The PNS

Peripheral Nervous System (PNS) All neural structures outside the brain and spinal cord Sensory receptors Peripheral nerves and associated ganglia Motor endings

Sensory Receptors Specialized to respond to changes in their environment (stimuli) Sensation (awareness of stimulus) perception (interpretation of the meaning of the stimulus) occur in the brain

Classification by Stimulus Type Mechanoreceptors Thermoreceptors Photoreceptors Chemoreceptors Nociceptors

Classification by Location Exteroceptors outside the body skin for touch, pressure, pain, and temperature Most special sense organs Interoceptors (visceroceptors) internal viscera and blood vessels Sensitive to chemical changes, tissue stretch, and temperature changes

Classification by Location 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

Unencapsulated Dendritic Endings (Free) Thermoreceptors Cold receptors (10–40ºC); in superficial dermis Heat receptors (32–48ºC); in deeper dermis Nociceptors Pinching,Chemicals,Temperatures,Capsaicin Light touch receptors

Table 13.1

Encapsulated Dendritic Endings All are mechanoreceptors Meissner’s corpuscles—discriminative touch Pacinian corpuscles—deep pressure and vibration Ruffini endings—deep continuous pressure Muscle spindles—muscle stretch Golgi tendon organs—stretch in tendons

Table 13.1

Levels of neural integration in sensory systems: Sensory Integration Levels of neural integration in sensory systems: Receptor level Circuit level Perceptual level

Perceptual level (processing in cortical sensory centers) 3 Perceptual level (processing in cortical sensory centers) Motor cortex Somatosensory cortex Thalamus Reticular formation Cerebellum Pons Medulla 2 Circuit level (processing in ascending pathways) Spinal cord Free nerve endings (pain, cold, warmth) Muscle spindle 1 Receptor level (sensory reception and transmission to CNS) Joint kinesthetic receptor Figure 13.2

Adaptation of Sensory Receptors Change in sensitivity in the presence of a constant stimulus Receptor membranes become less responsive Receptor potentials decline in frequency or stops

Warns of actual or impending tissue damage Perception of Pain Warns of actual or impending tissue damage Impulses travel on fibers that release neurotransmitters glutamate and substance P Some pain impulses are blocked by inhibitory endogenous opioids Morphine

Structure of a Nerve Cordlike organ of the PNS Bundle of myelinated and unmyelinated peripheral axons enclosed by connective tissue

Connective tissue coverings include: Structure of a Nerve Connective tissue coverings include: Endoneurium Perineurium Epineurium

Axon Myelin sheath Endoneurium Perineurium Epineurium Fascicle Blood vessels (b) Figure 13.3b

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 Peripheral nerves classified as cranial or spinal nerves

Contain neuron cell bodies associated with nerves Ganglia Contain neuron cell bodies associated with nerves Dorsal root ganglia (sensory, somatic) Autonomic ganglia (motor, visceral)

“On occasion, our trusty truck acts funny—very good vehicle anyhow” Cranial Nerves Twelve pairs of nerves associated with the brain Most are mixed in function; two pairs are purely sensory “On occasion, our trusty truck acts funny—very good vehicle anyhow”

Hypoglossal nerve (XII) Filaments of olfactory nerve (I) Frontal lobe Olfactory bulb Olfactory tract Optic nerve (II) Temporal lobe Optic chiasma Infundibulum Optic tract Facial nerve (VII) Oculomotor nerve (III) Vestibulo- cochlear nerve (VIII) Trochlear nerve (IV) Trigeminal nerve (V) Glossopharyngeal nerve (IX) Abducens nerve (VI) Vagus nerve (X) Cerebellum Accessory nerve (XI) Medulla oblongata Hypoglossal nerve (XII) (a) Figure 13.5 (a)

Yes (hearing and balance) Cranial nerves I – VI Sensory function Motor function PS* fibers I Olfactory Yes (smell) No No II Optic Yes (vision) No No III Oculomotor No Yes Yes IV Trochlear No Yes No V Trigeminal Yes (general sensation) Yes No VI Abducens No Yes No Cranial nerves VII – XII Sensory function Motor function PS* fibers VII Facial Yes (taste) Yes Yes VIII Vestibulocochlear Yes (hearing and balance) Some No IX Glossopharyngeal Yes (taste) Yes Yes X Vagus Yes (taste) Yes Yes XI Accessory No Yes No XII Hypoglossal No Yes No *PS = parasympathetic (b) Figure 13.5 (b)

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Spinal Nerves 31 pairs of mixed nerves named according to their point of issue from the spinal cord 8 cervical (C1–C8) 12 thoracic (T1–T12) 5 Lumbar (L1–L5) 5 Sacral (S1–S5) 1 Coccygeal (C0)

Cervical plexus Cervical nerves C1 – C8 Brachial plexus Cervical enlargement Thoracic nerves T1 – T12 Intercostal nerves Lumbar enlargement Lumbar nerves L1 – L5 Lumbar plexus Sacral plexus Sacral nerves S1 – S5 Cauda equina Coccygeal nerve Co1 Figure 13.6

Each spinal nerve connects to the spinal cord via two roots Spinal Nerves: Roots Each spinal nerve connects to the spinal cord via two roots Ventral roots Contain motor (efferent) fibers from the ventral horn motor neurons. Fibers innervate skeletal muscles) Dorsal roots Contain sensory (afferent) fibers from sensory neurons in the dorsal root ganglia

Gray matter White matter Dorsal and Ventral root ventral rootlets of spinal nerve Ventral root Dorsal root Dorsal root ganglion Dorsal ramus of spinal nerve Ventral ramus of spinal nerve Spinal nerve Rami communicantes Sympathetic trunk ganglion Anterior view showing spinal cord, associated nerves, and vertebrae. The dorsal and ventral roots arise medially as rootlets and join laterally to form the spinal nerve. Figure 13.7 (a)

Cervical Plexus Innervates skin and muscles of the neck, ear, back of head, and shoulders Phrenic nerve Major motor and sensory nerve of the diaphragm (receives fibers from C3–C5)

Segmental branches Hypoglossal Ventral nerve (XII) rami: Ventral rami Segmental branches Hypoglossal nerve (XII) Ventral rami: Lesser occipital nerve C1 Greater auricular nerve C2 Transverse cervical nerve C3 Ansa cervicalis C4 Accessory nerve (XI) Phrenic nerve C5 Supraclavicular nerves Figure 13.8