4/5/2015. Peripheral nervous system (PNS)

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1 Peripheral Nervous System Learn and Understand: Peripheral nerves connect the edges of the body and outside world to the CNS. Most nerves carry specific types of information to/from specific locations. PNS includes nervous-like tissue: sensitive and excitable. Humans possess both instinctual and learned reflexes. Many subconscious and involuntary, many protective, many postural/positional, most homeostatic. Figure 13.1 Place of the PNS in the structural organization of the nervous system. Provides neuronal link to and from body and outside world Central nervous system (CNS) Peripheral nervous system (PNS) Sensory (afferent) division Motor (efferent) division Somatic nervous system Autonomic nervous system (ANS) Sympathetic division Parasympathetic division Includes all neural structures outside brain Sensory receptors Peripheral nerves and associated ganglia, plexuses Efferent motor endings Figure 13.4b Structure of a nerve. Axon Myelin sheath Endoneurium Perineurium Epineurium Fascicle Blood vessels 1

2 Peripheral Nervous Tissue: Classification of Nerves Most nerves are mixtures of afferent and efferent fibers and somatic and autonomic fibers Classified according to direction of impulses Mixed nerves both sensory and motor fibers; impulses both to and from CNS Sensory (afferent) nerves impulses only toward CNS Motor (efferent) nerves impulses only away from CNS Peripheral nerves classified as cranial or spinal nerves Spinal Nerves 31 pairs of mixed nerves named for point of issue from spinal cord Supply all body parts but head and part of neck 8 cervical (C 1 C 8 ) 12 thoracic (T 1 T 12 ) 5 Lumbar (L 1 L 5 ) 5 Sacral (S 1 S 5 ) 1 Coccygeal (C 0 ) Figure 13.8a Formation of spinal nerves and rami distribution. Dorsal root ganglion Dorsal ramus of spinal nerve Ventral ramus of spinal nerve Spinal nerve Rami communicantes Sympathetic trunk ganglion Ventral root Dorsal root Dorsal and ventral rootlets of spinal nerve Anterior view showing spinal cord, associated nerves, and vertebrae. The dorsal and ventral roots arise medially as rootlets and join laterally to form the spinal nerve. 2

3 Figure 13.8b Formation of spinal nerves and rami distribution. Dorsal ramus Ventral ramus Spinal nerve Rami communicantes Sympathetic trunk ganglion Intercostal nerve Dorsal root ganglion Dorsal root Ventral root Branches of intercostal nerve Lateral cutaneous Anterior cutaneous Sternum Cross section of thorax showing the main roots and branches of a spinal nerve. Figure Map of dermatomes Specificity of sensory information carried by individual spinal nerves C2 T2 C5 C7 C8 L1 L2 C4 C5 T1 T2 T3 T4 T5 T6 T7 T8 T9 T10 T11 T12 S2 S3 L1 C2 C3 L2 T2 C5 C7 C8 C7 C8 L2 L4 C3 C4 C5 C7 C8 T1 T2 T3 T4 T5 T6 T7 T8 T9 T10 S1 S4 S5 S2 S3 T11 T12 L1 L3 C5 C7 C8 S1 S2 S2 S1 L3 L3 L1 L4 L4 L2 L3 L4 S1 S1 L4 L4 S1 Anterior view Posterior view Cranial Nerves Twelve pairs of nerves associated with brain Ten attach to brain stem Most are mixed nerves; two pairs purely sensory Each numbered (I through XII) and named from rostral to caudal "On occasion, our trusty truck acts funny very good vehicle anyhow" "Oh once one takes the anatomy final, very good vacations are heavenly" 3

