книги студ / color atlas of physiology 5th ed[1]. (a. despopoulos et al, thieme 2003)

11 Hormones and Reproduction

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Sexual Response, Intercourse and

Fertilization

Sexual response in the male (!A1). Impulses from tactile receptors on the skin in the genital region (especially the glans penis) and other parts of the body (erogenous areas) are transmitted to the erection center in the sacral spinal cord (S2–S4), which conducts them to parasympathetic neurons of the pelvic splanchnic nerves, thereby triggering sexual arousal. Sexual arousal is decisively influenced by stimulatory or inhibitory impulses from the brain triggered by sensual perceptions, imagination and other factors. Via nitric oxide (! p. 278), efferent impulses lead to dilatation of deep penile artery branches (helicine arteries) in the erectile body (corpus cavernosum), while the veins are compressed to restrict the drainage of blood. The resulting high pressure (!1000 mmHg) in the erectile body causes the penis to stiffen and rise (erection). The ejaculatory center in the spinal cord (L2 –L3) is activated when arousal reaches a certain threshold (!A2). Immediately prior to ejaculation, efferent sympathetic impulses trigger the partial evacuation of the prostate gland and the emission of semen from the vas deferens to the posterior part of the urethra. This triggers the ejaculation reflex and is accompanied by orgasm, the apex of sexual excitement. The effects of orgasm can be felt throughout the entire body, which is reflected by perspiration and an increase in respiratory rate, heart rate, blood pressure, and skeletal muscle tone. During ejaculation, the internal sphincter muscle closes off the urinary bladder while the vas deferens, seminal vesicles and bulbocavernous and ischiocavernous muscles contract rhythmically to propel the semen out of the urethra.

Semen. The fluid expelled during ejaculation (2–6 mL) contains 35–200 million sperm in a nutrient fluid (seminal plasma) composed of various substances, such as prostaglandins (from the prostate) that stimulate uterine contraction. Once semen enters the vagina during intercourse, the alkaline seminal plasma increase the vaginal pH to increase sperm motility. At least one sperm cell must reach the ovum for fertilization to occur.

Sexual response in the female (!A2). Due to impulses similar to those in the male, the erectile tissues of the clitoris and vestibule of the vagina engorge with blood during the erection phase. Sexual arousal triggers the release of secretions from glands in the labia minora and transudates from the vaginal wall, both of which lubricate the vagina, and the nipples become erect. On continued stimulation, afferent impulses are transmitted to the lumbar spinal cord, where sympathetic impulses trigger orgasm (climax). The vaginal walls contract rhythmically (orgasmic cuff), the vagina lengthens and widens, and the uterus becomes erect, thereby creating a space for the semen. The cervical os also widens and remains open for about a half an hour after orgasm. Uterine contractions begin shortly after orgasm (and are probably induced locally by oxytocin). Although the accompanying physical reactions are similar to those in the male (see above), there is a wide range of variation in the orgasmic phase of the female. Erection and orgasm are not essential for conception.

Fertilization. The fusion of sperm and egg usually occurs in the ampulla of the fallopian tube. Only a small percentage of the sperm expelled during ejaculation (1000–10 000 out of 107 to 108 sperm) reach the fallopian tubes (sperm ascension). To do so, the sperm must penetrate the mucous plug sealing the cervix, which also acts as a sperm reservoir for a few days. In the time required for them to reach the ampullary portion of the fallopian tube (about 5 hours), the sperm must undergo certain changes to be able to fertilize an ovum; this is referred to as capacitation (!p. 302).

After ovulation (!p. 298ff.) the ovum enters the tube to the uterus (oviduct) via the abdominal cavity. When a sperm makes contact with the egg (via chemotaxis), species-specific sperm-binding receptors on the ovum are exposed and the proteolytic enzyme acrosin is thereby activated (acrosomal reaction). Acrosin allows the sperm to penetrate the cells surrounding the egg (corona radiata). The sperm bind to receptors on the envelope surrounding the ovum (zona pellucida) and enters the egg. The membranes of both cells then fuse. The ovum now undergoes a second meiotic division, which concludes the act of fertilization. Rapid proteolytic changes in the receptors on the ovum (zona pellucida reaction) prevent other sperm from entering the egg. Fertilization usually takes place on the first day after intercourse and is only possible within 24 hours after ovulation.

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

12 Central Nervous System and Senses

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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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12 Central Nervous System and Senses

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