SUBJECT: PHYSIOLOGY TOPIC: GENERAL PRINCIPLES OF SENSORY PHYSIOLOGY LECTURER: DR. SIMBULAN DATE: FEBRUARY, 2011 I. Definitions: Sensory Physiology; Sensation, Perception, Sensory System Sensory physiology - concerned with the operation of individual sensory systems which generate specific sensations, and the subjective perceptions perceptions that result. Sensation - a subjective impression, feeling or awareness of a bodily state or condition arising from a stimulation of a sensory receptor site. Perception - a conscious recognition and interpretation of sensations by fitting them into what we have previously experienced and learned; perception also serve as a basis for understanding, learning, and knowing or for the motivation of a particular action or reaction.
Arterial blood pressure
Nerve endings
Stretch receptors in carotid sinus and aortic arch
Central venous pressure
Nerve endings
Stretch receptors in walls of great veins, atria
Inflation lung
of Nerve endings
Stretch receptors in lung parenchyma
Temperature of blood in head
Neurons hypothalamus
Sensory system is a part of the nervous system that consists of:
Arterial P O2
Glomus cells
a) sensory receptors that receive stimuli from the external or internal environment,
pH of CSF
Receptors on ventral surface of medulla oblongata
b) the neural pathways that conduct information from the receptors to the brain, c) those parts of the brain that deal primarily with information processing. II. Principal Sensory Modalities
Osmotic pressure plasma
Sensory Modality
Receptor
Sense Organ
Vision
Rods and cones
Eye
Hearing
Hair cells
Ear (organ of Corti)
Smell
Olfactory neurons
Olfactory mucous membrane
Taste
Taste receptor cells
Taste bud
Rotational acceleration
Hair cells
Ear (semicircular (semicircular canals)
Linear acceleration
Hair cells
Ear (utricle saccule)
Touchpressure
Nerve endings
Various2
Warmth
Nerve endings
Various2
Cold
Nerve endings
Pain
Naked endings
Joint position and movement
Nerve endings
Muscle length
Nerve endings
Muscle spindle
Muscle tension
Nerve endings
Golgi tendon organ
1
Carotid and aortic bodies
Cells in OVLT and of possibly other circumventricular organs in anterior hypothalamus
Arteriovenous blood glucose difference
Table 5-1. Principal sensory modalities.
in
Cells hypothalamus (glucostats)
in
The first 11 are conscious sensations. sensations.
Conscious senses:
and
vision
touch-pressure
hearing
warmth
smell
cold
taste
pain
rotational acceleration
and
linear
joint position and movement
Sensory information which do not reach “consciousness”:
muscle length
arterial PO2
muscle tension
pH of CSF
arterial blood pressure
osmotic pressure of
central venous pressure
plasma
Various2
lung inflation
arteriovenous arteriovenous blood glucose difference.
Various
III. Sensory receptors/ sensors of the Body
nerve
o
GENERAL PRINCIPLES OF SENSORY PHYSIOLOGY
Cells, or section of the membrane of a cell, which respond to a specific stimulus, and develop generator or receptor potentials , which are encoded in the associated afferent nerves as action potentials, or affect the release of a neurotransmitter to excite an associated nerve ending. Page 1
*Sensors are in some sense like Bar-Code Readers, but more specialized. They are connected to a Central Processing Unit, and they cannot operate without these higher centers.
2.
3.
4.
Polymodal receptors – no specific stimulus is needed as long as the stimulus is damaging (chemical, mechanical or thermal)
3.
Histological features:
1.
Tactile receptors (mechanoreceptors) - touch
Thermoreceptors - Heat and cold
Nociceptors - Pain
Auditory receptors - Sound
Gustatory receptors - Taste
Olfactory receptors - Smell
Free nerve endings of afferents o
Nociceptors
o
Polymodal receptors
o
thermoreceptors
Encapsulated specialized nerve endings o
Muscle spindles: intrafusal skeletal fibers
o
Golgi tendon organ: stretching
o
Pacinian corpuscle
o
Ruffini’s corpuscle
o
Meissner’s corpuscle
4.
