Cutaneous sensory receptors Smooth skin (glaborous ) Hairy skin Free nerve endings The receptor location and its associated structure can alter the stimulus and influence the response Subcutis Dermis Epidermis Merkel disks detect steady pressure & are slowly adapting Free nerve endings around hair root can be either rapid or slowly adapting - depends on hair type Meissner’s corpuscles detect flutter & are rapidly adapting Pacinian corpuscles detect vibration & are very rapidly adapting Ruffini corpuscles detect steady pressure at higher threshold & are slowly adapting Free nerve endings in the skin are modality specific and can detect either pain or touch or pressure or temperature
arise by activation encapsulated mechanoreceptors
arise by activation of free nerve endings
ability to perceive that something contacted skin
exact location, shape, size, or texture cannot be detected
provides specific information about location, shape, size, and texture of stimuli
Intrinsic knowledge of limb position is known as kinaesthesia.
Information is provided by sensory input from muscle spindles (Ia & II) and Golgi tendon organs.
These are mechanoreceptors and provide the CNS with information on muscle length, position and tension.
Muscle spindle group Ia afferent fibres are rapidly adapting (dynamic) and are sensitive to rapid changes in muscle length.
Muscle spindle group II afferent fibres are slowly adapting (static) and single the fixed length of the muscle.
Joint Kinesthetic Receptors
located within and around articular capsules of synovial joints
perception of body movements
free nerve endings and type II cutaneous mechanoreceptors
in capsules of joint and respond to pressure
small lamellated corpuscles
respond to acceleration and deceleration of joints during movement
articular ligaments contain receptors similar to tendon organs
adjust the contraction of adjacent muscles when excessive strain is placed on joint
Processing at Receptor Level
Receptor must have specificity for stimulus activity
Stimulus must be applied to receptive field
Transduction must occur
Generator potential in 1 st order neuron must reach threshold
How is information about stimulus encoded?
Tonic receptors (slow adapting)
Phasic receptors (fast adapting)
Rapidly adapting cutaneous mechanoreceptors signal the onset and offset (phasic) of a stimulus and give rise to sensations such as vibration, touch, and movement
Slowly adapting mechanoreceptors continuously signal (tonic) the intensity of the stimulus and give rise to the sensation of pressure.
Cutaneous receptors with small receptive fields are involved in fine spatial discrimination, whereas receptors with larger receptive fields are less spatially precise.
Overlap of receptive fields allows lateral inhibition to occur in the ascending pathways and increase sensory acuity.
Somatosensory system receptor classification
Sensory neuron delivers information to CNS
Cell body is located in dorsal root ganglion
Synapses with interneurons in CNS
Usually interneurons receiving information from first order neurons
Axons cross to opposite sides of body
May be located in the spinal cord or brain stem
Located in thalamus
Carries information to cerebrum
Synapses with neurons of the primary sensory cortex
Ascending Pathways to Brain
What type of information do the neurons carry?
3 chains of successive neurons to brain
1 st order neurons
Where is cell body?
Conduct impulses from cutaneous receptors and proprioceptors
Synapse w/ 2 nd order
2 nd order neurons
Cell bodies in dorsal horn or in medullary nuclei
Transmit impulses to thalamus or cerebellum; synapse
3 rd order neurons
Where do they conduct information to?
No 3 rd order neurons in cerebellum
First order neurons
Sensory neurons that deliver sensory information to the CNS
Second order neurons
First order neurons synapse on these in the brain or spinal cord
Third order neurons
Found in the thalamus
Second order neurons synapse on these
Only 1% of incoming sensory impulses actually reach the cerebrum.
First, second, and third order neurons The Organization of Sensory Pathways
Posterior Column Pathway
Carries sensations of highly localized (fine) touch, pressure, vibration, proprioception
Spinal Tracts Involved:
Left/right fasciculus gracilis
Left/right fasciculus cuneatus
Peripheral nerves system
Sensory Pathways and Ascending Tracts in the Spinal Cord
dorsal cloumn pathway
Dorsal column pathway
Dorsal-column leminiscal pathway
Principally conveys tactile discrimination, vibratory and position senses (A ,large fibres).
1st order sensory neurones run on the same side & synapse with 2nd order neurones in the dorsal column nuclei .
2nd order neurones integrate the input and their axons cross to the opposite side. These ascend through the medial leminiscus
Further integration in the thalamus & 3rd order neurones project to the cortex.
The Posterior Column Pathway and the Spinothalamic Tracts The area of sensory cortex devoted to a body region is relative to the number of sensory receptors.