3Sensory ReceptorsSensory receptors are structures that are specialized to respond to changes in their environmentSuch environmental changes are called stimuliTypically activation of a sensory receptor by an adequate stimulus results in depolarization or graded potentials that trigger nerve impulses along the afferent fibers coursing to the CNS
7تقسیم بندی انواع گيرنده ها؛ بر اساس: تقسیم بندی انواع گيرنده ها؛ بر اساس:1- مكان قرارگيری (Location)2- نوع تحريك(Type of stimulus detected)3- ساختار(Structure)4- تطابق پذيری (Adaptation)7
81- Classification by Location Proprioceptors : monitor degree of stretchInteroceptors (visceroceptors) : receive stimuli from internal visceraMonitor a variety of stimuliExteroceptors : sensitive to stimuli arising from outside the bodyLocated at or near body surfacesInclude receptors for touch, pressure, pain, and temperature (, …)
12Proprioceptors provide information about the position of body parts neuro4e-fig jpg
13Muscle Spindles Encapsulated fibers within the muscle belly Monitor changes in muscle lengthMonitor the rate of change in muscle lengthRespond by causing muscle contraction1313
14Structure of Muscle Spindle Each spindle is 3 to 10 mm long.It is built around 3 to 12 tiny intrafusal muscle fibers that are pointed at their ends and attached to the glycocalyx of the surrounding large extrafusal skeletal muscle fibers.
15Structure of Muscle Spindle Each intrafusal muscle fiber is a tiny skeletal muscle fiber.central region of each of these fibers has few or no actin and myosin filaments.Therefore, this central portion does not contract when the ends do. Instead, it functions as a sensory receptor.
16Motor Innervation of Muscle Spindle The end portions that do contract are excited by small gamma motor nerve fibers (gamma efferent fibers) that originate from small type A gamma motor neurons in the anterior horns of the spinal cord.the large alpha efferent fibers (type A alpha nerve fibers) innervate the extrafusal skeletal muscle.
17Sensory Innervation of the Muscle Spindle The receptor portion of the muscle spindle is its central portion.In this area, the intrafusal muscle fibers do not have myosin and actin contractile elements.sensory fibers originate in this area.They are stimulated by stretching of this midportion of the spindle.
18Muscle spindle, showing its relation to the large extrafusal skeletal muscle fibers. Note also both motor and sensory innervation of the muscle spindle.
19Sensory Innervation of the Muscle Spindle muscle spindle receptor can be excited in two ways:Lengthening the whole muscle stretches the midportion of the spindle and, therefore, excites the receptor.contraction of the end portions of the spindle's intrafusal fibers stretches the midportion of the spindle and therefore excites the receptor.
20Sensory Innervation of the Muscle Spindle Two types of sensory endings are found in this central receptor area of the muscle spindle :primary endingsecondary ending
21Primary EndingIn the center of the receptor area, a large sensory nerve fiber encircles the central portion of each intrafusal fiber, forming the so-called primary ending or annulospiral ending.This nerve fiber is a type Ia fiber averaging 17 micrometers in diameterit transmits sensory signals to the spinal cord at a velocity of 70 to 120 m/sec, as rapidly as any type of nerve fiber in the entire body.
22Secondary EndingUsually one but sometimes two smaller sensory nerve fibers- type II fibers with an average diameter of 8 micrometers-innervate the receptor region on one or both sides of the primary ending, This sensory ending is called the secondary ending;sometimes it encircles the intrafusal fibers in the same way that the type Ia fiber does, but often it spreads like branches on a bush.
24Division of the Intrafusal Fibers into Nuclear Bag and Nuclear Chain Fibers- Dynamic and Static Responses of the Muscle Spindle
25Division of the Intrafusal Fibers two types of muscle spindle intrafusal fibers:nuclear bag muscle fibers (one to three in each spindle), in which several muscle fiber nuclei are congregated in expanded "bags" in the central portion of the receptor areanuclear chain fibers (three to nine), which are about half as large in diameter and half as long as the nuclear bag fibers and have nuclei aligned in a chain throughout the receptor area.
