Sensory Systems

General Properties

• each sensory receptor responds to an environmental stimulus by causing an action potential in a sensory neuron

• receptors change different forms of energy into energy that can be interpreted by the brain

• thus vision and sound stimulate the brain the same way, but interpreted differently

Special Senses

• vision

• hearing

• taste

• smell

• equilibrium

Somatic Senses

• touch

• temperature

• pain

• itch

• proprioception

Somatic Stimuli

• muscle length and tension

• proprioception

Visceral Stimuli

• blood pressure

• distension of gastrointestinal tract

• blood glucose concentration

• internal body temperature

• osmolarity of body fluids

• lung inflation

• pH of cerebrospinal fluid

• pH and oxygen content of blood

Chemoreceptors

• respond to chemicals externally and internally

• have no change to their shape

• externally through taste and smell

• internally through O2, CO2, pH, glucose

Mechanoreceptors

• respond to stimuli that deform the plasma membrane of receptor

• includes pressure, vibration, acceleration and sound

• eg. the cochlea has about 1600 of these receptors

Photoreceptors

• respond to photons of light

• rods and cones

• eg. the eye has about 126 million of these receptors

Thermoreceptors

• respond to varying degrees of heat

• most of these are found in the skin but some are internal to regulate body temperature

Sensory Adaptation

• receptors respond with a burst of energy when stimulus is first applied

• some quickly decrease the firing rate (phasic receptors) and cease paying attention to constant stimuli such as odour, touch or temperature

• tonic receptors continue firing (eg. pain)

Tonic Receptors

• receptors that slowly adapt and respond for the duration of a stimulus such as pain

• one sharp increase in potential that plateaus overtime

Phasic Receptors

• receptors that rapidly adapt to a constant stimulus and turn off, firing once more when the stimulus turns off

• two sharp increases and decreases in potential overtime

Skin Sensory Receptors

• each receptor is designed to be maximally sensitive to one modality of sensation (touch, pressure, heat, cold and pain)

• in some cases, sensations are picked up by nerve ending

• in other cases, nerve endings are encapsulated

Free Nerve Endings

• temperature is sensed by thermoreceptors in top part of the dermis by heat and cold receptors

• there are many more cold receptors than heat receptors

Pain Receptor Activation

* above 45℃

Cold Receptor Activation

* less than 45℃ and above 5℃

Warm Receptor Activation

* above or equal to 45℃

Nociceptors

• free sensory dendrites that can be myelinated (pin prick-fast response) or nonmyelinated (dull ache- slow response)

• activated by a variety of noxious stimuli (chemical, mechanical and thermal) and have the potential to cause tissue damage

• often referred to as pain receptors (pain is a perception not a stimulus)

• itching sensation activated by this receptor

Nociceptor Location

• found in skin, muscles and joints, almost everywhere except the brain (threaten integrity)

• internally frequent in hollow organs including the GI tract and bladder where they are more likely to come into contact with noxious substances

• located near the surface of these areas

Touch and Pressure

1. Free nerve endings around hair follicles

2. a) Ruffini Endings

   b) Merkel’s Discs

   c) Meissener’s Corpuscles

   d) Pacinian Corpuscles

Ruffini Endings

• touch and pressure related to low frequency vibration

• adapt slowly to stretching

• found in lower layers of area responding to deeper touch

Merkel’s Discs

• touch and pressure related to sustained pressure or indentation

• found in upper layers of area to localize gentle touch

Meissner’s Corpuscles

• fine touch and pressure related to vibration

• also called tactile receptors

• found in upper layers of area to localize gentle touch

Pacinian Corpuscles

• related to touch and deeper pressure

• quickly adapt

• found in lower layers of area responding to deeper touch

• is the lowest receptor

Density of Mechanoreceptors

• some types of receptors have wide receptive fields providing less precise reception (back of legs have fewer receptors with wider fields)

• other types have smaller, denser and more sensitive receptive fields (fingertips have up to 50 receptors per cm3 and small receptive fields)

Proprioceptors

• receptors in muscles, joints, tendons and ligaments that provide a sense of body position

• allows fine control of the body positions and sends information about stretching contraction and positioning

• all sensory information from these receptors goes to the cerebellum

• two types: muscle spindles and golgi tendon organs

Muscle Spindles

• small sensory organs enclosed within a capsule found throughout the body of a muscle that detect changes in muscle length

• the stretching of muscle fibers triggers action potentials and motor fibers activate muscle fibers

Golgi Tendon Organs

• located in tendons that connect bone to muscle that are interwoven with collagen fibers

• provides information on tension and is activated by muscle contraction

• stretching of the muscle absorbed by the muscle itself

• more likely to innervate the muscle spindle

Chemoreceptors

• interoceptors in the internal environment

• exteroceptors in the external environment

• involved in both taste and smell which are some of the oldest senses from an evolutionary perspective

• smell is sensed by gaseous molecules in the air

• taste is sensed by chemicals dissolved in food and drinks

• distinction is arbitrary as they both must be dissolved in water

Taste

• receptor cells clustered together in taste buds, primarily on the surface of the tongue

• each taste cell is a non-neural epithelial cell that can become depolarized under appropriate stimulation

• taste buds are covered in saliva and have microvilli projecting from the surface

