By E. Falk. University of Tulsa.
Some free nerve end- ings are particularly sensitive to tickle and itch cheap levitra 20mg mastercard. Receptors for Heat, Cold, and Pain Root Hair Plexuses The principal receptors for heat and cold (thermoreceptors) and for pain (nociceptors) are the free nerve endings. Several million of Root hair plexuses are a specialized type of free nerve ending. The They are coiled around hair follicles, where they respond to free nerve endings responsible for cold sensations are closer to the movement of the hair. Lamellated Corpuscles Pain receptors respond to damage to tissues and are acti- Lamellated (pacinian) corpuscles are large, onion-shaped recep- vated by all types of stimuli. They are sparse in most visceral or- tors composed of the dendritic endings of several sensory nerve gans and absent entirely within the nervous tissue of the brain. They are commonly Although the free nerve endings are specialized to respond to found within the synovial membranes of synovial joints, in the tissue damage, all of the cutaneous receptors will relay impulses perimysium of skeletal muscle tissue, and in certain visceral or- that are interpreted as pain if stimulated excessively. Lamellated corpuscles are also abundant in the skin of the The protective value of pain receptors is obvious. Unlike palms and fingers of the hand, soles of the feet, external geni- other cutaneous receptors, free nerve endings exhibit little accom- talia, and breasts. They respond to heavy pressures, generally modation, so impulses are relayed continuously to the CNS as long those that are constantly applied. Muscle spasms, muscle fatigue, or an inadequate supply of blood to an organ may also cause pain. Organs of Ruffini Impulses for pain are conducted to the spinal cord through sen- sory neurons.
As the peripheral nervous system is easily accessible and end-points are simple to measure (salivation buy discount levitra 20 mg on-line, blood pressure, etc. Thus, VIP and acetylcholine interact to produce the full integrated salivation and vasodilatation that occur following stimulation of the parasympathetic supply to the salivary glands. Stimulation of the nerve causes secretion and vasodilatation Ð the former is muscarinic since it is blocked by atropine. When administered exogenously the same pattern is seen; ACh causes secretion and VIP dilates. When co-applied, both secretion and vasodilatation are potentiated and this may partly be due to VIP increasing muscarinic binding of ACh, an effect that has also been seen in the cortex. Noradrenaline and neuropeptide Y also have similar interactive effects on the vasculature. Thus, the vasoconstriction produced by both sympathetic stimulation and exogenous noradrenaline is enhanced in the presence of NPY. Co-existence of more than one neurotransmitter in a neuron is likely to be common in CNS neurons but only a small proportion of the billions of neurons have been investigated. The consequences of co-existence are considerable since the two or more transmitters can:. Alter breakdown 256 NEUROTRANSMITTERS, DRUGS AND BRAIN FUNCTION The combinations of co-existence within a single neuron that have been seen in the CNS are: (1) Different peptides from the same gene product (met and leu enkephalin, substance P and neurokinin A). The former two act on the same receptor, the delta opioid receptor, whereas the latter act on different receptors, the neurokinin 1 and 2 receptors. Despite this, the receptors for the neurokinins produce the same direction of effect, a slow depolarisation, even though their distribution differs. These combinations are intriguing in that TRH is excitatory yet enkephalins are inhibitory Ð complex postsynaptic effects can be envisaged. Substance P is excitatory and CCK acts on two receptors, A and B, with the former being the predominant CNS form.
It does this at a region of the receptor membrane (4) called the impulse initiation region (or coding region) be- Figure 4 order levitra 10 mg otc. The threshold (colored line) is a critical produce action potentials at a frequency related to the level of depolarization; membrane potential changes be- strength of the current caused by the stimulus. These cur- low this level are caused by the local excitatory currents rents, called local excitatory currents, provide the link be- and vary in proportion to them, while the membrane ac- tween the formation of the generator potential and the ex- tivity above the threshold level consists of locally pro- citation of the nerve fiber membrane. The lower trace shows a series of In complex sensory organs that contain a great many in- different stimuli applied to the receptor, and the upper dividual receptors, the generator potential may be called a trace shows the resulting electrical events in the impulse receptor potential, and it may arise from several sources initiation region. Often the receptor potential is given a No stimulus is given at A, and the membrane voltage is at special name related to the function of the receptor; for ex- the resting potential. At B, a small stimulus is applied, pro- ample, in the ear it is called the cochlear microphonic, ducing a generator potential too small to bring the impulse while an electroretinogram may be recorded from the eye. As in all ex- importance in the transduction process because it is the citable and all-or-none nerve membranes, the action poten- step in which information related to stimulus intensity and tial is immediately followed by repolarization, often to a duration is transduced. The strength (intensity) of the stim- level that transiently hyperpolarizes the membrane poten- ulus applied (in Fig. Since the brief stimulus has been removed by this time, no Varying the intensity of the stimulation will correspond- further action potentials are produced. A longer stimulus of ingly vary the generator potential, although the changes the same intensity (D) produces repetitive action potentials will not usually be directly proportional to the intensity. C, A brief, but intense, stimulus can cause a single action po- lus intensities and durations. D, Maintaining this stimulus leads to a train of action po- stimulus, the membrane is at rest.
These nuclei within the hypothalamus are ter gland buy 10mg levitra mastercard, because secretion of its hormones is in turn controlled thus endocrine glands; the hormones they produce are trans- by hormones secreted by the hypothalamus. Endocrine System © The McGraw−Hill Anatomy, Sixth Edition Coordination Companies, 2001 464 Unit 5 Integration and Coordination TABLE 14. Neurons in the hypothalamus secrete hormones into explain how the hypothalamus controls the secretion of the region of the median eminence, where they enter a net- each hormone. Which hormone secreted by the anterior pituitary does not through the pituitary stalk transports the hypothalamic hor- affect some other endocrine gland? Which hormone secreted by the pituitary gland affects scribed in chapter 16. Endocrine System © The McGraw−Hill Anatomy, Sixth Edition Coordination Companies, 2001 Chapter 14 Endocrine System 465 FIGURE 14. Endocrine System © The McGraw−Hill Anatomy, Sixth Edition Coordination Companies, 2001 466 Unit 5 Integration and Coordination FIGURE 14. Hypothalamic hormones (shown as dots) enter this system in the first set of capil- laries of the median eminence and are transported through the portal veins of the pituitary stalk to a second set of capillaries of the anterior pituitary. Description of the Thyroid Gland THYROID AND PARATHYROID GLANDS The thyroid gland is located in the neck, just below the larynx (fig. The thyroid is the are critically important for proper growth and development. It thyroid also secretes calcitonin, which may antagonize the action receives an abundant blood supply (80–120 ml/min) through the of parathyroid hormone in the regulation of calcium and phos- paired superior thyroid branches of the external carotid arteries phate balance. The venous return is through the paired superior and middle of the thyroid gland and list the actions of the thyroid hormones.