13 The Peripheral Nervous System and Reflex Activity: Part A Peripheral Nervous System (PNS) • All neural structures outside the brain • Sensory receptors • Peripheral nerves and associated ganglia • Motor endings Sensory Receptors • Specialized to respond to changes in their environment (stimuli) • Activation results in graded potentials that trigger nerve impulses • Sensation (awareness of stimulus) and perception (interpretation of the meaning of the stimulus) occur in the brain Classification of Receptors • Based on: • Stimulus type • Location • Structural complexity Classification by Stimulus Type • Mechanoreceptors—respond to touch, pressure, vibration, stretch, and itch • Thermoreceptors—sensitive to changes in temperature • Photoreceptors—respond to light energy (e.g., retina) • Chemoreceptors—respond to chemicals (e.g., smell, taste, changes in blood chemistry) • Nociceptors—sensitive to pain-causing stimuli (e.g. extreme heat or cold, excessive pressure, inflammatory chemicals) Classification by Location 1.Exteroceptors • Respond to stimuli arising outside the body • Receptors in the skin for touch, pressure, pain, and temperature • Most special sense organs Classification by Location 2.Interoceptors (visceroceptors) • Respond to stimuli arising in internal viscera and blood vessels • Sensitive to chemical changes, tissue stretch, and temperature changes Classification by Location 3.Proprioceptors • Respond to stretch in skeletal muscles, tendons, joints, ligaments, and connective tissue coverings of bones and muscles • Inform the brain of one’s movements Classification by Structural Complexity 1.Complex receptors (special sense organs) • Vision, hearing, equilibrium, smell, and taste (Chapter 15) 2.Simple receptors for general senses: • Tactile sensations (touch, pressure, stretch, vibration), temperature, pain, and muscle sense • Unencapsulated (free) or encapsulated dendritic endings Unencapsulated Dendritic Endings • Thermoreceptors • Cold receptors (10–40ºC); in superficial dermis • Heat receptors (32–48ºC); in deeper dermis Unencapsulated Dendritic Endings • Nociceptors • Respond to: • Pinching • Chemicals from damaged tissue • Temperatures outside the range of thermoreceptors • Capsaicin Unencapsulated Dendritic Endings • Light touch receptors • Tactile (Merkel) discs • Hair follicle receptors Encapsulated Dendritic Endings • All are mechanoreceptors • • • • • • Meissner’s (tactile) corpuscles—discriminative touch Pacinian (lamellated) corpuscles—deep pressure and vibration Ruffini endings—deep continuous pressure Muscle spindles—muscle stretch Golgi tendon organs—stretch in tendons Joint kinesthetic receptors—stretch in articular capsules From Sensation to Perception • Survival depends upon sensation and perception • Sensation: the awareness of changes in the internal and external environment • Perception: the conscious interpretation of those stimuli Sensory Integration • Input comes from exteroceptors, proprioceptors, and interoceptors • Input is relayed toward the head, but is processed along the way Sensory Integration • Levels of neural integration in sensory systems: 1. Receptor level—the sensor receptors 2. Circuit level—ascending pathways 3. Perceptual level—neuronal circuits in the cerebral cortex Processing at the Receptor Level • Receptors have specificity for stimulus energy • Stimulus must be applied in a receptive field • Transduction occurs • Stimulus energy is converted into a graded potential called a receptor potential Processing at the Receptor Level • In general sense receptors, the receptor potential and generator potential are the same thing stimulus receptor/generator potential in afferent neuron action potential at first node of Ranvier Processing at the Receptor Level • In special sense organs: stimulus receptor potential in receptor cell release of neurotransmitter generator potential in first-order sensory neuron action potentials (if threshold is reached) Adaptation of Sensory Receptors • Adaptation is a change in sensitivity in the presence of a constant stimulus • Receptor membranes become less responsive • Receptor potentials decline in frequency or stop Adaptation of Sensory Receptors • Phasic (fast-adapting) receptors signal the beginning or end of a stimulus • Examples: receptors for pressure, touch, and smell • Tonic receptors adapt slowly or not at all • Examples: nociceptors and most proprioceptors Processing at the Circuit Level • Pathways of three neurons conduct sensory impulses upward to the appropriate brain regions • First-order neurons • Conduct impulses from the receptor level to the second-order neurons in the CNS • Second-order neurons • Transmit impulses to the thalamus or cerebellum • Third-order neurons • Conduct impulses from the thalamus to the somatosensory cortex (perceptual level) Processing at the Perceptual Level • Identification of the sensation depends on the specific location of the target neurons in the sensory cortex • Aspects of sensory perception: • Perceptual detection—ability to detect a stimulus (requires summation of impulses) • Magnitude estimation—intensity is coded in the frequency of impulses • Spatial discrimination—identifying the site or pattern of the stimulus (studied by the two-point discrimination test) Main Aspects of Sensory Perception • Feature abstraction—identification of more complex aspects and several stimulus properties • Quality discrimination—the ability to identify submodalities of a sensation (e.g., sweet or sour tastes) • Pattern recognition—recognition of familiar or significant patterns in stimuli (e.g., the melody in a piece of music) Perception of Pain • Warns of actual or impending tissue damage • Stimuli include extreme pressure and temperature, histamine, K+, ATP, acids, and bradykinin • Impulses travel on fibers that release neurotransmitters glutamate and substance P • Some pain impulses are blocked by inhibitory endogenous opioids Structure of a Nerve • Cordlike organ of the PNS • Bundle of myelinated and unmyelinated peripheral axons enclosed by connective tissue Structure of a Nerve • Connective tissue coverings include: • Endoneurium—loose connective tissue that encloses axons and their myelin sheaths • Perineurium—coarse connective tissue that bundles fibers into fascicles • Epineurium—tough fibrous sheath around a nerve Classification of Nerves • Most nerves are mixtures of afferent and efferent fibers and somatic and autonomic (visceral) fibers • Pure sensory (afferent) or motor (efferent) nerves are rare • Types of fibers in mixed nerves: • Somatic afferent and somatic efferent • Visceral afferent and visceral efferent • Peripheral nerves classified as cranial or spinal nerves Ganglia • Contain neuron cell bodies associated with nerves • Dorsal root ganglia (sensory, somatic) (Chapter 12) • Autonomic ganglia (motor, visceral) (Chapter 14) Regeneration of Nerve Fibers • Mature neurons are amitotic • If the soma of a damaged nerve is intact, axon will regenerate • Involves coordinated activity among: • Macrophages—remove debris • Schwann cells—form regeneration tube and secrete growth factors • Axons—regenerate damaged part • CNS oligodendrocytes bear growth-inhibiting proteins that prevent CNS fiber regeneration