4 Figure 13.6a Location and function of cranial nerves. Filaments of olfactory nerve (I) Frontal lobe Olfactory bulb Olfactory tract Optic nerve (II) Optic chiasma Optic tract Oculomotor nerve (III) Trochlear nerve (IV) Trigeminal nerve (V) Abducens nerve (VI) Cerebellum Medulla oblongata Temporal lobe Infundibulum Facial nerve (VII) Vestibulocochlear nerve (VIII) Glossopharyngeal nerve (IX) Vagus nerve (X) Accessory nerve (XI) Hypoglossal nerve (XII) Primary Functions of Cranial Nerves Nerve Sensory Function Somatic Motor Function I olfactory Smell None None II optic Vision None None III oculomotor None Extrinsic eye muscles (rectus and inf. oblique); elevates eyelid IV trochlear None Superior oblique None V trigeminal General Senses: anterior scalp, nasal cavity, nasopharynx, face, most of oral cavity, anterior tongue, part of auricle, meninges Muscles of mastication Parasympathetic Motor Function Intrinsic eye muscles control pupil and lens None VI abducens None Lateral rectus None VII facial Taste: anterior tongue Muscles of facial expression Secretion of tears, saliva VIII vestibulocochlear IX glossopharyngeal X vagus Hearing, equilibrium None None General Sensory and taste: posterior tongue; General Sensory: part of pharynx; Visceral Sensory: carotid bodies Visceral Sensory: heart, lungs, abdominal organs; General Sensory: ear canal, pharynx, larynx One pharyngeal muscle Most pharyngeal muscles all laryngeal muscles XI accessory None Trapezius, sternocleidomastoid XII - hypoglossal none Intrisic and extrinsic tongue muscles Secretion of parotid salivary glands Smooth muscle and glands: heart, lungs, larynx, trachea, abdominal organs None None Peripheral Nervous Tissue: Sensory Receptors Specialized to respond to changes in environment (stimuli) Activation results in graded potentials that trigger nerve impulses Sensation (awareness of stimulus) and perception (interpretation of meaning of stimulus) occur in brain 4

5 Classification by Stimulus Type Mechanoreceptors respond to touch, pressure, vibration, and stretch Thermoreceptors sensitive to changes in temperature 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) Classification by Location Exteroceptors respond to stimuli arising outside body Receptors in skin for touch, pressure, pain, and temperature and 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 May cause discomfort but usually 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 movements, body part position/location Table 13.1 General Sensory Receptors Classified by Structure and Function 5

6 Table 13.1 General Sensory Receptors Classified by Structure and Function (2 of 3) Processing at the Receptor Level In general sense receptors, graded potential called generator potential Stimulus Generator potential in afferent neuron Action potential Processing at the Receptor Level In special sense receptors: Stimulus Graded potential in receptor cell called receptor potential Affects amount of neurotransmitter released Neurotransmitters generate graded potentials in sensory neuron 6

7 Figure 13.2 Three basic levels of neural integration in sensory systems. 3 Perceptual level (processing in cortical sensory centers) Motor cortex Somatosensory cortex Thalamus 2 Circuit level (processing in ascending pathways) Reticular formation Pons Medulla Spinal cord Cerebellum Free nerve endings (pain, cold, warmth) Muscle spindle 1 Receptor level (sensory reception and transmission to CNS) Joint kinesthetic receptor Peripheral Nervous Function: Basic Reflexes Basic functional unit of nervous system and simplest portion capable of receiving a stimulus and producing a response Automatic response to a stimulus that occurs without conscious thought. Quick, protective, homeostatic. Variety of Reflexes Some integrated within spinal cord; some within brain (those related to head) using cranial nerves Some involve excitatory neurons yielding a response; some involve inhibitory neurons that prevent an action Higher brain centers can influence, suppress, or exaggerate reflex responses Functional classification Somatic reflexes Activate skeletal muscle Autonomic (visceral) reflexes Activate visceral effectors (smooth or cardiac muscle or glands) 7

8 Figure The five basic components of all reflex arcs Stimulus Receptor site of stimulus action Sensory neuron transmits afferent impulses to CNS Integration center either monosynaptic or polysynaptic region within CNS Skin Motor neuron conducts efferent impulses from integration center to effector organ Effector muscle fiber or gland cell that responds to efferent impulses by contracting or secreting Receptor Interneuron Sensory neuron Integration center Motor neuron Effector Spinal cord (in cross scetion) Stretch and Tendon Reflexes To smoothly coordinate skeletal muscle nervous system must receive proprioceptor input regarding Length of muscle From muscle spindles Amount of tension in muscle From tendon organs Flower spray endings (secondary sensory endings) Anulospiral endings (primary sensory endings) Muscle spindle Capsule (connective tissue) Efferent (motor) fiber to muscle spindle Efferent (motor) fiber to extrafusal muscle fibers Extrafusal muscle fiber Intrafusal muscle fibers Sensory fiber Tendon organ Tendon Figure Anatomy of the muscle spindle and tendon organ. 8

