (In this book, sensory receptors are called sensors to distinguish them from binding sites for hormones and transmitters.) The sensory system extracts four stimulatory elements: modality, intensity, duration, and localization. Each type of sensor is specific for a unique or adequate stimulus that evokes specific sensory modalities such as sight, sound, touch, vibration, temperature, pain, taste, smell, as well as the body’s position and movement, etc. Each modality has several submodalities, e.g., taste can be sweet or bitter, etc.

In secondary sensors (e.g., gustatory and auditory sensors), sensor and afferent fibers are separated by a synapse, whereas primary sensors (e.g., olfactory sensors and nocisensors) have their own afferent fibers.

A stimulus induces a change in sensor potential (transduction), which results in depolarization of the sensor cell (in most types; !B1) or hyperpolarization as in retinal sensors. The stronger the stimulus, the greater the amplitude of the sensor potential (!C1). Once the sensor potential exceeds a certain threshold, it is transformed into an action potential, AP

(!B1; p. 46ff.).

Coding of signals. The stimulus is encoded in AP frequency (impulses/s = Hz), i.e., the

higher the sensor potential, the higher the AP frequency (!C2). This information is decoded at the next synapse: The higher the frequency of arriving APs, the higher the excitatory postsynaptic potential (EPSP; !50ff.). New APs are fired by the postsynaptic neuron when the EPSP exceeds a certain threshold (!B2).

Frequency coding of APs is a more reliable way of transmitting information over long distances than amplitude coding because the latter is much more susceptible to change (and falsification of its information content). At the synapse, however, the signal must be amplified or attenuated (by other neurons), which is better achieved by amplitude coding.

Adaptation. At constant stimulation, most sensors adapt, i.e., their potential decreases. The potential of slowly adapting sensors becomes proportional to stimulus intensity (P sensors or tonic sensors). Fast-adapting sensors respond only at the onset and end of a stimulus. They sense differential changes in the stimulus intensity (D sensors or phasic sensors). PD sensors have both characteristics (!p. 314).

Central processing. In a first phase, inhibitory and stimulatory impulses conducted to the CNS are integrated—e.g., to increase the contrast of stimuli (!D; see also p. 354). In this case, stimulatory impulses originating from adjacent sensor are attenuated in the process (lateral inhibition). In a second step, a sensory impression of the stimuli (e.g. “green” or “sweet”) takes form in low-level areas of the sensory cortex. This is the first step of subjective sensory physiology. Consciousness is a prerequisite for this process. Sensory impressions are followed by their interpretation. The result of it is called perception, which is based on experience and reason, and is subject to individual interpretation. The impression “green,” for example, can evoke the perception “There is a tree” or “This is a meadow.”

Absolute threshold (!pp. 340ff., 352, 362), difference threshold (!pp. 340ff., 352, 368), spatial and temporal summation (!pp. 52, 352), receptive field (!p. 354), habituation and sensitization are other important concepts of sensory physiology. The latter two mechanisms play an important role in learning processes (!p. 336).

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Nociception and Pain

(desensitization) and stimulated by prostaglandin E2 or bradykinin, which is released in response to inflammation (sensitization; !A). Endogenous opioids (e.g., dynorphin, enkephalin, endorphin) and exogenous opioids (e.g., morphium) as well as inhibitors of prostaglandin synthesis (!p. 269) are therefore able to alleviate pain (analgesic action).

Inflammatory sensitization (e.g., sunburn) lowers the threshold for noxious stimuli, leading to excessive

318sensitivity (hyperalgesia) and additional pain resulting from non-noxious stimuli to the skin (allody-

nia), e.g., touch or warm water (37 #C). Once the

nocisensors are stimulated, they start to release neuropeptides such as substance P or CGRP (calcitonin gene-related peptide) that cause inflammation of the surrounding vessels (neurogenic inflammation).

Projected pain. Damage to nociceptive fibers causes pain (neurogenic or neuropathic) that is often projected to and perceived as arising from the periphery. A prolapsed disk compressing a spinal nerve can, for example, cause leg pain. Nociceptive fibers can be blocked by cold or local anesthesia.

Nociceptive tracts (!C1). The central axons of nociceptive somatic neurons and nociceptive afferents of internal organs end on neurons of the dorsal horn of the spinal cord. In many cases, they terminate on the same neurons as the skin afferents.

Referred pain (!B). Convergence of somatic and visceral nociceptive afferents is probably the main cause of referred pain. In this type of pain, noxious visceral stimuli cause a perception of pain in certain skin areas called Head’s zones. That for the heart, for example, is located mainly in the chest region. Myocardial ischemia is therefore perceived as pain on the surface of the chest wall (angina pectoris) and often also in the lower arm and upper abdominal region.

In the spinal cord, the neuroceptive afferents cross to the opposite side (decussation) and are conducted in the tracts of the anterolateral funiculus—mainly in the spinothalamic tract— and continue centrally via the brain stem where they join nociceptive afferents from the head (mainly trigeminal nerve) to the thalamus (!C1). From the ventrolateral thalamus, sensory aspects of pain are projected to S1 and S2 areas of the cortex. Tracts from the medial thalamic nuclei project to the limbic system and other centers.

Components of pain. Pain has a sensory component including the conscious perception of site, duration and intensity of pain; a motor component (e.g., defensive posture and withdrawal reflex; !p. 320), an autonomic component (e.g., tachycardia), and an affective component (e.g., aversion). In addition, pain assessments based on the memory of a previous pain experience can lead to pain-related behavior (e.g., moaning).

In the thalamus and spinal cord, nociception can be inhibited via descending tracts with the aid of various transmitters (mainly opioids). The nuclei of these tracts (!C2, blue) are located in the brain stem and are mainly activated via the nociceptive spinoreticular tract (negative feedback loop).