The Sense of Touch: Tactile Receptors

• The sense of touch, or tactile sense, is the perception of objects through skin contact.
• Mechanical stimuli like pressure, stretch, vibration, or movement physically deform tactile receptors.
• Tactile receptors are nerve endings of sensory neurons.
• Deformation of tactile receptors leads to the opening of ion channels, causing cellular depolarization.
• Strong stimuli generate action potentials that are sent to the brain.

Types of Tactile Receptors

• Tactile receptors are located in different layers of the skin and deeper tissues.
• Anatomically, receptors can be encapsulated or unencapsulated.
• Encapsulated receptors are wrapped in connective tissue, while unencapsulated receptors are not.
• Functionally, receptors can be rapidly-adapting or slowly-adapting.
• Rapidly-adapting receptors generate action potentials upon initial stimulation but quickly adapt and reduce/stop signaling despite continued stimulus.
• Rapidly-adapting receptors detect movements and sequences of events.
• Slowly-adapting receptors maintain signal generation for longer durations.
• Slowly-adapting receptors convey information about steady pressure, texture, edges, and shapes.
• Touch receptors vary in sensitivity, with some responding to light touch and others to heavy pressure only.

Receptive Fields

• A sensory neuron receives signals from an area called its receptive field.
• Any touch within a receptive field stimulates a single sensory neuron, transmitting one signal to the brain.
• Being touched at two points within a single receptive field feels like a single touch.
• Sensitivity varies across the body; for example, two points on the back must be farther apart to be felt separately compared to the finger.
• Receptors with small receptive fields are more sensitive and respond to lower frequencies of vibration.
• Receptors with large receptive fields typically respond to higher frequencies of vibration.

Touch Sensory Pathway

• The touch sensory pathway involves three neurons: first-order, second-order, and third-order.
• Touch receptors are first-order neurons, and their axons form sensory fibers entering the spinal cord via the dorsal root of the spinal nerve.
• Inside the spinal cord, first-order neurons synapse with second-order neurons near the entry point or in the medulla of the brainstem.
• Second-order neurons cross over to the opposite side of the spinal cord before ascending to the thalamus.
• Sensory information from the left side of the body is transmitted to the right side of the brain and vice versa.
• Third-order neurons conduct information from the thalamus to the sensory cortex.
• Sensory neurons from the face and head follow cranial nerves, mainly the trigeminal nerve, to the brainstem, where they synapse with second-order neurons, which then cross over and continue to the thalamus.

Sensory Cortex

• The sensory cortex is spatially organized, with adjacent areas representing neighboring regions of the body.
• The size of a cortical area representing a body region is proportional to the number of sensory receptors in that region.
• Sensitive regions with high receptor density, like fingers or the face, have a larger cortical representation than less sensitive areas like the trunk.

The sense of touch, or tactile sense, is the perception of objects through skin contact. Tactile receptors are nerve endings of sensory neurons that detect mechanical stimuli. These receptors can be encapsulated or unencapsulated and are located in different layers of the skin and deeper tissues.