10/7/2010

Sensory Receptors Sensory Receptors (some basic properties)

Organs which receive information from outside or within the body and send it to the central nervous system for processing

Sensory Receptors Sensory Receptors Are Transducers (some basic properties)

Specialised nerve ending Definition of a Transducer or Transduction Conversion of one form of energy (e.g. heat, Activated by pressure, chemical) into another form: e.g. electrical various forms of energy: energy or depolarisation light, heat, mechanical The depolarisation occurring at the sensory terminal is chemical etc. a). Graded – Non-propagating, confined to terminal b). Called the Receptor Potential (R.P.)

Specialised cells The Receptor Potential: If large enough will evoke all or nothing action Sensory receptors are either specialized endings of afferent potentials that propagate along the nerve fibre neurones or separate cells that signal to the afferent neurone

The Receptor Potential Intensity of Stimulus is Linearly Encoded

Variable stimulus intensity

10mV Produces graded receptor potentials

Adequate stimulus

Produces variable patterns of action potentials in the CNS

The number of action potentials generated by a pressure- sensitive sensory afferent neuron is directly proportional More Stimulus = More Transmitter Released to the stimulus intensity.

1 10/7/2010

Receptor Potential: more or less proportional to stimulus Who can guess how a membrane of a neurone could react to stimulus receptor potential pressure or temperature by a change in membrane potential?

adaptation mV

Adaptation to stimulus depends on receptor type

Adaptation: A Mechanism Preventing Specific types of sensory receptors for Sensory Information Overload different mechanosensory stimulation The off response Skin: Paccinian Corpuscles pressure/touch

Muscle: stretch receptors

Different receptors vary in their speed of adaptation to stimuli

Receptor Specificity Sensory Processing: spatial discrimination Receptors have a differential sensitivity to different energies or stimuli

MechanoreceptorsPhotoreceptors Chemoreceptors Sensory afferent (A) nerve ending has a more spatially limited receptive field than sensory afferent neuron (B).

A receptor is specific to one energy (or stimulus), because it is most sensitive to that energy. Other energies may stimulate but only at high intensities.

2 10/7/2010

Sensory Processing: Sensory Processing: spatial discrimination – the paired point test spatial discrimination Lips Back

Overlapping stimulation between neighboring receptive fields provides general i nf ormati on about the location of a stimulus.

Sensory Processing: Sensory Processing: density of sensory afferent terminals descending modulation from higher centres Painful Condition

Stimulus A occurs in an area of greater nerve ending density

Stimulus A will generate Painful Condition Descending input a greater number & Fight or Flight of action potentials than stimulus B.

Context and Sensory Perception Sensory Processing: sensory discrimination

Deliver information about location- Provide background specific changes in touch and information about Monty Python & the Holy Grail temperature in the periphery. touch and temperature

3 10/7/2010

Sensory Modality Representation is Location Convergence of sensory pathways is one of the reasons Specific in the Cerebral Cortex for “referred” pain from visceral organs

Sensory Representation is Cortex

Summary:

1. Sensory receptors allow our body to interact with the external environment Fine discrimination 2. Sensory receptors in internal organs are essential for homeostasis and is highly represented also alert the body in case of some anomaly 3. Some receptors adapt fast, others slow and some – almost not at all. 4. The intensity of the stimulus is encoded by the frequency of the action potentials. 5. Most receptors detect the stimulus via a subset of highly specialised ion channels which open (Na+) or close (K+) to evoke depolarisation of the membrane of the sensory ending in response to the stimulus. 6. Information sent from the receptors is called AFFERENT information. In the cortical homunculus CNS it is usual that several afferent neurones contact the same postsynaptic cell. This is the convergence, which is also a reason for referred pain. 7. The organs with the highest tactile sensitivity have the largest representations in the “sensory homunculus”

4