L1 Sensory Receptor Types PDF

t o r s e c ep r y R e s s o i t i Sen Modaaltic Afferennttss & General Somsceral Affere neral Vi l Senses Ge Specia DCOM OMS-I...

t o r s e c ep r y R e s s o i t i Sen Modaaltic Afferennttss & General Somsceral Affere neral Vi l Senses Ge Specia DCOM OMS-I Dr. Stan Kunigelis DO SYS 715: Med Neuro II [email protected] Lecture 1H: Somatosensory Modalities Office: DCOM 217 (6818) January 09, 2025 IAC Lab: Hamilton 120 (6385) 1  Learning Objectives … 1. Describe how sensors act as transducers. 2. Differentiate between the 4 types of mechanoreceptors, paying attention to the significance of slow and fast adapting modalities. 3. Describe the structure-function relationships between the different classes of nerves. 4. Differentiate between polymodal and mechanical nocioceptors (i.e., compare and contrast). 5. Distinguish between receptive fields of warm and cold thermoreceptors. 6. Differentiate between Golgi Tendon Organ and Spindle Fiber mechanisms of action. 7. Describe a-, b-, g-motoneuron innervation (i.e., location and action) of skeletal muscle and associated spindles. Kunigelis: DCOM NeuroAnatomy 2 2  Learning Topics Slide # Learning Objectives 2 Sensory Receptor Types 5 Sensory Receptor Classification 6 Sensory Receptor Functional Properties 7 Peripheral Axon Classification 8 Mechanoreceptors 9 Nociceptors 17 First and Second Pain 20 Thermoreceptors 23 Proprioceptors 27 Muscle Spindles 31 Golgi Tendon Organs 48 Brilliant End Slide 53 Kunigelis: DCOM NeuroAnatomy 3 3  Sensory Receptor Types 1. Special Senses 3. General Somatic a) Olfaction Afferent (GSA) b) Gustation a) Skeletal muscle c) Vision b) Joints d) Audition c) Skin surface e) Vestibular d) Position 2. General Visceral Afferent e) Touch (GVA) f) Pressure a) Cardiovascular g) Pain b) Respiratory h) Temperature c) Digestive d) Urinary e) Reproductive Kunigelis: DCOM NeuroAnatomy 4 Kunigelis: PA (Patho) Physiology 4  Sensory Receptor Classification 1. Exteroceptors – Provide information about the external environment – Include GSA receptors associated with the body’s surface Heat, touch, pressure, and vibration Vision, hearing, smell 2. Proprioceptors – Position and movement of the body – Includes GSA receptors in muscles, tendons, joint capsules – Includes vestibular apparatus of the inner ear 3. Interoceptors – Provide information from internal organs – Includes GVA receptors Kunigelis: DCOM NeuroAnatomy 5 Kunigelis: PA (Patho) Physiology 5  Sensory Modality Classification This classification is based on the type of stimulation to which a GSA receptor responds 1. Mechanoreceptors – Mechanical deformation of tissue Stretch, vibration, pressure, touch – Includes both exteroceptors and proprioceptors 2. Thermoreceptors – Warmth or cold 3. Nociceptors (pain receptors) – Noxious or painful stimuli Kunigelis: DCOM NeuroAnatomy 6 Kunigelis: PA (Patho) Physiology 6 Sensory Afferent Functional Properties ① Axon Diameter. ③ Two-point Discrimination. The a. Proprioception (muscle spindle) use minimum distance between twin largest diameter axons and have the stimuli required to discern them as fastest conduction velocity. distinct. b. Touch (Merkel, Meissner, Pacinian, a. Fingers and upper lip can discern stimuli 2 Ruffini) use slightly smaller diameter mm apart. axons with slower conduction velocity. b. Cheek, nose, and toes can discern stimuli c. Pain and temperature use free nerve 10 mm apart. endings with myelinated axons. c. Shoulder, back, breast, thigh, calf can d. Pain, temperature, and itch use free discern stimuli 40 mm apart. nerve endings with unmyelinated axons yielding the slowest conduction ④ Temporal Dynamics. velocities a. Rapidly