Anatomy and Physiology HAP101 Chapter Notes - Chapter 16: Somatic Nervous System, Sensory Neuron, Stimulus Modality

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Published on 9 Dec 2016
School
Seneca College
Department
Anatomy and Physiology
Course
Anatomy and Physiology HAP101
Professor
HAP101 Week 11/Chapter 16: Sensory, Motor and Integrative Systems
LO 16.1: Define sensation and discuss the components of sensation
Sensation: the conscious or subconscious awareness of changes in the external or internal environment. The nature and
type of reaction generated vary according to the ultimate destination of nerve impulses that convey sensory info to the
CNS.
Perception is the conscious interpretation of sensations and is primarily a function of the cerebral cortex. Sometimes,
we do not have perception of sensory info because it does not reach the cerebral cortex
Sensory modality: each unique type of sensation (e.g. touch, pain, vision, hearing, etc.). A sensory neuron only carries
information for one sensory modality. They are grouped into two classes
o General Senses: includes both somatic and visceral senses. Somatic senses include tactile sensations, thermal
sensations, pain sensations, and proprioceptive sensations (perception of static and non-static limbs). Visceral
senses provide information about conditions within internal organs (e.g. pressure, temp., etc.).
o Special Senses: sensory modalities of smell, taste, vision, hearing and equilibrium/balance
The process of sensation: begins in a sensory receptor (either a specialized cell or the dendrites of a sensory neuron)
and it responds to a stimulus (a change in environment that activates sensory receptors). A sensory receptor will
respond weakly or not at all to other stimuli selectivity. There are four events for a sensation to arise
1) Stimulation of the sensory receptor: an appropriate sensory stimulus must occur within the sensory
receptor’s receptive field (the body region where stimulation activates the receptor and produces a response)
2) Transduction of the stimulus: the receptor will transduce energy in a stimulus into a graded potential.
Each type of sensory receptor exhibits selectivity: it can transduce only one kind of stimulus
3) Generation of nerve impulses: when a graded potential in a sensory neuron reaches threshold, it triggers
one or more nerve impulses that propagate towards the CNS. Sensory neurons that conduct impulses from the PNS into the
CNS are called first-order neurons
4) Integration of sensory input: a specific region of the CNS receives/integrates the impulses.
LO 16.2: Explain the different ways to classify sensory receptors
Structural/functional characteristics of sensory receptors can be used to classify sensory receptors
o Microscopic Structure: sensory receptors can be one of the following (1) free nerve endings (bare dendrites
that lack structural specializations; they are receptors for pain and other sensations) of first-order sensory
neurons, (2) encapsulated nerve endings (receptors for other somatic and visceral sensations; their dendrites are
enclosed in a connective tissue capsule) of first-order neurons, or (3) separate cells that synapse with first-order
sensory neurons.
Sensory receptors produce two kinds of graded potentials: generator potentials and receptor
potentials. A generator potential is produced when the dendrites of free-nerve endings, encapsulated
nerve endings, and the receptive part of the olfactory are stimulated. Receptor potentials are
produced when sensory receptors are separate cells; they trigger the release of NT through exocytosis,
the NT diffuse across the synaptic cleft to produce a postsynaptic potential, thus the PSPs may trigger
one or more nerve impulses that propagate along the axon into the CNS
o Location of Receptors and Origin of Activating Stimuli:
Exteroceptors: located at or near the external surface of the body; are sensitive to stimuli originating
outside the body and provide information about the external environment (e.g. hearing, vision, smell,
taste, touch, pressure, vibration, temperature, and pain)
Interoceptors: i.e. visceroceptors, located in blood vessels, visceral organs, muscles and the NS;
regulates conditions in the internal environment. These nerve impulses are not consciously received
o Proprioceptors: located in muscles, tendons, joints, and the inner ear; provide information about body
position, muscle length/tension and position/movement of joints
o Type of Stimulus Detected: most stimuli are in the form of mechanical energy, electromagnetic energy or
chemical energy
Mechanoreceptors: sensitive to mechanical stimuli (e.g. deformation, stretching, bending of cells);
provide sensations of touch, pressure, vibration, proprioception, hearing and equilibrium; monitor
stretching of blood vessels and internal organs
Thermoreceptors: detect changes in temperature
Nociceptors: respond to painful stimuli resulting from physical or chemical damage to tissue
Chemoreceptors: detect chemicals in the mouth (taste), nose (smell), and bodily fluids
Osmoreceptors: detect the osmotic pressure of body fluids
Adaptation in Sensory Receptors: this is a characteristic of most sensory receptors, in which the generator potential
or receptor potential decreases in amplitude during a maintained, constant stimulus. This cause first-order neuron
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frequency to decreases. Also, the perception of a sensation may fade even though the stimulus persists (e.g. when your
first step into a hot shower, the water feels very hot, but soon the sensation decreases to a comfortable warmth, even the
stimulus hot water does not change)
o Rapidly adapting receptors: receptors adapt very quickly; are specialized for signaling changes in a stimulus
e.g. pressure, touch, smell receptors
o Slowly adapting receptors: receptors that adapt slowly and continue to trigger nerve impulses as long as the
stimulus persists; associated with pain, body position, and chemical composition of the blood
LO 16.3: Describe the location and function of the somatic sensory receptors for tactile, thermal and pain sensations
Tactile Sensations: these sensations include touch, pressure, vibration, itch and tickle. They arise by activation of
some of the same types of receptors.
o Touch: such sensations result from stimulation of tactile receptors in the skin or subcutaneous layer.
