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abundance relative to each other.• For example, there are far more
pain receptors than temperature receptors in the body.
• Receptors also vary in terms of the concentration of their distribution over the surface of the body
• The fingertips having far more touch receptors than the skin of the back of the hand. The figure shows the distribution of temperature receptors in the skin by area.
From Sensation to Perception§ Survival depends upon sensation and perception
§ Sensation is the awareness of changes in the internal and external environment
§ Perception is the conscious interpretation of those stimuli and of the external world from a pattern of different sensory nerve impulses via the sensory receptors.
§ Some perceptions are indeed integrated compound sensations such as for example “wetness” ( touch, pressure and thermal input…. there is no such thing as a “wet-receptor”)
Processing at the Receptor Lever§ The receptor must have specificity for the stimulus energy
( temperature, touch, pressure, light,…)
§ The receptor’s receptive field must be stimulated
§ Stimulus energy must be converted into a graded potential
§ If the receptive field is in the same neuron that generates the action potential, we call it a generator potential.
§ If the receptive field is in a separate cell, it is called a receptor potential. If summed up to reach threshold, hhis will then release neurotransmitters in order to excite the associated sensory neuron.
Processing at the Receptor Lever§ The steps in formation of a generator potential are not known
for every receptor, but where it has been studied the start of the generator potential usually results from an increase in the permeability of the membrane of the receptor to all small ions
§ Usually, the ion furthest from its electrochemical equilibrium and in greatest concentration, namely sodium, contributes the greatest current. ( and thus results in EPSP’s)
Example : Muscle spindle• The figure shows the graded
responses of the muscle spindles when the muscle is stretched.
• The amplitude of the generator potentials increase with increasing stimulus strength. Different amounts of muscle stretch ( as shown by the heights in the lower trace) resulted in the graded series of generator potentials shown in the upper trace.
Example : Pacinian Corpuscle§ Pacinian corpuscles are
present in the skin, some mucous membranes etc. They are mechanoceptors, responding to pressure, or any kind of mechanical stimulus causing a deformation of the corpuscle.
§ The Pacinian corpuscle has a single afferent nerve fiber. Its end is covered by a sensitive receptor membrane whose sodium channels will open when the membrane is deformed in any way.
§ It is surrounded by several concentric capsules of connective tissue, with a viscous gel between them.
Adaptation of Sensory Receptors§ To a certain extent, the duration of the generator potential
depends upon the duration of the stimulus.
§ However, some receptors have generator potentials that last only a short time, no matter how long the stimulus is maintained.
§ We refer to a decrease in the amplitude of the generator potential or the frequency of discharge of the sensory fiber in the face of a persisting, constant stimulus as adaptation.
Tonic receptors are always active and generate actionpotentials at a frequency that reflects the background level of stimulation. When the stimulus increases or decreases, therate of action potential generation changes accordingly.
Tonic (slowly-adapting) receptors are important in situations where constant information about a stimulus is important ( they thus send information about ongoing stimulation)
Example : internal blood pressure, muscle spindles, injuries (pain)
Phasic (rapidly-adapting) receptors send information related to changing stimuli. They stop responding to a maintained stimulus, but when the stimulus is removed, they respond again
Any stimulus contains within it certain features that are of interest to the body. Stimuli have
• intensities or strengths• locations or sites of application• frequencies of application• rates of application• modalities
Modality, broadly speaking, is a class of sensations that are referred to a single type of receptor. Vision, hearing, touch, smell, and taste are all modalities ( energy forms).
Sensory receptors may be sensitive to different kind of energies. For example, putting pressure on the eye cause you to see light flashes, although the function of the eye receptors is to detect light.
• Doctrine of Specific Nerve Energies, as formulated by Johannus Müller, says that, although a sense organ may be sensitive to many forms of stimulus energy other than its real stimulus (called the adequate stimulus), the sensation evoked is always like that associated with the adequate stimulus, no matter what kind of energy was applied.
• For example : electrical stimulation of the optic nerve, does not result in an electric shock; the sensation evoked is one of seeing light.
• The doctrine of specific nerve energies implies that the modality or submodality of a sensation is determined not by the stimulus, but by what specific receptor or nerve fiber is stimulated. The doctrine also implies that the subjective qualities of a modality are determined, not in the receptors themselves, but in the central nervous system. (in this case for the optic nerve, it is determined by the visual cortex).
Processing at the Circuit Level• Neuronal signals from skin and deeper structures are segregated in the
spinal cord.
• For pain, temperature and the less discriminative aspects of touch, neurons in the dorsal horn have axons that cross in the spinal cord and ascend via the spinothalamic tract
• For discriminative touch and for conscious proprioception, the axons of primary sensory neurons ascend ipsilaterally ( do not cross over) in the dorsal funiculus (either gracile or cuneate fasciculus) and end in the gracile or cuneate nucleus. Fibers arising in these nuclei cross in the medulla and ascend in the medial lemniscus, which is near the midline in the medulla and shifts to a lateral location in the midbrain.
• The differences between the two main ascending somatosensory pathways are important functionally and clinically. Specific lesions within the spinal cord can thus results in specific loss of sensations in the body.