Adapting Afferents become e. See next slide for details. quiet in light of continuous stimulation→provides information about changes in ongoing stimulation→moving ② Receptive Field Size (Innervation stimulus Density). Reflects the density of b. Slowly Adapting Afferents provide afferent fibers supplying a particular information about the spatial attributes (e.g., area of skin. Smaller arborizations size and shape) of the stimulus yield smaller receptive fields. a. Fingers, lips, and toes have small receptive fields. b. Forearm and back are innervated by smaller number of afferent fibers. Kunigelis: DCOM NeuroAnatomy 7 7  Peripheral Axon Classification CLASS Myelination Diameter Cond. Vel. Example Aα Heavily 13-20 µm dia. 80-120 m/s Muscle spindle length and ∆length. (Type Ia) ~270mph myelinated Highly myelinated 13-20 µm 80-120 m/s Golgi tendon organs; (Type Ib) Aβ Well myelinated 5-15 µm 33-75 m/s Pacinian and Meissner corpuscles, (Type II) Merkel disks, Ruffini endings in skin. Aγ myelinated 3-8 µm 15-40 m/s Motor to intrafusal fibers of muscle spindles Aδ Poorly myelinated 1-5 µm 10-30 m/s Sensory from small hair follicles and from free nerve endings for (Type III) temperature and pain sensations (mechanical nociceptors). B myelinated 1-3 µm 5-15 m/s Preganglionic autonomic (white rami and cranial nerves 3, 7, 9, 10) C unmyelinated 0.2-1.5 µm 0.5-2.5 m/s -Pain, temperature, and itch (found at ~5.5mph junction of epidermis and dermis (80% (Type IV) of all skin axons)) -Olfaction; postganglionic autonomic Letters are used for any nerve group. Roman numerals are for sensory fiber types in dorsal spinal roots. Sensory nerves form dorsal root ganglia (dorsal spinal Kunigelis: nerve). Motor nerves DCOM form ventral roots (ventral horn of spinal cord). NeuroAnatomy 8 Kunigelis: PA (Patho) Physiology 8 “Afferent fibers are often encapsulated by specialized receptor cells called mechanoreceptors that help tune the afferent fiber to particular features of somatic stimulation. Those lacking receptor cells are free nerve endings and are especially important to pain sensation. Afferents with encapsulated endings are more sensitive to sensory stimulation.” Purves et al. Neuroscience 5th edition. Sinauer Associates, Inc. Kunigelis: DCOM NeuroAnatomy 9 9  Mechanoreceptors Body surface is covered by hairy and hairless (glabrous) skin Four types of mechanoreceptors are embedded to detect light mechanical stimuli Each receptor is a transducer, where a mechanical stimulus – opens/closes ion channels to form graded potentials that – stimulate / inhibit action potential formation Kunigelis: DCOM NeuroAnatomy 10 Kunigelis: PA (Patho) Physiology 10  Mechanoreceptors Vertical section of glabrous skin from the human hand showing four types of mechanoreceptors: Meissner Corpuscles Merkel Disks Ruffini Corpuscles Pacinian Corpuscles Kunigelis: DCOM NeuroAnatomy 11 Kunigelis: PA (Patho) Physiology 11  Mechanoreceptors 10- 15% 1. Pacinian Corpuscles (Deep) – Deep pressure and high frequency vibration – Comprises a central sensory axon surrounded by a fluid- Modified from “Fundamental Neuroscience” 2nd edition. Elsevier Science. filled capsule to filter 20% sustained stimuli. 