PHYL141 C13 Cranial Nerves, Peripheral Nervous System & General Senses

Structural Classification of Sensory Receptors

1) modified sensory neuron with free nerve ending 2) modified sensory neuron with nerve endings covered by cupola (or capsule); sensitive to pressure and touch. 3) modified sensory neuron with a separate receptor cell; there are two or more cells involved in the processing of the stimulus = found with special senses like retina.

Regeneration of Nerve Fiber in a peripheral nerve

1) the axon becomes fragmented at the injury site. 2) macrophages clean out the dead axon distal to the injury. 3) axon sprouts, or filaments, grow through a regeneration tube formed by Schwann cells. note: for this to happen successfully, the gap between the developing axon and the neuron downstream cannot be too large; otherwise the developing axon will never find the correct tube. 4) the axon regenerates and a new myelin sheath forms.

Classification of Nerves

Classified according to direction transmit 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 Most nerves are mixtures of afferent and efferent fibers and somatic and autonomic (visceral) fibers

Sensation

Conscious or sub-conscious awareness of changes in external or internal environment (stimuli). • can occur in the thalamus

Trigeminal Nerve (V)

Divisions 1) Ophthalmic nerve (eye) 2) Maxillary nerve (upper jaw) 3) Mandibular nerve (lower jaw) Sensory Function: Touch, Pain, Temperature, & Proprioception of 3 divisions Motor Function: Mandibular portion innervates chewing muscles

Main Aspects of Sensory Perception

Feature abstraction: identification of more complex aspects and several stimulus properties Quality discrimination: ability to identify submodalities of a sensation (e.g., sweet or sour tastes) Pattern recognition: recognition of familiar or significant patterns in stimuli (e.g., melody in piece of music)

Transmission

Generation of action potentials.

Three nerves innervate the extrinsic muscles of the eye, providing both Proprioception (sensory) and movement (motor) of the eye

III Oculomotor IV Trochlear VI Abducens

Processing at the Perceptual Level

Interpretation of sensory input depends on specific location of target neurons in sensory cortex Aspects of sensory perception: • Perceptual detection: ability to detect a stimulus (requires summation of impulses) • Magnitude estimation: intensity coded in frequency of impulses • Spatial discrimination: identifying site or pattern of stimulus (studied by two-point discrimination test)

Sensory Integration

Levels of neural integration in sensory systems: 1. Receptor level—sensory receptors 2. Circuit level—processing in ascending pathways 3. Perceptual level—processing in cortical sensory areas

Alternate Classifications of Sensory Receptors

Location of receptors & origin of stimuli Type of stimuli they detect Structural classification

Regeneration of Nerve Fibers

Mature neurons are amitotic but if soma of damaged nerve is intact, peripheral axon may regenerate If peripheral axon damaged • Axon fragments (Wallerian degeneration); spreads distally from injury • Macrophages clean dead axon; myelin sheath intact • Axon filaments grow through regeneration tube • Axon regenerates; new myelin sheath forms • Greater distance between severed ends-less chance of regeneration Most CNS fibers never regenerate because CNS does not have neuroglemma. CNS oligodendrocytes bear growth: inhibiting proteins that prevent CNS fiber regeneration Astrocytes at injury site form scar tissue containing chondroitin sulfate that blocks axonal regrowth Treatment: Neutralizing growth inhibitors, blocking receptors for inhibitory proteins, destroying chondroitin sulfate promising

Processing at the Circuit Level

Pathways of three neurons conduct sensory impulses upward to appropriate cortical regions First-order sensory neurons: Conduct impulses from receptor level to spinal reflexes or second-order neurons in CNS Second-order sensory neurons: Transmit impulses to third-order sensory neurons Third-order sensory neurons: Conduct impulses from thalamus to the somatosensory cortex (perceptual level)

Proprioceptive Sensations

Proprioceptors detect changes in body position and movement. Adapts SLOWLY. 1) Muscle Spindles: Monitor changes in muscle length 2) Tendon Organs: Monitor tendon stretch 3) Joint Kinesthetic Receptors: Located in synovial capsule; adjust muscle tone when excessive strain is placed on joint

