CHAPTER 13a THE PERIPHERAL NERVOUS SYSTEM REFLEX ACTIVITY

Peripheral Nervous System (PNS) • All neural structures outside the brain • Sensory receptors

Central nervous system (CNS) Peripheral nervous system (PNS) Sensory (afferent) division Copyright © 2010

Sensory Receptors • Specialized to respond to changes in their environment (stimuli) • Activation

Classification of Receptors • Based on: • Stimulus type • Location • Structural complexity

Classification by Stimulus Type • Mechanoreceptors—respond to touch, pressure, vibration, stretch, and itch •

Classification by Location 1. Exteroceptors • Respond to stimuli arising outside the body •

Classification by Location 2. Interoceptors (visceroceptors) • Respond to stimuli arising in internal viscera

Classification by Location 3. Proprioceptors • Respond to stretch in skeletal muscles, tendons, joints,

Classification by Structural Complexity 1. Complex receptors (special sense organs) • Vision, hearing, equilibrium,

Unencapsulated Dendritic Endings • Thermoreceptors • Cold receptors (10– 40ºC); in superficial dermis •

Unencapsulated Dendritic Endings • Nociceptors • Respond to: • Pinching • Chemicals from damaged

Unencapsulated Dendritic Endings • Light touch receptors • Tactile (Merkel) discs • Hair follicle

Copyright © 2010 Pearson Education, Inc. Table 13. 1

Encapsulated Dendritic Endings • All are mechanoreceptors • Meissner’s (tactile) corpuscles—discriminative touch • Pacinian

From Sensation to Perception • Survival depends upon sensation and perception • Sensation: the

Sensory Integration • Input comes from exteroceptors, proprioceptors, and interoceptors • Input is relayed

Sensory Integration • Levels of neural integration in sensory systems: 1. Receptor level—the sensor

Perceptual level (processing in cortical sensory centers) 3 Motor cortex Somatosensory cortex Thalamus Reticular

Processing at the Receptor Level • Receptors have specificity for stimulus energy • Stimulus

Processing at the Receptor Level • In general sense receptors, the receptor potential and

Processing at the Receptor Level • In special sense organs: stimulus receptor potential in

Adaptation of Sensory Receptors • Adaptation is a change in sensitivity in the presence

Adaptation of Sensory Receptors • Phasic (fast-adapting) receptors signal the beginning or end of

Processing at the Circuit Level • Pathways of three neurons conduct sensory impulses upward

Processing at the Perceptual Level • Identification of the sensation depends on the specific

Main Aspects of Sensory Perception • Feature abstraction—identification of more complex aspects and several

Perception of Pain • Warns of actual or impending tissue damage • Stimuli include

Structure of a Nerve • Cordlike organ of the PNS • Bundle of myelinated

Structure of a Nerve • Connective tissue coverings include: • Endoneurium—loose connective tissue that

Endoneurium Axon Myelin sheath Perineurium Epineurium Fascicle Blood vessels (b) Copyright © 2010 Pearson

Classification of Nerves • Most nerves are mixtures of afferent and efferent fibers and

Ganglia • Contain neuron cell bodies associated with nerves • Dorsal root ganglia (sensory,

Regeneration of Nerve Fibers • Mature neurons are amitotic • If the soma of

Endoneurium Schwann cells Droplets of myelin 1 The axon becomes fragmented at the injury

Schwann cell Copyright © 2010 Pearson Education, Inc. Macrophage 2 Macrophages clean out the

Aligning Schwann cells form regeneration tube 3 Axon sprouts, or filaments, grow through a

Schwann cell Site of new myelin sheath formation 4 The axon regenerates and a

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Central nervous system (CNS) Peripheral nervous system (PNS) Sensory (afferent) division Copyright © 2010 Pearson Education, Inc. Motor (efferent) division Somatic nervous system Autonomic nervous system (ANS) Sympathetic division Parasympathetic division Figure 13. 1

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 Copyright © 2010 Pearson Education, Inc.

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) Copyright © 2010 Pearson Education, Inc.

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 Copyright © 2010 Pearson Education, Inc.

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 Copyright © 2010 Pearson Education, Inc.

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 Copyright © 2010 Pearson Education, Inc.

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 Copyright © 2010 Pearson Education, Inc.

Unencapsulated Dendritic Endings • Nociceptors • Respond to: • Pinching • Chemicals from damaged tissue • Temperatures outside the range of thermoreceptors • Capsaicin Copyright © 2010 Pearson Education, Inc.

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 Copyright © 2010 Pearson Education, Inc.

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 Copyright © 2010 Pearson Education, Inc.

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 Copyright © 2010 Pearson Education, Inc.

Perceptual level (processing in cortical sensory centers) 3 Motor cortex Somatosensory cortex Thalamus Reticular formation Pons 2 Circuit level (processing in Spinal ascending pathways) cord Cerebellum Medulla Free nerve endings (pain, cold, warmth) Muscle spindle Receptor level (sensory reception Joint and transmission kinesthetic to CNS) receptor 1 Copyright © 2010 Pearson Education, Inc. Figure 13. 2

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 Copyright © 2010 Pearson Education, Inc.

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 Copyright © 2010 Pearson Education, Inc.

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) Copyright © 2010 Pearson Education, Inc.

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 Copyright © 2010 Pearson Education, Inc.

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 Copyright © 2010 Pearson Education, Inc.

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) Copyright © 2010 Pearson Education, Inc.

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) Copyright © 2010 Pearson Education, Inc.

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) Copyright © 2010 Pearson Education, Inc.

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 Copyright © 2010 Pearson Education, Inc.

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 Copyright © 2010 Pearson Education, Inc.

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 Copyright © 2010 Pearson Education, Inc.

Ganglia • Contain neuron cell bodies associated with nerves • Dorsal root ganglia (sensory, somatic) (Chapter 12) • Autonomic ganglia (motor, visceral) (Chapter 14) Copyright © 2010 Pearson Education, Inc.

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 Copyright © 2010 Pearson Education, Inc.

Aligning Schwann cells form regeneration tube 3 Axon sprouts, or filaments, grow through a regeneration tube formed by Schwann cells. Fine axon sprouts or filaments Copyright © 2010 Pearson Education, Inc. Figure 13. 4 (3 of 4)