2. Ruffini Corpuscles (Deep) – Slippage of objects along the surface of skin grip – Axon endings in a collagen network 40% 3. Meissner’s Corpuscles (Surface) – Light touch – Comprises axonal loops separated by supporting cells 4. Merkel’s Discs (Surface) 25% – Pressure and texture – Axons and Merkel cells 12 interspersed Kunigelis: DCOM NeuroAnatomy Kunigelis: PA (Patho) Physiology 12  Small Receptive Field Superficial: RA Superficial: SAI Perception of movement Form and texture perception. along the skin. Sensitive to edges, points, corners, Grip adjustment to slippage. and object curvature that are pressed into the skin. Deep: RA Large Deep: SAII Deep pressure & high Receptive Movement of digits and limbs, frequency vibration. Field unidirectional skin stretching. Large receptive field over Large and diffuse receptive field. several digits. From “Fundamental Neuroscience” 2nd edition. Elsevier Science. Kunigelis: DCOM NeuroAnatomy 13 Kunigelis: PA (Patho) Physiology 13  I http://neurobiography.info/images/teaching/cutaneous_receptors_adaptation_1.gif Rapidly Adapting Afferents (RA) Slowly Adapting Afferents (SA) become quiet in light of continuous stimulation provide information about the spatial attributes → provides information about changes in ongoing (e.g., size and shape) of the stimulus. stimulation (e.g., on/off detectors) → moving stimulus. Kunigelis: DCOM NeuroAnatomy 14 14  Modality Receptor Fiber Conduction Role in Perception Type Velocity (m/s) Mechano- Merkel cell Aß 42-72 Pressure, form, reception (Type II) texture Meissner Aß 42-72 Flutter, motion corpuscle (Type II) Ruffini Aß 42-72 Possibly, corpuscle (Type II) skin stretch Pacinian Aß 42-72 Vibration corpuscle (Type II) ~161mph Modified from “Fundamental Neuroscience” 2nd edition. Elsevier Science. Kunigelis: DCOM NeuroAnatomy 15 Kunigelis: PA (Patho) Physiology 15  Superficial/Cutaneous Receptors Deep Mechanoreceptors (small receptor field) (large receptor field) 1. Meissner Corpuscles 1. Pacinian Corpuscles – Rapid adapting (RA) – RA – Perception of movement along the – Sensitive to minute vibrations skin (esp. when objects are grasped) – Detect low-frequency vibration – Large receptive fields that (flutter; 50 Hz)) cover several digits / entire – Afferent input lead to adjustment of hand grip force when objects are first – Sensation of high frequency lifted and tend to slip vibration (200-300Hz) 2. Merkel Disks 2. Ruffini Corpuscles – Slowly adapting type I (SAI) – Form and texture perception – SA II – Present in high density in digits – Responds to the lateral (50/mm2 decreasing with age) and movement or stretching of around mouth; medium density in skin in one direction glabrous areas; low density in hairy skin – Large and diffuse receptive – Sensitive to edges of objects fields pressed into the skin – Respond to movements of – Initial and prolonged response limbs and digits unlike the Meissner Corpuscles which was rapidly adapting Object slippage adjustment can be regarded as being a 2 stage process with superficial Meissner corpuscles detecting the start and stop of slippage while the deeper Ruffini corpuscles lead to finger adjustments (grip) to counter slippage. Kunigelis: DCOM NeuroAnatomy 16 Kunigelis: PA (Patho) Physiology 16 https://i.ytimg.com/vi/wpVTORX_ifk/maxresdefault.jpg Kunigelis: DCOM NeuroAnatomy 17 17  Nociceptors Interpretation of pain is With no change in afferent dependent upon input to the CNS, the i. Mood interpretation of pain can ii. Attentiveness be reduced with training iii. Personality Therefore, pain is iv. Past experience interpreted in two manners: Objectively, the pain of 1. Discriminative properties of childbirth is more intense location and intensity than that associated with A single grain of sand under the eyelid verses several solid malignant tumors, but grains under the feet is subjectively far less Tissue is damaged at 45oC … unpleasant. this is the threshold for thermal pain 2. Behavioral expectations and reactions Kunigelis: DCOM NeuroAnatomy 18 Kunigelis: PA (Patho) Physiology 18  Mechanical Nociceptors Polymodal Nociceptors Free nerve endings Free nerve endings High threshold Respond to