Sensory SystemJohn Paul L. Oliveros, MD, DPPS

General Principles

Awarenesss of our external and internal world is brought about by neural mechanisms that process afferent information

Stimulus energy receptor potentials (graded potentials) action potentials (Nerve fibers)

Sensory system› Part of the nervous system that consists of

sensory receptors Neural pathways Processing areas of the brain

Sensory information› Information processed by a sensory system› May or may not lead to conscious awareness of the stimulus

Sensation› Sensory information that reaches consciousness

Perception› A persron’s understanding of the sensation’s meaning

Receptors Sensory Receptors

› Initiates neural activity at the border betwee the nervoussystem and the outside world

› Change stimulus energy (pressure, temperature, light, soundwaves, etc)

› Can either be: Specialized ending s of afferent neurons Separate cellthat affect the ends of

afferent neurons

Stimulus› Energy that impinges upon and

activates a sensory receptor Stimulus transduction

› The process by which stimulus is transformed into an electrical response

Adequate stimulus› The type of energy to which a receptor

responds in normal functioning› Receptors respond best to only a very

narrow range of stimulus energy (lowest threshold)

Receptor Potential Transduction process in all sensory

receptors involve the opening and closing of ion channels that receive information about the outside world

Receptor potential› A change in the membrane

potential on a specialized receptor membrane

› It is a Graded potential separate receptors:

graded potential causes release of neurotransmitter

Receptors on afferent neuons: A local current must flow to a part of

an axon that can produce an action potential

1st node of Ranvier Part of myelinated afferent neurons

capable of producing action potentials

Receptor Potential Graded potential magnitude

determines action potential frequency but not action potential magnitude

Factors controlling receptor potential magnitude› Stimulus strength› Rate of change of stimulus

strength› Temporal summation of

successive receptor potentials› Adaptation

Decrease in receptor sensitivity Results in decrease in frequency

of action potentials in an afferent neuron despite maintenance of the stimulus at a constant strength

Neural pathways in sensory system Sensory pathway

› A bundle of parallel 3-neuron chains

Sensory units› A single afferent

neuron with all its receptor endings

Receptive field› Portion of the body

that when stimulated leads to activity in a particular afferent neuron

Ascending pathways

Central processes› Part of afferent neurons

that enter the brain or spinal cord and synapse with interneurons

2nd order neurons› Interneurons that

synapse with afferent neurons

› Synapse with 3rd, 4th, etc interneurons until the cerebral cortex is reached

Ascending pathways Specific ascending pathways

› Ascending pathways in the brain and spinal cord that carry information about single types of stimuli

› Somatosensory cortex Lies in the parietal lobe of the brain

behind the junction of the parietal and frontal lobes

Where information from somatic recepotrs are transmitted

Information from skin, skeletal muscles, tendon and joints

› Visual cortex At the occipital lobe Where spefic pathways from the eyes

transmit

› Auditory cortex Where specific pathways from the

ears transmit Loacted at the temporal lobe

Ascending pathways

Nonspecific ascending pathways› Activated by sensory

units of several different types

› Signal general information

Polymodal neurons› 2nd order neurons that

respond to inputs from several afferent neurons, each activated by a different stimulus

Association Cortex and Perceptual Processing Cortical Association Areas

› Areas of the brain outside the primary cortical sensory areas but are adjacent to them

› Elaborates perception information from the primary sensory cortical areas

› Regions closests to the primary sensory cortical areas process information in fairly simple ways and serves basic sensory function

› Regions farther from the primary sensory cortical areas process information in more complicated ways Arousal Attention Memory Language Emotional and motivational

significance (frontal lobe/ limbic system)

Association Cortex and Perceptual Processing Factors that affect perception

1.Afferent information is influenced by sensory receptor mechanisms and by processing of the information along afferent pathways

2. Factors such as emotions, personality, experience and social background can influence perceptions so that 2 persons can witness the same events and yet perceive them differently

3. Not all informationentering the CNS give rise to conscious sensation* e.g. carotid/aortic bodies

4. We lack suitable receptors for many energy forms* x-ray, radio and TV waves

5. Damaged neural networks may give rise to faulty perceptions * phantom limb phenomenon

6. Some drugs alter perceptions* drugs* diseases

In summary:

*3 processes needed for perception to occur1. transducing stimulus energy into action potentials by receptor2. transmitting data through the CNS3. Interpreting the data

• 3 iportant organizational principles of the sensory system

• 1. there is heirarchical processing of afferent information along individual pathways

• 2. information is processed by parallel pathways, each of which handles a limited aspect of neural signals generated by the sensory transducers

• 3. information at each stage along the pathway is modified by “topdown” influences serving emotions, attention, memory and language

Primary sensory coding

The sensory system codes 4 aspects of a stimulus› Stimulus type› Intensity› Location› duration

Stimulus type› AKA stumulus modality

(temp, sound, pressure)› Submodalites:› A receptor type is

particularly sensitive to one stimulus modality (adequate stimulus) Due to the signal

transduction mechanisms and ion channels in the receptor’s plasma membrane

e.g. Vision receptors have pigments whose shape is transformed by light

Primary sensory coding

Stimulus intensity› Distinguishing intensity

Frequency of action potentials Inc. Stimulus strengthinc. Receptor potential inc. Action

potential frequency single receptor Other receptors of the same neuron

Recruitment Calling in of receptors on additional afferent neurons

Primary sensory coding

Stimulus location› Factors:

Main factor: Site of the stimulated receptor

amount of convergence of neuronal input in ascending pathways: inversely related to acuity/precision

Size of the receptive field covered by a receptor

Overlap of nearby receptive fields

Primary sensory coding

Primary sensory coding Lateral inhibition

› More important in localization than the different sensitivites of receptors throughout the receptor field

› Information from afferent neurons whose receptors are at the edge of the stimulus is inhibitted compared to information from the stimulus’ center

› Increases contrast between relevant and irrelevant information

› May occur at any levels of the pathway but mostly on the early stages

Primary sensory coding Stimulus duration

› Receptors differ in the way they respond to a constantly maintained stimulus adaptation

› Rapidly adapting receptors: Important in signaling rapid

change On response On-off response

› Slowly adapting receptors: Maintain response at or near

the initial level of firing regardless of the stimulus duration

For prolonged events (posture)

Central control of afferent information Reticular formation

and cortex: main control

End