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PowerPoint® Lecture Slides prepared by Janice Meeking, Mount Royal College CHAPTER 13 The Peripheral Nervous System and Reflex Activity: Part A Copyright © 2010 Pearson Education, Inc. Peripheral Nervous System (PNS) • All neural structures outside the CNS • Sensory receptors • Peripheral nerves and associated ganglia • Motor endings Copyright © 2010 Pearson Education, Inc. Sensory Receptors • Specialized to respond to changes in their environment (stimuli) • Activation results in graded potentials that trigger nerve impulses • Sensation (awareness of stimulus) and perception (interpretation of the meaning of the stimulus) occur in the brain Copyright © 2010 Pearson Education, Inc. Classification of Receptors • Based on: • Stimulus type • Location • Structural complexity Copyright © 2010 Pearson Education, Inc. Classification by Stimulus Type • Mechanoreceptors — respond to touch, pressure, vibration, stretch, and itch • Thermoreceptors temperature — sensitive to changes in • Photoreceptors — respond to light energy (e.g., retina) • Chemoreceptors — respond to chemicals (e.g., smell, taste, changes in blood chemistry) • Nociceptors — sensitive to pain-causing stimuli (e.g. extreme heat or cold, excessive pressure, inflammatory chemicals) Copyright © 2010 Pearson Education, Inc. Classification by Location 1. Exteroceptors • Respond to stimuli arising outside the body • Receptors in the skin for touch, pressure, pain, and temperature • Most special sense organs 2. Interoceptors (visceroceptors) • Respond to stimuli arising in internal viscera and blood vessels • Sensitive to chemical changes, tissue stretch, and temperature changes Copyright © 2010 Pearson Education, Inc. Classification by Location 3. 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 Copyright © 2010 Pearson Education, Inc. Classification by Structural Complexity 1. Complex receptors (special sense organs) • Vision, hearing, equilibrium, smell, and taste (Chapter 15) 2. Simple receptors for general senses: • Tactile sensations (touch, pressure, stretch, vibration), temperature, pain, and muscle sense • Unencapsulated (free) or encapsulated dendritic endings Copyright © 2010 Pearson Education, Inc. From Sensation to Perception • Survival depends upon sensation and perception • Sensation: the awareness of changes in the internal and external environment • Perception: the conscious interpretation of those stimuli Copyright © 2010 Pearson Education, Inc. Sensory Integration • Levels of neural integration in sensory systems: 1. Receptor level — the sensor receptors 2. Circuit level — ascending pathways 3. Perceptual level — neuronal circuits in the cerebral cortex Copyright © 2010 Pearson Education, Inc. Processing at the Receptor Level • The receptor must have specificity for the stimulus energy • The receptor’s receptive field must be stimulated • The smaller the receptive field the greater the ability of the brain to localize the site • Stimulus energy must be converted into a graded potential also called receptor potential or transduction • A generator potential is a depolarization that leads to action potential in the afferent fibers • Associated sensory neuron (neuron of first order close to the receptor) must reach threshold • Information is encoded in the frequency of the stimuli – the greater the frequency, the stronger is the stimulus. Copyright © 2010 Pearson Education, Inc. Adaptation of Sensory Receptors • Adaptation is the change in sensitivity in the presence of constant stimulus • Adaptation occurs when • Receptor membranes become less responsive, or • Receptor potentials decline in frequency or stop • Receptors responding to pressure, touch, and smell adapt quickly • Receptors responding slowly include Merkel’s discs, Ruffini’s corpuscles (light and deep pressure respectively) • Pain receptors and proprioceptors do not exhibit adaptation (WHY?) Copyright © 2010 Pearson Education, Inc. Processing at the circuit Level • The circuit level role is to deliver the impulses to the appropriate region in the cerebral cortex. • The