multiple stimuli – Intense Mechanical Broad range of conduction velocities … Aδ … 10-30 m/s – Thermal – Chemical Receptive fields = 5 - 20 spots (2-3 mm dia.) Most are peripheral nerve, C fibers (≤1 m/s) unaccompanied by other nociceptor types Graded response to minute High initial threshold to skin punctures thermal stimulation 40-60oC thermal input with Threshold decreases with graded response above 46oC prolonged stimulation (NO Overlapping mechanical and sensitization) thermal receptive fields Kunigelis: DCOM NeuroAnatomy 19 Kunigelis: PA (Patho) Physiology 19  First and Second Pain Group Aδ Fibers Group C Fibers (Type III Sensory) (Type IV Sensory) Rapidly conducting fibers Slowly conducting fibers First Pain = cutaneous Second pain = burning pain pricking pain Slower conductions means Well localized it is perceived later than Aδ Easily tolerated (even with fiber (1s delay at fingers or reflex withdrawl) toes) sharp pain. Initiated by mechanical nociceptor Kunigelis: DCOM NeuroAnatomy 20 Kunigelis: PA (Patho) Physiology 20  Modality Receptor Fiber Conduction Role in Perception Type Velocity (m/s) Nociception Bare nerve Aδ 12-36 Sharp pain endings (Type III) Bare nerve C 0.5-1.2 Burning pain endings (Type IV) Modified from “Fundamental Neuroscience” 2nd edition. Elsevier Science. Kunigelis: DCOM NeuroAnatomy 21 Kunigelis: PA (Patho) Physiology 21  Perception of Pain: Cancer Patients Tumors Pressing on Body Parts ≈Type and stage of cancer, pain threshold 1. Nerve Pain: burning, 4. Phantom pain: shooting, tingling, or as a – pain in a part of the body that has feeling of something been removed crawling under their skin – Felt by 60-70% patients after limb removal (most difficult pain type to – “brain pain” treat). – unbearable 2. Bone Pain: aching, dull or 5. Referred pain: throbbing. pain from one organ may be felt in a different part of the body 3. Visceral (soft tissue) pain: e.g., liver may cause pain in the sharp, cramping, aching, right shoulder → Liver presses on or throbbing. nerves that end in the shoulder. Palliative vs Hospice pain management??? Palliative care can begin at diagnosis, and at the same time as treatment. Hospice care begins after treatment of the disease is stopped and when it is clear that the person is not going to survive the illness. Both provide comfort. Kunigelis: DCOM NeuroAnatomy 22 22 Kunigelis: DCOM NeuroAnatomy 23 23  Thermoreceptors Sustained response over a narrow range of skin temperatures Do not respond to skin indentation Farside by Gary Larson Cold-blooded vs warm- blooded? Why dinosaurs went extinct! Kunigelis: DCOM NeuroAnatomy 24 Kunigelis: PA (Patho) Physiology 24  Thermoreceptors have small receptive fields (1 mm in glabrous skin; 3-5 mm in hairy skin). One axon innervates 3-4 fields. Both types fire at 30-35oC. Below, cold receptors will dominate. Above, warm receptors dominate. Warm Receptors Cold Receptors Unmyelinated Lightly myelinated Slow conducting C fibers Faster conducting Aδ fibers (majority of fibers in 25% axons in peripheral peripheral nerve) nerve End in dermis or deep epidermis Unencapsulated terminals Experiment: One hand in cold water, the other in warm water. Transfer both to tepid water. Result: Chilled hand feels warm. Warmed hand feels cool. Conclusion: Thermoreceptors respond to change, not absolute temperature. Kunigelis: DCOM NeuroAnatomy 25 Kunigelis: PA (Patho) Physiology 25  Modality Receptor Fiber Conduction Role in Perception Group Velocity (m/s) Thermo- Bare nerve Aδ 12-36 cold reception Ending Bare nerve C 0.5-1.2 warmth Ending Modified from “Fundamental Neuroscience” 2nd edition. Elsevier Science. Kunigelis: DCOM NeuroAnatomy 26 Kunigelis: PA (Patho) Physiology 26  ors ept ioc pr o Pr Position and movement of the body Includes GSA receptors in muscles, tendons, joint capsules Includes vestibular apparatus of the inner ear Kunigelis: DCOM NeuroAnatomy 27 27  Proprioceptors Inventory of limb position at rest and during movement (kinesthesia) can be done without audition or vision – Position of joints at rest – Direction and velocity of movement Input from – Cutaneous mechanoreceptors – Joint receptors – Muscle spindles – Golgi tendon organs Kunigelis: DCOM NeuroAnatomy 28 Kunigelis: PA (Patho) Physiology 28  Proprioception Mechanoreceptor Role Ruffini SAII afferents are the Role is location dependent only type of cutaneous – Locally anesthetized skin mechanoreceptor to play an around knee has no influence active role in proprioception on joint proprioception – Locally anesthetized skin – Encode joint position around mouth, hand, or feet, – Role depends on location impairs proprioception Kunigelis: DCOM NeuroAnatomy 29 Kunigelis: PA (Patho) Physiology 29  Joint Receptors Muscle Spindles Several receptor types Complex peripheral receptor to present in joint capsules* (a measure muscle length and its synovial joint) rate of change to stretch: – Ruffini Endings – Comprises two types of encapsulated fibers (i.e., – Pacinian corpuscles Nuclear Bag and Nuclear – Ligament GTO Chain), 0.5 -7mm in length. Anesthetization or surgical – Has two types of sensory removal of joint capsules fiber innervation: does NOT impair limb Large diameter 1o sensory axon (Type Ia fiber) position sense, indicating a Thinner 2o sensory axon protective role to prevent (Type II fiber) hyperextension or – Has two types of motor fibers hyperflexion. (g- and b-motor neurons). * Concentration of mechanoreceptors is greater in areas related to the extremes of movement ® represents the first line of defense in sensing impending injury. Kunigelis: DCOM NeuroAnatomy 30 Kunigelis: PA (Patho) Physiology 30  Muscle Spindles Muscle spindles comprise 3- 12 intrafusal muscle fibers enclosed in a fusiform connective tissue capsule Intrafusal fibers are ¼ size of extrafusal fibers Bag and Chain are the two types of intrafusal fibers. A typical spindle contains 2 nuclear bag fibers and 4-5 nuclear chain fibers. Nuclear chain fibers are ½ size of nuclear bag fibers Only end regions are contractile, containing actin and myosin Kunigelis: DCOM NeuroAnatomy 31 Kunigelis: PA (Patho) Physiology 31  How Do Spindle Fibers Work? Normally, extrafusal However, when the load muscle fibers generate exceeds the applied tension tension to counter a load then the muscle is at risk of Note that load and tension being damaged by over are both forces that act in stretching. opposite directions Muscle spindles are sensory ∴ a loaded muscle is said receptors within the belly of to be under tension a muscle, which primarily detect changes in the length of this muscle Tension The muscle spindle has both sensory and motor Load components. Kunigelis: DCOM NeuroAnatomy 32 Kunigelis: PA (Patho) Physiology 32  Intrafusal Spindle Fibers Motor Component Sensory Component the motor component is provided by Primary and secondary – up to a dozen gamma sensory nerve fibers spiral motoneurons and – to a lesser extent by one or two around and terminate on beta motoneurons. the central portions (Type g (gamma) and b (beta) Ia) and ends (Type II) of the motoneurons are called fusimotor neurons, because they intrafusal muscle fibers, activate the intrafusal muscle providing the sensory fibers. component of the structure b (beta) motoneurons innervate via stretch-sensitive ion- both extrafusal and intrafusal muscle fibers and so are referred channels of the axons. to as skeletofusimotor neurons. 