1. MECHANORECEPTORS

1. Cutaneous Receptors

Discriminative Touch- information travels to cortex via the dorsal columns of the spinal cord.

Functions:

Distinguish between different types of stimuli to the skin, including features such as direction, intensity, and frequency.

Receptors for touch are located in the skin.

Physical contact with the skin causes mechanical alteration or deformation, which is detected by specialized receptors, that are found in the epidermis and dermis layers of the skin.

The axons arising from these various receptors are classified based on temporal and spatial properties.

Temporal

1. Rapidly adapting (RA)- indicating that they are quick to adapt to an object in contact with the skin; axons respond only to the onset and offset of mechanical deformation.
2. Slowly adapting (SA)-discharge when a tactile stimulus is applied and continue to respond during stimulus presentation.

1. Based on the amount of skin innervated by a single unit. A single nerve unit distributes to a specific region, this region, or spatial extent of skin that activates the nerve fiber is called the receptive field of that unit. There are two broad categories, small (type I) and large (type II).

Types of Mechanoreceptors

a. Pacinian Corpuscles – largest sensory end organ, most deeply situated, transduces only high frequency components of touch sensations (vibration); associated sensation vibration; is unresponsive to sustained deformation of the skin, ratio of nerve axon to receptor is 1:1, low activation threshold. (RA type II)

b. Meissner’s Corpuscles – superficially located; stimulus-vibration; associated sensation -flutter or contact; low activation threshold; axon to receptor 1:20 ratio; (RA type I)

c. Merkel’s discs – lie near epidermis-dermis junction, stimulus-deformation of the skin; sensation-light touch or pressure, low activation threshold; 1 fiber innervates 3-4 complexes (complex contains up to 30 cells); (SA type 1)

d. Ruffini Endings – located deep in the dermis but more superficial that Pacinian corpuscle, stimulus-skin distortion; sensation touch, moderate activation threshold; (SA type II)

e. Hair Follicle – stimulus- displacement of the hair shaft, sensation- contact, touch; low threshold (RA type I)

2. Musculoskeletal Receptors

a. Joint Receptors- located in the joint capsule, 4 different types, stimulus-joint movement and pressure; sensation-proprioception, (SA)

b. Muscle Spindles- elongated structures, found, in muscle belly, composed of bundles of small intrafusal muscle fibers, lie in parallel to the extrafusal fibers, stimulus- spindle stretch; sensation – proprioception; has afferent (sensory) and efferent (motor) innervation (SA)

Two types of intrafusal muscle fibers- nuclear bag fibers and nuclear chain fibers

c. Golgi Tendon Organs – found at the musculotendinous junction, arranged in series with 15-20 extrafusal fibers, have no motor innervation, only sensory; stimulus – tendon tension; sensation – proprioception; (SA)

1. Thermoreceptors

Information travels to the cortex via the anterolateral system (spinothalamic and spinoreticular tract)

 

1. Free nerve endings – stimulus hot and cold, sensation – hot and cold, adaptation – intermediate, extreme temperatures are perceived as pain and are carried by nocioceptors.

 

2. Nocioceptors

1. Free nerve endings - stimulus- noxious; sensation – pain; (SA)

Information travels to the cortex via the anterolateral system

Clinical Correlations

Tabes Dorsalis – affects dorsal roots and thus axons in dorsal columns, poor position sense, 2-point discrimination, ataxic or steppage gait.

Brown–Sequard Syndrome – hemi-section of the spinal cord due to vascular insult or knife wound. Ipsilateral deficits except for loss of pain and temperature loss which will be contralateral to side of lesion.

Phantom Limb – occurs after amputation, sensation that limb is still there and they can feel it moving, related to changes in somatosensory cortex (?)