SENSORY SYSTEM LECTURE 1 RECEPTORS

SENSORY SYSTEMRECEPTORS & SENSORY PATHWAYSLECTURE 3DR. ZAHOOR ALI SHAIKH11Sensory system or Input system What type of sensation do you know?TouchFine CrudePressurePositionVibrationTwo point discriminationPainTemperatureCold Warm Stereognosis 2How we feel the sensation?Sensory receptorSensory pathwaySensory cortex3Cont.COMPONENTSReceptorsPeripheral nervesSpinal cordTractsBrain stemThalamusThalamocortical projectionSomatosensory cortex4Sensory ReceptorsThese are transducers that convert various forms of energy into action potentials .The particular form of energy to which a receptor is most sensitive is called as its adequate stimulus.56Adequate stimulusEach type of receptor is most sensitive to a specific form of energy, called adequate stimulus, the receptor is almost non-responsive to the normal intensities of other forms of energy.e.g. rods & cones are stimulated by light not heat. Touch receptors are stimulated by touch.

Sensory receptors Sensory receptors can be subdivided into:

1) Mechanoreceptors. 2) Thermoreceptors. 3) Nociceptors. 4) Proprieceptors. 5) Visceral receptors 6) Special senses.8

Pressure

Tactile Receptors in the Skin9Skin Receptors10

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Receptor Properties1.Receptor potential or Generator PotentialWhen we apply pressure, we generate depolarization in the receptor it is called Generator potential or Receptor potential.Non propagated depolarizing potential.

1213What is the difference between generator potential and action potential?Receptor potential

In the ReceptorGradedDoesnt obey all or none ruleCan be summatedUnpropagatedAction potential

In the Sensory Nerve fiberNot GradedObeys all or none rule

Not summatedPropagated 14

RECEPTOR POTENTIAL & ACTION POTENTIALNote: if stimulus to the receptor is sub threshold then no action potential is generated in sensory nerve fiber15

Relation between receptor potential and action potential of sensory nerve16

Sensory Transduction2.Adaptation or DesensitizationWhen stimulus of constant strength is applied to a receptor, some receptors can diminish the extent of their depolarization inspite of sustained stimulus. This is called adaptation. Degree of adaptation varies in different receptors.17Classification of receptorsRapidly adapting or phasic receptors e.g. pacinian corpuscles, Meissners corpusle. Because of that we are not aware of sitting on the chair, wearing watch, wearing clothes.

Slowly or non adapting or tonic receptors e.g. Nociceptors, muscle spindles.1819

ADAPTATION OF RECEPTORPhasicTonic20

RESPONSE OF PHASIC & TONIC MECHANORECEPTORWhy we feel touch, pain, warm sensation when all sensory nerve carry the information to brain as action potential?213.Coding of sensory informationCoding of sensory informationDue to the Doctrine of specific nerve energies.We have;Specific receptors.Specific sensory pathways.Specific part of the brain they activate.22Law of projectionIf we stimulate sensory pathway along its course to the sensory cortex, the conscious sensation produced is refered to the location of receptors. This principle is called as Law of projection.If we stimulate sensory cortex area which receives impulses from left hand, patient reports sensation in the left hand, not in the head. 23Cont..Q. In patients whose limb was amputed (cut off) they complained of pain in the absent limb (Phantom limb), Why?Due to law of projection, the ends of nerves cut at the time of amputation when stimulated, sensation evoked are projected to where the receptor used to be present,i.e. in the limb.2425Receptor field Receptor field of a sensory unit is the area from which a stimulus produces response in that unit. Smaller the receptive field More precise the information e.g. Finger tips Larger the receptive field- less precise the information e.g. arms, legs.

Discriminative ability of regions with small versus large receptive field26Recruitment of sensory unitsWeak stimulus activates receptors with lowest threshold.Strong stimulus activates those receptors also which have high threshold .2728

SPATIAL SUMMATION29

Physiological classification of nerve fibers that transmit different types of sensations30Physiological classification of nerve fibers that transmit different types of sensationsIa

Ib

II

III

IVMuscle spindle, annulo-spiral ending.Golgi tendon organ.

Muscle spindle, flower-spray ending, touch, pressure

Pain and cold receptors; some touch receptors

Pain, temperature, and other receptorsA

A

A

A

Dorsal root CNumberOriginFiber typeSensory Pathways Somatosensory pathways are THREE Neuron system31How Sensation are carried?32From receptors impulses are carried by sensory nerves, the sensory nerve have cell bodies in dorsal root ganglia.Primary afferent fibers which carry cutaneous sensations are Large myelinated fiber A and A propriception, touch, pressure.Small myelinated fiber A fast painSmall unmyelinated C fibers slow painSensory fiber go to dorsal horn in the spinal cord.Dorsal horn represent a gate in which impulses in the sensory nerve fiber are translated into impulses in ascending tracts.Dorsal horns are divided into laminae I VII. (I superficial, VII deepest).

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Sensory pathways or tracts areThree Neuron system1. Dorsal column or posterior column or Gracilis and cuneatus or leminscal system.It carries sensations of fine touch, position, viberation, two point discrimination & stereognosis.2. Anterolateral systemVentral spinothalamic tract carries crude touch and pressure,Lateral spinothalamic tract carries pain and temperature.

35Spinal Tracts

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MedullaDorsal Column System39

Anterolateral System40

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Anterolateral SystemDorsal Column42

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DORSAL COLUMN PATHWAYCARRIES FINE TOUCH, POSITION, PRESSURE, VIBRATION, TWO POINT DESRIMINATION stereognosisAFFERENT SENSORY FIBERS A TYPE.VERY FAST VELOCITY 30 70 m/s3 NEURON SYSTEM(SEE THE DIAGRAM)ANTEROLATERAL PATHWAYCARRIES PAIN & TEMPRATURE (lat. Sp.Th)CRUDE TOUCH & PRESSURE (VENT, Sp. Th)AFFERENT SENSORY FIBERS A 6 30 m/s (MYELINATED) FAST PAIN C FIBERS 0.5 2 m/s (UNMYELINATED) SLOW PAIN3 NEURON SYSTEM(SEE THE DIAGRAM)

44Different SensationsTEMPERATURE Skin has cold sensitive area & heat sensitive area. Receptors are free nerve endings.Cold receptors are 4 10 times more than warm sensitive spots.Cold receptor responds from 10 38 oC.Warm receptor responds from 30 45 oC.Afferent from cold receptors A & C fibers.Afferent from warm receptors C fibers.Temperature sensation is carried via lateral spinothalamic tract. 4546

THERMORECPTOR Different SensationsPAIN Pain receptors or nociceptors free nerve endings.Pain sensation are carried by two types of nerve fibers.A (myelinated) -2 5 m.in diameter, conduction velocity 12 30 m/sec.For fast pain (sharp localized)C fibers (unmyelinated) 0.4 1.2m. in diameter, conduction velocity 0.5 2 m/sec. For slow pain (dull, diffuse)Both A & C fibers terminate in dorsal horn.Pain is carried via lateral spinothalamic tract. 47PERCEPTION Perception is conscious interpretation of external world by Brain due to sensory impulses delivered to Brain from sensory receptors. We perceive sounds , colors, smell.

Why we do not perceive X-Ray, light waves,MRI. Because we do not have receptors to respond. 48Thank you49