1a sensory fibers Kunigelis: DCOM NeuroAnatomy 33 33 Kunigelis: DCOM NeuroAnatomy 34 34  A typical spindle contains Sensory nerve fibers are associated 2 nuclear bag fibers and with intrafusal muscle fibers 4-5 nuclear chain fibers. 1. Nuclear bag fibers have spiral dendritic endings Large type Ia sensory fibers respond to rate and degree of stretch " located on center of bag fibers 2. Nuclear chain fibers have "flower spray" sensory / dendritic endings Sensory (Afferent). – Small type II sensory fibers measure length of resting muscle (static) " located on end of chain fibers Motor (Efferent). – End regions innervated by γ-motoneurons and a few b-motoneurons http://fig.cox.miami.edu/~lfarmer/BIL265/locomotion.html Kunigelis: DCOM NeuroAnatomy 35 35  There are two types of intrafusal fibers: 1. Bag1 and Bag2, differing in their contraction speed and motor innervation – dynamic nuclear bag fibers (Bag1 fibers) (~8 mm long). Bag1 " fast contracting. – static nuclear bag fibers (Bag2 fibers) (~10 mm). Bag2 " slow contracting. Both bag fiber types extend beyond the spindle capsule 2. Nuclear chain fibers (static; nuclei are linearly (slow) stacked in a chain) (much smaller at 4-5 mm long). (fast) Static vs Dynamic Intrafusal Fibers g-, b-, and a-motoneurons. Static Axons (used in simpler movements) innervate the chain and Bag2 fibers. increase the firing rate of Type Ia (center: rate and degree of stretch) and Type II (ends: resting length) afferents at a given muscle length Dynamic axons (used in more complex movements). innervate the Bag1 intrafusal muscle fibers only. increase the stretch-sensitivity of the Type Ia A nuclear bag cell with central Type Ia sensory and terminal γ-motor fibers. afferents by stiffening the Bag1 intrafusal fibers. Kunigelis: DCOM NeuroAnatomy 36 Kunigelis: PA (Patho) Physiology 36 Kunigelis: DCOM NeuroAnatomy 37 37  b-Innervation In addition to the g-motor axons, the intrafusal fibers are also innervated by b- motor axons. These axons are collaterals from the axons that innervate extrafusal slow twitch muscle fibers. The axons of the b-motor axon innervate all intrafusal motor fiber. These axons are the pathways for coactivation of intrafusal and extrafusal fibers. Figure: the innervation of each type of spindle is represented by the color-match symbol on the side of the figure; red: Type 1a sensory endings; green: sensory type II fibers; purple: motor static fibers; light purple: motor dynamic fibers; blue: beta axons. Kunigelis: DCOM NeuroAnatomy 38 http://pediatricneuro.com/alfonso/tip%20muscle%20spindle.htm 38 S U - Bag cells have a large number M - - of nuclei in bags. Annulospiral nerve endings Nuclei but no striations in middle M - region. Bag2 (static= slow contracting) A cells are larger than bag1 (dynamic = rapid contraction) cells b-motor innervation R of nuclear chain fiber central region. Y Type Ia: responds to stretch velocity/rate and length of intrafusal fibers. Type II: responds only to length changes of static intrafusal fibers. Kunigelis: DCOM NeuroAnatomy 39 39  Bag and Chain Fiber Summary Fiber Fiber Move- Afferents Afferents Efferents Name Type ments Type 1a Type II (ends only) Nuclear Dynamic Fast/ Center with Annulospiral endings: none g - motor Bag 1 (smaller) complex stretch velocity neurons 8mm and absolute (few b) length. Nuclear Static Slow/ Center with Annulospiral endings: none g - motor Bag 2 (larger) simple stretch velocity neurons 10mm and absolute (few b) length. Nuclear Static Slow / Equatorial: Flower-spray endings: g - motor Rate and degree Chain 4-5mm simple resting neurons of stretch muscle (few b) length g-efferent fibers