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

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