The Peripheral Nervous System

1. 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

2. Functional Organization of the PNS Figure 14.1

3. 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

4. Structural Organization of PNS in Region of a Spinal Nerve Peripheral Nerve Endings: • Afferent: Sensory Receptors • Efferent: Somatic Motor • Efferent: Autonomic Nervous System

5. Peripheral Sensory Receptors • Structures that pick up sensory stimuli • Initiate signals in sensory axons • Two main categories of sensory receptors • Special nerve endings of sensory neurons • Monitor general sensory information • Independent receptor cells – specialized epithelial cells or small neurons • Sensory receptors can be classified in three ways: • Type of stimulus detected • Location • Structural Complexity • Monitor most types of special sensory information

6. Classification by Type of Stimuli • 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

7. 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 within the body • internal viscera & blood vessels • Monitor a variety of stimuli such as chemical changes, stretching of tissue & temperature • Proprioceptors: – monitor degree of stretch • Located in musculoskeletal organs

8. Classification by Structure • Simple: • Vast majority • Modified dendritic extensions of sensory neurons • Found in skin, mucous membranes, muscles & connective tissue • Complex: • Associated with the special senses (vision, hearing, smell & taste)

9. General Sensory Receptors (simple receptors) • Widely distributed • Nerve endings of sensory neurons monitor: • Touch, pressure, vibration, stretch • Pain, temperature, proprioception • Divided into two groups • Free dendritic (nerve) endings • Encapsulated dendritic (nerve) endings

10. Free Nerve Endings • Abundant in epithelia and underlying connective tissue • Most are unmyelinated with a small diameter • 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

11. Encapsulated Nerve Endings • Consist of one or more end fibers of sensory neurons • Enclosed in connective tissue • Mechanoreceptors • Include four main types: • Meissner’s corpuscles • Pacinian corpuscles • Ruffini’s corpuscles • Proprioceptors – monitor stretch in locomotory organs • Muscle spindles • Golgi tendon organs • Joint kinesthetic receptors

12. 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

13. Structure of Receptors in Skin

14. Innervation of Skeletal Muscle • Motor axons innervate skeletal muscles • Neuromuscular junctions (motor end plates) • Similar to synapses between neurons • Acetylcholine diffuses across the synaptic cleft • Binds with molecules on the sarcolemma • Motor axons branch to innervate muscle fibers • Motor unit – a motor neuron and all the muscle fibers it innervates

15. The Neuromuscular Junction Figure 14.5a, b

16. Innervation of Visceral Muscle and Glands • Simpler than neuromuscular junctions of skeletal muscle • Near the smooth muscle or gland it innervates • Visceral motor axon swells into a row of varicosities • Visceral motor responses • Slower than somatic motor reflexes

17. 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 • The vagus nerve (X) extends into the abdomen

18. The 12 Pairs of Cranial Nerves Figure 14.8

19. CN I: Olfactory Nerves • Sensory nerves of smell Table 14.2

20. CN II: Optic Nerve • Sensory nerve of vision Table 14.2

21. CN III: Oculomotor Nerve • Innervates four of the extrinsic eye muscles Table 14.2

22. CN IV: Trochlear Nerve • Innervates an extrinsic eye muscle Table 14.2

23. CN V: Trigeminal Nerve • Provides sensory innervation to the face • Motor innervation to chewing muscles

24. CN VI: Abducens Nerve • Abducts the eyeball Table 14.2

25. CN VII: Facial Nerve • Innervates muscles of facial expression • Sensory innervation of face • Taste Table 14.2

26. CN VIII: Vestibulocochlear Nerve • Sensory nerve of hearing and balance Table 14.2

27. CN IX: Glossopharyngeal Nerve • Sensory and motor innervation of structures of the tongue and pharynx • Taste Table 14.2

28. CN X: Vagus Nerve (pneumogastric nerve) • A mixed sensory and motor nerve • Main parasympathetic nerve • “Wanders” into thorax and abdomen Table 14.2

29. CN XI: Accessory Nerve • An accessory part of the vagus nerve • Somatic motor function of pharynx, larynx, neck muscles Table 14.2

30. CN XII: Hypoglossal Nerve • Runs inferior to the tongue • Innervates the tongue muscles Table 14.2

31. 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) • 5 pairs of sacral nerves (S1-S5) • 1 pair of coccygeal nerves (Co1)

32. Spinal Nerves Posterior View Figure 14.9

33. 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 (cell bodies in gray matter of spinal cord – no ganglia)

