Nervous System 3-PNS- Cranial Nerves and Reflexes PDF

1 Learning Objectives The Peripheral Nervous System: Sensory Describe the stimuli transduced by the major types of sensory receptors (e.g., mechanoreceptor, thermoreceptor, photoreceptor, chemoreceptor, baroreceptor, osmoreceptor, proprioceptor, and nociceptor [pain recep...

1 Learning Objectives The Peripheral Nervous System: Sensory Describe the stimuli transduced by the major types of sensory receptors (e.g., mechanoreceptor, thermoreceptor, photoreceptor, chemoreceptor, baroreceptor, osmoreceptor, proprioceptor, and nociceptor [pain receptor]). Describe the locations and functions of the first-, second-, and third-order neurons in a sensory pathway The Peripheral Nervous System: Motor Describe the locations and functions of the upper and lower motor neurons in a motor pathway Describe the major functions of each cranial nerve Identify and describe the formation, structure, and branches of a typical spinal nerve Define spinal plexus. Name the four major spinal plexuses and describe the distribution and function of the peripheral nerves arising from each plexus. Name the components of a reflex arc. Diagram or describe the neural pathway of the flexor (withdrawal) reflex: a painful stimulus to the right hand results in a response of the target muscle.  The Peripheral Nervous System PNS provides links from and to world outside our body Consists of all neural structures outside brain and spinal cord that can be broken down into four parts: Part 1 – Sensory Receptors Part 2 – Transmission Lines: Nerves and Their Structure and Repair Part 3 – Motor Endings and Motor Activity Part 4 – Reflex Activity © 2016 Pearson Education, Ltd.  Part 1 – Sensory Receptors and Sensation 13.1 Sensory Receptors Sensory receptors: specialized to respond to changes in environment (stimuli) – Activation results in graded potentials that trigger nerve impulses Awareness of stimulus (sensation) and interpretation of meaning of stimulus (perception) occur in brain Three ways to classify receptors: by type of stimulus, body location, and structural complexity © 2016 Pearson Education, Ltd.  Classification by Stimulus Type Mechanoreceptors—respond to touch, pressure, vibration, and stretch Thermoreceptors—sensitive to changes in temperature Photoreceptors—respond to light energy (example: retina) Chemoreceptors—respond to chemicals (examples: smell, taste, changes in blood chemistry) Nociceptors—sensitive to pain-causing stimuli (examples: extreme heat or cold, excessive pressure, inflammatory chemicals) © 2016 Pearson Education, Ltd.  Classification by Location Exteroceptors – Respond to stimuli arising outside body – Receptors in skin for touch, pressure, pain, and temperature – Most special sense organs Interoceptors (visceroceptors) – Respond to stimuli arising in internal viscera and blood vessels – Sensitive to chemical changes, tissue stretch, and temperature changes – Sometimes cause discomfort but usually person is unaware of their workings Proprioceptors – Respond to stretch in skeletal muscles, tendons, joints, ligaments, and connective tissue coverings of bones and muscles – Inform brain of one's movements © 2016 Pearson Education, Ltd.  13.2 Sensory Processing Survival depends upon: – Sensation: the awareness of changes in the internal and external environment – Perception: the conscious interpretation of those stimuli © 2016 Pearson Education, Ltd.  General Organization of the Somatosensory System Somatosensory system: part of sensory system serving body wall and limbs Receives inputs from: – Exteroceptors, proprioceptors, and interoceptors Input is relayed toward head, but processed along the way © 2016 Pearson Education, Ltd. Organization of the Somatosensory System Three main levels of neural integration in the somatosensory system: Receptor level – the sensor receptors Circuit level – ascending pathways Perceptual level – neuronal circuits in the cerebral cortex  Part 2 – Transmission Lines: Nerves and Their Structure and Repair 13.3 Nerves and Associated Ganglia Structure and Classification Nerve: cordlike organ of PNS Bundle of myelinated and nonmyelinated peripheral axons enclosed by connective tissue Two types of nerves: spinal or cranial, depending on where they originate © 2016 Pearson Education, Ltd.  