ascending tract typically consists of 3 neurons • First order neurons • cell bodies in a ganglion (dorsal or cranial) • Impulses from skin and proprioceptors to spinal cord or brain stem to a 2nd order neuron • Second order neuron • In the dorsal horn of the spinal cord or in the medulary nuclei • Transmit impulses to thalamus or cerebellum • Third order neurons • Cell bodies in the thalamus (no 3rd-order neurons in the cerebellum) • Transmit signals to the somatosensory cortex of the cerebrum Copyright © 2010 Pearson Education, Inc. Processing at the circuit Level • Impulses ascend in : • Non specific pathway that in general transmit pain, temperature and touch • Give branches to reticular formation and thalamus on the way up • Sends general information that is also involved in emotional aspects of perception • Specific ascending pathways involve in more precise aspect of sensation Copyright © 2010 Pearson Education, Inc. Processing at the Perceptual Level • Interpretation of sensory input occurs in the cerebral cortex • The ability to identify the sensation depends on the specific location of the target neurons in the sensory cortex not on the nature of the message (all messages are action potentials) Copyright © 2010 Pearson Education, Inc. Main Aspects of Sensory Perception • Perceptual detection – detecting that a stimulus has occurred and requires summation • Magnitude estimation – the ability to detect how intense the stimulus is • Spatial discrimination – identifying the site or pattern of the stimulus • Feature abstraction – used to identify a substance that has specific texture or shape • Quality discrimination – the ability to identify submodalities of a sensation (e.g., sweet or sour tastes) • Pattern recognition – ability to recognize patterns in stimuli (e.g., melody, familiar face) Copyright © 2010 Pearson Education, Inc. Motor ending and motor activity • Motor ending are the PNS elements that activate effectors by releasing neurotransmitters • Innervation of skeletal muscle • Somatic motor fibers innervate voluntary muscles and form neuromuscular junction. • At this junction the NT that is released is the Ach • Innervation of visceral muscle and glands • Junctions between autonomic motor endings and their effectors – smooth and cardiac muscles and glands • Acetylcholine and norepinephrine are used as neurotransmitters • Tend to response slower than the somatic motor endings Copyright © 2010 Pearson Education, Inc. Levels of Motor Control • The three levels of motor control are • Segmental level • Projection level • Precommand level Copyright © 2010 Pearson Education, Inc. Segmental Level • The segmental level is the lowest level of motor hierarchy • It consists of segmental circuits of the spinal cord • A segmental circuit activates network of ventral horn neurons in a certain segment causing the activation of a specific group of muscles • These circuits are called central pattern generators (CPGs) • A central pattern generator is a network of neurons which is able to exhibit rhythmic behavior in the absence of sensory input. • locomotion, breathing, chewing Copyright © 2010 Pearson Education, Inc. Projection Level • The projection level controls the spinal cord and consists of: • Upper motor neurons - Cortical motor areas that produce the direct (pyramidal) system • Voluntary movements of skeletal muscles • Brain stem motor areas that oversee the indirect system • Control reflex and CPG-controlled motor actions • Pass on information to lower motor neurons and send a copy of this information to higher command levels Copyright © 2010 Pearson Education, Inc. Precommand Level • The cerebral cortex is the highest level of conscious motor pathway but it is not the ultimate planner and coordinator of complex motor activities • Cerebellum • Acts on motor pathways through projection areas of the brain stem • Acts on the motor cortex via the thalamus • Basal nuclei • Inhibit various motor centers under resting conditions Copyright © 2010 Pearson Education, Inc. Structure of a Nerve • Nerve – cordlike organ of the PNS consisting of peripheral axons enclosed by connective tissue • Connective include: tissue coverings • Endoneurium – connective tissue surrounds axons loose that • Perineurium – coarse connective tissue that bundles fibers into fascicles • Epineurium – tough fibrous sheath around a nerve Copyright © 2010 Pearson Education, Inc. 