Chpt 13 handout/ Homeostatic imbalance/ T and F

Terms in this set (44)

The main aspects of sensory perception include the following:
■ Perceptual detection is the ability to detect that a stimulus has occurred. This is the simplest level of perception. As a general rule, inputs from several receptors must be summed for perceptual detection to occur.
■ Magnitude estimation is the ability to detect how intense the stimulus is. Because of frequency coding, perception in- creases as stimulus intensity increases
■ Spatial discrimination allows us to identify the site or pattern of stimulation. A common tool for studying this quality in the laboratory is the two-point discrimination test. The test determines how close together two points on the skin can be and still be perceived as two points rather than as one. This test provides a crude map of the density of tactile receptors in the various regions of the skin. The distance between perceived points varies from less than 5 mm on highly sensitive body areas (tip of the tongue) to more than 50 mm on less sensitive areas (back).
■ Feature abstraction is the mechanism by which a neuron or circuit is tuned to one feature in preference to others. Sensation usually involves an interplay of several stimulus properties or features. For example, one touch tells us that velvet is warm, compressible, and smooth but not completely continuous, each a feature that contributes to our perception of "velvet." Feature abstraction enables us to identify more complex aspects of a sensation.
■ Quality discrimination is the ability to differentiate the submodalities of a particular sensation. Each sensory modality has several qualities, or submodalities. For example, the sub-modalities of taste include sweet and bitter.
■ Pattern recognition is the ability to take in the scene around us and recognize a familiar pattern, an unfamiliar one, or one that has special significance for us. For example, a figure made of dots may be recognized as a familiar face, and when we listen to music, we hear the melody, not just a string of notes.
Short version:
1. The axon becomes fragmented at the injury site. (the injury site begin to disintegrate because they cannot receive nutrients from the cell body this process, Wallerian degeneration,)
2. Macrophages clean out the dead axon distal to the injury. (Schwann cells proliferate in response to mitosis-stimulating chemicals released by the macrophages)
3. Axon sprouts, or filaments, grow through a regeneration tube formed by Schwann cells.
4. The axon regenerates and a new myelin sheath forms.

Longer version:
Almost immediately after a peripheral axon has been severed or crushed, the separated ends seal themselves off and then swell as substances being transported along the axon begin to accu- mulate in the sealed ends. Within a few hours, the axon and its myelin sheath distal to the injury site begin to disintegrate because they cannot receive nutrients from the cell body (Fig- ure 13.4, ). This process, Wallerian degeneration, spreads distally from the injury site, completely fragmenting the axon. Generally, the entire axon distal to the injury is degraded by phagocytes within a week, but the neurilemma remains intact within the endoneurium (Figure 13.4, ). After the debris has been disposed of, surviving Schwann cells proliferate in response to mitosis-stimulating chemicals released by the macrophages, and migrate into the injury site. Once there, they release growth factors and begin to express cell adhesion mole- cules (CAMs) that encourage axonal growth. Additionally, they form a regeneration tube, a system of cellular cords that guide the regenerating axon "sprouts" across the gap and to their original contacts (Figure 13.4, and ). The same Schwann cells protect, support, and remyelinate the regenerating axons.
Changes also occur in the neuronal cell body after the axon has been destroyed. Within two days, its chromatophilic substance breaks apart, and then the cell body swells as protein synthesis revs up to support regeneration of its axon.
Axons regenerate at the approximate rate of 1.5 mm a day. The greater the distance between the severed endings, the less the chance of recovery because adjacent tissues block growth by pro- truding into the gaps, and axonal sprouts fail to find the regener- ation tube.
I. Olfactory. These are the tiny sensory nerves (filaments) of smell, which run from the nasal mucosa to synapse with the olfactory bulbs.
Olfactory nerve fibers arise from olfactory receptor cells located in olfactory epithelium of nasal cavity and pass through cribriform plate of ethmoid bone to synapse in olfactory bulb.

II. Optic. Because this sensory nerve of vision develops as an outgrowth of the brain, it is really a brain tract.

Origin and course: Fibers arise from retina of eye to form optic nerve, which passes through optic canal of orbit. The optic nerves converge to form the optic chiasma where fibers partially cross over, continue on as optic tracts, enter thalamus, and synapse there. Thalamic fibers run (as the optic radi- ation) to occipital (visual) cortex, where visual interpretation occurs

III. Oculomotor. The name oculomotor means "eye mover." This nerve supplies four of the six extrinsic muscles that move the eyeball in the orbit.
Origin and course: Fibers extend from ventral midbrain (near its junction with pons) and pass through bony orbit, via superior orbital fissure, to eye.

IV. Trochlear.
The name trochlear means"pulley"and it innervates an extrinsic eye muscle that loops through a pulley-shaped ligament in the orbit.

Origin and course: Fibers emerge from dorsal midbrain and course ventrally around midbrain to enter orbit through superior orbital fissure along with oculomotor nerves.