Facial Nerve (VII)

Sensory Function: Proprioception and Taste on first 2/3 tongue Motor Functions: • Innervation of Facial Muscles (Allows for Facial Expression) • Secretion of Saliva and Tears (ANS)

Abducens Nerve (VI)

Sensory Function: Proprioception of eye muscles Motor Function: Innervates external eye muscles (moves eye)

Processing at the Receptor Level

To produce a sensation • Receptors have specificity for stimulus energy • Stimulus must be applied in receptive field • Transduction occurs: Stimulus changed to graded potential (Generator potential or receptor potential) Graded potentials must reach threshold of Action Potential

What types of anesthetics/analgesics stop/reduce processes in the pain pathway?

Transduction: NSAIDs and local anesthetics Transmission: opioids and glutamate & aspartate blocker Modulation: tricyclic antidepressants Perception: opioids and non-pharmacological therapies

Cranial Nerves

Twelve pairs of cranial nerves originate from the brain Exit the skull through formanina. Named primarily on the basis of distribution and numbered by order of attachment to the brain. Some cranial nerves (I, II, and VIII) contain only sensory fibers and are called sensory nerves. The rest contain both sensory and motor fibers and are called mixed nerves.

What is meant by "mixed" nerves?

both sensory and motor fibers; impulses both to and from CNS

sensory (afferent) division

brings nerve impulses from outside the body to the brain/spinal chord.

Chemoreceptors

classification by stimulus type respond to chemicals (e.g., smell, taste, changes in blood chemistry)

Photoreceptors

classification by stimulus type respond to light energy (e.g., retina)

Mechanoreceptors

classification by stimulus type respond to touch, pressure, vibration, and stretch

Exteroceptors

classified by location Respond to stimuli arising outside body Receptors in skin for touch, pressure, pain, and temperature Most special sense organs

Bell's Palsy

condition that causes the paralysis of facial muscles on one side; affects facial nerve (CN 7). loss of sensation on the skin of that side of the face. loss of taste in that area of the mouth. occurs very abruptly, sometimes caused by herpes virus or trauma or unknown reason. sometimes its temporary or other times its permanent.

receptor potentials

graded potential that occurs in a specialized sensory receptor that is not derived from a sensory neuron. sensory neuron is responsible for transmission of nerve impulses.

Special Senses

located exclusively in the head: • Taste • Smell • Vision • Hearing • Balance

Vagus Nerve (X)

most important in the autonomic nervous system because 90% of our parasympathetic axons travel by the vagus nerve. enervates nerves all over the body. Sensory Functions: • Taste & somatic sensations from epiglottis and pharynx • Monitors: O2, CO2, BP, & Breathing (rate & depth) • Sensations from viscera in thorax and abdomen Motor Functions: • Swallowing, Coughing, & Voice • Stimulation and relaxation of GI tract muscles (ANS) • Stimulate digestive fluids (ANS) • Heart Rate (ANS) Controls: digestive system, heart, lungs, and some glands

motor (efferent) division

nerve impulses traveling away from the central nervous system to effectors like muscles and glands. motor division can be divided into somatic and autonomic nervous systems.

What is the difference between a nerve and a neuron?

nerve is a bundle of cells providing an environment for transporting signals from neurons. neuron is a single cell responsible for generating electrochemical signals

Referred Pain

pain from internal organs is often perceived to originate from other areas of the body. since action potentials from internal pain receptors are carried by same spinal nerve segments that transmit action potentials from skin and skeletal muscles. results in confusion about the origin of pain. *green is overlapping referred pain.