stimulate motor ends to control overall sensitivity of muscle spindle. Kunigelis: DCOM NeuroAnatomy 40 40  Stretch Reflex 2o Afferents: Type II 1o Afferents: Type Ia Stretch a muscle When the muscle is stretched, 1o – Load > tension so entire muscle afferent (type Ia) firing increases stretches* (external stretch) resulting in monosynaptic activation of – Stretching of 2o afferent fibers α-motor neurons to contract extrafusal (type II) activate γ-motor fibers, resisting further muscle stretch neurons leading to the Further, activation of sensory afferents contraction of intrafusal fibers can inhibit antagonistic muscles (via distal ends → this contraction interneuron) to cause relaxation which stretches the middle region of ends stretching of the antagonistic pair intrafusal fibers, taking out the partner = reciprocal inhibition slack, restoring optimal sensory γ-motoneurons only innervate sensitivity. intrafusal muscle fibers (= fusimotor), ------ whereas β-motor neurons innervate * Braking action of gastrocnemius when walking down hill stretches a muscle OR both extrafusal and intrafusal muscle in antagonistic muscle pairs, contraction of one muscle stretches the other e.g. biceps and triceps! fibers (= skeletofusimotor neurons). Type Ia responds to stretch velocity and absolute length of intrafusal fibers. Type II responds only to length changes. Kunigelis: DCOM NeuroAnatomy 41 Kunigelis: PA (Patho) Physiology 41  Muscle Spindle Function Tension on the central Stretching a muscle stretches a spindle which increases region of an intrafusal afferent firing fiber during normal Contraction of a muscle reduces spindle stretch to muscle contraction, decrease action potential firing mechanically distorts its γ-motor activation causes plasma membrane to contraction of spindle fiber ends, shortening the overall open cation channels → fiber length which maintains this leads to increased tension in the mid-region allowing them to operate in an action potential firing optimal range of sensitivity regardless of muscle length Kunigelis: DCOM NeuroAnatomy 42 Kunigelis: PA (Patho) Physiology 42 Compare the rate of sensory, Type 1a firing, during stretch, rest, and contraction. Kunigelis: DCOM NeuroAnatomy 43 43  Weak or Slow Stimulation Strong or Sharp Stimulation α-motor neurons stimulate With a strong or sharp muscle contraction, (γ- stimulus, both the α- and γ- motor neurons don't fire) fibers fire together (see As the muscle shortens next slide). during contraction, spindle The γ-fibers help to "take sensory/afferent fibers also the slack" out of the spindle decrease in length and so that sensory fibers from therefore, firing rate. the spindle do not inhibit α- This decrease helps to stimulated firing and the reduce stimulation to the α- muscle contraction motor neurons so the response is stronger. muscle doesn't contract too much. Kunigelis: DCOM NeuroAnatomy 44 Kunigelis: PA (Patho) Physiology 44 Kunigelis: DCOM NeuroAnatomy 45 45  Intrafusal Muscle Fibers Extrafusal Muscle Fibers Innervation gamma motor neurons alpha motor neurons Sensory deep muscle spindle fibers. Spindle fibers are stretched as muscle lengthens. Increased firing of sensory neurons (group Ia) monosynaptically stimulates α-motor neurons, causing muscle contraction. γ- motor neuron co- activation keep intrafusal fibers taut. Function proprioceptors that detect the amount Force generating units and rate of change of length in a muscle Kunigelis: DCOM NeuroAnatomy 46 Kunigelis: PA (Patho) Physiology 46  Sensory Fiber Types Conduction Type Diameter Response Velocity Respond to the rate of change in muscle length Type Ia 13-20 µm 8-120 m/s (velocity), as well to absolute