V. Trigeminal. Three (tri) branches spring from this, the largest of the cranial nerves. It supplies sensory fibers to the face and motor fibers to the chewing muscles.
Largest of cranial nerves; fibers extend from pons to face, and form three divisions (trigemina threefold): ophthalmic, maxillary, and mandibular divisions. As major general sensory nerves of face, transmit afferent impulses from touch, temperature, and pain receptors. Cell bodies of sensory neurons of all three divisions are located in large trigeminal ganglion.

VI. Abducens. This nerve controls the extrinsic eye muscle that abducts the eyeball (turns it laterally).
Origin and course: Fibers leave inferior pons and enter orbit via superior orbital fissure to run to eye.

VII. Facial. A large nerve that innervates muscles of facial expression (among other things).
Origin and course: Fibers issue from pons, just lateral to abducens nerves (see Figure 13.5), enter temporal bone via internal acoustic meatus, and run within bone (and through inner ear cavity) before emerging through stylomastoid foramen; nerve then courses to lateral aspect of face.

VIII. Vestibulocochlear. This sensory nerve for hearing and balance was formerly called the auditory nerve.
Origin and course: Fibers arise from hearing and equilibrium apparatus located within inner ear of temporal bone and pass through internal acoustic meatus to enter brain stem at pons-medulla border. Afferent fibers from hearing receptors in cochlea form the cochlear division; those from equilibrium receptors in semicircular canals and vestibule form the vestibular division (ves- tibular nerve); the two divisions merge to form vestibulocochlear nerve

IX. Glossopharyngeal. The name glossopharyngeal means "tongue and pharynx," and reveals the structures that this nerve helps to innervate.
Origin and course: Fibers emerge from medulla and leave skull via jugular foramen to run to throat.
Function: Mixed nerves that innervate part of tongue and pharynx. Provide somatic motor fibers to, and carry proprioceptor fibers from, a superior pharyngeal muscle called the stylopharyngeus, which ele- vates the pharynx in swallowing. Provide parasympathetic motor fibers to parotid salivary glands (some of the nerve cell bodies of these parasympathetic motor neurons are located in otic ganglion).

X. Vagus.
This nerve's name means "wanderer" or "vagabond," and it is the only cranial nerve to extend beyond the head and neck to the thorax and abdomen.
Origin and course: The only cranial nerves to extend beyond head and neck region. Fibers emerge from medulla, pass through skull via jugular foramen, and descend through neck region into thorax and abdomen.

XI. Accessory. Considered an accessory part of the vagus nerve, this nerve was formerly called the spinal accessory nerve.
Origin and course: Unique in that they are formed from ventral rootlets that emerge from the spinal cord, not the brain stem. These rootlets arise from superior region (C1-C5) of spinal cord, pass upward along spinal cord, and enter the skull as the accessory nerves via fora- men magnum. The accessory nerves exit from skull through jugular foramen together with the vagus nerves, and supply two large neck muscles. Until recently, it was thought that the accessory nerves also received a contribution from cranial rootlets, but it has now been de- termined that in almost all people, these cranial rootlets are instead part of the vagus nerves. This raises an interesting question: Should the accessory nerves still be considered cranial nerves? Some anato- mists say "yes" because they pass through the cranium. Others say "no" because they don't arise from the brain. Stay tuned!

XII. Hypoglossal. The name hypoglossal means under the tongue. This nerve runs inferior to the tongue and innervates the tongue muscles.
Origin and course: As their name implies (hypo below; glossal tongue), hypoglossal nerves mainly serve the tongue. Fibers arise by a series of roots from medulla and exit from skull via hypoglossal canal to travel to tongue.
Major Plexuses: (nerves)
1. The cervical plexus is formed by the ventral rami of the first four cervical nerves.ts branches are summa- rized in Table 13.3. Most branches are cutaneous nerves that supply only the skin. They transmit sensory impulses from the skin of the neck, the ear area, the back of the head, and the shoulder. Other branches innervate muscles of the anterior neck.

2. The brachial plexus is situated partly in the neck and partly in the axilla and gives rise to virtually all the nerves that innervate the upper limb. It can be palpated (felt) in a living person just superior to the clavicle at the lateral border of the sternocleidomastoid muscle.

3. The sacral and lumbar plexuses overlap and because many fibers of the lumbar plexus contribute to the sacral plexus via the lumbosacral trunk, the two plexuses are often referred to as the lumbosacral plexus.

Lumbar Plexus The lumbar plexus arises from the spinal nerves L1-L4 and lies within the psoas major muscle (Figure 13.10). Its proximal branches innervate parts of the abdominal wall muscles and the psoas muscle, but its major branches descend to in- nervate the anterior and medial thigh.

Sacral Plexus The sacral plexus arises from spinal nerves L4-S4 and lies immediately caudal to the lumbar plexus (Figure 13.11). Some fibers of the lumbar plexus contribute to the sacral plexus via the lumbosacral trunk, as mentioned earlier. The sacral plexus has about a dozen named branches. About half of these serve the buttock and lower limb; the others innervate pelvic structures and the perineum. The most important branches are described here. Table 13.6 summarizes all but the smallest ones.