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 • Types of fibers in mixed nerves: • Somatic afferent and somatic efferent • Visceral afferent and visceral efferent • Peripheral nerves classified as cranial or spinal nerves Copyright © 2010 Pearson Education, Inc. Ganglia • Contain neuron cell bodies associated with nerves • Dorsal root ganglia (sensory, somatic) (Chapter 12) • Autonomic ganglia (motor, visceral) (Chapter 14) Copyright © 2010 Pearson Education, Inc. Cranial Nerve I: Olfactory • Arises from the olfactory epithelium • Passes through the cribriform plate of the ethmoid bone • The axons will synapse on the olfactory bulb. • From the olfactory bulbs, Olfactory nerve continues through the olfactory tracts and can synapse on the • 1. Olfactory cortex (Rhinencephalon) for conscious interpretation • 2. Hypothalamus (Limbic system) for emotional response • 3. Autonomic Nervous System for other responses (digestive system) • Functions solely by carrying afferent impulses for the sense of smell Copyright © 2010 Pearson Education, Inc. Cranial Nerve II: Optic • Arises from the retina of the eye • Optic nerves pass through the optic canals and converge at the optic chiasm • They continue to the thalamus where they synapse • From there, the optic radiation fibers run to the visual cortex • Functions solely by carrying afferent impulses for vision Copyright © 2010 Pearson Education, Inc. Cranial Nerve III: Oculomotor • Fibers extend from the ventral midbrain, pass through the superior orbital fissure, and go to the extrinsic eye muscles • Functions in raising the eyelid, directing the eyeball, constricting the iris, and controlling lens shape Copyright © 2010 Pearson Education, Inc. Cranial Nerve IV: Trochlear • Fibers emerge from the dorsal midbrain and enter the orbits via the superior orbital fissures; innervate the superior oblique muscle • A motor nerve that directs the eyeball Copyright © 2010 Pearson Education, Inc. Cranial Nerve V: Trigeminal • Fibers run from the face to the pons, to the thalamus and to the primary somatosensory cortex • Three divisions: • ophthalmic (V1), sensory from face • maxillary (V2), • mandibular (V3) supplies motor fibers (V3) for mastication Copyright © 2010 Pearson Education, Inc. Cranial Nerve V: Trigeminal Copyright © 2010 Pearson Education, Inc. Figure V from Table 13.2 Cranial Nerve VI: Abdcuens • Fibers leave the inferior pons and enter the orbit via the superior orbital fissure • Primarily a motor nerve innervating the lateral rectus muscle Copyright © 2010 Pearson Education, Inc. Figure VI from Table 13.2 Cranial Nerve III, IV and VI: extrinsic eye muscles control Copyright © 2010 Pearson Education, Inc. http://www.neuroanatomy.wisc.edu/virtualbrain/Images/13N.jpg Cranial Nerve VII: Facial • Fibers leave the pons to the lateral aspect of the face • Mixed nerve with five major branches • Motor functions include facial expression, and the transmittal of autonomic impulses to lacrimal and salivary glands (subconscious) • Sensory function is taste from the anterior two-thirds of the tongue (taste buds to pons, to the thalamus, to the insula and parietal cortex for taste perception) Copyright © 2010 Pearson Education, Inc. Cranial Nerve VIII: Vestibulocochlear • Fibers arise from the hearing and equilibrium apparatus of the inner ear to enter the brainstem at the pons-medulla border • Two divisions – cochlear (hearing) and vestibular (balance) • Functions are solely sensory – equilibrium and hearing Copyright © 2010 Pearson Education, Inc. Cranial Nerve IX: Glossopharyngeal • Fibers emerge from the medulla and run to the throat • Nerve IX is a mixed nerve • Motor – innervates part of the tongue and pharynx, and provides motor fibers to the parotid salivary gland (autonomic) • Sensory – fibers conduct taste