Detailed, and in-depth lecture notes, combined with online and textbook resources used by the university to deliver the lectures. Principles of Anatomy and Physiology is a core module in first year Biomedical Sciences and Life Sciences degrees and will be assessed at the end of the year through mul...
Lecture Eight – Senses
Perception: the conscious awareness and interpretation of a sensation – memories
of perceptions are stored in the cortex.
Sensation: awareness of any stimulus (conscious or unconscious) – through
receptors.
Events of a sensation:
- Receptor is stimulated
- Transduction (conversion) of stimulus to a graded potential
- Generation of impulses when graded potential reaches threshold
- Integration of sensory input by CNS
General senses can be either somatic or visceral (receptors distributed across the
body)
Special senses include smell, taste, vision, hearing and equilibrium which happens
through special receptors.
Sensory modality: property by which one sensation is distinguished from another.
1. Chemoreceptors: detect changes in chemical concentration
2. Thermoreceptors: detect changes in temperature
3. Nociceptors: respond to tissue damage, excess exposure to various stimuli
4. Mechanoreceptors:
Baroreceptors: detect changes in fluid pressure
Proprioceptors: detect changes in muscle/tendon tension
Stretch receptors: detect degree of inflammation in lungs
5. Photoreceptors: detect change in light energy and intensity
General Senses
Tactile sensations:
1. Touch: Crude, which is the ability to perceive something has touched the
skin. Discriminative, which provides location and texture of the source.
2. Pressure: sustained sensation over a large area
3. Vibration: rapidly repetitive sensory signal
4. Itching: chemical stimulation of free nerve endings – heat can be used to
relieve itching as it stimulates the nerve endings beyond the threshold.
5. Tickle: stimulation of free nerve endings (only by someone else)
Thermal sensations:
There are free nerve endings with 1mm diameter receptive fields on the surface of
the skin.
- Cold receptors in the stratum basale respond to temperature between 10-
40ºC
- Warm receptors in the dermis respond to temperatures between 32-48ºC
They both adapt rapidly at first but then generate low frequency impulses.
Pain is produced below 10ºC and over 48ºC
Pain receptors:
We are pain-avoiding creatures so pain receptors (nociceptors) are free endings that
are located in nearly every tissue – except the brain.
, They are stimulated by excessive distension, muscle spasm and inadequate blood
flow. Tissue injuries release chemicals like K+, kinins or prostaglandins which
stimulate nociceptors.
- Visceral pain is felt deep to the skin overlying the stimulated organ or in a
surface area far from the organ
Skin area and organs are served by the same segment of the spinal cord.
E.g. A heart attack can be felt in the skin along the left arm since both are
supplied by the spinal cord segment T1-T5
Special Senses
Smell
Olfaction (smell) and gustation (taste) both project to the cerebral cortex and limbic
system (emotions) which evokes strong emotional reactions.
The olfactory epithelium holds 10-100 million receptors. Nasal epithelium
(connected to olfactory epithelium) covers superior nasal cavity and cribriform plate.
- Olfactory receptors are bipolar neurons with cilia or olfactory hairs.
- Supporting cells are columnar epithelium.
- Basal (stem) cells replace receptors monthly.
- Olfactory glands produce mucus
Process:
1. Odorant (smell) binds to receptors
2. Na+ channels open
3. Depolarisation occurs
4. A nerve impulse is triggered and passed along olfactory receptor axons (1 st
order neurones) to their termini within the olfactory bulb.
5. They are then transmitted through synapses to dendrites and cell bodies of
neurones (2nd order)
6. Eventually they get transmitted to the olfactory tract.
Nerve bundles (olfactory receptor axons) can be damaged as a result of skull
fractures or other pathology – resulting in partial/complete anosmia (loss of
sense or smell)
The temporal lobe (primary olfactory area) is the conscious awareness of smells.
The frontal lobe has the secondary olfactory cortex.
Within the brain is the limbic system and hypothalamus. The limbic system is the
emotional part of the brain and strong emotions increase efficiency of memory.
Once olfactory receptors are stimulated, impulses travel through:
Olfactory nerves –> Olfactory bulbs –> Olfactory tracts –> Limbic system
(emotions) –> Olfactory cortex (interpretation)
Additional:
COVID-19 patients do suffer from anosmia as a symptom of the virus. There are
three mechanisms reported to explain this loss of smell/taste.
1. Local infection of support cells and vascular pericytes in the nose and
olfactory bulb which may affect function of bipolar neurones mitral cells.
2. Damage to support cells in the sensory epithelium that may indirectly
influence the signalling pathway from the sensory neurones to the brain.
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