34. Spinal Nerves • Branch into dorsalramus and ventralramus • Rami communicantes connect to the base of the ventral ramus • Lead to the sympathetic chain ganglia (gray and white ramus) • Dorsal and ventral rami contain sensory and motor fibers

35. Innervation of the Skin: Dermatomes • Dermatome – an area of skin • Innervated by cutaneous branches of a single spinal nerve • Upper limb – skin is supplied by nerves of the brachial plexus • Lower limb • Lumbar nerves – anterior surface • Sacral nerves – posterior surface

36. Map of Dermatomes – Anterior & Posterior View

37. Reflex Activity • A reflex is a rapid, predictable motor response to a stimulus • The basic reflex is unlearned, unpremeditated and involuntary, • Some reflexes we are aware of (pulling away from hot pot) and others, usually visceral, we are not • In addition to basic reflexes there are also learned or acquired reflexes (automatic actions of an experienced driver) • Reflexes occur in highly specific neural paths called reflex arcs

38. The Reflex Arc • Receptor: site of stimulus action • Sensory Neuron: transmits afferent impulses to the CNS • Integration Center: synapse between sensory & motor neuron or multiple synapses within a chain of interneurons • Motor Neuron: conducts the afferent impulses from integration center to effector organ • Effector: the muscle fiber or gland cell that response to the efferent impulses in a characteristic way

39. Spinal Reflexes • Spinal reflexes are somatic reflexes mediated by the spinal cord • In the stretch reflex the muscle spindle is stretched and excited by either an external stretch or an internal stretch • The Golgi tendon reflex produces muscle relaxation and lengthening in response to contraction • The flexor, or withdrawal, reflex is initiated by a painful stimulus and causes automatic withdrawal of the threatened body part from the stimulus • They crossed extensor reflex is a complex spinal reflex consisting of an ipsilateral withdrawal reflex and a contralateral extensor reflex • Superficial reflexes are elicited by gentle cutaneous stimulation

40. The Stretch Reflex

41. Deep Golgi Tendon Reflex

42. Disorders: Anencephaly • Various degrees of absence of the various of the skull bones, cerebrum and the brain stem • One of the most common neural tube defects. Neural tube. • Occurs early in the development when the upper part of the neural tube fails to close. • Possible causes include environmental toxins and low intake of folic acid by the mother during pregnancy. • Often results in miscarriage, if child survives to birth he/she will be totally vegetative and unable to see, hear or process sensory input. Death will occur soon after birth.

43. Disorders: Shingles • Viral infection • Childhood chicken pox dormant in spinal ganglia • Often brought on by stress • Mostly experienced by those over 50 • Unilateral patch of skin blisters and discoloration along the path of one or more spinal nerves • Most commonly around the waist. The symptoms may persist for 3 to 6 months.

44. Disorders: Cerebal Palsy • Group of neuromuscular disorders in which the voluntary muscles are poorly controlled or paralyzed due to brain damage • Can result in spasticity, speech difficulties, seizures, deafness or reduced cognitive ability • Can be due to lack of oxygen during delivery, smoking or drug use, or maternal infection during pregnancy

45. Disorders: Spina Bifida & Lumbar Myelomeningocele • Incomplete formation of vertebral arches, usually in lumbosacral region • Laminae & spinal processes missing in at least one vertebrae • In the severe form, spina bifida cyctica, it can result in a sac-like cyst protruding dorsally • The cyst may contain meninges and CSF and possibly portiosns of the spinal cord and spinal root nerves • primary functional deficits are lower limb paralysis and sensory loss, bladder and bowel dysfunction, and cognitive dysfunction

46. Disorders :Migraine Headache • Migraine headache • Relates to sensory innervation of cerebral arteries • Arteries dilate • Compresses and irritates sensory nerve endings

47. Disorders: Poliomyelitis • “polio” means gray matter • caused by Poliovirus hominis. • CNS infection, but its major effect is on the peripheral nerves and the muscles they supply. • virus infects the motor neurons in the anterior horn of the central gray matter of the spinal cord. • The infection causes degeneration of the motor neurons, which results in paralysis and atrophy of the muscles innervated by those nerves.

48. Disorders: Anesthetic leprosy • Bacterial infection of the peripheral nerves caused by Mycobacterium leprae. • Infection results in anesthesia, paralysis, ulceration, and gangrene

49. Disorders :Myasthenia Gravis • Myasthenia gravis • Progressive weakening of the skeletal muscles • An autoimmune disorder • Antibodies destroy acetylcholine receptors Ptosis due to weakness of eyelid muscles