The Peripheral Nervous System Nervous structures outside the brain and spinal cord Nerves allow the CNS to receive information and take action Functional components of the PNS Sensory inputs and motor outputs Categorized as somatic or visceral Sensory inputs also classified as general or special

The Peripheral Nervous System Autonomic nervous system (ANS) General visceral motor part of the PNS ANS has two divisions Parasympathetic Sympathetic

Functional Organization of the PNS Figure 14.1

Basic Structural Components of the PNS Sensory receptors – pick up stimuli from inside or outside the body Motor endings – axon terminals of motor neurons Innervate effectors (muscle fibers and glands) Nerves and ganglia Nerves – bundles of peripheral axons Ganglia – clusters of peripheral neuronal cell bodies

Basic Anatomical Scheme of the PNS in the Region of a Spinal Nerve Figure 14.2

Peripheral Sensory Receptors Structures that pick up sensory stimuli Initiate signals in sensory axons Two main categories of sensory receptors Free nerve endings of sensory neurons Monitor general sensory information Complete receptor cells – specialized epithelial cells or small neurons Monitor most types of special sensory information

Peripheral Sensory Receptors Sensory receptors also classified according to: Location Type of stimulus detected Structure

Classification by Location Exteroceptors – sensitive to stimuli arising from outside the body Located at or near body surfaces Include receptors for touch, pressure, pain, and temperature Interoceptors – (visceroceptors) receive stimuli from internal viscera Monitor a variety of stimuli Proprioceptors – monitor degree of stretch Located in musculoskeletal organs

Classification by Stimulus Detected Mechanoreceptors – respond to mechanical forces Thermoreceptors – respond to temperature changes Chemoreceptors – respond to chemicals in solution Photoreceptors – respond to light – located in the eye Nociceptors – respond to harmful stimuli that result in pain

Classification by Structure General sensory receptors Widely distributed Nerve endings of sensory neurons monitor: Touch, pressure, vibration, stretch Pain, temperature, proprioception Divided into two groups Free nerve endings Encapsulated nerve endings

Abundant in epithelia and underlying connective tissue Free Nerve Endings Abundant in epithelia and underlying connective tissue Respond to pain and temperature Monitor affective senses Two specialized types of free nerve endings Merkel discs – lie in the epidermis Slowly adapting receptors for light touch Hair follicle receptors – wrap around hair follicles Rapidly adapting receptors

Unencapsulated Nerve Endings Table 14.1

Encapsulated Nerve Endings Consist of one or more end fibers of sensory neurons Enclosed in connective tissue Mechanoreceptors Include four main types

Encapsulated Nerve Endings Meissner’s corpuscles Spiraling nerve ending surrounded by Schwann cells Occur in the dermal papillae Rapidly adapting receptors for discriminative touch Occur in sensitive, hairless areas of the skin

Meissner’s Corpuscles Table 14.1

Encapsulated Nerve Endings Pacinian corpuscles Single nerve ending surrounded by layers of flattened Schwann cells Occur in the hypodermis Sensitive to deep pressure – rapidly adapting receptors Ruffini’s corpuscles Located in the dermis and respond to pressure Monitor continuous pressure on the skin – adapt slowly

Pacinian Corpuscles and Ruffini’s Corpuscles Table 14.1

Encapsulated Nerve Endings Proprioceptors Monitor stretch in locomotory organs Three types of proprioceptors

Three Types of Proprioceptors Muscle spindles – measure the changing length of a muscle Imbedded in the perimysium between muscle fascicles Golgi tendon organs – located near the muscle-tendon junction Monitor tension within tendons Joint kinesthetic receptors Sensory nerve endings within the joint capsules

Proprioceptors Table 14.1

Structure of Proprioceptors Figure 14.4