length Type Ib 13-20 µm 8-120 m/s in Golgi tendon organ Respond only to length Type II 6-12 µm 33-75 m/s changes (cutaneous mechanoreceptors) Sensory receptors are innvervated by type Ia and Ib sensory fibers, while cutaneous receptors (mechanoreceptos, nociceptors, thermoreceptors, chemoreceptors) activate Aβ (Type II), Aδ and C fibers. Type I are also called 1o afferent fibers. Type II are also called 2o afferent fibers. Kunigelis: DCOM NeuroAnatomy 47 Kunigelis: PA (Patho) Physiology 47  Proprioceptors: Golgi Tendon Organs A second type of Since GTO is located in muscle series (muscle-GTO- tendon; spindle fibers are mechanoreceptor in parallel), it can be Encapsulated stretch activated by passive receptor located at the stretch or shortening junction between during active contraction – dendrites of type 1b afferent muscle and tendon fibers are woven between 1 mm length X 100µm collagen fibers of tendon – Action potential frequency dia. ≈ ∆force Kunigelis: DCOM NeuroAnatomy 48 Kunigelis: PA (Patho) Physiology 48  Golgi Tendon Organ made up of strands of collagen between muscle fibers and the tendon proper. Each tendon organ is innervated by a single afferent type Ib sensory fiber that branches and terminates as spiral endings around the collagen strands. http://www.lab.anhb.uwa.edu.au/mb140/CorePages/Connective/Images/mtj040vg.jpg http://www.arn.org/docs/glicksman/040105%20fig3.jpg Kunigelis: DCOM NeuroAnatomy 49 Kunigelis: PA (Patho) Physiology 49 Note the simplicity Name the of the nerves: Are GTO. they sensory or motor … function? Kunigelis: DCOM NeuroAnatomy 50 50  ap firing ap firing decreases increases during during contraction stretch Prevents damage due to over- contraction. A muscle is stretched by As muscle shortens during contraction of its antagonistic contraction, there is increased partner or when its load exceeds tension in the tendon. This info is max producible tension. This info coded as an increase in GTO is coded as an decrease in GTO firing. firing. Kunigelis: DCOM NeuroAnatomy 51 51  Modality Receptor Fiber Conduction Role in Perception Type Velocity (m s-2) Proprio- Ruffini-like Aβ 42-72 Protection against reception Paciniform-like hyperextension Bare nerve endings Muscle spindles Aα 72-120 Muscle absolute length Type Ia and velocity (rate of length change) Golgi tendon Aα 72-120 Muscle tension organs Type Ib Muscle spindles Aβ 42-72 Δ Muscle length Type II Ruffini endings Aβ 42-72 Joint angle Modified from “Fundamental Neuroscience” 2nd edition. Elsevier Science. Letters are used for any nerve group. Roman numerals are for sensory fiber types in dorsal spinal roots. Kunigelis: DCOM NeuroAnatomy 52 Kunigelis: PA (Patho) Physiology 52 Kunigelis: DCOM NeuroAnatomy 53 53 Kunigelis: DCOM NeuroAnatomy 54 54  Spindle Motor Input Spindle Sensory Output The end regions of nuclear Both nuclear chain and bag bag fibers (only ends are fibers have large 1o sensory, contractile) and nuclear chain type Ia, afferent output. fibers receive γ-motor and All static fibers, nuclear some b-motor innervation. chain cells and Bag2, have (The central region of nuclear small 2o sensory, type II, chain fibers may receive β- afferent output. motor innervation???) Sensory Output Motor Input Nuclear Bag Cells Type Ia (central γ to contractile end region annulospiral endings) β to end region Nuclear Chain Cells Types Ia (center) and II γ to contractile end region (flower spray at ends) β to end region Type Ia responds to stretch velocity and absolute length Control overall sensitivity of of intrafusal fibers muscle spindle Type II responds only to length changes. Kunigelis: DCOM NeuroAnatomy 55 Kunigelis: PA (Patho) Physiology 55