Independent cells of neural origin o
Rods and cones of retina
o
Olfactory cells
o
Hair cells of cochlea: epithelial & neuronal
o
Taste bud cells: neuron-like epithelial cells
o
Merkel’s disk
According to anatomical location:
Special senses – vision, smell, taste
Cutaneous – skin and nerve endings
Visceral senses – chemoreceptors and barroreceptors 5.
Specialized non-neural elements innervated by afferents
According to type of sensation:
According to rate of adaptation:
Slowly adapting (tonic or static) o
ex. pain receptors, baroreceptors, chemoreceptors of carotid and aortic branches, Golgi tendon apparatus, muscle spindle and joint capsule receptors receptors
Functions:
They convert the adequate stimulus energy into electrical impulses, conveying appropriate information about the stimulus to the central nervous system. This information is used to:
Receptors fire action potentials continuously during stimulus application
Rapidly adapting (or phasic or dynamic or movement receptor) o
(1) elicit a reflex response;
meant for sudden changes in stimulus application ex. pacinian corpuscle and hair
(2) alter behavior; (3) and/or produce a conscious sensation (not in all cases). Classification: 1.
According to stimulus source:
Teloreceptors – distance reviewers Exteroreceptors – provide external environment
information
about
the
ex. cutaneous receptors and those of the eye, ear and taste receptors receptors
Interoreceptors – provide internal environment
information
about
the
ex. those in the mucous linings and smooth muscle walls of the respiratory, digestive, urinary tracts and CVS (baroreceptors) (baroreceptors)
o
Stimulus source? exteroreceptor/teleroreceptor
o
Stimulus energy? mechanoreceptor
o
Type of sensation? auditory receptor
o
Rate of adaptation? slowly adapting
IV. Classification of sensory neurons Table IV.a Nerve fiber types in mammalian nerve. Fiber type
Function
TYPE
Proprioreceptors – measure the position of the body in space, muscle length and stretching of tendons
2.
According to type of stimulation energy:
Mechanoreceptors – tissue deformation/pressure
Thermoreceptors – temperature
Photoreceptors – detect light
HOW DO WE CLASSIFY A HAIR CELL OF THE COCHLEA ACCORDING TO:
Chemoreceptors – sensations of smell and taste, chemical stimuli, ph, pO 2
GENERAL PRINCIPLES OF SENSORY PHYSIOLOGY
B ** C - *** Dorsal root
Proprioception; somatic motor Fine Touch, pressure Motor to muscle spindles Pain, cold, crude touch Preganglionic autonomic Pain, temperature, some
12 20 5 – 12 3–6
<3
70 120 30 70 15 30 12 30 3-15
0.4 – 1.2
0.5 2
2–5
0.4 0.5
0.4 1
1.2
1.2
2
2
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C - *** sympathe
mechanorecepti on, (axon reflex responses) Postganglionic sympathetic
0.3 – 1.3
0.7 – 2.3
2
2
**A and B are myelinated; *** C fibers are unmyelinated. Table IV.b. Numerical classification system used for sensory neurons.
Table IV.b. Numerical classification system used for sensory neurons. Group
o
o
Dorsal column of SC
o
Thalamus
o
Postcentral gyrus of CC
o
Pacinian corpuscle P
o
Brachial plexus tumor can also lead to touch.
Stimulation of these structures will all lead to the sensation of touch
This overturned/contradicted the so-called “overstimulation hypothesis” (we can feel different types of sensation just by increasing intensity)
Fiber Type
Origin
(Number) Ia Ib
Muscle spindle, annulo-spiral Aα ending. Golgi tendon organ (more in Aß Lecture on reflexes)
II
Muscle spindle, flowerspray ending; fine touch, pressure receptors
Aγ
III
Pain and cold receptors; some crude touch receptors
AΔ
IV
Pain, temperature, and other receptors
Dorsal root C
VII. Law of Projection No matter where a particular sensory pathway is stimulated along its course to cortex, the conscious sensation produced is referred to the location of the receptor. Like for when the cortex is stimulated in the area receiving the impulses for the right foot, the sensation is projected in the right foot not in the head where the brain is found.
