The Peripheral Nervous System

1. The Peripheral Nervous System Chapter 14

2. Introduction to the PNS • Nervous structures outside the brain and spinal cord - sensory and motor connections to the outside world - nerves thread throughout the body to allow the CNS to receive information and take action • Functional components of the PNS - sensory inputs and motor outputs - categorized as somatic or visceral - also classified as general or special

3. Functional Components of the PNS • Basic structural components: 1. Sensory receptors – pick up stimuli from inside and outside the body, then initiate impulses in sensory axons 2. Motor endings – the axon terminals of motor neurons that innervate the effectors 3. Nerves – bundles of peripheral axons and Ganglia - clusters of peripheral neuronal cell bodies - most are mixed nerves, contain both sensory and motor axons - some cranial nerves are purely sensory or purely motor in function

4. Autonomic Nervous System • General visceral motor part of the PNS • Has 2 divisions (with opposite effects): - Parasympathetic: ‘housekeeping’ activities (rest and digest) - Sympathetic: extreme situations (fight or flight)

5. Functional Organization of the PNS Figure 14.1

6. Peripheral Sensory Receptors • Most fit into 2 main categories: 1. free nerve endings of sensory neurons - monitor general sensory information such as touch, pain, pressure, temperature, and proprioception 2. complete receptor cells – specialized epithelial cells or small neurons that transfer sensory information to sensory neurons - monitor most special sensory information such as taste, vision, hearing, and equilibrium

7. Basic Anatomical Scheme of the PNS in the Region of a Spinal Nerve A nerve is composed of numerous nerve fibers Figure 14.2

8. Sensory Receptors of the PNS • Also classified according to: a) Location – based on body location or location of stimuli to which they respond b) Type of stimulus detected – kinds of stimuli that most readily activate them c) Structure – divided into 2 broad categories free or encapsulated nerve endings

9. Classification by Location • Exteroceptors – sensitive to stimuli arising from outside the body - located at or near body surfaces - include receptors for touch, pressure, pain, temperature, and most receptors of the special sense organs • Interoceptors (visceroceptors) – receive stimuli from internal viscera (digestive tube, bladder, lungs) - monitor a variety of stimuli such as changes in chemical concentration, taste stimuli, stretching of tissues, and temperature - activation causes visceral pain, nausea, hunger, or satiety

10. Classification by Location • Proprioceptors – monitors degree of stretch and sends input on body movements to the CNS - located in musculoskeletal organs such as skeletal muscles, tendons, joints, and ligaments

11. Classification by Stimulus Detected • Mechanoreceptors – respond to mechanical forces - such as touch, pressure, stretch, vibrations, and itch • Thermoreceptors – respond to temperature changes • Chemoreceptors – respond to chemicals in solution (molecules tasted or smelled) and to change in blood chemistry • Photoreceptors in the eye – respond to light • Nociceptors – respond to harmful stimuli that result in pain (noci = harm)

12. Classification by Structure • General sensory receptors – widely distributed • Nerve endings of sensory neurons moniter: - Touch - Pressure - Vibration - Stretch - Pain - Temperture - Proprioception

13. Classification by Structure • General sensory receptors are divided into 2 groups - Free nerve endings - Encapsulated nerve endings Note: there is no perfect ‘one receptor – one function’ - one receptor can respond to several kinds of stimuli and different receptor types can respond to similar stimuli

14. Free Nerve Endings • Abundant in epithelia and underlying CT • Respond to pain and temperature • Monitor affective senses – those to which people have an emotional response (pain) • 2 specialized types of free nerve endings: - Merkel discs: lie in the epidermis - Hair follicle receptors: wrap around hair follicles

15. Free Nerve Endings • Merkel discs – a disc-shaped epithelial cell innervated by a sensory nerve ending - slowly adapting receptors for light touch (respond and send out action potentials even after continual stimulation) • Hair follicle receptors – receptors for light touch - monitor the bending of hairs - rapidly adapting (sensation disappears quickly even if the stimulus is maintained) • Itch receptor – in the dermis (newly discovered)

16. Table 14.1 (1 of 4)

17. Encapsulated Nerve Endings • Consist of one or more end fibers of sensory neurons enclosed in connective tissue • All seem to be mechanoreceptors – capsules either amplify the stimulus or filter out the wrong types of stimuli • 4 main types - Meissner’s corpuscles - Pacinian corpuscles - Ruffini endings - Proprioceptors

18. 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

19. Meissner’s Corpuscles Table 14.1 (2 of 4)

20. Pacinian Corpuscles and Ruffini Endings • Pacinian corpuscle - single nerve ending - surrounded by layers of flattened Schwann cells - occur in the hypodermis - sensitive to deep pressure - rapidly adapting receptors • Ruffini endings – located in the dermis - monitor continuous pressure on the skin (adapt slowly)

