-​ Usually the current is inward, which produces depolarization of the receptor.

-​ The exception is in the photoreceptor, where light causes decreased inward current or hyperpolarization
(take note that it is a special exception to the rule)
-​ The photoreceptors in the retina after receiving the light, what happens is hyperpolarization compared to the usual
depolarization in other receptors.
-​ As an example, we have a receptor in the taste bud and this would trigger the opening of sodium channels or epithelial
sodium channels.
-​
3) The change in membrane potential produced by the stimulus is the receptor potential/generator potential.

-​ Image shows receptor potential compared to the action potential

Steps in sensory transduction

-​ If the receptor potential is depolarizing, it brings the membrane potential closer to threshold. If large enough, the
membrane potential will exceed threshold and an action potential will fire in the sensory neuron.
-​ It will eventually cause the firing of an action potential.
-​ Receptor potential is different from an action potential because an action potential are all or none while receptor
potentials are graded in size depending on the size of the stimulus

I.​ SENSORY RECEPTORS

Adaptation of Sensory Receptors:
Slowly Adapting/ Tonic Receptors
-​ Examples of slowly adapting or tonic receptors are muscle spindle, pressure and slow pain receptors
-​ These receptors respond repetitively to a prolonged stimulus and detect a steady stimulus
-​ Continues to produce action potentials throughout the duration of the stimulus
-​ It can be seen in the image the slow adapting receptor’s stimulus strength (pink) and the receptor
potential (green). The receptor potential (green) is continuous to the production of the action potential
throughout the duration of the stimulus.
-​ As long as there is stimulation, there is a production of the receptor potential.
-​ Provide the CNS with information about the duration and intensity of the stimulus
 Adaptation of Sensory Receptors:
Rapidly Adapting/ Phasic Receptors
-​ Examples of rapidly adapting or phasic receptors include the Pacinian corpuscle and light touch receptors
-​ This primarily detects the onset and offset of a stimulus
-​ It can be seen in the image that there is receptor potential (green) at the onset of the stimulus and
there is an off response (green) at the end of the stimulus. This is the difference between a rapidly
adapting and slowly adapting receptor.
-​ Responds to the application or removal of a stimulus but action potential do not continue throughout the
duration of the stimulus
-​ Reflects the rate of application of the stimulus, and the timing of movements

I.​ Sensory Receptors

Sensory Pathways from Receptor to the Cerebral Cortex
1)​ Sensory Receptors
-​ Activated by environmental stimuli
-​ May be specialized epithelial cells
-​ Photoreceptors
-​ Taste receptors
-​ Auditory hair cells
-​ May be primary afferent neurons such as olfactory chemoreceptors in the nose
-​ Transduce the stimulus into electrical energy aka receptor potential

2)​ First-Order Neurons

-​ Primary afferent neurons that receive the transduced signal and send the information to the CNS
 -​ The cell bodies of the primary afferent neurons are in dorsal root or spinal cord ganglia
-​ It can be seen in the image that sensory stimuli travels from the receptors going to the cell body or
the first-order neurons located in the dorsal root or spinal cord ganglia

3)​ Second-order Neurons

-​ Located in the spinal cord or brain stem
-​ Receive information from one or more primary afferent neurons in relay nuclei and transmit it to the
thalamus
-​ Axons may cross the midline in a relay nucleus in the spinal cord before they ascend to the
thalamus.
-​ An example of immediate crossing would be the spinothalamic tract compared to the
dorsal column medial lemniscus which ascends first before it decussates in the medulla
-​ Therefore, sensory information originating on one side of the body ascends to the contralateral
thalamus.

4)​ Third-Order Neurons

-​ Located in the relay nuclei of the thalamus
-​ Encodes sensory information and eventually ascends to the cerebral cortex.

5)​ Fourth-Order Neurons

-​ Located in the appropriate sensory area of the cerebral cortex.
-​ The information received results in a conscious perception of the stimulus.
-​ From the receptor up to conscious perception of the stimulus in the cortex
​
II. Dorsal Column Medial Lemniscus
-​ Consists primarily of group II fibers
-​ It may also consist of group I fibers but mostly group II
-​ Group II fibers: heavily myelinated therefore there is rapid conduction of action potential.
-​ The dorsal column has to be accurate in localization of touch which is important in maintaining balance and
maintaining an upright position, thus it has to be accurate all the time
-​ The accuracy is brought about by the following:
-​ Somatic afferent fibers (heavily myelinated group I and group II fibers) with fast conduction velocities.
-​ Limited number of synaptic relays
-​ Precise somatotropic organization.
-​ Limited convergence along the pathway which leads to high fidelity and degree of spatial and temporal resolution.
-​ It is very important to maintain our posture and upright position for proprioception

Different Types of Sensation Detected by the Dorsal Column Medial Lemniscus

●​ Sensations of fine touch and pressure
●​ Two-point discrimination
●​ Vibration
●​ Size, shape and texture discrimination
●​ Stereognosis (recognition of three-dimensional shape)
○​ Astereognosis: patient not being able to distinguish common items placed on their hand
●​ Motion detection
●​ Conscious awareness of body position (Proprioception)
●​ Limb movement in space (Kinesthesia)

Course of Dorsal Column Medial Lemniscus

-​ Starts with the primary afferent neuron (Pseudounipolar neuron) cell bodies in the dorsal root (1° neuron)
-​ Ipsilaterally to the nucleus gracilis (located medially) and nucleus cuneatus (located laterally) in the medulla (2°
neuron)
-​ Cross the midline and ascend as the medial lemniscus going to the contralateral Ventroposterolateral (VPL) portion
of the thalamus which houses the third order neuron (3° neuron)
-​ Finally, it will arrive at the fourth-order neuron (4° neuron) in the Somatosensory cortex.
 Course of Dorsal Column Medial Lemniscus

Dorsal Column Medial Lemniscus. Receptors coming from the legs are more medial compared to the receptors from the upper trank and from the upper
extremities which are more lateral. They ascend ipsilaterally through the fasciculus cuneatus and fasciculus gracilis and terminate in the nucleus gracilis and nucleus cuneatus
in the medulla (second-order neuron). Finally, it will ascend to the VPL nucleus part of the thalamus (third-order neuron) before going to the cortex where the fourth-order
neuron is located.