lecture 3 sensory

8/9/2019 Lecture 3 Sensory

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Lecture 3 Sensory/Motor

SENSORY SYSTEMS

1. General Principles of Sensory Transduction

Adequate stimulus

a specific stimulus that activates sensoryreceptor(s)

e.g. Pacinian corpuscle

Change in membrane conductance

Generator potential

Action potential


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A. Stimulus intensity and frequency of action

potentials

frequency code of stimulus intensity:

stimulus intensity p generator potential p

frequency of action potential This allows the receptor to have a extreme

range of response, from very weak to very

intense

population codeMore sensory receptors are activated as

stimulus gets greater

INTENSITY (%)

FREQUENCY (%)


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B. Modality of sensation

Each type of sensation (pain, touch, sound, etc.)is called modality of sensation

The quality of a stimulus is encoded by the

pathway of transmission Different modality of sensation is

transmitted by a specific nerve fiber and

determined by where the nerve fiber

terminates in the brain Labeled-line code principle

The specificity of nerve fibers for transmitting

only one modality of sensation


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B. Mechanisms of adaptation

can occur at the brain and the receptor sensory adaptation at receptor site


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1. Somatic Sensory System

A. Types of somatic sensory receptors

tactile receptors (mechanoreceptors)

stimuli: touch, pressure, vibration

e.g. Pacinian corpuscle

hair receptor

Ruffini ending


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thermal receptors

stimuli: heat and cold

nociceptors (pain receptors)

stimuli: painful touch, cut, extreme

temperatures

proprioceptive receptors detect position and movement

static proprioceptors

- tonic discharge

dynamic proprioceptors/kinesthesia

- phasic discharge


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joint capsules

muscle spindle receptors

general principles for somatic sensory receptorsmediated transmission

specificity

cross-over of most pathways

topographical organization

B. The dorsal root ganglion

a cluster of cell bodies

different size of nerve fibers


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large fibers: D 13-22 Qm, glutamate as

neurotransmitters

myelinated, fast action potential

conduction: 70-120m/sec

small fibers: D: 1-5 Qm, substance P as

neurotransmittersunmyelinated, slow conduction of

action potential: 2-15m/sec

two major pathways: dorsal column and

anterolateral

C. The spinal cord in somatic sensory function

anatomy of the spinal cord


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useful anatomic terms: rostro-caudal axis;

dorsal/ventral; medial/lateral; anterior/posterior transverse section: gray matter and white

matter

segments of the spinal cord

31 segments: 8 cervical (C1-C8)

12 thoracic (T1-T12)

5 Lumbar (L1-L5)

5 sacral (S1-S5)

1 sacro-coccygeal

dermatomes

lesions of the spinal cord


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D. The thalamus in somatic sensory function

One of the brain areas that receives most types of

sensory information

VPL receives somatic sensory information

E. The somatic sensory cortex

somatotopic organizationF. Receptive field of neurons

The area monitored by a single receptor cell iscalled the receptive field for that cell

The receptive fields differ in different parts of body. small in more sensitive areas like fingertips,

tongue (1 mm in diameter)

large in less sensitive areas such as in theback (7 cm)


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Lateral inhibition

Two-points threshold discrimination test

G. Pain sensation

Pathway: anterolateral pathway

Neurotransmitter: substance P Modification of pain sensation

Brain stem

Other sensory fibers (touch, pressure)

Higher brain center

The gating theory

Referred pain


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Referred pain. Nociceptors from

several locations converging on

a single ascending tract in the

spinal cord may cause referred

pain.

lecture 3 sensory

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