Somatic senses Flashcards
Somatic senses
- Part of the sensory division of the PNS
- As somatic senses are part of the sensory division, they rely on the presence of receptors
Receptors
Structure specialised to detect a specific stimulus
Generate a signal in response to stimulus which is carried to CNS informing it of the event
- Cluster of free nerve endings (myelinated or unmyelinated); senses pain, heat and cold
- Meissner’s corpuscle (unmyelinated nerve endings in CT capsule); senses light touch
- Pacinian/pressure corpuscle (single unmyelinated nerve ending surrounded by layers of CT separated by a gel); senses pressure
- Merkel/tactile discs (combination of Merkel cell + enlarged myelinated axon terminals); senses light touch
- Ruffini’s corpuscle (bundle of collagen fibrils with a myelinated sensory axon branching between fibrils, enclosed in a flattened capsule); senses heavy touch and pressure
Signal transduction
Process of converting the stimulus into an AP
- Receptor potential = MP potential change when receptor responds to stimulus
- When this potential reaches threshold, AP produced
- Receptor potentials occur due to ions crossing cell membrane; result of ion channel opening (mechanical deformation, release of chemicals) or increased membrane permeability (changes in tissue temp)
- When a continuous stimulus is applied most receptors undergo ADAPTION; a decrease in AP generation over time
- Some types are rapidly adapting eg. Meissner’s corpuscles, however is the steady state changes these receptors are quick to respond
- Other types are very slow to adapt eg. Baroreceptors (monitor BP) and are therefore good at continually monitoring stimuli
- Pain receptors adapt very little and sometimes become more sensitive to stimuli, responding to non-noxious stimuli (hyperalgesia)
Labelled line principle
As APs are an all-or-nothing phenomenon, the labelled line principle explains how the brain is able to know what kind of signal it’s receiving
- Describes how the type of sensation felt when a nerve fibre is stimulated is dictated by the point to which the fibre leads
- Interpretation of the signal depends on the location of the end point of the AP in the cortex
If AP is all-or-nothing, how does the brain know how intense the sensation is?
- Spatial summation; stimulus intensity increases, number of individual neurones generating an AP increases
- Temporal summation; stimulus intensity increases, number of APs generated by individual neurones per unit time increases
Touch and pressure
- Stimuli is mechanical deformation; receptors involved are referred to as mechanoreceptors
- Found throughout the skin and subcutaneous tissue
- Most numerous in the skin of fingers and lips
Temperature
- Perception of temp gradation results from stimulation of different proportions of each receptor type; warmth, cold, heat-pain and cold-pain
- Thermoreceptors are found in the skin and other tissues eg. cornea and urinary bladder
- Stimulated by changes in metabolic rates; temp alters rate of intracellular reactions more than two-fold for every 10ºc shift
- Stimulated by changes in membrane permeability; if molecules have more energy (due to heat increase), they will be moving around more and making contact with the plasma membrane, therefore opening ion channels
- Thermoreceptors adapt fairly rapidly, therefore thermal senses respond to changes in temp rather than steady states of temp
Pain
- Associated with actual or potential tissue damage; has a protective function
Nociceptor:
- Receptor type responsible for the sensation of pain
- Free nerve ending
- Widespread throughout skin, arterial walls and joint surfaces, and less widespread in deeper tissues
Fast pain:
- Sharp, immediate, highly localised sensation
- Conveyed to SC by thin, myelinated nerve fibres with rapid conduction velocity (6-30m/s)
- Aδ fibres
Slow pain:
- Delayed, dull pain that is hard to locate
- Conveyed to SC by unmyelinated nerve fibres with slow conduction velocity (0.5-2m/s)
- Type C fibres
Stimuli which excite nociceptors:
- Serotonin
- Bradykinin
- K+ (which would be released from damaged cells + intensity of pain felt correlates with local K+ conc)
- Heat/cold
- Lactic acid
- Do not need tissue damage to feel pain; once mast cells recognise foreign antigens, they release histamine which stimulates pain receptors
Somatosensory pathways
Somatic senses originating in the head region:
- Travel via the trigeminal nerve to an area of the parietal lobe called the somatosensory cortex
Somatic senses originating below the head:
- Signals travel via a tract called the spinothalamic tract (touch, pressure, pain, temp)
- Signals pass up the spinal cord, into the brainstem, the thalamus and then into the somatosensory cortex
- Sensory fibres travelling via the spinothalamic tract cross over in the SC, so the cortex receives sensory info from the opposite side of the body
- Dorsal columns of SC carry proprioceptive signals and the sensation of fine touch
Somatosensory cortex
- Distinct areas dealing with specific parts of the body
- The more sensitive the body part, the greater the area of the cortex allocated for that part
- This is because, the more sensitive the body part, the greater the number of sensory receptors there are, and essentially more brain tissue is required to deal with the greater amount of incoming data
