Week 3 - General Senses - Touch, proprioception, vibration

General senses Breakdown

Senses that are spread throughout the entire body.

• Do not have a dedicated organ like special senses

3 Types:

• General sense → receptor

• Touch, proprioception, vibration → mechanoreceptors

• Temperature → thermoceptors

• Pain → nociceptors

Touch, proprioception, vibration: Skin anatomy: Mechanoreceptor types

Mechanoreceptors help distinguish between different touch types

Types of mechanoreceptors:

• Merkel’s discs - detects edges & points

• Lamellated (Pacinian) corpuscles - detects vibration

• Tactile (Meissner’s) corpuscles - detects skin motion & movement

• Bulbous (Ruffini’s) corpuscles - skin stretching

Touch, proprioception, vibration: Skin anatomy: Mechanoreceptor types: Merkel’s discs

Touch, proprioception, vibration: Sensation

1. As mechanoreceptors transduce touch, physical distortion of the sensory cell membranes stretch the membrane.

2. Mechanically gated-ion channels of Na+ & Ca2+ open, allowing them to travel down the concentration gradient into the cell

3. Results in cell depolarisation

4. Second messenger cascades occur

5. If graded receptor potential is strong enough, action potential is sent

Touch, proprioception, vibration: Skin anatomy: Receptive fields

When a certain receptive field is touched, respective mechanoreceptor type is activated.

Each mechanoreceptor type has a varying size in their receptive fields.

Small receptive fields:

• High receptor density, many cells found in one general location

• Provides precise information about the contours of an object

• Found mostly in face, lips, hands

Large receptive fields:

• Low receptor density, less cells found in one general location

  • Can be large enough to cover part of the palm

• Provides precise information about the contours of an object

• Found mostly in face, lips, hands

Touch, proprioception, vibration: Sensation: Two-point discrimination

• When a receptive field is touched, the neuron sends an action potential to the CNS

  • No. of action potentials sent based on the number of touches is based on the size and density of the receptive fields stimulated

    • i.e

      • Skin with smaller, but more numerous receptive fields can distinguish between two close touches because different receptive fields are stimulated

      • Skin with larger, and less numeroud receptive fields cannot distinguish between two close touches because the same receptive field is stimulated

Touch, proprioception, vibration: Sensation: Acuity

Acuity describes how accurately our brains can understand a stimulus.

• Greater acuity with smaller, more numerous receptive fields, more accurate informations

• Less accuracy with larger receptive fields (provide “coarse information” a.k.a rough information)

Touch, proprioception, vibration: Sensation: Adaption

Skin mechanoreceptor types adapt at different rates.

• Phasic receptors: Rapidly adapting receptors, Fire only when stimulus changes (i.e taking a watch off after wearing it for a day)

• Tonic receptors: Slow adapting receptors, Fire continuously, produces a sensation of pressure (i.e too tight socks)

Touch, proprioception, vibration: Sensation: Neural pathways; Dorsal column pathway

1. Mechanoreceptor stimulated

First-order neuron:

1. Action potential travels up the dorsal column in the spinal cord on the same side as the sensation

2. Synapses at the medulla

Second-order neuron:

1. Decussates in the medulla, crossing over to the other side (travels contralateral to the first sensory neuron)

2. Synapses in the thalamus

Third-order neuron:

1. Travels to the primary somatosensory cortex

Touch, proprioception, vibration: Sensation to perception

1. Information arrives to the primary somatosensory cortex inside the parietal lobe

2. Each region in the cross-section of the primary somatosensory cortex receives input from a specific section of the body (homunculi)

3. Due to the decussation of the second-order neuron, information received in one hemisphere, originated from the opposite side of the body

   1. Hence, damage to the left primary somatosensory cortex effects info arriving from the right side of the body