What are the three levels of sensory integration? Where does each occur?

receptor level occurs at sensory reception circuit level processed in ascending pathways perceptual level processed in the cortical sensory centers

Pacinian corpuscles

respond to deep preassure

Meissner's corpuscles

responsible for fine touch and stereogenesis.

trochlear nerve (IV)

sensory function: Proprioception of eye muscles Motor Function: Innervates external eye muscles/ moves eye

oculomotor nerve (III)

sensory function: proprioception of eye muscles motor function: innervates external and internal eye muscles (moves eye and dilates pupil)

olfactory nerve (I)

sensory function: sense of smell no motor functions

optic nerve (II)

sensory function: vision (attaches to retina) no motor functions

receptor

structure specialized to receive stimuli. converts stimuli to electrical potentials (nerve impulses).

Slow pain (chronic)

type of pain begins more slowly & increases in intensity aching or throbbing pain of toothache in both superficial and deeper tissues smaller C nerve fibers

Fast pain (acute)

type of pain occurs rapidly after stimuli (.1 second) sharp pain like needle puncture or cut not felt in deeper tissues larger A nerve fibers

autonomic nervous system (ANS)

under motor division controls automatic body functions. involuntary control of glands, smooth muscle, and cardiac muscle. ANS can be divided into sympathetic and parasympathetic, which are antagonistic (opposing one another).

somatic nervous system

under motor division controls voluntary actions of the body. voluntary contraction of skeletal muscle.

Sensory Receptors can Produce 2 Types of graded potentials

way the sensory receptors transduce a stimulus and the way they generate the action potentials of the nerve impulses 1) generator potentials 2) receptor potentials

Describe how the cerebellum coordinates muscle movement.

stores memories of previously learned movements receives input from cerebral motor cortex, brain stem nuclei, and sensory receptors. provides precise timing and appropriate patterns of skeletal muscle contraction for-smooth, coordinated movements.

Thermoreceptors

classification by stimulus type sensitive to changes in temperature Free nerve endings with 1mm diameter receptive fields on the skin surface • Cold receptors in the stratum basale respond to temperatures between 50-105 degrees F • Warm receptors in the dermis respond to temperatures between 90-118 degrees F Both adapt rapidly at first, but continue to generate impulses at a low frequency Pain receptors are activated below 50 and over 118 degrees F.

Nociceptors (pain receptors)

classification by stimulus type sensitive to pain-causing stimuli (e.g. extreme heat or cold, excessive pressure, inflammatory chemicals). free endings that are located in nearly every body tissue except the brain • adaptation is slight if it occurs at all. Stimulated by excessive distension, muscle spasm, & inadequate blood flow Tissue injury releases chemicals such as K+, kinins or prostaglandins that stimulate nociceptors

Interoceptors (visceroceptors)

classified by location Respond to stimuli arising in internal viscera and blood vessels Sensitive to chemical changes, tissue stretch, and temperature changes Sometimes cause discomfort but usually unaware of their workings generally not consciously perceived.

stimulus

any type of sensory input

Perception

the conscious interpretation of a stimulus. • Occurs in the cerebral cortex (where perception occurs) • Not all stimuli are perceived

Proprioceptors

classified by location Respond to stretch in skeletal muscles, tendons, joints, ligaments, and connective tissue coverings of bones and muscles Inform brain of one's movements

Meninges

three layers of connective tissue in which the brain and spinal cord are wrapped

Phasic (fast-adapting) receptors

signal beginning or end of stimulus examples: receptors for pressure, touch, and smell

free nerve endings

responsible for transducing temperature, pain, tickling, and itching.

General Senses

spread out fairly evenly all over the body: • Pain • Temperature • Light touch • Pressure • Sense of body and limb position

Ganglia

Contain neuron cell bodies associated with nerves in PNS 1) Dorsal Root Ganglia 2) Autonomic Ganglia

Ascending Sensory Pathways

1) First-order neurons conduct impulses to the CNS (brainstem or spinal cord): either spinal or cranial nerves 2) Second-order neurons conducts impulses from brain stem or spinal cord to thalamus: cross over to opposite side of body 3) Third-order neurons conducts impulses from thalamus to primary somatosensory cortex (postcentral gyrus of parietal lobe)

Sensation: Sequence of events

1) Stimulation of sensory receptor 2) Transduction of stimulus (Graded Potential) 3) Transmission of Nerve Impulses (Action Potentials) 4) Integration of sensory input in brain = sensation then perception modulation occurs in the spinal cord, a way for the CNS to amplify or dampen the magnitude of the sensory information. sensation and perception occurs in the cerebral cortex.

