Understanding Sensory Receptors and Peripheral Nerves

1. Chapter 13 PNS

2. PNS • Cranial nerves • Spinal nerves

3. Sensory Receptors • Specialized cells that monitor specific conditions in the body or external environment • Activation of sensory receptors results in depolarizations that trigger impulses to the CNS • Sensation: activation of sensory receptor cells

4. Sensation vs. Perception • Sensation: • Activity in sensory cells • Perception: • Conscious awareness of a sensation (in cortex)

5. Signaling • Stimulation of a receptor produces action potentials along the axon of a sensory neuron • Action potentials are all the same so: • The frequency and pattern of action potentials contains information about the strength, duration, and variation of the stimulus

6. General senses: temperature pain touch pressure vibration proprioception Special senses Olfaction (smell) Vision (sight) Gustation (taste) Equilibrium (balance) Hearing Senses

7. General receptor types • Exteroceptors • Provide information about the external environment • Proprioceptors • Report the positions of skeletal muscles and joints • Interoceptors • Monitor visceral organs and functions

8. Modality • Your perception of the nature of a stimulus (its modality) depends on the path it takes inside the CNS, especially, where in the brain the information ends up.

9. Adaptation of Sensory Receptors • Adaptation occurs when sensory receptors are subjected to an unchanging stimulus • Receptor membranes become less responsive • Receptor potentials decline in frequency or stop • Receptors responding to pressure, touch, and smell adapt quickly • Pain receptors and proprioceptors do not exhibit adaptation

10. Spinal Nerves Figure 13–6

11. Spinal Nerves • 31 pairs • one per segment on each side of the spine • dorsal and ventral roots join to form a spinal nerve • Carry both afferent (sensory) and efferent (motor) fibers = mixed nerves

12. Spinal Nerve Organization • Every spinal nerve is surrounded by 3 connective tissue layers that support structures and contain blood vessels (just like muscles) • Epineurium: • outer layer • dense network of collagen fibers • Perineurium: • middle layer • divides nerve into fascicles (axon bundles) • Endoneurium: • inner layer • surrounds individual axons

13. Spinal nerves: start where dorsal and ventral roots unite (just lateral to the vertebral column), then branch and form pathways to destination Peripheral Distribution of Spinal Nerves

14. Spinal Nerves: Rami • The short spinal nerves branch into three or four mixed, distal rami • Small dorsal ramus • Larger ventral ramus • Rami communicantes at the base of the ventral rami in the thoracic region

15. Nerve Plexuses • All ventral rami except T2-T12 form interlacing nerve networks called plexuses • Plexuses are found in the cervical, brachial, lumbar, and sacral regions • Each resulting branch of a plexus contains fibers from several spinal nerves • Each muscle receives a nerve supply from more than one spinal nerve • Damage to one spinal segment (gray matter) cannot completely paralyze a muscle

16. Peripheral Distribution of Spinal Nerves Motor fibers Figure 13–7a

17. Motor fibers: First Branch • From the spinal nerve, the first branch (blue): • carries visceral motor fibers to sympathetic ganglion of autonomic nervous system (More about this later)

18. Communicating Rami • Also called Rami Communicantes, means “communicating branches” • made up of gray ramus and white ramus together

19. Communicating Rami • White Ramus: • Preganglionic branch • Myelinated axons (hence: white) • Going “to” the sympathetic ganglion • Gray Ramus • Unmyelinated nerves (so: gray) • Return “from” sympathetic ganglion • Rejoin spinal nerve, go to target organ

20. Dorsal and Ventral Rami Both are somatic and visceral outflow to the body • Dorsal ramus: • contains somatic and visceral motor fibers that innervate the back • Ventral ramus: • larger branch that innervates ventrolateral structures and limbs

21. Peripheral Distribution of Spinal Nerves Sensory fibers Figure 13–7b

22. Sensory Nerves • Dorsal, ventral, and whiterami (but not gray) also carry sensory information in addition to motor efferent outflow.

