Proprioception & Touch: Sensory Mechanisms
Proprioception and Touch: Sensory Mechanisms
Proprioception
Proprioception refers to the sense of the position of joints, tension in muscle fibers, and the overall position of the body.
Touch
Touch involves the perception of spatial and temporal patterns of pressure on the skin.
- Deformation of the capsule surrounding sensory nerve fiber endings leads to stretching of the membrane.
- This stretching increases the probability of opening stretch-sensitive cation channels.
- A net influx of Na+ leads to depolarization of the nerve fiber ending.
- If the deformation is intense enough, an action potential (AP) will be generated.
Receptive Fields
A receptive field is the area of skin from which a touch elicits activity in a particular nerve fiber.
- Measuring the discriminability of two-point stimulation provides insight into the size of receptive fields in different parts of the body.
- Discrimination is highest in body parts where fine discrimination is most needed.
- Discriminability is highest, and receptive fields are smallest, in the fingers and areas of the face around the mouth.
Types of Sensory Receptors
- Fast-adapting receptors: Used to perceive vibration and rapidly changing patterns.
- Slow-adapting receptors: Code for aspects like the pressure in your hand when gripping an object.
- Free nerve endings: Responsible for nociception (tissue damage, chemical stimuli) and temperature sensation.
Specific Receptor Types
- Meissner’s corpuscles: Rapidly adapting, located in the superficial skin, with highest sensitivity.
- Merkel disks: Slow-adapting, located in the superficial skin. Composed of Merkel cells and nerve endings. Provide the highest resolution.
- Ruffini endings: Slowly adapting, located deep in the skin.
- Pacinian corpuscles: Rapidly adapting, located deep in the skin.
Functional Roles of Receptors
- Merkel disk receptors: Slow-adapting, high acuity, responsible for shape and edge discrimination.
- Meissner’s corpuscles: Fast-adapting, high acuity, detect edges and textures during hand movement.
- Pacinian corpuscles: Fast-adapting, high sensitivity, low acuity, used for perception of vibration and speed of hand movement across surfaces.
- Ruffini endings: Slow-adapting, low acuity, sensitive to stretch, possibly involved in the perception of lifting objects.
Neural Pathways for Touch
- From the skin to the brainstem (dorsal column nuclei, where the signal crosses to the opposite side) via dorsal root ganglion cells.
- From the brainstem to the thalamus (ventral posterior lateral nucleus).
- From the thalamus to the primary somatosensory cortex.
Cortical magnification of important areas mirrors the overrepresentation of the fovea in vision.
Somatosensory Cortex Areas
- Area 3a: Input stage for proprioceptive information.
- Area 3b: Input stage for fine tactile discrimination information.
- Area 1: Higher association area, receiving inputs from 3a and 3b, as well as direct inputs from the thalamus.
- Area 2: Higher association area, receiving inputs from all lower levels and the thalamus.
All cells within a column have a similar receptive field location. Center-surround organization features an excitatory center region and a surrounding inhibitory region.
Lateral Inhibition
Lateral inhibition is implemented in the somatosensory system, likely at several different levels. It significantly improves the discrimination of two-point stimuli and enhances the detection of edges, similar to its function in the visual cortex.
Cortical Plasticity
The cortical map of a hand was mapped before and three months after the amputation of one finger. The cortical area originally responding to digit 3 developed new responses to the adjacent fingers (2 and 4). Normal experience can also induce plasticity in cortical maps. For example, a monkey trained for several months to perform a task requiring extensive use of fingers 2 and 3, and sometimes finger 4, showed enlargement of the cortical areas dedicated to those fingers.