Chapter 13 The Peripheral Nervous System and Reflex Activity Part 1 Sensory Receptors and Sensation I Sensory Receptors pp 491 494 Fig 13 1 Table 13 1 A Sensory receptors are specialized to respond to changes in their environment called stimuli pp 491 494 1 Receptors may be classified according to the activating stimulus 2 Receptors may be classified based on their location or the location of the activating stimulus 3 Receptors may be classified based on their overall structural complexity B Free or naked nerve endings are present everywhere in the body and respond primarily to pain and temperature p 492 C Encapsulated Dendritic Endings pp 492 494 Table 13 1 1 Meissner s corpuscles are receptors for discriminatory and light touch in hairless areas of the body 2 Pacinian or lamellated corpuscles are stimulated when deep pressure is first applied 3 Ruffini endings respond to deep and continuous pressure 4 Muscle spindles detect when a muscle is being stretched and initiate a reflex that resists the stretch 5 Golgi tendon organs are stimulated when the associated muscle stretches the tendon 6 Joint kinesthetic receptors monitor the stretch in the articular capsules of synovial joints II Overview From Sensation to Perception pp 494 498 Fig 13 2 A The somatosensory system the part of the sensory system serving the body wall and limbs involves the receptor level the circuit level and the perceptual level 1 Processing at the receptor level involves a stimulus that must excite a receptor in order for sensation to occur 2 Processing at the circuit level is involved with delivery of impulses to the appropriate region of the cerebral cortex for stimulus localization and perception 3 Processing at the perceptual level involves interpretation of sensory input in the cerebral cortex Part 2 Transmission Lines Nerves and Their Structure and Repair I Nerves and Associated Ganglia pp 498 500 Figs 13 3 13 4 A A nerve is a cordlike organ consisting of parallel bundles of peripheral axons enclosed by connective tissue wrappings B Ganglia are collections of neuron cell bodies associated with nerves in the PNS C If damage to a neuron occurs to the axon and the cell body remains intact cut or compressed axons can regenerate II Cranial Nerves pp 500 508 Fig 13 5 Table 13 2 A Olfactory nerves are responsible for smell B Optic nerves are responsible for vision C Oculomotor nerves play a role in eye movement D Trochlear nerves play a role in eye movement E Trigeminal nerves are general sensory nerves of the face F Abducens nerves play a role in eye movement G Facial nerves function as the chief motor nerves of the face H Vestibulocochlear nerves are responsible for hearing and equilibrium I Glossopharyngeal nerves innervate part of the tongue and pharynx J Vagus nerves innervate the heart lungs and the abdominal organs K Accessory nerves move structures associated with the head and neck L Hypoglossal nerves are mixed nerves that arise from the medulla and serve the tongue III Spinal Nerves pp 508 518 Figs 13 6 13 12 Tables 13 3 13 6 A Thirty one pairs of mixed spinal nerves arise from the spinal cord and serve the entire body except the head and neck B Innervation of Specific Body Regions 1 Each spinal nerve connects to the spinal cord by a dorsal root and a ventral root 2 Rami lie distal to and are lateral branches of the spinal nerves that carry both motor and sensory fibers 3 The back is innervated by the dorsal rami with each rami innervating the muscle in line with the point of origin from the spinal column 4 Only in the thorax are the ventral rami arranged in a simple segmental pattern corresponding to that of the dorsal rami 5 The cervical plexus is formed by the ventral rami of the first four cervical nerves 6 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 7 The sacral and lumbar plexuses overlap and because many fibers of the lumber plexus contribute to the sacral plexus via the lumbosacral trunk the two plexuses are often referred to as the lumbosacral plexus 8 The area of skin innervated by the cutaneous branches of a single spinal nerve is called a dermatome 9 Hinton s law states that any nerve serving a muscle that produces movement at a joint also innervates the joint and the skin over the joint Part 3 Motor Endings and Motor Activity I Peripheral Motor Endings p 519 A Peripheral motor endings are the PNS element that activates effectors by releasing neurotransmitters B The terminals of the somatic motor fibers that innervate voluntary muscles form elaborate neuromuscular junctions with their effector cells and they release the neurotransmitter acetylcholine C The junctions between autonomic motor endings and the visceral effectors involve varicosities and release either acetylcholine or epinephrine as their neurotransmitter II Overview of Motor Integration From Intention to Effect pp 519 521 Fig 13 13 A Levels of Motor Control 1 The segmental level is the lowest level on the motor control hierarchy and consists of the spinal cord circuits 2 The projection level has direct control of the spinal cord 3 The precommand level is made up of the cerebellum and the basal nuclei and is the highest level of the motor system hierarchy Part 4 Reflex Activity I The Reflex Arc pp 521 522 Fig 13 14 A Reflexes are unlearned rapid predictable motor responses to a stimulus and occur over highly specific neural pathways called reflex arcs pp 521 522 Fig 13 14 II Spinal Reflexes pp 522 527 Figs 13 15 13 19 A Spinal reflexes are somatic reflexes mediated by the spinal cord pp 522 527 Figs 13 15 13 19 1 In the stretch reflex the muscle spindle is stretched and excited by either an external stretch or an internal stretch 2 The Golgi tendon reflex produces muscle relaxation and lengthening in response to contraction 3 The flexor or withdrawal reflex is initiated by a painful stimulus and causes automatic withdrawal of the threatened body part from the stimulus 4 The crossed extensor reflex is a complex spinal reflex consisting of an ipsilateral withdrawal reflex and a contralateral extensor reflex 5 Superficial reflexes are elicited by gentle cutaneous stimulation 6 Plantar reflex test L4 S4 spinal integrity a Normal response is downward flexion or curling of toes b Babinski s Sign is fanning of toes and dorsiflexion of big toe i normal for infants up to about 1 year due to incomplete myelination of NS III