and general sensory impulses from the tongue and pharynx Copyright © 2010 Pearson Education, Inc. Cranial Nerve X: Vagus • The only cranial nerve that extends beyond the head and neck • Fibers emerge from the medulla The vagus is a mixed nerve • Most motor fibers are parasympathetic fibers to the heart, lungs, and visceral organs • Its sensory function is in taste Copyright © 2010 Pearson Education, Inc. Cranial Nerve XI: Accessory • Formed from a cranial root emerging from the medulla and a spinal root arising from the superior region of the spinal cord • A motor nerve • Supplies fibers to the larynx, pharynx, and soft palate • Innervates the trapezius and sternocleidomastoid, which move the head and neck Copyright © 2010 Pearson Education, Inc. Cranial Nerve XII: Hypoglossal • Fibers arise from the medulla • Innervates both extrinsic and intrinsic muscles of the tongue, which contribute to swallowing and speech Copyright © 2010 Pearson Education, Inc. Spinal Nerves: Rami • The short spinal nerves branch into three or four mixed, distal rami • Small dorsal ramus • Larger ventral ramus • Tiny meningeal branch – innervate the meninges and blood vessels within the vertebral canal • Rami communicantes at the base of the ventral rami in the thoracic region that contain autonomic nerve fibers Copyright © 2010 Pearson Education, Inc. Nerve Plexuses • All ventral rami except T2-T12 form 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 • Fibers travel to the periphery via several different routes • Each muscle receives a nerve supply from more than one spinal nerve • Damage to one spinal segment cannot completely paralyze a muscle Copyright © 2010 Pearson Education, Inc. Plexus Main spinal nerves Regions innervated Major nerves Cervical C1-C5 Skin and muscles of head & Phrenic (diaphragm) neck. Superior chest and shoulder Brachial C5-C8, T1 Shoulder and upper limbs Axillary Musculocutaneous Radial Median Ulnar Lumbar L1-L4 Antero-lateral abdominal Femoral wall, external genitalia, part of lower limbs Sacral L4-L5, S1-S4 Buttocks, perineum, lower limbs Copyright © 2010 Pearson Education, Inc. Sciatic Dermatomes • A dermatome is the area of skin innervated by the cutaneous branches of a single spinal nerve • All spinal nerves except C1 participate in dermatomes Copyright © 2010 Pearson Education, Inc. Reflexes • A reflex is a rapid, predictable motor response to a stimulus • Reflexes may be: • inborn (intrinsic)/basic: • breathing. • Putting a hand on a hot stove and quickly removing it, • learned (acquired) • Pavlov's dogs, every time Pavlov would feed the dogs he would ring a bell, before long even if there was no food if he rang the bell the dogs began to salivate. • Example: driving skills • Reflexes may • Involve only peripheral nerves and the spinal cord • Involve higher brain centers as well Copyright © 2010 Pearson Education, Inc. Copyright © 2010 Pearson Education, Inc. Reflex Arc • There are five components of a reflex arc • Receptor – site of stimulus • Sensory neuron – transmits the afferent impulse to the CNS • Integration center – either monosynaptic or polysynaptic region within the CNS • Motor neuron – conducts efferent impulses from the integration center to an effector • Effector – muscle fiber or gland that responds to the efferent impulse Copyright © 2010 Pearson Education, Inc. Spinal Reflexes • Spinal somatic reflexes • Integration center is in the spinal cord • Effectors are skeletal muscle • Testing of somatic reflexes is important clinically to assess the condition of the nervous system Copyright © 2010 Pearson Education, Inc. Stretch and Golgi Tendon Reflexes • For skeletal muscle activity to be smoothly coordinated, proprioceptor input is necessary • Muscle spindles inform the nervous system of the length of the muscle • Golgi tendon organs inform the brain as to the amount of tension in the muscle and tendons Copyright © 2010 Pearson Education, Inc. Stretch Reflexes • Maintain muscle tone in large postural muscles • Cause muscle contraction in response to increased muscle length (stretch) Copyright © 2010 Pearson Education, Inc.