Appunti di Fisiologia (Human physiology, ENG)
4. Stimulus location and modality (i.e. type of perception) are coded according to which kind of
receptors in the body area are activated.
5. Each somatosensory pathway projects to the specific area of the somatosensory cortex,
dedicated to a particular receptive field along the body. Then, brain can tell the origin of each
incoming signal and interpret the stimulus in perception.
For what concerns the third point, for an individual sensory neuron intensity and duration are
respectively coded in two information: frequency of action potentials coming from a given receptor,
called firing frequency, and time-window in which this burst of action potentials is prolonged. In fact,
once the stimulus provokes a depolarization over the threshold in the receptor, the primary sensory
neuron begins to fire action potentials, with a frequency of discharge and duration over time
proportional respectively to the amplitude (intensity) and duration of the receptor’s membrane
According to what as being said so far, a longer stimulus generates a longer series of action
potentials in the primary sensory neuron. However, if a stimulus persists, some receptors adapt, or
cease to respond. This kind codification is a tuneable feature of the receptive element. It happens
that identical stimuli, continuously repeated over time, can be coded in different action potential
patterns over: this property is called sensory adaptation. Depending on how receptors adapt to
continuous stimulation, they are divided in two class:
• Tonic receptors are slowly adapting receptors that fire rapidly when first activated, then slow
and maintain their firing as long as the stimulus is present. In general, the stimuli that activate
tonic receptors are parameters that must be monitored continuously by the body.
• Phasic receptors are rapidly adapting receptors that fire when they first receive a stimulus
but cease firing if the strength of the stimulus remains constant. Phasic receptors are attuned
specifically to changes in a physiological parameter. Once a stimulus reaches a steady
intensity, phasic receptors adapt to the new steady state and turn off. This type of response
allows the body to ignore information that has been evaluated and found not to threaten
homeostasis or well-being.
In general, once adaptation of a phasic receptor has occurred, the only way to create a new signal
is to either increase the intensity of the excitatory stimulus or remove the stimulus entirely and allow
the receptor to reset. For what concerns the fourth point, human sensory system has a special
arrangement, called lateral inhibition, that enhances precise localization of the stimulated area for
most receptive units. In order to isolate the location of a stimulus, this property works on increasing
the contrast between activated receptive field and their inactive neighbours.
The working principle is based on the inhibitory outputs of secondary sensory neuron synapsing with
the close tertiary order neurons. In reference of the previous picture, a pin stimulates the primary
sensory neuron B, which release the higher amount of neurotransmitter with respect to A and C.
Their respective second order neurons activated in turn, and their lateral inhibitory branches starts
as well to release inhibitory neurotransmitters to the tertiary order neurons each other. Nevertheless,
A and C inhibition is not enough to stop their neighbour tertiary neurons, while B, which receives a
higher stimulation, release enough inhibitory neurotransmitter to inactivate both A and C. In this
scenario, only B is capable to transmit the action potential to the higher order neurons, while A and
C are neglected, with a precise localization of the zone of maximum detection of the stimulus, which
presumably coincides with its point of application.
As already said, receptors for the somatic senses are found both in the skin and in the viscera.
Receptor activation triggers action potentials in the associated primary sensory neuron, whose cell
bodies are gathered in one of the dorsal root ganglion. In the spinal cord, many primary sensory
neurons synapse onto interneurons that serve as the secondary sensory neurons. The location of
the synapse between a primary neuron and a secondary neuron varies according to the type of
receptor. Neurons associated with receptors for nociception, temperature, and coarse touch synapse
onto their secondary neurons, shortly after entering the spinal cord.
In contrast, most fine touch, vibration, and proprioceptive neurons have very long axons that project
up the spinal cord all the way to the medulla. All secondary sensory neurons cross the midline of the
body at some point, so that sensations from the left side of the body are processed in the right
hemisphere of the brain and vice versa. The secondary neurons for nociception, temperature, and
coarse touch cross the midline in the spinal cord, then ascend to the brain. Fine touch, vibration, and
proprioceptive neurons cross the midline in the medulla. In the thalamus, all secondary sensory
neurons synapse onto tertiary sensory neurons, which in turn project to the somatosensory region
of the cerebral cortex. In addition, many sensory pathways send branches to the cerebellum so that
it can use the information to coordinate balance and movement.