There are two types of rapidly adapting touch receptors
Corpuscles of Touch: receptors located in the dermal papillae of hairless skin. They generate
impulses at the onset of a touch; are abundant in the fingertips, hands, eyelids, tip of tongue,
lips, nipples, soles, clitoris, and tip of penis.
Hair Root Plexuses: receptors found in hairy skin; detect movements on the skin surface that
disturb hairs/
There are two types of slowly adapting touch receptors
Type I Cutaneous Mechanoreceptors: i.e. tactile discs; plentiful in the fingertips, hands,
lips, external genitalia
Type II Cutaneous Mechanoreceptors: are present in the hands, soles and are most
sensitive to stretching that occurs as digits or limbs are moved
o Pressure: this is a sustained sensation that is felt over a larger area than touch which occurs with deformation
of deeper tissues. Corpuscles of touch, type I cutaneous mechanoreceptors, and lamellated corpuscles
(these adapt quickly; widely distributed in the body)
o Vibration: these sensations occur from rapidly repetitive sensory signals from tactile receptors. The receptors
are corpuscles of touch (detect lower-frequency vibrations) and lamellated corpuscles (detect higher-
frequency vibrations)
o Itch: sensations result from stimulation of free nerve endings by certain chemicals, often because of a local
inflammatory response
o Tickle: sensations are created by free-nerve endings. This sensation arises only when someone else touches
you.
Thermal Sensations: these are free-nerve endings that have receptive fields on the skin surface. Two distinct thermal
sensations are detected by different receptors. They both adapt rapidly at the onset of a stimulus, but continue to
generate impulses at a lower frequency throughout a prolonged stimulus
o Cold Receptors: are activated between 10 degrees and 40 degrees Celsius. They are located in the stratum
basale of the epidermis and are attached to myelinated A-fibers
o Warm Receptors: are not abundant as cold receptors; located in the dermis and are attached to unmyelinated
C-fibers; are activated by temperatures between 32-48 degrees Celsius
Pain Sensations: pain serves as a protective function by signaling the presence of noxious, tissue-damaging conditions.
The subjective description and indication of the location of pain helps pinpoint the underlying cause of disease
o Nociceptors: receptors of pain; are free nerve endings found in every somatic tissue, except for brain; are
activated by intense thermal, mechanical, or chemical stimuli and are also stimulated by tissue irritation/injury,
as chemicals (prostaglandins, kinins, potassium) are released.
o Pain persists even after the pain-causing stimulus is removed because pain-radiating chemicals are still
lingering and because nociceptors exhibit very little adaption. Distention, prolonged muscle contractions and
spasm, or ischemia
o There are two types of pain:
Fast Pain: this perception occurs very rapidly (within 0.1 second after stimulus is applied, since nerve
impulses propagate along the myelinated A fibers); known as acute, sharp, prickling pain; not felt in
deeper tissue
Slow Pain: begins a second or more after stimulus is applied; gradually increases in intensity over a
period of several seconds or minutes; nerve impulses conduct unmyelinated C fibers; pain is
excruciating, chronic, burning, aching, or throbbing; occurs in skin and in deeper tissue/internal
organs
o Pain that arises from stimulation of receptors in the skin is called superficial somatic paint
o Stimulation of receptors in skeletal muscles joints, tendons and fascia causes deep somatic pain
o Stimulation of nociceptors in visceral organs causes visceral pain (if stimulation is diffuse involves a larger
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Document Summary

Hap101 week 11/chapter 16: sensory, motor and integrative systems. Lo 16. 1: define sensation and discuss the components of sensation. Sensation: the conscious or subconscious awareness of changes in the external or internal environment. The nature and type of reaction generated vary according to the ultimate destination of nerve impulses that convey sensory info to the. Perception is the conscious interpretation of sensations and is primarily a function of the cerebral cortex. Sometimes, we do not have perception of sensory info because it does not reach the cerebral cortex. Sensory modality: each unique type of sensation (e. g. touch, pain, vision, hearing, etc. ). A sensory neuron only carries information for one sensory modality. They are grouped into two classes: general senses: includes both somatic and visceral senses. Somatic senses include tactile sensations, thermal sensations, pain sensations, and proprioceptive sensations (perception of static and non-static limbs).