What happens when an adequate stimuli interacts with a receptor?
Sensory transduction -> receptor potential -> local graded potential -> depolarization -> threshold reached -> generate action potentials
Labeled Line Principle
tells you the sensory modality since there is a chain of connected neurons from the sensory receptor which responds to the stimulus to the brain area that perceives the stimulus
How does nervous system code for stimulus duration?
1. Maintain continuous signal during stimulus
2. Have on-signal and off-signal
How does the nervous system code for stimulus strength?
1. By increasing frequency of nerve impulses - frequency code.
2. by increasing the number of fibers activated - population code
Adaptation
When a maintained stimulus of constant strength is applied to a receptor, the frequency of the action potentials in the receptor's sensory nerve decreases over time
-general property of sensory receptors
Non-adapting receptors
example: nociceptors
-often lump with slowly adapting receptors
Slowly Adapting receptors
a. sometimes called tonic receptors
b. example: Merkel cell ending or Merkel's disks
c. useful for giving continuous information about stimulus strength and signaling stimulus duration
d. not as useful for signaling stimulus duration at low stimulus inten
Rapidly adapting receptors
a. sometimes called phasic receptors or rate receptors
b. examples:
Pacinian corpuscles
Meissner's corpuscles
c. disadvantage - cannot give continuous signal in response to continuous stimulus
d. useful for signaling beginning and end of stimulus
e. can s
free nerve endings
responsible for "crude" touch
low threshold (high sensitivity) mechanoreceptors
encapsulated endings of myelinated fibers (Group II or A?) or for Merkel cell afferent Group II fiber that contacts specialized receptors.
Merkel's disks
� found at the base of the epidermis
� Several Merkel's disks can be grouped together into the Iggo dome receptor which is innervated by just one large myelinated nerve fiber (A?), the Merkel cell afferent
- slowly adapting; highest spatial resolution of
Slowly adapting afferent II
� Innervates a receptor that responds to stretch of the skin
Meissner's corpuscles
- rapidly adapting
~ 40% of the mechanosensory innervation of the human hand
� Very high density of these receptors in fingertips
� Closer to the skin surface than Merkel's disks
� very sensitive to low frequency vibrations; 30-50 Hertz
- necessary for ef
Pacinian Corpuscles
� rapidly adapting receptor
� found in subcutaneous tissue
� adapt more rapidly than Meissner's corpuscles so sensitive to higher frequencies (250-350 Hz)
� can detect vibrations transmitted through objects held in the hand thus contributing to skilled to
hair end-organ
� hair follicles in the skin are innervated by nerve endings which are stimulated by any slight movement of the hair
� adapts rapidly, slower A? conduction velocity
� detects mainly movement of objects on the body surface and the initial contact of an obj
Active tactile exploration
Need relative movement between skin and surface for accurate discrimination of texture
Proprioception
� Proprioceptors provide information about joint angles, muscle length and muscle tension
� is important for
1. maintaining balance
2. controlling limb movement
3. evaluating the shape of a grasped object
Receptors Involved in Proprioception
a. joint receptors
b. muscle spindles and Golgi tendon organs
c. skin tactile receptors
Dorsal Column function
- fine tactile discriminations, vibratory sense and proprioceptive sense
- responsible for two-point discrimination ability
- Information crosses the midline in the brain stem
Dorsal column lesions
can cause deficits in:
a. tactile discrimination
b. vibratory sense
c. proprioception
Romberg Test
- test of balance
- The patient is asked to stand with his feet together. If the patient is steady and does not fall, the examiner can ask patient to close eyes. If patient is unsteady with eyes open this could indicate a cerebellar disease or vestibular
Anterolateral System (Spinothalamic tracts) Function
1. responsible for conveying nociceptive and thermal sensations.
2. Some tactile information is also carried by these pathways so lesions of the dorsal column system do not totally destroy ability to sense touch.
3. Information crosses the midline in the
Somatotopic Maps
� Each of the four cortical areas in SI contains a separate and complete representation of the contralateral side of the body.
� Foot area always in medial cortex.
� Plasticity and reorganization
graphesthesia
sense through which figures or numbers drawn on the skin can be recognized
stereognosis
ability to recognize objects through touch alone
Somatosensory area II (SII)
- neurons in this area show attention modulated responses
- the responses of the neurons in this area to a stimulus depend on whether the subject is paying attention to the stimulus
Spinal Root Injuries
� Could cause impairment of cutaneous sensation over an entire dermatome
� Since there is overlap of dermatomes a patient does not lose all sensation unless two or more adjacent spinal roots are affected.
sever one peripheral nerve
� sensory loss in fragments of adjacent dermatomes
peripheral neuropathies
� stocking - glove pattern
A ? fibers
Axon of alpha motor neuron; muscle spindle primary ending (Ia); Golgi tendon organ afferent (Ib)
A ? fibers
Muscle spindle secondary ending (II) And the axons of cutaneous mechanoreceptors
A ? fibers
Axon of gamma motor neuron to muscle spindle fibers
A ? fibers
Fast pain, some temperature receptors
B fibers
Sympathetic preganglionic axons
C
- Slow pain, some temperature (Group IV)
- Sympathetic, postganglionic axons