Special senses

• Special senses are those senses that have specialized organs devoted to them. These include vision, hearing, taste, smell, and balance. Each of these senses has specific receptors and dedicated neural pathways that process and interpret sensory information.
• The gustatory pathway is the neural pathway responsible for the perception of taste. It involves the detection of taste stimuli by taste receptors on the tongue and the transmission of this sensory information to the brain where it is processed and perceived as taste
• Taste perception is mediated by gustatory receptors, also known as taste buds, responding to chemical stimulation on the dorsum of the tongue and in parts of the larynx, pharynx and epiglottis.
• Taste perception is mediated by gustatory receptors (taste buds).

  • There are approximately 4000 taste buds in humans which are distributed throughout the oral cavity and upper GIT.
  • About 3/4 of all taste buds are found on the dorsal surface of the tongue in small raised structures called papillae.
  • There are 4 types of papillae but only 3 of them contains taste buds (fungiform, foliate, and circumvallate)

  
• There are 3 types of cells found in taste buds

  • Type I, II, III,
  • Gustatory receptor cells: the actual sensory cell responsible for detecting taste stimuli. Have microvili that extend into taste pore where they come in contact with dissolved food molecules
  • Supporting Cells: structural + nutritional support to gustatory receptor cell. Maintain integrity of taste bud.
  • Basal cell: serve as precursor cell that can differentiate into new gustatory receptor and supporting cells. Located at base of taste bud.

  
• Taste sensation can be categorised as

  • sweetness, sourness, saltiness, bitterness, and umami.
  • These basic tastes can either have a stimulatory effect and encourage intake (sweet, salty, and savory) or inhibit it (bitter and perhaps sour).
• Sweet (glucose)

  Salty (NaCl)

  Sour (H+, fats)

  Bitter (alkaline type)

  Umami (glutamate, L-amino acids)
• Sensations perceived by the body are organized into two major groups: general sensations (touch, pressure, pain, and temperature) and special sensations (vision, hearing, taste, smell, and the sense of body position and movement.)
• Structures making part of olfactory system

  • Olphactory epithelium
  • Olphactory nerve
  • Olphactory bulb
  • Olphactory tract
  • Olphactoru striae
  • Olphactory cortical areas

  
• Olfactory epithelium

  • Specialised nasal epithelium
  • Lines superior part of nasal cavity
  • Present just bellow cribriform plate of ethmoid bone
  • Contains : olfactory receptor cells, supporting cells, and basal (stem) cells, + bowman glands (secretes mucous), + mucous film (rich in cl ions; humidifies/warms inhaled air, traps and dissolves chemicals for smell sensation)

  
• olfactory receptor cells

  • bipolar: they have two projections from their cell body
  • One projection, the dendrite, extends to the surface of the olfactory epithelium to become knob-like.
  • At dendrite’s surface there are non motile cilia that extend into the fluid layer covering the epithelium in the nose.
  • cilia contains receptors for odour molecules that pass into the nasal cavity and are captured in the fluid covering the olfactory epithelium.
• Pathway for odourants

  • odor particles enter nose through nostrils
  • Pass through nasal cavity
  • odourants dissolve in mucous of olfactory epithelium
  • Activate immotile cilia of olfactory receptor cell
  • Stimulate olfactory nerve which stimulates olfactory bulb.

  
• Olfactory nerve

  • Each receptor cell has an axon extending from its basal surface
  • As the axons project from the cell body, they combine with other receptor cell axons, making up bundles of nerve fibers. All of these axonal bundles can collectively be thought of as the olfactory nerve (CNI).

  
• Olfactory bulb

  • The axons projecting from the olfactory receptor cells via the olfactory nerve terminate within the olfactory bulb. The olfactory bulb is the main relay station within the olfactory pathway
  • Within the olfactory bulb are bundles of nerve fibers known as glomeruli; where incoming receptor cell axons make connections with the dendrites of mitral relay neurons.
  • Axons of mitral cells leaving olfactory bulb forms olfactory tract. Olfactory tract divided into Medial and latetal olfactory striae near anterior perforated substance

  
• Olfactory striae

  • Posterior and anterior to the optic chiasm, the olfactory tract on both sides divides into medial and lateral olfactory striae.
  • The medial stria projects to the anterior commissure, and subsequently, to contralateral olfactory structures. Associated emotions with smell through its connection with limbic system
  • The lateral stria continues on to structures associated with the olfactory cortex. Consciously aware/perceive olfaction.

  
• The olfactory pathway is the neural pathway responsible for the sense of smell. It involves the detection of odorant molecules by olfactory receptors in the nasal cavity, the transmission of this sensory information to the brain, and the processing and perception of odors.
• Olfactory Pathway

  1. Odorant enter nasal cavity

  2. Dissolve in mucus layer covering olfactory epithelium

  3. Bind to specific receptors on cilia

  4. Causes activation of G-protein-coupled receptor mechanism

  5. Generation of electrical signal in ORNs

  6. Olfactory nerve fibers pass through cribriform plate and synapse in olfactory bulb

  7. Within olfactory bulb, ORN axons synapse in structures called glomeruli with dendrites of mitral and tufted cells

  8. Olfactory bulb processes incoming signals

  9. Mitral and tufted cells carry processed signals via their axons through the olfactory tract

  10. Signal travels along olfactory nerve

  11. Olfactory tract projects to 1 olfactory cortex (piriform cortex, entorhinal cortex, parts of amygdala)
• This cortex is not a single structure, rather, it is defined as the combined areas of the cerebral cortex (generally within the temporal lobe) that receive input directly from the olfactory bulb. These regions include the:

  • Piriform cortex: which is located below the lateral olfactory stria.
  • Amygdala: which is located anterior to the temporal/inferior horn of the lateral ventricle, and is associated with the emotion of fear.
  • Entorhinal cortex: which is the anterior part of the parahippocampal gyrus, and is involved in the formation of memory.
• From the olfactory cortex, information about smell is sent to the orbitofrontal cortex via the dorsal medial nucleus of the thalamus. The orbitofrontal cortex is a portion of the prefrontal cortex that is located on the underside of the frontal lobe and situated over the eye orbit. Lesions of this cortical region can result in an inability to distinguish different odors. Odor information is also sent to portions of the hypothalamus and brainstem that trigger autonomic responses involved in appetite, salivation, and gastric contraction.
• Olfactory pathway

  
• • Anosmia: temporary or permanent loss of sense of smell (inflammation, blockage of nasal passages, trauma to orbit or nasal cavity, pathological lesion, degenerative CNS disease.
  • Dysosmia: distorted senses of smell
• Optic nerve pathway

  
• The primary function of the optic nerve is to transmit visual information from the retina to the brain. This includes

  • Visual Acuity: Sharpness and clarity of vision.
  • Vision: Differentiation of colors.
  • Peripheral Vision: Detection of objects outside the direct line of sight.
  • Visual Fields: The entire area that can be seen when the eye is directed forward.
• Pathway of Visual Information

  • Light Detection: Photoreceptor cells (rods and cones) in retina detect light and convert it into electrical signals.
  • Bipolar cells relay the signals to retinal ganglion cells.
  • Optic Nerve: Ganglion cell axons optic nerve and transmit signals to the brain.
  • Optic Chiasm: Nasal fibers cross to opposite side, while temporal fibers remain ipsilateral
  • Optic Tracts: Carry visual information to LGN of thalamus.
  • Optic Radiations: LGN neurons project to 1 visual cortex.
  • Visual Cortex: Processes and interprets visual information, resulting in perception of images.

  
• Visual pathway

  
• Olfactory bulb

  
• Olfactory nerve

  
• Olfactory nerve