Structure and Classification (cont.) Most nerves are mixtures of afferent and efferent fibers and somatic and autonomic (visceral) fibers Nerves are classified according to the direction they transmit impulses – Mixed nerves: contain both sensory and motor fibers Impulses travel both to and from CNS – Sensory (afferent) nerves: impulses only toward CNS – Motor (efferent) nerves: impulses only away from CNS © 2016 Pearson Education, Ltd. Peripheral Is the part of the nervous system outside the CNS. nervous Consists of the nerves that extend from the brain (cranial nerves) and from the spinal system cord (spinal nerves). Function: Carries messages to and from the CNS Sensory Conducts impulses from receptors to the CNS. Consists of: (afferent) Somatic afferent fibers (from skin, skeletal muscles and joints) division Visceral afferent fibers (from internal organs). Motor Transmits impulses from the CNS to effector organs (efferent) Somatic nervous system (from CNS to skeletal muscles) division Autonomic nervous system (smooth muscles, cardiac muscles and glands). Receptors Structures specialized to respond to stimuli - provide sensation (awareness of changes in the internal and external environments) Receptors by Mechanoreceptors – respond to mechanical stimuli (touch, pressure, vibration, stretch) stimulus type Thermoreceptors – sensitive to temperature changes Photoreceptors – in the retina, respond to light energy Chemoreceptors – respond to chemicals in solution (blood, interstitial fluid chemistry) Nociceptors – respond to potentially damaging stimuli that result in pain. Receptors by Exteroceptors – sensitive to stimuli arising outside the body location Interoceptors – respond to stimuli within the body (from internal organs and blood vessels); Proprioceptors – respond to stimuli in skeletal muscles, tendons, joints and ligaments. Dorsal roots – Afferent (to the CNS) sensory fibers that carry impulses from peripheral sensory receptors axons enter the (both somatic and visceral organs) to the spinal cord. spinal cord Ascending fiber tracts in the spinal cord - conduct sensory impulses to the brain. Dorsal root ganglia or cranial ganglia - contain the cell bodies of sensory neurons. The realization of these stimuli and perception (conscious interpretation of the stimuli) - occur in the brain (Somatosensory cortex and Sensory association areas). Nerve Cordlike organ of the PNS consisting of axons enclosed by connective tissue. Classification Sensory (afferent) – carry impulse to the CNS; of nerves Motor (efferent) – carry impulses from CNS; Mixed – sensory and motor fibers carry impulses to and from CNS; most common type of nerve. Cranial and Peripheral nerves originate from the brain (cranial nerves -12 pairs). Two Spinal Nerves attach to forebrain; rest with brain stem: or spinal column (spinal nerves 31 pairs). Summary Cranial Nerves (pp. 492–501; Fig. 13.6; Table 13.2) A. There are twelve pairs of cranial nerves that originate from the brain and, with the exception of the vagus nerve, serve areas of the head and neck (pp. 492–501; Fig. 13.6; Table 13.2). 1. Olfactory nerves (cranial nerve I) detect odors. 2. Optic nerves (cranial nerve II) are responsible for vision. 3. Oculomotor, trochlear, and abducens nerves (cranial nerves III, IV, and VI) allow movement of the eyeball. 4. Trigeminal nerves (cranial nerve V) allow sensation of the face and motor control of chewing muscles. 5. Facial nerves (cranial nerve VII) allow movement of muscles creating facial expression. 6. Vestibulocochlear nerves (cranial nerve VIII) are responsible for hearing and balance. 7. Glossopharyngeal nerves (cranial nerve IX) control the tongue and pharynx. 8. Vagus nerves (cranial nerve X) control several visceral organs. 9. Accessory nerves (cranial nerve XI) have a relationship with the vagus nerves. 10. Hypoglossal nerves (cranial nerve XII) innervate muscles of the tongue. 13 13.5 Spinal Nerves 31 pairs of spinal nerves All are mixed nerves named for point of issue from spinal cord Supply all body parts except head and part of neck 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 tiny coccygeal nerves (C0) © 2016 Pearson Education, Ltd. Figure 13.7 Spinal nerves. Cervical plexus Cervical nerves C1 – C8 Brachial plexus Cervical enlargement Intercostal Thoracic nerves nerves T1 – T12 Lumbar enlargement Lumbar plexus Lumbar nerves L1 – L5 Sacral plexus Sacral nerves S1 – S5 Cauda equina Coccygeal nerve © 2016 Pearson Education, Ltd. Co1 13.5 Spinal Nerves Each spinal nerve is connected to spinal cord via two roots: Ventral roots Contain motor (efferent) fibers from ventral horn motor