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- property by which one sensation is distinguished from another
ModalitIes: Touch-pressure, Posture-movement, Temperature, Pain
Submodalities: Warmth, cold (Temperature)

-The type of sensory receptor activated by a stimulus plays the primary role in coding the stimulus modality

Modality of sensation


Frequency - Increased stimuli, increased action potential
Recruitment - “calling in” or activation of receptors on additional afferent neurons

Intensity of stimulation


True or False

The more the receptor potential rises above the threshold level, the greater action potential frequency



- the magnitude of a subjective sensation increases proportional to a power of the stimulus intensity
- the more power the stimulus gives you, the more sensation you feel
- stimulus is directly proportional to sensation

Stevens’ power Law


- the magnitude of a subjective sensation increases proportional to the logarithm of the stimulus intensity
- Very intense stimulation causes progressively less and less additional increase in amplitude of receptor potentials
- Allows the receptors to have an extreme range of response
- From very weak to very intense
- ratio would matter and not the absolute weight

Weber-Fechner Law


-Receptors adapt either partially or completely to any constant stimulus after a period of time.

When a continuous sensory stimulus is applied,
- The receptor responds at a high impulse rate at first
- Then progressively slower rate until
- Finally the rate of action potentials decreases to very few to none at all

Adaptation of Receptors


- Muscle spindle; pressure; slow pain
- Slowly adapting
- Respond repetitively to a prolonged stimulus
- Detect a steady stimulus

Adaptation: Tonic receptors


- Pacinian corpuscle; light touch
- Rapidly adapting
- Action potential frequency declines with time in response to a constant stimulus
- Primarily detect onset and offset of a stimulus

Adaptation: Phasic receptors


- where the stimulus is being applied
- Acuity - precision in locating the stimulus; small receptive field size, more precise localization

Localization of stimuli


receptors are at the edge of a stimulus is strongly inhibited compared to information from stimulus' center

Lateral inhibition


- Transmit signals in varying frequencies
- Diameter is proportional to conduction velocity
- Labeled line principle
- General and Sensory nerve classification

Sensory Nerve Fibers


Nerve fibers are specific in transmitting only one modality of sensation

Labeled line principle


- Signals are subject to modification at the various synapses along the sensory pathways before they reach higher levels of the CNS
- Information is reduced or even abolished by inhibition from collaterals from other ascending neurons (e.g., lateral inhibition) or by pathways descending from higher brain centers

Control of Incoming Sensory Signals


- Consists of a bundle of 3-afferent sensory neuron chains that run parallel to each other in the CNS and carry information to the cerebral cortex*
- Specific ascending – carry a single type of stimulus
- Nonspecific ascending – different stimuli

Ascending pathway (Sensory)


- Transmit information from somatic receptors pass the brainstem and thalamus into the Somatosensory cortex
- Processing of afferent information does not end in the primary cortical receiving areas but continues to association areas of the cerebral cortex

Specific ascending pathway


- Polymodal neurons – different stimuli
- Convey information from more than one type of sensory unit to the brainstem reticular formation and regions of the thalamus that are not part of the specific ascending pathways

Nonspecific ascending pathway


- Specific regions of the Primary Somatosensory area (postcentral gyrus, posterior to the central sulcus) receive somatic sensory input from different parts of the body
- The major somatosensory areas of the cerebral cortex are SI and SII

Somatosensory cortex


Sensory pathway: Receptors to the Cortex

First-order neurons
Second-order neurons
Third-order neurons
Fourth-order neurons


- Primary afferent neurons that receive the transduced signal and send the information to the CNS
- Cell bodies are in the dorsal root or spinal cord ganglia

First-order neurons


- Located in the spinal cord or brain stem
- Receive information from 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 - sensory information originating on one side of the body ascends to the contralateral thalamus.

Second-order neurons


- located in the relay nuclei of the thalamus
- information ascends to the cerebral cortex

Third-order neurons


- located in the appropriate sensory area of the cerebral cortex
- information received results in a conscious perception of the stimulus

Fourth-order neurons


- Ascending Anterolateral pathway/ Spinothalamic pathway
- Dorsal column pathway
- Pathways cross from the side where the afferent neurons enter the central nervous system to the opposite side either in the spinal cord (Anterolateral system) or in the brainstem (Dorsal column system)

Neural pathways of the Somatosensory system


- Fine touch, pressure, two-point discrimination, vibration, and proprioception
- Consists primarily of group II fibers

- Primary afferent neurons: cell bodies in the dorsal root, axons ascend ipsilaterally to the nucleus gracilis and nucleus cuneatus of the medulla
- Second-order neurons cross the midline and ascend to the contralateral thalamus
- Third-order neurons ascend to the somatosensory cortex, where they synapse on fourth-order neurons

Dorsal column system


- Temperature, pain, and light touch
- Group III and IV fibers enter the spinal cord and terminate in the dorsal horn

- Second-order neurons cross the midline to the anterolateral quadrant of the spinal cord and ascend to the contralateral thalamus
- Third-order neurons ascend to the somatosensory cortex, where they synapse on fourth-order neurons

Anterolateral pathway


- Information from different parts of the body is arranged somatotropically
- Destruction of the thalamic nuclei results in loss of sensation on the contralateral side of the body



- “Little man”
- SI has a somatotopic representation similar to that in the thalamus
- The largest areas represent the face, hands, and fingers, where precise localization is most important.

Sensory homunculus


- Fast pain
- Mechanical (intense pressure), thermal pain stimuli (>45° or

Neospinothalamic tract


- Touch sensations:
- High degree of localization of stimulus.
- Fine graduations in intensity of stimulus.
- Phasic sensations (vibrations)
- Sensations of movement against the skin.
- Fine positional and pressure sensations

Dorsal Lemniscal System


- Thermal sensations: Cold, warm
- Pain sensations
- Crude pressure and touch sensations
- Tickle and itch sensations
- Sexual sensations

Anterolateral Spinothalamic System