26Division of the Intrafusal Fibers The primary sensory nerve ending (17-micrometer) is excited by both the nuclear bag intrafusal fibers and the nuclear chain fibers.Conversely, the secondary ending ( 8 micrometer) is usually excited only by nuclear chain fibers.
27Details of nerve connections from the nuclear bag and nuclear chain muscle spindle fibers.
29Golgi Tendon Organ Helps Control Muscle Tension is an encapsulated sensory receptorAbout 10 to 15 muscle fibers are usually connected to each Golgi tendon organis stimulated when this small bundle of muscle fibers is "tensed" by contracting or stretching the muscle.
30Thus, the major difference in excitation of the Golgi tendon organ versus the muscle spindle is that the spindle detects muscle length and changes in muscle length,whereas the tendon organ detects muscle tension as reflected by the tension in itself.
31Golgi Tendon Organs (GTO) Encapsulated receptorsLocated at the musculotendinous junctionMonitor tension within the tendonRespond by causing the muscle to relax3131
32Golgi Tendon Organ Helps Control Muscle Tension The tendon organ, like the primary receptor of the muscle spindle, has both a dynamic response and a static response :dynamic response : reacting intensely when the muscle tension suddenly increasesstatic response : settling down within a fraction of a second to a lower level of steady-state firing that is almost directly proportional to the muscle tension.Thus, Golgi tendon organs provide the nervous system with instantaneous information on the degree of tension in each small segment of each muscle.
33Transmission of Impulses from the Tendon Organ into the Central Nervous System are transmitted through large, rapidly conducting type Ib nerve fibers that average 16 micrometers in diameter.These fibers, like those from the primary spindle endings, transmit signals both into local areas of the cord and, after synapsing in a dorsal horn of the cord, through long fiber pathways such as the spinocerebellar tracts into the cerebellum and through still other tracts to the cerebral cortex
34The local cord signal excites a single inhibitory interneuron that inhibits the anterior motor neuron (by releasing Glycine).This local circuit directly inhibits the individual muscle without affecting adjacent muscles.
35Inhibitory Nature of the Tendon Reflex and Its Importance When the Golgi tendon organs of a muscle tendon are stimulated by increased tension in the connecting muscle, signals are transmitted to the spinal cord to cause reflex effects in the respective muscle.This reflex is entirely inhibitory.Thus, this reflex provides a negative feedback mechanism that prevents the development of too much tension on the muscle and a protective mechanism to prevent tearing of the muscle or avulsion of the tendon from its attachments to the bone.
37Classification by Location InteroceptorsDetect stimuli from inside the body and include receptors that respond to pH, oxygen level in arterial blood, carbon dioxide concentration, osmolality of body fluids, distention and spasm (e.g., gut), and flow (e.g., urethra)37
38Classification by Location ExteroceptorsSensitive to stimuli arising from outside of the bodyTypically located near the surface of the bodyInclude receptors forTouchPressurevibrationPainTemperatureSpecial sense receptors…..
402- Classification by Stimulus Detected Mechanoreceptors – respond to mechanical forcesThermoreceptors – respond to temperature changesChemoreceptors – respond to chemicals in solutionPhotoreceptors – respond to light – located in the eyeNociceptors – respond to harmful stimuli that result in pain
413- Classification by Structure General sensory receptorsWidely distributedNerve endings of sensory neurons monitor:Touch, pressure, vibration, stretchPain, temperature, proprioceptionDivided into two groupsFree nerve endingsEncapsulated nerve endings
54Tonic & Phasic Receptors Transduction: translation of a stimulus into an action potential.Transferred to an afferent fiber which then travels to the CNS.Tonic Receptors-always sending signals.Phasic Receptors: only when the conditions they monitor change.Fast-adapting receptors are phasic and slow-adapting are tonic.
55Receptor Adaptation Slowly adapting (SA) Rapidly adapting (RA) Different rates of adaptation¯ response to continued stimulationSlowly adapting (SA)steady pattern of firingRapidly adapting (RA)fire only at onset of stimulus