IX. How is a stimulus energy converted into electrical impulses of the nerve ? Stimulus transduction
Table IV.c. Relative susceptibility of mammalian Type A, B,
and C nerve fibers to conduction block produced by various agents (from Ganong, Table 2-3,Chapter 2). Susceptibility
Most
Intermediate
Least
to:
susceptible
Hypoxia
B
A
susceptible C
Pressure
A
B
C
Local
C
B
A
anesthetics
V. Adequate stimulus o
o
o
It is the type of energy to which a receptor responds in normal functioning and to which the receptor has a low t hreshold.
Body sensors in response to a specific stimulus, develop generator or receptor potentials which are encoded in the associated afferent nerves as action potential, or effect the release of a neurotransmitter to excite an associated nerve ending An adequate stimulus is necessary. Each receptor is specialized to receive a particular type of stimulus. They can also respond to other forms of energy if the intensity is high enough. This ability by receptors to respond and convert particular physical events into membrane responses, which initiate nerve impulses is known as transduction. Conversion of stimulus energy. Receptors cells have 2 parts 1.
Form of energy converted to electrical impulse to be carried to the afferent pathway If the intensity is enough, it can produce receptor or generator potentials
Ex: Photoreceptors (light) Hair cells of cochlea (sound, waves, mechanical energy) Warm or cold receptors (heat energy) Taste bud cell (chemical substances)
2.
Transducer region o Process of converting stimulus energy into an electrical impulse known as receptor/generator potentials o Receptor potentials increase in magnitude, can activate 1 st node of Ranvier o Located in receptor membrane Spike generator region / AP generator reg’n o
o o o
VI. Law of Specific Nerve Energies o
o
o
Converts the receptor potential to an actual action potential Sensitive to depolarization st node of Ranvier Action potentials (spikes) are produced if the threshold is reached
For every kind of sensation there is a special type of receptor (and pathway) whose activation always gives rise to that sensation. This is regardless of how the receptor is stimulated. Sensation depends on the part of the brain they activate because sense organs are connected to their corresponding area in the cortex by specific sensory pathways. Stimulation of sensory nerve pathways from a particular sense organ are stimulated, the sensation evoked is that for which the receptor is specialized no matter how or where along the pathway the activity is initiated. For example,
GENERAL PRINCIPLES OF SENSORY PHYSIOLOGY
When a receptor potential reaches a threshold, an AP is produced. Page 3
except photoreceptors which exhibit hyperpolarization in response to a photon stimulation It is produced in the: o nerve ending itself, o in specialized neural cells, or o specialized non-neural non-neural cells such as in the hair cells of the cochlea It is a graded response. o the receptor potential reflects the magnitude of the stimulus local potential (generator or electronic potential) o which spreads over the membrane electronically and not actively conducted like nerve impulses can undergo spatial and temporal summation in order to modify the gradations of intensity. spatial – when 2 weak stimuli are applied o simultaneously, they can elicit suprathreshold depolarization using increasing numbers of parallel fibers to carry the signal o temporal – when 2 weak stimuli occur in rapid succession, the second potential is added to the first, causing suprathershold depolarization sending more action potentials along a single fiber o
The picture shows the central fiber of a Pacinian corpuscle after all the capsule layers but one have been removed. The tip of the central capsule inside the capsule is unmyelinated, but eventually, the fiber becomes myelinated shortly before leaving the corpuscle to enter a peripheral sensory nerve. The receptor potential is produced by the initial stimulation of the adequate stimulus of compression of the corpuscle (observe the deformed area). This deformed area opens the ion channels in the membrane allowing for the positive sodium ions to diffuse inside the fiber producing an increased positivity which is the receptor potential. This in turn induces a local circuit of flow upto the first node of Ranvier inside the capsule. The membrane of the node is depolarized by the local current flow which then sets off AP along the nerve fiber toward the CNS.