• release neurotransmitters that stimulate associated sensory neurons

• taste buds innervate one of two cranial nerves

Sour

* presence of hydrogen ion

Salty

* presence of sodium ion

Sweet and Umami

* organic molecules associated with nutritious food

Bitter

* organic molecules associated with toxic effects

Smell

• receptors consist of dendrites with several million bipolar sensory neurons

• axons form the first cranial nerve the olfactory nerve

• olfactory nerve synapses with secondary neurons in the olfactory bulb which leads to the olfactory tract and cortex

• receptors are unique among neurons of an adult and replace themselves every 1-2 months

• the location of the receptor affects its mechanism however size is more important than the mechanism

• each sensory neuron has multiple cilia that bind to odorant molecules

• about 400 different receptor that can detect about 10,000 smells

Olfactory Bulb

* part of the limbic system and plays an important role in generating motions and in memory

Amygdala

* important in generating emotional responses to smell such as remembering the first time you smell something

Hippocampus

* stores memories associated with particular smells

Equilibrium

• established by the vestibular apparatus of the inner ear including the otolith organs (utricle and saccule) and ampullae of the semicircular canals

• sensory cells and hair cells are located within these

Hair Cells

• modified epithelial cells that have about 50 hair-like extensions

• the largest one is a kinocilium and the rest are sterocillia

What happens when stereocilia bend in the direction of the kinocilium?

* the cell membrane depolarizes and a neurotransmitter is released stimulating the dendrites of the vestibulocochlear nerve

What happens when the stereocilia bend away from the kinocilium?

* the cell membrane hyperpolarizes causing lower neurotransmitter release and therefore less stimulation

Utricle and Saccule

• each has a patch of specialized epithelium containing macula, hair cells and support cells

• hair cells are embedded in otolithic membrane that contains microscopic crystals (otoliths)

• the utricle detects horizontal movements and the saccule detects vertical movements

Semicircular Canals

• three project at different angles with an ampulla at each base

• the ampulla contain sensory hairs embedded in a gelatinous membrane called the capula

• hairs can be pushed in both directions to detect rotational movements

Hearing

• sound causes vibrations of the tympanic membrane producing movements in the inner ear ossicles and pressing against the oval window of the cochlea

• pressure waves in the cochlea cause movements of the basilar membrane where sensory cells are located (damage usually occurs here)

Cochlea

• has three chambers

• vibrations at the oval window displace the fluid in the scala vestibuli travelling to the end of the scala vestibuli and back to the scala tympani through the helicotrema which displace the round window and dissipate

• different regions pick up different frequencies

Low Sound Frequency

* pressure waves travel a long way up the scala vestibuli and back down the scala tympani

High Sound Frequency

• transmitted through the vestibular membrane and basilar membrane to the scala tympani

• cause maximum vibration of the basilar membrane closer to the oval window

Organ of Corti

• sensory hairs are located on the basilar membrane and project into the cochlear duct, embedded in the tectorial membrane

• is the functional unit of hearing

• the greater the displacement of the basilar membrane, the greater the transmitter release by the hair cells

Photoreceptor

* light from the observed object is focused on the retina and the back of the eye

How do we see?

• the sensory cells in the retina convert electromagnetic energy into nerve impulses

• light of longer wavelengths (IR-infared) does not have sufficient energy to excite receptors

• light of shorter wavelengths (UV) are filtered out by the lens but can be seen by animals

Wavelengths

• shorter wavelengths have more energy than longer wavelengths

• length and energy work inversely

Lens

* focuses rays of light to be refracted to a point on the retina

Accomadation

• the ability to keep objects at different distances focused

• results from the contraction of the ciliary muscle (sphincter like)

Ciliary Muscle

• when an object is far away, the muscle is relaxed and tension on the suspensory ligaments increases pulling the lens taught

• when an object gets closer, the muscle contracts and tension fibers decrease

Retina

• contains two types of photoreceptive cells called cones and rods

• also have other neuron layers

• objects usually focused on the fovea which only has cones

Rods

* black and white vision under low intensity light

Cones

* sharp colour vision when light intensifies greater

Effect of Light on Rods

• contain a purple pigment called rhodopsin that dissociates into two components when stimulated by light

• initiates changes in cell membrane permeability which ultimately results in the production of nerve impulses

Effect of Light on Cones

• less sensitive to light but provide colour vision and greater visual acuity

• three types, red, green and blue that contain proteins called photospins which each absorb different wavelengths

Photoreceptors and Nerve Cells

• approximately 120 million rods, 6 million cones and 1.2 million nerve fibers in the retina (105 photoreceptors for each nerve cell)

• there are fewer nerves for cones except in the fovea where visual acuity is the highest and the ratio is 1:1

Neural Pathways

• in the eye, the lens focusses light on the retina activating the rods and cones

• a signal is sent through bipolar cells to ganglion cells, which axons make up the optic nerve

• a right visual field goes to the left side of both retinas (and vice versa) to get the same image, crossing to the left thalamus from the right eye

• the left geniculate nucleus in the thalamus receives the input from both eyes that relates to the right half of the visual field

• detection in the thalamus is genetic

Interoceptors

• provide information about the internal environment including temperature, chemicals and pressure

• important for homeostasis