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Chapter 13 The Peripheral Nervous System and Reflex Activity

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1 Chapter 13 The Peripheral Nervous System and Reflex Activity
J.F. Thompson, Ph.D. & J.R. Schiller, Ph.D. & G. Pitts, Ph.D.

2 Overview of PNS Modality Selectivity of Receptors
each input is a specific type of sensation: temperature, pain, pressure, touch, body position, equilibrium, hearing, vision, smell, taste individual sensory neurons generally carry only one modality Selectivity of Receptors sensory neurons respond strongly to one type of stimulus and weakly or not at all to other types some respond accidentally to other types of stimuli – rubbing one’s eyes mechanically stimulates the eyes’ light receptors in the retina

3 Components of Sensation
stimulation - stimulus or change in the environment transduction requires a sensory receptor cell or organ which responds to specific stimuli and converts them into receptor/generator potentials impulse generation and conduction if a receptor/generator potential (graded potential) reaches a threshold, then the neuron’s action potential will be sent to the CNS integration some region of the CNS must receive and translate nerve impulses into sensations and perceptions this generally occurs in the cerebral cortex

4 Sensory Receptor Classification
Classified by the type of stimulus Mechanoreceptors mechanical pressure or stretching generate action potentials when deformed Thermoreceptors - changes in temperature Nociceptors - pain due to physical or chemical damage to nearby tissue Photoreceptors light strikes retinal receptor cells generates action potentials in response to light energy Chemoreceptors – certain specific chemical molecules are detected in the mucous fluids of the GI & respiratory tracts, or in the blood or other body fluids

5 Sensory Receptor Classification
Classified by location Exteroceptors located at or near the body’s surface provide information about the external environment Interoceptors (visceroceptors) found in blood vessels, connective tissues, and organs provide information about the internal environment Proprioceptors located in muscles, tendons, joints and the internal ear provide information about gravity, body and limb positions and skeletal muscle movements

6 Adaptation by Sensory Receptors
a change in sensitivity to a long-lasting stimulus primarily by rapidly-adapting phasic receptors pressure, touch, hearing, smell adapt very quickly, i.e., respond less if the stimulus remains constant allows us to shut out background “noise” little adaptation by slowly-adapting tonic receptors pain, body position, chemicals in the blood or CSF adapt slowly, continue to respond even when the stimulus remains constant continuous input is useful for some modalities because the body needs to make continuous responses to that kind of information

7 Anesthesia a partial or complete loss of sensation
General anesthesia – gas agents act in the CNS through poorly understood mechanisms Local anesthesia – drugs injected near peripheral nerves inhibit the opening of gated sodium channels, preventing local transmission of action potentials Analgesia – reduced perception of pain without loss of other sensory information or loss of consciousness: biochemical interference with local stimulus (NSAIDs) or mimic endogenous endorphins in CNS (opiates) Paresthesias – abnormal sensations (burning, tingling, numbness) not related to normal stimulation, e.g., mechanical pressure on nerves in your leg puts your foot “to sleep”

8 Functional Types of Nerves
mixed nerves – contain both sensory and motor fibers motor (efferent) nerves [Note: so-called pure “motor” nerves do also carry proprioceptive sensory signals back to the CNS from the skeletal muscles, joints, and tendons being served by that nerve.] sensory (afferent) nerves

9 Anatomy of Nerves bundles/fascicles of axons & dendrites
endoneurium – around individual processes perineurium around fascicles individual nerve fibers with their endoneurium epineurium - outermost covering around entire peripheral nerve

10 Cranial Nerves Twelve pairs of nerves which originate from the brain and exit through foramina of the skull First 2 pairs originate from the forebrain (olfactory, optic) Remaining 10 pairs originate from the brain stem

11 What You Should Know About the Cranial Nerves
name and number general region(s) served main functional roles modality (sensory, motor*, mixed (m/s)) (* motor nerves carry proprioception sensory information back to the CNS) See Table 13.2 pp and slides after end slide in this PPT for details

12 Spinal Nerves 31 pairs of spinal nerves originate from the spinal cord
All are mixed (m/s) nerves Thousands of fibers per spinal nerve Each pair serves a particular region of the body Each pair also provides some service to the region supplied by the spinal nerve above it and the spinal nerve below it (redundancy)

13 Spinal Nerve Anatomy Spinal nerves are very short, they divide almost immediately dorsal ramus - supplies posterior body trunk ventral ramus - supplies the rest of body trunk and the limbs meningeal branch - supplies the meninges and blood vessels within meninges

14 Dorsal and Ventral Rami of a Typical Spinal Nerve

15 Dermatomes Areas of skin innervated by the cutaneous branches of each pair of spinal nerves Each pair also provides some service to the region of the spinal nerve above and the spinal nerve below (redundancy)