Sensitive membrane of afferent receptor that responds to stimulus is either: 1.
An ending of an afferent neuron
2.
A separate cell adjacent to the afferent neuron
X. Overview of Specific transduction mechanisms
a. Mechanoreceptor – the ion channel of a mechanoreceptor is opened in response to the application of a mechanical force along the membrane; this opens the ion channel and allows an influx of current that that depolarizes the sensory receptor. b. Chemoreceptor – responds when a molecule of a chemical stimulant reacts with “receptor molecules” on the plasma membrane of the sensory receptor. The reaction between the chemical stimulant and the membrane receptor molecules opens ion channels, which enables the influx of ionic current that depolarizes the sensory receptor. c. Photoreceptor – in the dark, the photoreceptor cell is “open”, depolarizing. In response to light, the photoreceptor cell is closed when a photon is absorbed by pigment on the disc membrane. In this case, an influx of current occurs in the dark (called the dark current); the current ceases when light is applied. When the current stops, the photoreceptor hyperpolarizes hyperpolarizes (detected by retinal ganglion). d. Thermoreceptors metabolic rate
change in heat energy changes
Spatial stimulus whereby increasing signal strength is transmitted by using progressively greater numbers of fibers. The pinprick in the center of the receptive field is far greater than in the periphery because of the number of free nerve endings that are more abundant in the center.
**chemo and photoreceptors require G-protein coupled membrane receptors for their signal transduction. XI. Receptor Potential General properties of receptor potential:
usually a depolarizing event
GENERAL PRINCIPLES OF SENSORY PHYSIOLOGY
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Its function is to primarily encode information about rate of stimulus application. Also, it helps decrease amount of information reaching the brain within tolerable limits. Mechanisms:
Sensory adaptation takes place via two major mechanisms: 1.
The transducer mechanism fails to maintain a receptor potential despite continued stimulus application.
2.
The spike generator fails to sustain a train of action potentials. Although a receptor potential is present, the excitability of the spike generator membrane is diminished. An increase in the membrane conductance to K+ or the activity of the activity of the electrogenic Na+-K+ pump, or the inactivation of Na+ channels may be responsible for the decreased excitability of the membrane.
Temporal summation summation is done by increasing the frequency of nerve impulses brought about by the strength of the signal.
Function of RP: to trigger an Action potential.
If the receptor potential is strong enough to reach the threshold an action potential is generated. The more it rises above the threshold – the more AP is produced (frequency). Unlike the action potential more specifically the Excitatory Postsynaptic Action Pot’l (EPSP), receptor potential is a nonpropagated depolarizing potential, meaning it is does not reach the CNS. XII. How Receptor Potentials Trigger Action Potentials (through the Transducer region and the spike generator region.) In the case where the sensitive membrane is on a separate cell, the receptor potential there causes the release of a neurotransmitter that diffuses across the synaptic cleft between the receptor cell (presynaptic terminal) and the afferent neuron (postsynaptic terminal). The neurotransmitter binds to specific sites on the afferent neuron to generate an excitatory postsynaptic potentials. (EPSPs) the magnitude of the graded receptor potential determines the action potential frequency in the afferent neuron ↑magnitude of receptor potential = ↑frequency of AP
XV. Neural pathways in sensory systems (also called ascending pathways to the brain) a. A sensory pathway is made up of a group of neuron chains, each chain consisting of 3 or more neurons connected end to end by synapses. ( 1st order neuron 2nd order neuron 3rd order neuron, etc.) b. A sensory pathway is a link from receptors to the cerebral cortex (which is responsible for conscious recognition of information). Bell-Magendie Law indicates that in the spinal cord the dorsal roots are sensory and the ventral roots are motor.
