BIO Lecture 5 - The Senses

Intro to Biological Psychology & The Senses

• The lecture covers the senses, drawing from Chapter 3 of Ellison and Chapters 10/11 of Kolb & Whishaw.

Sensation and Perception

• Sensation is the sensory organs registering physical stimuli from the environment.
• Perception involves the brain's interpretation of sensations.
• Perception is the brain's constructed version of reality; for example, sound is not the same as sound waves, and the color red is created from light waves.

Somatosensation

• Somatosensation includes:
• Hapsis (touch)
• Nocioception (pain and temperature)
• Proprioception (body awareness).
• Nocioception detects pain and temperature through free nerve endings, with adaptation to surrounding cells releasing chemicals to stimulate the dendrites and produce action potentials.
• Hapsis detects fine touch and pressure using pressure on encapsulated dendrites to mechanically stimulate action potentials. The capsules include:
  • Meissner's corpuscle (touch).
  • Pacinian corpuscle (flutter).
  • Ruffini corpuscle (vibration).
  • Merkel's receptor (steady skin indentation).
  • Hair receptors (flutter or steady skin indentation).
• Proprioception detects body awareness through movements stretching receptors to mechanically stimulate action potentials. This includes:
  • Muscle spindles (muscle stretch).
  • Golgi tendon organs (tendon stretch).
  • Joint receptors (joint movement).

Physical Stimulus Conversion

• Sensory receptors possess specialized dendrites that open ion channels when stimulated by an external stimulus.
• As an example, touch receptors wrapped around hair follicles open sodium channels when the hair moves, causing depolarization.
• A similar process can be noted in photoreceptors; the absorption of light resulting in changes in light-sensitive pigments and the opening of ion channels.

Signal Transmission to the Brain

• Pain and temperature signals travel differently than touch and pressure signals.
• The primary somatosensory cortex (areas 3-1-2) receives somatosensory information via the ventrolateral thalamus.
• The medial lemniscus carries sensory information to the ventrolateral thalamus.
• Dorsal-root ganglion neurons respond to fine touch, pressure, joint, tendon, muscle changes, pain, and temperature.
• Unilateral spinal cord damage leads to loss of fine-touch and pressure sensation on the same side of the body below the cut, and loss of pain and temperature sensation on the opposite side below the cut.

Somatic Sensation Components

• Somatic sensation includes touch (mechanical stimulation), pain (heat, chemical, or mechanical stimulation), and limb proprioception (muscle, joint stimulation).
• Signals from the body travel to the spinal cord and then to the brain, where they are processed by the opposite side.
• Pain is conveyed by unmyelinated axons, whereas other somatosensory neurons are myelinated; the stimulation of one blocks the other.

Receptive Fields

• Each sensory receptor responds to physical stimuli within a circumscribed area of the body, known as the receptive field.
• Receptive field size is an important factor in determining the sensitivity of a sensory system. Smaller receptive fields allow greater sensitivity.
• Glabrous skin exhibits the most sensitivity to touch.

Encoding Stimulus Intensity

• Sensory receptors indicate stimulus strength by the number of action potentials they fire.
• Stronger stimuli generate more action potentials per unit of time compared to weaker stimuli.

Constructing Sensations

• The majority of the cortex is devoted to constructing sensations, with each sense handled in a specific part of the brain.
• The main regions of the brain involved in processing the 5 major senses are:
• Vision (occipital/temporal/parietal)
• Hearing (temporal)
• Touch (parietal)
• Taste (frontal)
• Smell (frontal)
• Excessive use of a single sense can expand the relevant cortical area.

Brain Maps

• Sensory pathways from periphery to the cerebral cortex (except for taste and smell) are organized to maintain the spatial relations of adjacent receptors in the body.
• An example of this is seen in the somatosensory cortex (touch/pain), as it maps to stimulation of various body parts.
• Adjacent body parts are represented next to each other, but areas like hands and the face have more cortical space due to a higher concentration of receptors.
• Similar spatial distortion is found in brain maps for vision and hearing.

Olfaction

• Scent interacts with chemical receptors; chemicals dissolve in the mucosa and interact with cilia.
• Changes in the membrane potential are mediated by metabotropic ion channels.
• Neurons respond to a range of odors, and summed activity allows perception of a particular smell.
• This is controlled by a 350 strong gene family that allows for discrimination of 10,000 different smells
• Odors are processed by the same side of the brain.
• Unlike other senses, smell signals bypass the thalamus and proceed directly to the amygdala & pyriform areas.
• One projection goes to the limbic system (emotional response), another to the orbitofrontal region (conscious perception).
• Olfactory receptor cells can also detect pheromones, which are not consciously perceived but still influence behavior.

Taste

• Taste stimuli (tastants) are chemicals detected by taste receptors on the tongue.
• Different taste receptor neurons detect four basic tastes: salty, sour, sweet, and bitter.
• Umami (meaty or savory)
• Receptors specific to fat.
• Damage to the frontal lobe of the right hemisphere can lead to a Gourmand syndrome (fanatical seekers of fine food.)
• The gustatory nerve is formed by cranial nerves 7 (facial), 9 (glossopharangeal) and 10 (vagus), which form the solitary tract.
• Divides into two pathways: one for sensation, one for emotional and hormonal responses.

Flavour

• Mastication unlocks odors from food, mixing them with air, so holding your nose inhibits flavor.
• The brain can distinguish between orthonasal (smelling something through sniffing) and retronasal odors (smelling something through the back of your throat).

Synesthesia

• Synesthesia involves the experience of multiple senses together.
• It is an uncommon experience with only about one in 25,000 people sharing multiple senses.
• Signals from receptors in one sensory system may get diverted to the wrong part of the brain.