Lecture 14Outline of Last Lecture I. Cross-Sections through the braina. Generalb. Myelencephalonc. Metencephalond. Mesencephalone. Diencephalonf. Telencephalon Outline of Current LectureI. Overview and Gross Anatomy Systema. Divisionsb. Spinal nervesc. true proportionsd. nerve plexusII. Classifying Input and Output III. Sensory Part of the PNSIV. Motor Part of the PNSCurrent LectureI. Overview and Gross Anatomy Systema. Divisionsi. The PNS can be divided into cranial and spinal partsii. Cranial nerves: attach to the brain and mainly innervate the headiii. Spinal nerves: attach to the spinal cord and their branches mainly innervate bodyregions from the neck downb. Spinal nerves: components and branchesi. About 30 pairs of spinal nerves spaced at regular intervals along the length of thespinal cord, divided into series of cervical, thoracic, lumbar, sacral, and coccygealnerves.ii. Each spinal nerve is attached to the spinal cord by dorsal and ventral rootsiii. Each dorsal root is sensory, consisting of the central processes of many sensory neurons iv. Along the dorsal root is a dorsal ganglion, composed of cell bodies of sensory neurons v. Each ventral root is composed of axons of motor neurons vi. Lateral to spinal cord, the dorsal and ventral roots come together to form a spinal nerve , which contains both sensory and motor fibers vii. Spinal nerves are shortviii. Divide into 2 parts laterally, these rami each contain both sensory and motor fibers, they do go to different regions of the body1. dorsal rami  innervate the muscles and skin of the back BIOL_315 1nd Edition2. ventral rami  muscles of the side and anterior of the ramia. the limbs are nearer to the anterior of the body so they are therefore innervated by the ventral rami3. the dorsal and ventral rami and their branches of the outer somatic, part of the body4. the sensory and motor axons to the visceral organs, by contrast, run in different rami, which hang ventrally from the spinal nervesa. most are called rami communicantes to the sympathetic gangliac. true proportionsi. dorsal and ventral roots are rather short and confined within the vertebral canal of the vertebral column ii. spinal nerves lie close to the spinal cordiii. within the intervertebral foraminaiv. note the dorsal root ganglion is at the extreme lateral edge of each dorsal root, very near the spinal nerve and within the intervertebral foramen v. dorsal and ventral roots actually form from bundles of rootlets; these rootlets attach continuously along the length of the spinal cordd. nerve plexus i. nerve plexus: where nerves branch ii. the largest nerve plexuses occur where consecutive ventral rami exchange some nerve fibersiii. we have 3 plexus:1. cervical plexus (C1-C4)2. brachial plexus (C5-T1)3. lumbo-sacral plexus (T12-S4)iv. all dorsal rami remain separate from one another and show a clean segmental arrangementv. as do the ventral rami in the thorax, where there is no plexusvi. difficult to describe the functional significance of the plexus, but may be an efficient way to arrange the nerve fibers to the limbs II. Classifying inputs and outputs carried by the sensory and motor axons in the PNSa. Sensory and motor systems are further classified as general vs. special i. General inputs or outputs: are widely distributed, over the entire body or large regions of the bodyii. Special inputs or outputs: localized to small regionsb. General somatic senses can be experienced anywhere over large areasi. Such as all the skin sensesc. Special somatic sensesi. Have receptors in the outer body that are localized d. General visceral sensesi. Felt at many locations within the different visceral organs1. Stretch, pain, temperature withine. Special visceral senses i. Localized distribution1. Taste, because taste receptors only on the tongue and some surrounding structuresf. Smell is hard to classify, special somatic sense or special visceral senseg. Motor i. Special visceral motor innervation of the pharyngeal muscles is now called brachial motor and the special visceral sensory category for taste is simply tasteIII. Sensory Part of the PNSa. Receptorsi. Pick up, or receive, the sensory stimuli ii. Receptors for special senses differ from those for general sense iii. Receptors for special sense are distinct receptor cellsiv. Receptors for general sense are branched peripheral ending (dendrites) of sensory neurons v. General receptors are widely distributed in epithelia and the underlying connective tissue vi. Another way to classify the sensory receptors, their distribution1. Exteroceptors: monitor external stimuli from the outside world and have a superficial location on the body surface2. Enteroceptors: monitor internal stimuli and occur in the walls of visceral organs3. Proprioceptors: receptors that monitor stretch in the tendons, muscles, and jointsvii. The way to classify the general sensory receptors is by whether or not their nerveendings are surrounded by a distinct capsule of connective tissueviii. Non-encapsulated (unencapsulated) endings: penetrate an epithelium, lying between the individual epithelial cells, or they can end in the underlying connective tissue , usually the most sensitive to pain, itch, and perhaps, temperatureix. Encapsulated receptors: all types of such are sensory nerve endings surrounded by a bulbous layer of connective tissue, and each responds to its own type of mechanical stimulus1. Movement of the capsule may somehow act to amplify such stimuli and filter out stimuli to which the receptor is no designed to respond2. Meissner’s corpuscles: encapsulated receptors specialized for fine touch a. Confined to the dermis o the hairless skin, these occupy dermal papillae b. Within, the nerve endings spiral into a spring3. Pacinian corpuscle a. This receptor picks up deep-pressure and vibration stimuli b. Surrounding the nerve ending is connective-tissue capsule and many concentric layers of flattened Schwann cells c. Large, several millimeters in diameterd. Deeply located in hypodermis and other deep connective tissues of the body as well as in ligaments and joint capsules. 4. Proprioceptors: encapsulated nerve endings that monitor stretch in tendons, muscles, and joint capsules5. Golgi tendon organs: receptor for tendon stretch, function to keep muscles from pulling so hard that they rip the tendon out of its bony insertion6. Muscle spindles: measure the stretching of skeletal muscles, reasons to knowa. Moving the brain