XVI. Sensory Unit, Receptive fields of neurons; Convergence and Divergence of central processes of neurons.
a. Sensory unit – a single afferent neuron with all its (peripheral) receptor endings. b. Receptive field of a neuron – the portion of the body that, when stimulated, leads to activity in that particular afferent neuron. Activation of a single sensory unit almost never overlap. occurs because of c. Central processes of afferent neurons synapse upon interneurons or projection neurons in the brain or spinal cord. These central processes may exhibit: 1. Divergence of afferent neuron terminals. 2. Convergence of input from several afferent neurons onto single interneurons/ projection neurons.
a. In the case of the Pacinian corpuscle, when the receptor potential reaches about 10 mV, it gives rise to a sequence of action potentials. This transformation ordinarily takes place at the first node of Ranvier in the afferent nerve fiber. b. In unmyelinated afferents (nociceptors), the exact site of transformation transformation (spike generator region) is unknown.
XIII. Factors affecting magnitude of receptor potential
a. Stimulus strength b. Rate of change of stimulus application c. Temporal (and spatial) summation d. Adaptation XIV. Adaptation property of sensory receptors; mechanisms
Adaptation – it is a decrease in the frequency of action potentials (spike discharge frequency) in an afferent neuron despite maintenance of the stimulus at constant strength. GENERAL PRINCIPLES OF SENSORY PHYSIOLOGY
XVII. Sensory pathways are generally divided into SPECIFIC versus NON-SPECIFIC ascending pathways (know the 1st, 2nd, 3rd or 4th order neuronal arrangement)
A.
Specific ascending pathways – these are the ascending pathways in the spinal cord and brain that carry information about SINGLE types of stimuli. The information is finally fed into specific primary sensory receptive areas of the cerebral cortex.
Examples: i. Somatic receptors from inside and outside walls of the body (including skin, skeletal muscle, tendons, joints) via corresponding afferent nerves spinal cord brainstem thalamus somatosensory cortex (parietal lobe, postcentral gyrus).
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ii. Photoreceptors of the eye (retina) optic nerve brainstem thalamus visual or striate cortex (occipital lobe).
iii. Cochlear hair cells auditory nerve brainstem thalamus auditory cortex (temporal lobe) iv. Gustatory receptor cells (of taste buds) afferent neuron brainstem thalamus face region of somatosensory cortex known as the taste cortex in parietal lobe. v. Olfactory neurons olfactory bulb olfactory cortex (bypasses thalamus)
XX. Anatomical Location of the various cortical association areas 1.
Frontal lobe association area – in front of the premotor area .
2.
Parietal lobe association area
3.
Occipital lobe association area
4.
Temporal lobe association area – this is also known as the limbic association area; this extends from the lower portion of the temporal lobe to the limbic system.
B. Non-specific ascending pathways : Central neurons in these pathways are activated by sensory units of several different types (multimodal). i. Their function is to signify that something is happening, without specifying just what or where. In short, a signal for ALERTNESS/AROUSAL . ii. The multimodal sensory information is fed into the brainstem reticular, thalamus and various parts of the cerebral cortex.
Parieto-temporal Occipital association area : between the somesthetic and visual cortices extending into the posterior portion of the temporal lobe.
XXI. Primary sensory coding for stimulus type, intensity, location, duration
The sensory systems encode four aspects of a stimulus (TILD ) XVIII. Summary of common analytical plan for each sensory system before reaching the cortex; Role of the various sensory cortices; location of various sensory cortices
a.
– specific receptor types activated by an adequate stimulus play the primary role in encoding stimulus type. The stimulus is also encoded by the particular neural pathway that is stimulated (labeled line mechanism).
a. Axons in each sensory system cross the midline (decussations or cross-overs) on their way to the thalamus (either at the level of the medulla for fine touch information or at the level of the spinal cord (pain, temperature, and crude touch information), or more complex crossovers which exists in the visual and auditory systems.
