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PNS Chapter 13 • Cranial nerves • Spinal nerves PNS 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 Sensation vs. Perception • Sensation: – Activity in sensory cells • Perception: – Conscious awareness of a sensation (in cortex) 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 Senses • General senses: temperature – – – – – pain touch pressure vibration proprioception • Special senses – – – – – Olfaction (smell) Vision (sight) Gustation (taste) Equilibrium (balance) Hearing 1 Modality General receptor types • Exteroceptors – Provide information about the external environment • Proprioceptors • 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. – Report the positions of skeletal muscles and joints • Interoceptors – Monitor visceral organs and functions Adaptation of Sensory Receptors Spinal Nerves • 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 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 Figure 13–6 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: 2 Peripheral Distribution of Spinal Nerves • Spinal nerves: – start where dorsal and ventral roots unite (just lateral to the vertebral column), then branch and form pathways to destination 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 Peripheral Distribution of Spinal Nerves 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 Motor fibers Figure 13–7a 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) Communicating Rami • Also called Rami Communicantes, means “communicating branches” • made up of gray ramus and white ramus together 3 Communicating Rami Dorsal and Ventral Rami • White Ramus: Both are somatic and visceral outflow to the body • Dorsal ramus: – Preganglionic branch – Myelinated axons (hence: white) – Going “to” the sympathetic ganglion – contains somatic and visceral motor fibers that innervate the back • Gray Ramus • Ventral ramus: – Unmyelinated nerves (so: gray) – Return “from” sympathetic ganglion – Rejoin spinal nerve, go to target organ – larger branch that innervates ventrolateral structures and limbs Peripheral Distribution of Spinal Nerves Sensory fibers Sensory Nerves • Dorsal, ventral, and white rami (but not gray) also carry sensory information in addition to motor efferent outflow. Figure 13–7b Dermatomes Peripheral Neuropathy • Bilateral region of skin • Each is monitored by specific pair of spinal nerves • Regional loss of sensory or motor function • Due to trauma, compression, or disease Figure 13–8 4 Reflexes Reflexes 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 5 Steps in a Neural Reflex • 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 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) 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 Figure 13–14 5 Classification of Reflexes Monosynaptic Reflexes There are several ways to classify reflexes but most common is by complexity of the neural circuit: monosynaptic vs polysynaptic Monosynaptic Reflex • 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 Muscle Spindles • The receptors in stretch reflexes • Bundles of small, specialized muscle fibers • Sense passive stretching in a muscle A stretch reflex Figure 13–15 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 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 6 A Flexor Reflex 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 Figure 13–17 Reflex Arcs Crossed Extensor Reflexes • Crossed extensor reflexes: • Occur simultaneously and coordinated with flexor reflex • Example: flexor reflex causes leg to pull up: – involves a contralateral reflex arc – occurs on side of body opposite from the stimulus – crossed extensor reflex straightens other leg to receive body weight Integration and Control of Spinal Reflexes The Crossed Extensor Reflex • Though reflex behaviors are automatic, processing centers in brain can facilitate or inhibit reflex motor patterns based in spinal cord Figure 13–18 7 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 Inhibition of Spinal Reflexes • Higher centers can inhibit spinal reflexes: – by stimulating inhibitory neurons – creating IPSPs at reflex motor neurons – suppressing postsynaptic neurons 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 8