Sensory Cells: Types and Functions

Zintuigcellen (Sensory Cells)

• Sensory cells convert specific stimuli into impulses.
• Sensory cells transmit the impulse through sensory nerve cells.
• Adequate stimulus is what activates each sensory cell.
• Sensory cells are also called receptor cells, since they "catch" stimuli.
• Four types of sensory/receptor cells exist:
  • Mechanoreceptors: sensitive to mechanical stimulation (pressure, bending).
  • Thermoreceptors: sensitive to temperature changes.
  • Chemoreceptors: sensitive to specific molecules.
  • Photoreceptors: sensitive to light.
• Receptor cells change the existing membrane potential in response to the stimuli.
• Neurotransmitters from the receptor cell activate sensory nerve cells, which send impulses to the central nervous system.
• Continuous stimulation can raise the threshold value of a receptor cell, which makes it respond less to the adequate stimulus.
• This is called habituation or adaptation and a stronger stimulus is needed to achieve the same effect.

Het evenwichtszintuig (The Vestibular Sense)

• The vestibular sense gives data about acceleration and deceleration of the body.
• It gives data rotation of the head.
• The vestibular sense consists of two parts:
  • Vestibulum: registers acceleration or deceleration of linear movements: forward-backward; left-right; up-down.
  • Maculae: sensory cells are in two sensory organs of the vestibulum.
  • Maculae contain chalk stones that move when there is a change in speed or gravity.
  • Cilia (hairs) of hair cells (mechanoreceptors) in a macula bend to generate an action potential.
  • The three semicircular canals register rotational movements of the head and are positioned to be each sensitive to other direction of rotational movement.
  • The degree and direction of bending cilia provide a specific impulse frequency.
  • The brain interprets this information about the degree and direction of movement change.
  • Receptor cells in the vestibulum do not register constant speed movement.
  • Semicircular canals are filled with fluid (endolymph) containing a gelatinous mass (cupula).
  • Cilia of hair cells stick into the cupula and when the semicircular canal moves the endolymph moves slower.
  • Cupula moves in the endolymph in the opposite direction as the semicircular canal.
  • Bending of the cilia of hair cells causes a change in the membrane potential to cause the hair cells to go limp.
  • Impulses from the semicircular canals are sent to the equilibrium center in the brain stem, so the body/head rotations can be perceived.
• Information from the vestibular system combines with information from the eyes and skeletal muscle tension to create a full image of posture and spatial orientation.

Het gehoorzintuig (The Auditory Sense)

• The adequate stimulus for the ear is sound, created by air vibrations.
• Vibrations are transmitted through different parts of the ear in the following order:
  • Oorschelp (Auricle)
  • Gehoorgang (Auditory canal)
  • Trommelvlies (Eardrum)
  • Gehoorbeentjes (Ossicles)
  • Ovale venster (Oval window)
  • Voorhoftrap (Vestibular duct)
  • Perilymfe (Perilymph) travels into the vestibule towards the slakkenhuiscentrum (cochlear center)
  • Endolymfe (Endolymph) vibrates in the small middle canal at the location where the membrane of the vestibule vibrates
  • Small middle canal transmits the vibration to the perilymph in the tympanic duct
  • Trilling gaat naar het slakkenhuiscentrum

De receptor Cellen (The Receptor Cells)

• Vibrations are converted into impulses in the receptor cells, in the following steps:
  • The vibrations set the basilar membrane in motion through the middle canal
  • Hairs of the organ of Corti bend
  • Potassium channels of the sensory cells open
  • Potassium influx causes depolarization of the membrane
  • Neurotransmitter releases to the sensory nerve cells
  • Impulses travel to the primary auditory center via the auditory nerve

Het Gezichtszintuig (The Visual Sense)

• Pigment cells at the back of the retina contain pigment granules to protect eyes against bright light.
• These pigment granules scatter in the extensions of the pigment cells.
• In pupil reflex, ring muscles in the iris contract to make the pupil smaller when there is excessive light.
• Length muscles will contract making the pupil bigger when there is a lack of light.
• Light travels in the following pathway to the retina:
  • Hoornvlies (cornea)
  • Voorste oogkamer (anterior chamber, filled with fluid)
  • Pupil in iris
  • Achterste oogkamer (posterior chamber, filled with fluid)
  • Ooglons (Eye lens)
  • Glasachtig lichaam (vitreous body)
  • Netvlies (retina)
• Light converge at the follow points in the eye:
  • Hoornvlies (cornea, strong), oogkamers (eye chambers)
  • Ooglens (eye lens, strong)
  • Glasachtig lichaam (vitreous body, weak)
• When viewing distant objects:
  • The lens is flattened
  • Ring muscles of the ciliary body are relaxed
  • suspensory ligaments are tight
• When viewing close objects:
  • The lens is round
  • Ring muscles in the ciliary body are contracted
  • Suspensory ligaments are less tight
• Accommodating is changing the shape of the lens to focus
• Maximum sharpness comes when light falls on the yellow spot and because of the high density of cone cells in the yellow spot.
• If light falls on the blind spot you will not see anything, because no photoreceptors are located on the eye.

Oogafwijkingen (Eye disorders)

• Verziendheid (Farsightedness)
  • You cannot clearly see objects that are a short distance from eye
  • Eye length is too short
  • Light rays are not sufficiently bent converge
  • Corrected with convex/positive lenses
  • Lens elasticity decreases with age, leading to farsightedness
• Bijziendheid (Nearsightedness):
  • You cannot clearly see objects that are a greater distance from eye
  • Eye length is too long or the eye lens is too curved
  • Light rays are bent too much diverge
  • Corrected with concave lens
• Two methods used to estimate distances:

Diepte Zien (Depth perception)

• Images between left and right eye differ slightly because of the angle the eyes looks from shifts
• In the visual cortex the images are combined into on observed image, called stereoscopic seeing
• Stereoscopic helps assess depth because comparisons are made
• The visual cortex can also gauge the interpretation that a larger object is closer than a smaller object called perspective

Het Netvlies (The Retina)

• The retinal sensory cells are located within the eye
• The threshold for rods is lower than cones. This means that rods fire at a lower light intensity than cones.
• Cones are sensitive to the following light wavelengths:
  • Type 1: blue light (420 nm)
  • Type 2: green light (535 nm)
  • Type 3: red light (565 nm)
• In rods, the photopigment rhodopsin is found and the photopigment photopsin (in three variants) are in cone cells
• Many rods (up to 1000) are connected to one ganglion cell and each cone is only connected with 1 bipolar and 1 ganglion cell
• When no light falls on the sensory cells, no hyperpolarization takes place. Neurotransmitters release at the same level so the impulses are at the same level.
• When light does fall on the sensory cells, hyperpolarization takes place and less neurotransmitters are released and the impulses decrease.

Functie Neuroncellen (Function Of Neuron Cells)

• Bipolar cells neurons connect (switch cells) the cones to ganglion cells.

• Ganglion cells are the neurons that discharge the impulses to the brain.

• In our retina, we have the following cells:

  • Three cone cells
  • The level each cone cell is stimulated leads to additional or decreased hyperpolarization caused by stimulation of three types of cones of the retina
  • Every combination is linked to a fixed color that allows a large colors to be interpreted

Werking neuronen (Neurons In The Book)

• In Question 8, four while the spots are strongly stimulated causing horizontal cells to be strongly stimulated
• Next door, the receptor fields inhibit with the stimulation and visual information is not passed on