23. Dermatomes • Bilateral region of skin • Each is monitored by specific pair of spinal nerves Figure 13–8

24. Peripheral Neuropathy • Regional loss of sensory or motor function • Due to trauma, compression, or disease

25. Reflexes

26. Reflexes • Rapid, automatic responses to specific stimuli coordinated within the spinal cord (or brain stem) • Occurs via interconnected sensory, motor, and interneurons • Can be a movement, like a knee jerk, or visceral, like pupil dilation or swallowing

27. Functional Organization of Neurons in the NS • Sensory neurons: • about 10 million that deliver information to CNS • Motor neurons: • about 1/2 million that deliver commands to peripheral effectors • Interneurons: • about 20 billion that interpret, plan, and coordinate signals in and out = information processors

28. The Reflex Arc • The wiring of a single reflex • Begins at sensory receptor • Ends at peripheral effector (muscle, gland, etc) • Generally opposes original stimulus (negative feedback)

29. 5 Steps in a Neural Reflex • Step 1: Arrival of stimulus, activation of receptor • physical or chemical changes • Step 2: Activation of sensory neuron • graded depolarization • Step 3: Information processing by postsyn. cell • triggered by neurotransmitters • Step 4: Activation of motor neuron • action potential • Step 5: Response of peripheral effector • triggered by neurotransmitters

30. 5 Steps in a Neural Reflex Figure 13–14

31. Classification of Reflexes There are several ways to classify reflexes but most common is by complexity of the neural circuit: monosynaptic vs polysynaptic

32. Monosynaptic Reflexes • Have the least delay between sensory input and motor output: • e.g.,stretch reflex (such as patellar reflex) • Completed in 20–40 msec • No interneurons involved

33. Monosynaptic Reflex A stretch reflex Figure 13–15

34. The receptors in stretch reflexes Bundles of small, specialized muscle fibers Sense passive stretching in a muscle Muscle Spindles

35. Polysynaptic Reflexes • More complicated than monosynaptic reflexes • Interneurons involved that control more than 1 muscle group • Produce either EPSPs or IPSPs • Examples: the withdrawal reflexes

36. Withdrawal Reflexes • Move body part away from stimulus (pain or pressure): • flexor reflex: • pulls hand away from hot stove • crossed extensor reflex • Strength and extent of response depends on intensity and location of stimulus

37. A Flexor Reflex Figure 13–17

38. Key = Reciprocal Inhibition • For flexor reflex to work: • the stretch reflex of the antagonistic (extensor) muscles must be inhibited • reciprocal inhibition by interneurons in spinal cord causes antagonistic extensors to be inhibited

39. Reflex Arcs • Crossed extensor reflexes: • involves a contralateral reflex arc • occurs on side of body opposite from the stimulus

40. Crossed Extensor Reflexes • Occur simultaneously and coordinated with flexor reflex • Example:flexor reflex causes leg to pull up: • crossed extensor reflex straightens other leg to receive body weight

41. The Crossed Extensor Reflex Figure 13–18

42. Integration and Control of Spinal Reflexes • Though reflex behaviors are automatic, processing centers in brain can facilitate or inhibit reflex motor patterns based in spinal cord

43. Reinforcement of Spinal Reflexes • Higher centers can reinforce spinal reflexes: • by stimulating excitatory neurons in brain stem or spinal cord • creating EPSPs at reflex motor neurons • facilitating postsynaptic neurons

44. Inhibition of Spinal Reflexes • Higher centers can inhibit spinal reflexes: • by stimulating inhibitory neurons • creating IPSPs at reflex motor neurons • suppressing postsynaptic neurons

45. Voluntary Movements and Reflex Motor Patterns • Higher centers of brain incorporate lower, reflexive motor patterns • Automatic reflexes: • can be activated by brain as needed • use few nerve impulses to control complex motor functions • e.g. walking, running, jumping