neurons that innervate skeletal muscles Dorsal roots Contain sensory (afferent) fibers from sensory neurons in dorsal root ganglia that conduct impulses from peripheral receptors Both ventral and dorsal roots are branched medially as rootlets that then join laterally to form spinal nerve © 2016 Pearson Education, Ltd. Innervation of Specific Body Regions (cont.) Fibers from ventral ramus go to body periphery via several routes Means each limb muscle is innervated by more than one spinal nerve, so damage to one does not cause paralysis Cervical Brachial Lumbar Sacral © 2016 Pearson Education, Ltd. Innervation of Specific Body Regions (cont.) Innervation of skin: dermatomes Dermatome: area of skin innervated by cutaneous branches of single spinal nerve All spinal nerves except C 1 participate in dermatomes Extent of spinal cord injuries ascertained by affected dermatomes Most dermatomes overlap, so destruction of a single spinal nerve will not cause complete numbness Innervation of joints To remember which nerves serve which synovial joint, use: Hilton’s law: Any nerve serving a muscle that produces movement at a joint also innervates that joint and skin over that joint © 2016 Pearson Education, Ltd. Figure 13.13 Map of dermatomes. C2 C3 C2 C4 C3 C5 C6 C4 C7 C8 C5 T1 T1 T2 C5 T2 T3 T3 T4 T4 T2 T5 T2 T6 T5 T7 T6 T8 T9 T7 T10 T8 C6 C6 C5 T9 C5 T11 C7 T12 C7 T10 L1 C6 C6 L2 S1 T11 C8 L3 C8 L4 S2 L5 T12 S3 C6 L1 L1 C6 S4 S2 C7 S5 C7 C8 S3 C8 L2 L2 S1 S2 S2 S1 L3 L3 L1 L5 L2 L5 L4 L4 L3 L5 L5 L4 S1 S1 L4 L4 L5 L5 S1 Anterior view Posterior view © 2016 Pearson Education, Ltd. Part 3 – Motor Endings and Motor Activity Motor endings: PNS elements that activate effectors by releasing neurotransmitters These elements innervate skeletal muscle Takes place at neuromuscular junction Neurotransmitter acetylcholine (ACh) is released when nerve impulse reaches axon terminal ACh binds to receptors, resulting in: Movement of Na + and K+ across membrane Depolarization of muscle cell An end plate potential, spreads to adjacent areas of sarcolemma, which triggers opening of Na+ voltage-gated channels Results in an action potential, which leads to muscle contraction © 2016 Pearson Education, Ltd. Innervation of Visceral Muscle and Glands Autonomic motor endings and visceral effectors are simpler than somatic junctions Branches form synapses en passant (“synapses in passing”) with effector cells via varicosities Acetylcholine and norepinephrine act indirectly via second messengers Visceral motor responses are slower than somatic responses © 2016 Pearson Education, Ltd. Part 4 – Reflex Activity Inborn (intrinsic) reflex: rapid, involuntary, predictable motor response to stimulus Examples: maintain posture, control visceral activities Can be modified by learning and conscious effort Learned (acquired) reflexes result from practice or repetition Example: driving skills © 2016 Pearson Education, Ltd. Components of a Reflex Arc Components of a reflex arc (neural path) 1. Receptor: site of stimulus action 2. Sensory neuron: transmits afferent impulses to CNS 3. Integration center: either monosynaptic or polysynaptic region within CNS 4. Motor neuron: conducts efferent impulses from integration center to effector organ 5. Effector: muscle fiber or gland cell that responds to efferent impulses by contracting or secreting © 2016 Pearson Education, Ltd.  Reflex Arc There are five components of a reflex arc: Receptor Sensory neuron Integration center Motor neuron Effector Reflex arc Spinal Reflexes Spinal reflexes occur without direct involvement of higher brain centers Brain is still advised of spinal reflex activity and may have an effect on the reflex Testing of somatic reflexes important clinically to assess condition of nervous system If exaggerated, distorted, or absent, may indicate degeneration or pathology of specific nervous system regions Most commonly assessed reflexes are stretch, flexor, and superficial reflexes © 2016 Pearson Education, Ltd.  Summary Spinal 31 pairs of mixed (sensory and motor) nerves arise from the spinal cord and nerves innervate all parts of the body except the head. Nerve contain fibers from several spinal nerves. plexuses Reflex A rapid motor response to a stimulus. There are five components of a reflex arc: 1) Receptor, 2) Sensory neuron, 3) Integration center, 4) Motor neuron, 5) Effector. Motor Neuromuscular junctions at skeletal muscles Endings Synapses en passant via varicosities at smooth muscles and glands.