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Lectures Sensory Neurophysiology
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Notes from all the lectures about sensory physiology - Audition - Vision - Touch - Olfaction - Gustation
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Sensory PhysiologyLT Perception and sensation
The general senses are touch, hearing, vision, smell and taste. There is more; self,
temperature, pain and balance.
Touch Mechanoreceptors
Hearing Mechanoreceptors
Vision Electromagnetic receptors
Smell Chemoreceptors
Taste Chemoreceptors
Self Proprioceptors/Mechanoreceptors
Temperature Thermoreceptors
Pain Nociceptors
Balance Mechanoreceptors
Membrane potential is the difference between charge inside and outside the cell
membrane. Is around -70mV. A change in potential of the receptor membrane is called the
receptor potential. A receptor potential above a threshold (-40mV) is called an action
potential.
Touch
The skin has different kinds of sensory receptors; Merkel’s
disk, Meissner’s corpuscle, Pacinian corpuscle and Ruffini’s
ending. The Pacinian corpuscle is deep inside the skin. Is
encapsulated in a ball. Has large receptive fields. The
Pacinian corpuscle is a mechanoreceptor. Tactile stimuli on
the Pacinian corpuscle give compression on the outer layers
of the receptor body. It indents, but also indents and
stretches the membrane of the axon in it. Deformation of the
axon membrane will open the Na-channels. Because of the
capsule, the axon responds very differently. With the
capsule it is phasic. Without the capsule it is tonic.
There is a difference between the stimulus strength and
the receptor potential. The relation is not linear. The
receptor potential rises fast with increasing low stimulus
strength, but progressively less with increasing high
stimulus. The receptor is a non-linear transducer. Does not
start at zero because there is also a threshold for the stimulus. The receptor is a non-linear
transducer.
From receptor potential to action potential is linear. The frequency of action potentials in the
nerve fiber is directly proportional to the amplitude of the receptor potential. Action potential
is a linear transducer.
There is adaptation to equal pressure. Receptor potential will decrease so also the action
potential frequency will go down. Increasing pressure will lead to an increasing receptor
potential so higher action potential frequency. Sinusoidal pressure for rhythmic movements.
, Receptors can follow the stimulus quite well within a certain trajectory. There are borders. If
a stimulus is too low, there will be no reaction. If a stimulus is too high, there will be
saturation. Some receptors adapt slow and some fast.
The transduction of sensory stimuli into nerve impulses is always by a change in potential of
the receptor membrane (the receptor potential). Receptor potentials can be induced by
mechanical deformation of the receptor, applying chemical, temperature change and
electromagnetic radiation.
If the receptor potential rises above the threshold to elicit action potentials in the nerve
attached to the receptor, the action potentials will start to appear.
The action potential is a travelling wave. Entry of positive charge during the action potential
causes the membrane just ahead to depolarize. There is also inactivation of the sodium
channels.
There are different types of afferent neurons. Have different diameters, myelination so also
a different speed. Also use different sensory receptors. Different nerve conduction
velocities. Muscles need to respond fast so the thickest axons. Pain and temperature use to
slower afferent fibers. For pain there are also two different fibers with different velocities. The
Aδ fibers are quicker than the C fibers.
Two different somatosensory pathways. Dorsal column-medial lemniscal pathway and the
spinalthalamic pathway. Dorsal column-medial lemniscal pathway crosses in the medulla.
Spinothalamic pathway already in the spinal cord. Lesions in the spinal cord can lead to
different changes in the sensation.
Action potential and the labeled line principle; An action potential always is the same. Does
not make clear what kind of sensation it represents. We have different modalities of
sensation. The brain areas where the nerve are connected to are responsible for the
modalities.
LT Hearing
Sound is a change of air pressure. Tympanic membrane can respond to the sound. The
ossicles then move this sound to the oval window. Stapes are on the oval window. In the
cochlea there is fluid that can move there back and forward. Round window for the pressure
relief.
Basilar membrane starts to move up and down because of the fluid move. Does not do that
everywhere, but it depends on the frequency of the sound. At the base it is narrow and stiff,
so more for high pitch sound. To the end more wide and floppy so more for low pitch noise.
So; based on the place of the basilar membrane the pitch of sound can be distinguished.
Decomposes the complex wave of sounds into frequencies. Is fourier transformation. Also
different amplitudes.
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