The somatosensory cortex is the part of the brain that recognizes where ascending sensory tracts
originate, so that an exact portion on this cortical region on one side of the brain corresponds to the
somatic stimuli on a precise part of the body, on the contralateral side. Likewise to the motor cortex,
also in this case regions of the bodies are organically arranged along the somatosensory cortex, so
that this mapping is widely referred to as sensory homunculus. Within the cortical region for a
particular body part, columns of neurons are devoted to particular types of receptors. Experiments
shows that the more sensitive a region of the body is to touch and other stimuli, the larger the
corresponding region in the cortex. Interestingly, the size of the regions is not fixed. If a particular
body part is used more extensively, its topographical region in the cortex will expand. For example,
people who are visually handicapped and learn to read Braille with their fingertips develop an
enlarged region of the somatosensory cortex devoted to the fingertips. In contrast, if a person loses
a finger or limb, the portion of the somatosensory cortex devoted to the missing structure begins to
be taken over by sensory fields of adjacent structures. Reorganization of the somatosensory cortex
“map” is an example of the remarkable plasticity of the brain. Unfortunately, sometimes the
reorganization is not perfect and can result in sensory sensations, including pain, that the brain
interprets as being located in the missing limb (phantom limb pain).
Touch and thermoception
Touch is one of the fifth senses and is the response of the somatosensory system to mechanical
stimuli either on the external surface of the body or the inner layer of organs and insights. According
to the nature of the perceived stimulus, touch receptors are mechanoreceptors, capable to respond
to many forms of physical contact, such as stretch, steady pressure, fluttering or stroking
movements, vibration and texture. Touch receptors in the skin come in many forms. Some are free
nerve endings, such as those that respond to noxious (harmful) stimuli. The most well-studied touch
receptors are Pacinian corpuscles, composed of nerve endings encapsulated in layers of
connective tissue where elastic energy release mechanically gated ion channels. They respond
mainly to vibrations and high-frequency perturbations. Pacinian corpuscles are rapidly adapting
phasic receptors, and this property allows them to respond to a change in touch but then ignore it.
Other kind of mechanic and vibrational stimuli are transduced by Meissner’s corpuscles, Ruffini’s
corpuscles, and Merkel’s receptors. 96
Thermoception is the sense by which an organism perceives temperatures. Thermoreceptors are
free nerve endings that terminate in the subcutaneous layers of the skin. Mammals have at least two
types of sensor: those that detect decrease below the basal temperature, called cold receptors, and
those that detect increase above the basal temperature, called warm receptors, perceiving from 37°
C to 45° C. Above that temperature, nociceptors are activated, creating sensation of painful heat.
The receptive field for a thermoreceptor is about 1 mm in diameter, and the receptors are scattered
across the body. There are considerably more cold receptors than warm one. Temperature receptors
slowly adapts between 20 °C and 40 °C, allowing people sea bathing in summer’s early days or
being exposed to sun after a little initial discomfort. Outside this range, where the likelihood of tissue
damage is greater, the receptors cease to sense the stimuli, which are collected in bulk by the
Nociception is the sensory nervous system's response to certain harmful or potentially harmful
stimuli. In general, these receptors are addressed to inform the individual of any prospective or
concrete source of peril and harmfulness. For this reason, nociceptors are receptors that respond to
a variety of strong noxious stimuli (chemical, mechanical, or thermal) that cause or have the potential
to cause tissue damages. Activation of nociceptors initiates adaptive, protective responses, such as
the reflexive withdrawal of a hand from a hot stove touched accidentally. Nociceptors are not limited
to the skin. Discomfort from overuse of muscles and joints helps us avoid additional damage to these
structures. Two sensations may be perceived when nociceptors are activated: pain and itch.
Nociception triggers a variety of physiological and behavioural responses and usually results in a
subjective experience of pain in sentient beings.
Nociceptors are sometimes called pain receptors, even though pain is a perceived sensation rather
than a stimulus. Nociceptive pain is mediated by free nerve endings whose ion channels are
sensitive to a variety of chemical, mechanical, and thermal stimuli. Nociceptor activation is
modulated by local chemicals that are released upon tissue injury. They can activate two pathways:
reflexive protective responses that are integrated at the level of the spinal cord and ascending
pathways to the cerebral cortex that become conscious sensation (pain or itch). Primary sensory
neurons from nociceptors terminate in the dorsal horn of the spinal cord. There they synapse onto
secondary sensory neurons that project to the brain or onto interneurons for local circuits.
Irritant responses that are integrated in the spinal cord initiate rapid unconscious protective reflexes
that automatically remove a stimulated area from the source of the stimulus. For example, when
accidentally touching a hot stove, an automatic withdrawal reflex causes to pull back hand even
before being aware of the heat.
Afferent signals from nociceptors are carried to the CNS through A
+1 anno fa
I contenuti di questa pagina costituiscono rielaborazioni personali del Publisher Meliuk di informazioni apprese con la frequenza delle lezioni di Human Physiology e studio autonomo di eventuali libri di riferimento in preparazione dell'esame finale o della tesi. Non devono intendersi come materiale ufficiale dell'università Politecnico delle Marche - Univpm o del prof Fabri Mara.
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