Examples are heat, cold, sound and pressure ** Labeled line Principle
b. Specific thalamic relay nuclei exist for each sensory system on their way to their respective primary sensory receptive areas in the cortex (except for the olfactory system which bypasses the thalamus).
o
c. Role of the sensory cortices in the sensory system (somatosensory cortices; visual cortex; auditory cortex; taste cortex, olfactory cortex) d. Know the anatomical location of the various sensory cortices for the somatosensory system and the special senses. (Recall the Brodmann’s areas in neuroanatomy). XIX. Role of Association Cortex in Perception: Cortical association areas are outside the primary cortical sensory (or motor areas); they are not considered part of the sensory pathways, but connected to them. Function:
Stimulus TYPE/ or modality
o
b.
The sensation produced by the stimulation of the receptor and any part of its pathway is ultimately due to activation of a specific brain site. The brain site or the cortical area is responsible for type of sensation felt by the sense organ.
Stimulus INTENSITY this is encoded by the firing frequency of a sensory neuron -
The firing frequency of the sensory neuron is proportional to the magnitude of the receptor potential (Weber-Fechner (Weber-Fechner law or Steven’s power law function). Increasing intensity recruits more receptors to fire.
(a) complex analysis of incoming information from sensory cortices;
Consequently, relation between sensation and stimulus intensity is determined primarily by peripheral receptor.
(b) integration;
RECRUITMENT OF SENSORY UNITS:
(c) behavior ;
Increase in strength of stimulus spread over a large area and generally not only activates the sense organs immediately in contrast with it but also recruits those in the surrounding area.
(d) comparing information. These areas are the processing.
structural basis of perceptual
In summary, for perception of sensory stimuli to occur processes are required:
1. Transducing stimulus energy to action potentials by the receptor cell. 2. Transmitting sensory information through the nervous system; 3. Interpreting the information. GENERAL PRINCIPLES OF SENSORY PHYSIOLOGY
The receptors activated are of the same sensory neuron which leads to increase in AP frequency. Because of overlap and interdigitation of other units are also stimulated, more afferent pathways are activated which leads to intensity of sensation. c. Stimulus LOCATION
i. This is encoded primarily by the location of sensory projection in the cerebral cortex; this Page 6
topographic mechanism is known as representation; used by visual and somatosensory systems to localize the point of stimulus application.
ii. The receptive fields of primary afferent neurons aid in localizationprecision. THE SMALLER RECEPTIVE PRECISE LOCALIZATION.
FIELD,
the
more
iii. Stimulating the center of receptive field increases the firing frequency more than the periphery of receptive field, since there are more receptors in the center. iv. Overlapping receptive fields of primary afferents (sensory neurons) can aid in more precise localization by providing information on the differential firing frequency response of receptors. v. Lateral inhibition mechanism increases the contrast between “wanted” and “unwanted” information. Information from receptors at the edge of a stimulus is inhibited, while information from the stimulus center is enhanced. d. Stimulus DURATION : This is encoded primarily by rapidly adapting (phasic) and slowly adapting(tonic) receptors. i. Rapidly adapting receptors –signal change of stimulus frequency; the response declines during the continued application of a constant stimulus. (this is called ADAPTATION or DESENSITIZATION) ii. Slowly adapting receptors – signal slow changes or prolonged events; maintain their response at or near the initial level of firing regardless of stimulus duration. XXII. Central control of afferent information: a. Inhibition from collaterals from other neurons in ascending pathways (example: lateral inhibition , or gating in dorsal horn – to be discussed in pain system) b. Descending inhibitory controls (example: reticular formation and cerebal cortex control much afferent input; pain control systems also exist through these mechanisms)
GENERAL PRINCIPLES OF SENSORY PHYSIOLOGY
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