21. Table 14.1 (3 of 4)

22. Proprioceptors • 3 types - monitor stretch in locomotory organs: • 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

23. Table 14.1 (4 of 4)

24. Structure of Proprioceptors Figure 14.4

25. Cranial Nerves • Attach to the brain and pass through foramina of the skull • Numbered from I – XII • Cranial nerves I and II attach to the forebrain - all others attach to the brain stem • Primarily serve head and neck structures - except the vagus nerve (X) that extends into the abdomen

26. The 12 pairs of cranial nerves Figure 14.5

27. Olfactory Nerves • Sensory nerves of smell Table 14.3 (1 of 12)

28. Optic Nerve • Sensory nerve of vision Table 14.3 (2 of 12)

29. Oculomotor Nerve • Innervates four of the extrinsic eye muscles Table 14.3 (3 of 12)

30. Trochlear Nerve • Innervates the superior oblique muscle (extrinsic eye muscle) Table 14.3 (4 of 12)

31. Trigeminal Nerve Table 14.3 (5 of 12) • Provides sensory innervation to the face and motor innervation to chewing muscles

32. Abducens Nerve • Abducts the eyeball – innervates lateral rectus muscle Table 14.3 (6 of 12)

33. Facial Nerve • Innervates muscles of facial expression Table 14.3 (7 of 12)

34. Vestibulocochlear Nerve • Sensory nerve of hearing and balance Table 14.3 (8 of 12)

35. Glossopharyngeal Nerve • Innervates structures of the tongue and pharynx Table 14.3 (9 of 12)

36. Vagus Nerve • A mixed sensory and motor nerve • “Wanders” into thorax and abdomen • Parasympathetic innervation of organs Table 14.3 (10 of 12)

37. Accessory Nerve • An accessory part of the vagus nerve • Innervates trapezius muscle Table 14.3 (11 of 12)

38. Hypoglossal Nerve • Runs inferior to the tongue - innervates the tongue muscles Table 14.3 (12 of 12)

39. Spinal Nerves • 31 pairs – contain thousands of nerve fibers • Connect to the spinal cord • Named for point of issue from the spinal cord - 8 pairs of cervical nerves (C1 – C8) - 12 pairs of thoracic nerves (T1 – T12) - 5 pairs of lumbar nerves (L1 – L5) - 1 pair of coccygeal nerves (Co1)

40. Spinal Nerves Posterior View Figure 14.6

41. Spinal Nerves • Connect to the spinal cord by the dorsal root and ventral root • Dorsal root – contains sensory fibers - cell bodies located in the dorsal root ganglion • Ventral root– contains motor fibers arising from anterior gray column

42. Spinal Nerves • Branch into dorsal ramus and ventral ramus - dorsal and ventral rami contain sensory and motor fibers • Rami communicantes – connect to the base of the ventral ramus - lead to the sympathetic chain ganglia

43. Spinal Nerves Figure 14.7a

44. Innervation of the Back • Dorsal rami – innervate back muscles - follow a neat, segmented pattern - innervate a horizontal strip of muscle and skin (in line with emergence point from the vertebral column)

45. Innervation of the Back Figure 14.7b

46. Innervation of the Anterior Thoracic and Abdominal Wall • Thoracic region – ventral rami arranged in simple, segmented pattern • Intercostal nerves supply intercostal muscles, skin, and abdominal wall - each gives off lateral and anterior cutaneous branches

47. Introduction to Nerve Plexuses • A network of nerves • Ventral rami (except T2 – T12) - branch and join with one another - form nerve plexuses in the cervical, brachial, lumbar, and sacral regions - primarily serve the limbs - fibers from ventral rami crisscross

48. The Cervical Plexus • Buried deep in the neck under the sternocleidomastoid muscle • Formed by ventral rami of first 4 cervical nerves (C1 – C4) • Most are cutaneous nerves • Some innervate muscles of the anterior neck • Phrenic nerve – major nerve

49. The Brachial Plexus and Innervation of the Upper Limb • Brachial plexus lies in the neck and axilla • Formed by ventral rami of C5 – C8 • Cords give rise to main nerves of the upper limb Fig 14.9d

50. Nerves from Lateral and Medial Cords • Musculocutaneous – main branch of the lateral cord - innervates the biceps brachii and brachialis • Median – originates from both lateral and medial cords - innervates anterior forearm muscles and lateral palm • Ulnar – branches from the medial cord - innervates intrinsic hand muscles and skin of the medial hand • Radial – continuation of the posterior cord - largest branch innervates muscles of posterior upper limb • Axillary – innervates the deltoid and teres minor