Tonic receptors

adapt slowly or not at all examples: nociceptors (Pain receptors) and most proprioceptors

Receptor Adaptation

CNS can "ignore" some sensations to concentrate on others Touch and smell receptors adapt quickly, whereas pain receptors usually do not adapt Examples: 1) Your nose easily grows accustomed to strange smells 2) Skin adapts to continual presence of your clothes

transduction

Conversion of a stimulus into a graded potential by a sensory receptor.

Structure of a Nerve

Cordlike organ of PNS Bundle of myelinated and nonmyelinated peripheral axons enclosed by connective tissue Connective tissue coverings include • Endoneurium: loose connective tissue that encloses axons and their myelin sheaths • Perineurium: coarse connective tissue that bundles fibers into fascicles • Epineurium: tough fibrous sheath around a nerve

Vestibulocochlear Nerve (VIII)

Sensory: Cochlear branch begins in medulla • Function: hearing, if damaged deafness or tinnitus (ringing) is produced Vestibular branch begins in pons, receptors in vestibular apparatus • Function: sense of balance •• vertigo (feeling of rotation) •• ataxia (lack of coordination)

Peripheral Nervous System (PNS)

Provides links from and to world outside body All neural structures outside brain • Sensory receptors • Peripheral nerves and associated ganglia • Efferent motor endings

Somatosensory Map of Postcentral Gyrus

Relative sizes of cortical areas • proportional to number of sensory receptors • proportional to the sensitivity of each part of the body Can be modified with learning

Accessory Nerve (XI)

Sensory Function: Proprioception of larynx & pharynx muscles and soft palate Motor Functions: • Swallowing • Movement of head and shoulders

Hypoglossal Nerve (XII)

Sensory Function: Proprioceptors in tongue muscles Motor Function: Movement of tongue for speech and swallowing

Glossopharyngeal Nerve (IX)

Sensory Functions: • Taste & somatic sensations (touch, pain temp) of posterior 1/3 of tongue • Proprioception of swallowing muscles • Monitors: O2, CO2, BP, & Breathing (rate & depth) Motor Functions: • Elevates pharynx during swallowing & speech • Stimulates salivation (ANS)

proprioception

ability to tell where one's body is in space.

Adaptation of Sensory Receptors

Variability in tendency to adapt: 1) Rapidly adapting receptors smell, pressure, touch: specialized for detecting changes 2) Slowly adapting receptors pain & body position: nerve impulses continue as long as the stimulus persists; Pain is not easily ignored. • Phasic (fast-adapting) receptors • Tonic receptors

Be able to differentiate the Posterior Column-Medial Lemniscus and Spinothalamic pathways

What types of sensory information do they relay? • the posterior column-medial lemniscus is primarily responsible for touch sensations and proprioception • the spinothalamic is primarily responsible for pain and temperature sensations. Where do 1st , 2nd , and 3rd order neurons synapse? • 1st order neuron synapses with 2nd order neuron in nucleus gracilis and nucleus cuneatus of medulla • 2nd order neuron decussate and ascend to thalamus where it synapses with 3rd order neuron • 3rd order neuron transmits impulse to somatosensory cortex

Tactile Sensations (skin receptors)

touch, pressure, and vibration plus itch and tickle. Receptors include 1) corpuscles of touch (Meissner's corpuscles) 2) hair root plexuses 3) type I (Merkel's discs) 4) type II cutaneous (Ruffini's corpuscles) 5) mechanoreceptors 6) lamellated (Pacinian) corpuscles 7) free nerve endings

generator potentials

transduction: occurs in the free nerve ending. transmission: the propagation of nerve impulses down the axon.