Introduction of Sensory Physiology

and Physiology of Receptors

DR. KARISHMA R. PANDEYASSISTANT PROFESSOR

DEPARTMENT OF BASIC AND CLINICAL PHYSIOLOGY

Topics

1) Terms 2) Classification of receptors3) Physiology of receptors4) Properties of sensory system5) Pathways

Sensory system

Sensation: the conscious or subconscious awareness of changes in the external or internal environment.

Perception: the conscious awareness and interpretation of sensations and is primarily a function of the cerebral cortex.

Stimulus: the energy that impinges upon & activates a sensory receptor.

Sensory receptors: cells or regions of cells that respond to specific stimuli and perform sensory transduction.

Sensory transduction: the process by which a stimulus of any form is transformed into electrical response.

Sense organ: the receptor and the non neural structure

Adequate Stimulus: the form of energy the receptor is most sensitive

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*Teleceptors

Classification of Receptors

Sensory Receptors

Receptor potentials:An initial response of a receptor cell to a stimulus, consisting of a change in voltage across the receptor membrane proportional to the stimulus strength.

The intensity of the receptor potential determines the frequency of action potentials traveling to the nervous system.      

1. Area deformed by compression

2. Change in membrane permeability

3. Ion channels opened- Na ions diffused into the fiber4. Increased positivity inside fiber- Receptor potential

5. Induce a local circuit of current flow, spread along the nerve fiber6. At node of Ranvier local current flow depolarieszes---Sets off typical AP along the fiber to CNS

“Laws of Specific Sense Energies.” Muller (1835) proposed “each nerve fiber is activated primarily by a certain type of stimulus and each makes specific connections to structures in the central nervous system whose activity gives rise to specific sensations”. Labelled line coding specificity of nerve fibers for transmitting only one modality of sensation is called the labeled line principle

Adequate stimulus

Adaptation: When a stimulus of constant strength is applied continuously to a receptor, frequency of the action potentials in its sensory nerve declines. Phasic/ Rapidly adapting (pacinian corpuscles)Tonic/Slowly adapting (nociceptors, baroreceptors)

Properties of sensory

Location where the stimulus being applied

Sensory unit: a single afferent neuron with all its receptor endings.

Receptive field : the area of the body that when stimulated leads to activity in a particular afferent neurons.

Acuity the precision of stimulus localization or the ability to distinguish fine

details

Depends ona. Size of the receptive field (small field ⇒ better acuity)

b. Innervation density (higher density ⇒ better acuity)

c. Convergence along CNS pathways (less convergence ⇒ better acuity)

d. Lateral or surround inhibition (increases acuity)

The information from neuron A indicates the stimulus location more precisely than does that from neuron B because A’s receptive field is smaller.

2 points of calipers are simultaneously positioned

determines the minimum distance between 2 caliper points , perceived as separate points of stimulation

• back 65mm• finger tip 2mm.

TWO-POINT DISCRIMINATION

magnitude of two-point discrimination thresholds varies at different parts of the body and is smallest where touch receptors are most abundant

Two stimulus points, A and B, in the receptive field of a single afferent neuron.The density of nerve endings around area A is greater than around B, and the frequency of action potentials in response to a stimulus in area A will be greater than the response to a similar stimulus in B.

lateral inhibition Information from sensory neurons whose receptors are at the peripheral edge of the stimulus is inhibited compared to information from the sensory neurons at the center of the stimulus.

PresynpaticInhibition

Mildinhibition

Massiveinhibition

Law of projectionEach receptor (or group of the same receptor) is connected through a separate pathway to a certain point in the cerebral cortex. Stimulation of this point in the cerebral cortex (or any where in the pathway) projects the sensation to the location of the receptor.

PHANTOM LIMB SENSATION

Intensity

magnitude of the sensation felt is proportional to the log of the intensity of the stimulus

Power law power function more accurately describes this relation. R = KSA

R= sensation felt, S=intensity of stimulus, K & A= constant

When a stimulus is applied, receptors discharge depending on the strength of the stimulus

Weber Fechner Law

Sensory Pathways

Fiber type FunctionFiber

diameter (μm)

Conduction velocity (m/s)

Aα Proprioception 12-20 70-120Aβ Touch, pressure, 5-12 30-70Aδ Pain, cold, touch 2-5 12-30C Pain, temperature,

some mechanoception 0.3–1.3 0.7–2.3

Sensory pathways

Dorsal column Medial lemniscal pathways: fine touch, pressure, vibration, position

Anterolateral pathway : pain, thermal (warmth & cold), crude touch, tickle and itch

4 steps in sensation Stimulation application of stimulus Must be strong enough to induce AP in sensory neuron Sensors most sensitive to one particular stimulus modality (adequate stimulus)

Transduction induction of an action potential Stimulation of sensor induces graded potentials in sensors generator potentials/receptor potentials– If strong enough depolarization, AP results ↑ stimulus strength above threshold → ↑ AP firing rate

Conductionrelay of information through a sensory pathway to specific region of CNS Usually three neurons in sensory pathway• 1st order neuron– from stimulation point to CNS• 2nd order neuron– e.g., from entry into CNS to thalamus• 3rd order neuron – e.g., from thalamus to perception site

Perception Detection of environmental change by CNS Evaluation of nature of change and magnitude