Sensory neurons in
vertebrates are predominantly
pseudounipolar or
bipolar, and different types of sensory neurons have different
sensory receptors that respond to different kinds of
stimuli. There are at least six external and two internal sensory receptors:
External receptors External receptors that respond to stimuli from outside the body are called
exteroreceptors. Exteroreceptors include
chemoreceptors such as
olfactory receptors (
smell),
taste receptors,
photoreceptors (
vision),
thermoreceptors (
temperature),
nociceptors (
pain),
hair cells (
hearing and
balance). There are a number of other different
mechanoreceptors for
touch and
proprioception (
stretch,
distortion and
stress).
Smell The sensory neurons involved in
smell are called
olfactory sensory neurons. These neurons contain
receptors, called
olfactory receptors, that are activated by
odor molecules in the air. The molecules in the air are detected by enlarged
cilia and
microvilli. These sensory neurons produce action potentials. Their axons form the
olfactory nerve, and they synapse directly onto neurons in the cerebral cortex (
olfactory bulb). They do not use the same route as other sensory systems, bypassing the brain stem and the thalamus. The neurons in the olfactory bulb that receive direct sensory nerve input, have connections to other parts of the olfactory system and many parts of the
limbic system.
Taste Taste sensation is facilitated by specialized sensory neurons located in the taste buds of the tongue and other parts of the mouth and throat. These sensory neurons are responsible for detecting different taste qualities, such as sweet, sour, salty, bitter, and savory. When you eat or drink something, chemicals in the food or liquid interact with receptors on these sensory neurons, triggering signals that are sent to the brain. The brain then processes these signals and interprets them as specific taste sensations, allowing you to perceive and enjoy the flavors of the foods you consume. When taste receptor cells are stimulated by the binding of these chemical compounds (tastants), it can lead to changes in the flow of ions, such as sodium (Na+), calcium (Ca2+), and potassium (K+), across the cell membrane. In response to tastant binding, ion channels on the taste receptor cell membrane can open or close. This can lead to depolarization of the cell membrane, creating an electrical signal. Similar to
olfactory receptors,
taste receptors (gustatory receptors) in
taste buds interact with chemicals in food to produce an
action potential.
Vision Photoreceptor cells are capable of
phototransduction, a process which converts light (
electromagnetic radiation) into electrical signals. These signals are refined and controlled by the interactions with other types of neurons in the retina. The five basic classes of neurons within the retina are
photoreceptor cells,
bipolar cells,
ganglion cells,
horizontal cells, and
amacrine cells. The basic circuitry of the retina incorporates a three-neuron chain consisting of the photoreceptor (either a
rod or
cone), bipolar cell, and the ganglion cell. The first action potential occurs in the retinal ganglion cell. This pathway is the most direct way for transmitting visual information to the brain. There are three primary types of photoreceptors:
Cones are photoreceptors that respond significantly to
color. In humans the three different types of cones correspond with a primary response to short wavelength (blue), medium wavelength (green), and long wavelength (yellow/red).
Rods are photoreceptors that are very sensitive to the intensity of light, allowing for vision in dim lighting. The concentrations and ratio of rods to cones is strongly correlated with whether an animal is
diurnal or
nocturnal. In humans, rods outnumber cones by approximately 20:1, while in nocturnal animals, such as the
tawny owl, the ratio is closer to 1000:1. Issues and decay of sensory neurons associated with vision lead to disorders such as: •
Macular degeneration – degeneration of the central visual field due to either cellular debris or blood vessels accumulating between the retina and the choroid, thereby disturbing and/or destroying the complex interplay of neurons that are present there. •
Glaucoma – loss of retinal ganglion cells which causes some loss of vision to blindness. •
Diabetic retinopathy – poor blood sugar control due to diabetes damages the tiny blood vessels in the retina.
Auditory The
auditory system is responsible for converting pressure waves generated by vibrating air molecules or
sound into signals that can be interpreted by the brain. This mechanoelectrical transduction is mediated with
hair cells within the ear. Depending on the movement, the hair cell can either hyperpolarize or depolarize. When the movement is towards the tallest
stereocilia, the Na+ cation channels open allowing Na+ to flow into cell and the resulting depolarization causes the Ca++ channels to open, thus releasing its neurotransmitter into the afferent auditory nerve. There are two types of hair cells: inner and outer. The inner hair cells are the sensory receptors. Problems with sensory neurons associated with the auditory system leads to disorders such as: •
Auditory processing disorder – Auditory information in the brain is processed in an abnormal way. Patients with auditory processing disorder can usually gain the information normally, but their brain cannot process it properly, leading to hearing disability. •
Auditory verbal agnosia – Comprehension of speech is lost but hearing, speaking, reading, and writing ability is retained. This is caused by damage to the posterior superior
temporal lobes, again not allowing the brain to process auditory input correctly.
Temperature Thermoreceptors are sensory receptors, which respond to varying
temperatures. While the mechanisms through which these receptors operate is unclear, recent discoveries have shown that
mammals have at least two distinct types of thermoreceptors. The
bulboid corpuscle, is a
cutaneous receptor a
cold-sensitive receptor, that detects cold temperatures. While the other type is a warmth-sensitive receptor.
Mechanoreceptors Mechanoreceptors are sensory receptors which respond to mechanical forces, such as
pressure or
distortion. Specialized sensory receptor cells called mechanoreceptors often encapsulate afferent fibers to help tune the afferent fibers to the different types of somatic stimulation. Mechanoreceptors also help lower thresholds for action potential generation in afferent fibers and thus make them more likely to fire in the presence of sensory stimulation. Some types of mechanoreceptors fire action potentials when their membranes are physically stretched.
Proprioceptors are another type of mechanoreceptors which literally means "receptors for self". These receptors provide spatial information about limbs and other body parts.
Nociceptors are responsible for processing pain and temperature changes. The burning pain and irritation experienced after eating a chili pepper (due to its main ingredient, capsaicin), the cold sensation experienced after ingesting a chemical such as menthol or icillin, as well as the common sensation of pain are all a result of neurons with these receptors. Problems with mechanoreceptors lead to disorders such as: •
Neuropathic pain - a severe pain condition resulting from a damaged sensory nerve
Internal receptors Internal receptors that respond to changes inside the body are known as
interoceptors. These receptors are
polymodal responding to a number of different stimuli.
Nociceptors Nociceptors respond to potentially
damaging stimuli by sending signals to the spinal cord and brain. This process, called
nociception, usually causes the perception of
pain. They are found in internal organs as well as on the surface of the body to "detect and protect". == Connection with the central nervous system ==