Chapter 13 PNS and reflex activity I. Peripheral nervous system A. PNS links CNS to the body B. Sensory: the afferent division C. Motor: the efferent division D. Ganglia: collections of cell bodies in the PNS II. Sensory receptors A. Terminology 1. Stimulus: an environmental change (internal or external) 2. Sensation: CNS awareness of stimuli (these may be subconscious) 3. Perception: conscious interpretation of stimuli B. Classification by stimulus type 1. Mechanoreceptors: respond to mechanical deformation 2. Thermoreceptors: temperature 3. Photoreceptors: light 4. Chemoreceptors: chemicals 5. Nociceptors: respond to damaging stimuli C. Classification by location 1. Exteroceptors: sense external sensations 2. Interoceptors(visceroceptors): sense internal sensations a. Proprioceptors: info to cerebellum about position
D. Classification by structural complexity 1. Simple: found throughout body; they monitor general processes 2. Complex: special sense organs E. Simple sensory receptors 1. Free sensory endings (naked dendritic) a. Merkel discs: light touch found in deeper epidermis b. Hair follicle receptors: wrap hair follicles, light touch (bending) c. Itch receptors: responds to histamine 2. Encapsulated endings: C.T. capsule around dendritic endings a. Meisner corpuscles b. Pacinian corpuscles c. Ruffini's corpuscles d. Proprioceptors: report body position 1) Muscle spindle: reports skeletal muscle stretch 2) Golgi tendon organ: reports tendon stretch 3) Joint kinesthetic receptor: reports joint position III. Overview: from sensation to perception A. 3 processing levels in any sensory system 1. Receptor level: occurs at the level of sensory receptors 2. Circuit level: pathways leading to higher level processing circuits 3. Perceptual: complex circuits that allow us to form perceptions
B. Processing at the receptor level 1. Transduction: converting the stimulus to electrical potential 2. Receptor potential: a graded potential is produced 3. Generator potential: occurs when the receptor potential reaches threshold value 4. Stronger stimuli: cause the delivery of more impulses per second to the brain C. Processing at circuit level: 2 main routes to reach the sensory cortex 1. Nonspecific ascending (antero-lateral) pathways: used by many different receptor types 2. Specific ascending (posterior) pathways: few receptor types use these D. Processing at the perceptual level: the highest and final stage of sensory processing 1. Perceptual detection
2. Magnitude estimation 3. Spatial discrimination a. Two-point discrimination test: how close 2 points perceived as 2 distinct points 4. Feature abstraction: the interaction of several different stimuli coordinates the perception a. Texture of marble 5. Quality discrimination: a sensory mode that has several sub-modes or qualities a. Taste: has sub-modes: sweet, sour, salty, bitter, umami 6. Pattern recognition: the ability to recognize different patterns IV. Nerves and ganglia A. Terminology 1. Nerve: a bundle of axons in the PNS
2. Ganglion: a cluster of cell bodies in the PNS B. Connective tissue wrappings of a nerve 1. Endoneurium: surrounds individual nerve fibers (cells) 2. Perineurium: surrounds bundles of fibers (fascicles) 3. Epineurium: covers entire nerve (bundle of fascicles) C. Types of nerves 1. Some nerves are sensory only 2. Other nerves are motor only 3. Mixed nerves: contain both sensory and motor axons a. Types D. Ganglia 1) Somatic: afferent and efferent 2) Visceral: afferent and efferent 1. Afferent: dorsal root ganglion 2. Efferent: are part of autonomic N.S.: prevertebral, paravertebral, terminal E. Regeneration of nerve tissue: possible under certain circumstances 1. Repair steps: after an axon has just been damaged a. Cut ends: seal themselves spontaneously
b. Axon distal to cut dies: (Wallerian degeneration) c. Schwann cells and macrophages: clean up the debris d. Schwann cells: form a regrowth tunnel to guide the axon sprout back to its original site e. Growth rate is 1.5 mm per day 2. The farther the damage is from the cell body, the more likely successful repair will be made. 3. If sprout has to travel too far, may escape from tunnel into surrounding tissue (neuroma) 4. CNS axons do not repair: because oligodendrocytes do not form regrowth tunnels and they release growth inhibiting factors
V. Cranial nerves: 12 pair, all except vagus serve the head and neck only A. Olfactory (I) B. Optic (II) C. Occulomotor (III) D. Trochlear (IV) E.. Trigeminal (V) Opthalmic 2. Maxillary 3. Mandibular F. Abducens (VI) G. Facial nerves (VII) H. Vestibulocochlear (VIII) I. Glossopharyngeal (IX) J. Vagus (X): The only cranial nerve to extend beyond head to almost all internal organs
K. Accessory (XI) L. Hypoglossal (XII) VI. Spinal nerves: 31 pairs: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal A. Distribution of spinal nerves 1. Each spinal nerve: arises from the junction of a dorsal and a ventral root a. Dorsal root: sensory only b. Ventral root: motor only 1) Immediately after exiting intervertebral foramina: spinal nerves split into dorsal and ventral rami a) Ventral rami: serve most of the body b) Dorsal rami: serve muscles and skin of the back
B. Plexuses: branching networks formed by the junction of adjacent ventral rami 1. Cervical plexus: serves mostly the head and neck a. Phrenic nerve
2. Brachial plexus: serves the upper limb a. Median b. Ulnar c. Radial 3. Lumbar plexus: serves the lower limb a. Femoral 4. Sacral plexus: serves the lower limb a. Sciatic VII. Overview: Motor integration: from intent to effect A. Levels of motor control 1. Segmental level: the lowest level: spinal cord segments contain circuits a. Central pattern generators (CPGs): are circuits that produce fixed action patterns 1) Walking CPGs: reverbrating circuits 2. Projection level: the next higher level a. Command neurons: interneurons that start or inhibit a CPG b. Pathways that the projection level uses 1) Direct (pyramidal tract) system: command neurons most commonly use this pathway 2) Indirect (extrapyramidal) system: arise from several brain stem nuclei 3. Pre-command level: allows us to precisely start/stop motor activity a. Cerebellum: the main integration center for precise motor control b. Basal nuclei: relay information from cortex to cerebellum in the planning and initiation of motor activity
VIII. Reflex activity A. Terminology 1. Ipsilateral: control of the same side of the body 2. Contralateral: control of the opposite side of the body B. Spinal reflexes: the spinal cord is integration center 1. Somatic reflexes: activate skeletal muscle 2. Autonomic reflexes: activate smooth, cardiac, glands 3. Five components of a reflex arc a. Receptor: transduces the stimulus into an electrical impulse b. Sensory neuron: conducts the impulse to the CNS c. Integration center: within the CNS, this is the control center d. Motor neuron: conducts impulses from the CNS to muscle or gland e. Effector: muscle or gland 4. Stretch and deep tendon reflex: (receptors: muscle spindle and Golgi tendon organ) a. Stretch reflex: uses muscle spindles
b. Deep tendon: uses Golgi tendon organs 5. Flexor reflex: withdraw reflex (painful stimulus) 6. Crossed-extensor reflex