16 Reflex Activity a reflex is a rapid, predictable, automatic response to a stimulus a reflex is unlearned, unpremeditated, and involuntary one is conscious of somatic reflexes only after they occur reflexes are involved in homeostasis two fundamental types of reflexes somatic reflexes - produce contraction of skeletal muscle autonomic (visceral) reflexes generally, they are not perceived consciously produce responses by smooth muscle, cardiac muscle, glands to adjust conditions of the internal environment

17 Components of a Reflex Arc
5 Functional Components receptor - dendrites or other sensory structures respond to changes in the environment sensory neuron - conducts an impulse from a receptor to its axon terminals integrating center (some region within the CNS) simple - monosynaptic (2 cells only: sensory and motor neurons) complex – polysynaptic (> 2 cells: interneurons involved) motor neuron - impulses from integrating center to an effector effector - body part (muscle or gland) which responds to the motor nerve impulse

18 Stretch Reflexes receptors - muscle spindles and Golgi tendon organs
sensory mechanoreceptors which respond to stretching increased tension (stretching) stimulates the receptors sends proprioceptive inputs to spinal cord contraction of the skeletal muscle reduces tension on the muscle spindle lowers the rate of action potential generation decreases input to the spinal cord and higher centers: cortex and cerebellum

19 Stretch Reflexes Remember that if a muscle is being stretched, the stretch is caused by the contraction of its antagonist. This sensory proprioception information contributes to maintaining proper muscle tone.

20 Patellar Reflex monosynaptic ipsilateral (same side)
segmental (at one level of the spinal cord) polysynaptic component – for reciprocal inhibition of the antagonist

21 Golgi (Deep) Tendon Reflex
an increase in muscle tension activates receptors (Golgi tendon organ) in the tendon the muscle relaxes and lengthens in response to its antagonist’s contraction D-T reflex inhibits the agonist D-T reflex excites the antagonist helps to regulate a smooth start and stop for a contraction input from the Golgi tendon organs are sent to the cerebellum and the cortex polysynaptic, ipsilateral, and segmental

22 Flexor Reflex a pull on the limb, extending it, will trigger the reflex also a painful stimulus – a burn, pin prick, toe stub, etc. F-R causes an automatic withdrawal from the (dangerous) stimulus polysynaptic, ipsilateral, and segmental

23 Crossed Extensor Reflex
flexion of a body part is often balanced by extension of the same body part on the opposite side of the body polysynaptic contralateral segmental

24 End Chapter 13 [Note: Summary slides for the cranial nerves appear after this slide for your exam 4 review.]

25 Cranial Nerve I: Olfactory
Passes through the cribriform plate of the ethmoid bone Fibers run through the olfactory bulb and terminate in the primary olfactory cortex Functions solely by carrying afferent impulses for the sense of smell

26 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

27 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 Parasympathetic cell bodies are in the ciliary ganglia Proprioceptor afferents from extrinsic eye muscles

28 Cranial Nerve IV: Trochlear
Fibers emerge from the dorsal midbrain and enter the orbits via the superior orbital fissures; innervate the superior oblique muscle Primarily a motor nerve that directs the eyeball Proprioceptor afferents from extrinsic eye muscles

29 Cranial Nerve V: Trigeminal
Composed of three divisions: ophthalmic (V1), maxillary (V2), and mandibular (V3) Fibers run from the face to the pons via the superior orbital fissure (V1), the foramen rotundum (V2), and the foramen ovale (V3) Conveys sensory impulses from various areas of the face (V1) and (V2), and supplies motor fibers (V3) for mastication

30 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

31 Cranial Nerve VII: Facial
Fibers leave the pons, travel through the internal acoustic meatus, and emerge through the stylomastoid foramen 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 Sensory function is taste from the anterior two-thirds of the tongue

32 Cranial Nerve VIII: Vestibulocochlear
Fibers arise from the hearing and equilibrium apparatus of the inner ear, pass through the internal acoustic meatus, and enter the brainstem at the pons-medulla border Two divisions – cochlear (hearing) and vestibular (balance) Functions are solely sensory – equilibrium and hearing

33 Cranial Nerve IX: Glossopharyngeal
Fibers emerge from the medulla, leave the skull via the jugular foramen, and run to the throat Nerve IX is a mixed nerve with motor and sensory functions Motor – innervates part of the tongue and pharynx, and provides motor fibers to the parotid salivary gland Sensory – fibers conduct taste and general sensory impulses from the tongue and pharynx

34 Cranial Nerve X: Vagus The only cranial nerve that extends beyond the head and neck Fibers emerge from the medulla via the jugular foramen 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

35 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 The spinal root passes upward into the cranium via the foramen magnum The accessory nerve leaves the cranium via the jugular foramen Primarily a motor nerve Supplies fibers to the larynx, pharynx, and soft palate Innervates the trapezius and sternocleidomastoid, which move the head and neck

36 Cranial Nerve XII: Hypoglossal
Fibers arise from the medulla and exit the skull via the hypoglossal canal Innervates both extrinsic and intrinsic muscles of the tongue, which contribute to swallowing and speech

37 End Chapter 13 Cranial Nerve Slides

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