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Fleur sam ch. 10Special senses: vision, hearing, taste, smell, equilibrium
Somatic senses: touch, temp, pain, itch, proprioception.
Proprioception= awareness of body movement and position in space, mediated by muscle
and joint sensory receptors called proprioceptors, may be conscious or unconscious.
Somatic stimuli: muscle length and tension, proprioception.
Visceral stimuli: blood pressure, distension of gastrointestinal tract, blood glucose conc.,
internal body temp, osmolarity of body fluids, lung inflation, pH of cerebrospinal fluid, pH
and oxygen content of blood.
Sensory pathways begin with stimulus, in the form of physical energy that acts on sensory
receptor The receptor/sensor is a transducer that converts the stimulus into an
intracellular signal, which is usually a change in membrane potential. If stimulus is above
threshold, action potentials pass along a sensory neuron to the CNS, where incoming signals
are integrated.
Some stimuli pass upward to the cerebral cortex, where they reach conscious perception,
but others are acted on subconsciously, without our awareness.
At each synapse along the pathway, the nervous system can modulate/shape sensory info.
Sensory systems in human body vary widely in complexity, simplest systems are single
sensory neurons with branched dendrites that function as sensors bv. Pain/itch receptors.
most complex systems are multicellular sense organs, bv. Ears/eye.
Sensory neurons vary widely in complexity, ranging from branched endings of a single
sensory neuron to complex nonneural cells that act as sensors.
- Simplest neurons consist of neuron with naked (free) nerve endings.
- More complex receptors, nerve endings are encased in connective tissue.
Axons of both simple & complex neural receptors may be myelinated or unmyelinated.
The nonneural sensors include some of the most highly specialized receptors, bv. Hair cells
of ear. Nonneural sensors are usually highly organized cells that synapse onto sensory
neurons. When activated, nonneural sensor releases a chemical signal that initiates an AP in
the associated sensory neuron. Both neural and nonneural receptors develop from the same
embryonic tissue.
Nonneural accessory structures are critical to the operation of many sensory systems. Bv.
Lens and cornea of the eye help focus incoming light onto photoreceptors. Hairs on our arms
help somatosensory receptors sense movement in the air millimetres above the skin surface.
Accessory structures often enhance the info-gathering capability of sensory system.
Receptors can be divided within 4 major groups, based on type of stimulus to which they are
most sensitive:
- Chemoreceptors: respond to chemical ligands that bind to receptor (bv. Taste/smell,
oxygen, pH, various organic molecules like glucose)
, - Mechanoreceptors: respond to various forms of mechanical energy, including
pressure (baroreceptors), cell stretch (osmoreceptors), vibration, gravity,
acceleration, sound- bv. Hearing.
- Thermoreceptors respond to temperature.
- Photoreceptors for vision respond to light.
Receptors convert diverse physical stimuli, like light/heart, into electrical signals. First step is
transduction, conversion of stimulus energy into info that can be processed by nervous
system in many receptors, opening/closing of ion channels converts mechanical,
chemical, thermal, light energy directly into change in membrane potential. Some sensory
transduction mechanisms include signal transduction & second messenger systems that
initiate change in membrane potential.
Each sensory receptor has an adequate stimulus, a particular form of energy to which it’s
most responsive bv. Thermoreceptors more sensitive to temp. changed than to pressure,
mechanoreceptors respond to stimuli that deform cell membrane.
Although receptors are specific for 1 form of energy, they can respond to most other forms if
the intensity is high enough. Photoreceptors of the eye respond most readily to light, but a
blow to the eye can cause us seeing stars, example of mechanical energy of sufficient force
to stimulate photoreceptors.
Sensory receptors can be incredibly sensitive to their preferred form of stimulus. Bv. A single
photon of light already stimulates certain photoreceptors. And a single odorant(=geurstof)
molecule may activate chemoreceptors involved in the sense of smell.
Threshold= the minimum stimulus required to activate a receptor.
Like the minimum depolarization required to trigger an action potential is called the
threshold.
How is physical/chemical stimulus converted into a change in membrane potential? Stimulus
opens/closes ion channels in receptor membrane, directly or indirectly (through second
messenger). In most cases, channel opening leads to net influx of Na+ or other cations into
the receptor, depolarizing the membrane. In few cases, response to stimulus is
hyperpolarization when K+ leaves the cell.
In vision, the stimulus (light) closes cation channels to hyperpolarize the receptor.
Change in sensory receptor membrane potential is a graded potential, called receptor
potential.
- In some cells, the receptor potential initiates an action potential that travels along
the sensory fiber to the CNS.
- In other cells, receptor potentials influence neurotransmitter secretion by the
receptor cell, which in turn alters electrical activity in an associated sensory neuron.
Somatic sensory and visual neurons are activated by stimuli that fall within a specific physical
area known as the neuron’s receptive field. Bv. A touch-sensitive neuron in the skin
responds to pressure that falls within its receptive field.
, In simplest case, 1 receptive field is associated with 1 sensory neuron (the primary sensory
neuron in the pathway), which in turn synapses on 1 CNS neuron (the secondary sensory
neuron).
Primary neuron= first order
Secondary neuron= second order
Receptive fields frequently overlap with neighbouring receptive fields.
In addition, sensory neurons of neighbouring receptive fields may exhibit convergence, in
which multiple presynaptic neurons provide input to a smaller number of postsynaptic
neurons.
Convergence allows multiple simultaneous subthreshold stimuli to sum at the postsynaptic
(secondary) neuron.
When multiple primary sensory neurons converge on a single secondary neuron, their
individual receptive fields determine how sensitive a given area is to a stimulus.
The size of secondary receptive fields determines how sensitive a given area is to a stimulus.
Bv. Sensitivity to touch is demonstrated by a two-point discrimination test. In some regions
of skin, like arms/legs, 2 pins placed within 20 mm of each other are interpreted by the brain
as single pinprick. In these areas, many primary neurons converge on a single secondary
neuron, so the secondary receptive field is very large.
More sensitive areas of the skin (bv. Fingertips) have smaller receptive fields, as little as 1:1
relationship between primary and secondary sensory neurons.
Sensory info from body, enters spinal cord and travels through ascending pathways to the
brain. Some sensory info goes directly into brain stem via the cranial nerves.
Sensory info that initiates visceral reflexes are integrated in the brain stem or spinal cord and
usually does not reach conscious perception.
Unconscious visceral reflex is bv. the control of blood pressure by centres in brain stem.
Each major division of the brain processes 1 or more types of sensory info.
- Bv. midbrain receives visual info
- Bv. medulla oblongata receives input for sound and taste.
Info about balance and equilibrium is processed primarily in the cerebellum. These
pathways, along with those carrying somatosensory info project to the thalamus, which acts
as relay and processing station before passing the info on to the cerebrum.
Only olfactory info is not routed through the thalamus. Sense of smell, type of
chemoreception. Info about odors (geuren) travels from nose through first cranial nerve, and
olfactory bulb to olfactory cortex in the cerebrum.
Maybe its bc of this direct input to the cerebrum that odors (geuren) are closely linked to
memory and emotion.
When CNS is processing sensory info, the perceptual threshold, is the level of stimulus
intensity necessary to be aware of a particular sensation. Brain can filter out/ignore some
stimuli. When you ‘’zone out’’ the noise is adequate to stimulate sensory neurons in the ear,
but neurons in the pathway dampen the perceived signal so that it does not reach the
conscious brain.
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