9 Excited in two ways 1. External stretch of muscle and muscle spindle 2. Internal stretch of muscle spindle Stretch causes increased rate of impulses to spinal cord Adjustment for moving/contracting muscles: Contracting muscle reduces tension on muscle spindle Sensitivity lost unless muscle spindle shortened by impulses in motor neurons coactivation maintains tension and sensitivity of spindle during muscle contraction Figure 13.17a Operation of the muscle spindle. How muscle stretch is detected How muscle stretch is detected Muscle spindle Intrafusal muscle fiber Sensory fiber Extrafusal muscle fiber Time Unstretched muscle. Action potentials (APs) are generated at a constant rate in the associated sensory fiber. Time Stretched muscle. Stretching activates the muscle spindle, increasing the rate of APs. Stretch Reflexes How stretch reflex works Stretch activates muscle spindle Sensory neurons synapse directly with motor neurons in spinal cord motor neurons cause stretched muscle to contract All stretch reflexes are monosynaptic and ipsilateral 9

10 Stretch Reflexes Reciprocal inhibition also occurs IIa fibers synapse with interneurons that inhibit motor neurons of antagonistic muscles Example: In patellar reflex, stretched muscle (quadriceps) contracts and antagonists (hamstrings) relax Figure Stretch Reflex (2 of 2) Slide 6 The patellar (knee-jerk) reflex an example of a stretch reflex 2 Quadriceps (extensors) 1 Muscle spindle Excitatory synapse Inhibitory synapse Hamstrings (flexors) 3a 3b 3b Patella Spinal cord (L 2 L 4) 1 Tapping the patellar ligament stretches the quadriceps and excites its muscle spindles. Patellar ligament 2 Afferent impulses (blue) travel to the spinal cord, where synapses occur with motor neurons and interneurons. 3a The motor neurons (red) send activating impulses to the quadriceps causing it to contract, extending the knee. 3b The interneurons (green) make inhibitory synapses with ventral horn neurons (purple) that prevent the antagonist muscles (hamstrings) from resisting the contraction of the quadriceps. The Tendon Reflex Polysynaptic reflexes Helps prevent damage due to excessive stretch Important for smooth onset and termination of muscle contraction Produces muscle relaxation (lengthening) in response to tension 10

11 Figure The tendon reflex. Slide 5 1 Quadriceps strongly contracts. Tendon organs are activated. 2 Afferent fibers synapse with interneurons in the spinal cord. Interneurons Quadriceps (extensors) Tendon organ Spinal cord Excitatory synapse Inhibitory synapse Hamstrings (flexors) 3a Efferent impulses to muscle with stretched tendon are damped. Muscle relaxes, reducing tension. 3b Efferent impulses to antagonist muscle cause it to contract. The Flexor and Crossed-Extensor Reflexes Flexor (withdrawal) reflex Initiated by painful stimulus Causes automatic withdrawal of threatened body part Ipsilateral and polysynaptic Protective; important Brain can override E.g., finger stick for blood test Flexor and Crossed-Extensor Reflexes Crossed extensor reflex Occurs with flexor reflexes in weight-bearing limbs to maintain balance Consists of ipsilateral withdrawal reflex and contralateral extensor reflex Stimulated side withdrawn (flexed) Contralateral side extended e.g., step barefoot on broken glass 11

12 Figure The crossed-extensor reflex. Excitatory synapse Inhibitory synapse Afferent fiber Interneurons Efferent fibers Efferent fibers Extensor inhibited Flexor stimulated Arm movements Flexor inhibited Extensor stimulated Site of stimulus: A noxious stimulus causes a flexor reflex on the same side, withdrawing that limb. Site of reciprocal activation: At the same time, the extensor muscles on the opposite side are activated. 12