BME 5010, CHAPTER 9: THE SENSORY SYSTEMS  

 KEY WORDS MEANING

Sensory system Consists of (1) sensory receptors, (2) the neural pathways that conduct information from the receptors to the brain and (3) the parts of the brain that process this information.

Sensory information Information sent to the brain that may or may not reach consciousness (conscious awareness).

Sensation Information sent to the brain that reaches consciousness.

Perception Understanding the meaning of a sensation. 

RECEPTORS  

Sensory receptors Nerve endings that serve to initiate information from area around the nerves to the nervous system.

Stimulus The energy form that acts upon and activates a receptor.

Transduction

9-1

The process by which the receptors at the ends of afferent neurons translate energy into graded potentials that can initiate action potentials.

The receptors can be at the ends of afferent neurons or can be separate cells nearby that effect the ends of neurons.

 The energy input can include pressure, temperature, light, sound waves, etc.

Adequate stimulus The amount of stimulus energy required to generate a potential in the receptor.

Doctrine of specific energies The concept that for every type of sensation there is a specialized receptor type.

THE RECEPTOR POTENTIAL

(Also called a generator potential).

9-2

A graded potential in a receptor caused by a transduction process which opens or closes ion channels.

The magnitude of the receptor potential is increased with increasing stimulus strength.

Increasing receptor potential magnitude can increase action potential frequency. This does not increase action potential magnitude because the AP is constant in magnitude (all-or-none).

If the magnitude of the receptor potential is sufficient to bring the membrane potential to threshold an action potential will result.

Adaptation The decrease in frequency of action potentials despite the maintenance of a constant strength stimulus.

   

 KEY WORDS MEANING

NEURAL PATHWAYS IN SENSORY SYSTEMS

 

Sensory pathways A group of neuron chains, each chain made up of three or more neurons connected end to end by synapses. The chains in a given pathway run parallel to each other in the central nervous system and carry information to that part of the brain responsible for conscious recognition of the information. 

Also called "ascending pathways." 

Sensory unit A single afferent neuron with all its receptor endings.

Receptive field

9-4

A portion of the body, that when stimulated, leads to activity in a particular afferent neuron.

Receptive fields overlap - activation of a small area can activate several sensory units. 

Convergence/divergence in ascending pathways

9-5

Divergence of afferent neuron processes onto many neurons in the CNS. 

Convergence of several afferent neurons on a single interneuron.

Specific ascending pathways

9-7

 

Pathways that convey information about only a single type of sensory information (ie. pain pathways).

The pathways pass through the brainstem, synapse in the thalamus, and then the final neuron in the pathway goes to the cerebral cortex.

Somatic receptors Receptors in the skin, muscle, joints, bone and tendon.

Somatosensory cortex - 9-6A strip of cortex in the parietal lobe just behind the frontal lobe. It receives information that is conveyed by the somatic receptors.

Visual cortex - 9-6An area in the occipital lobe that receives the input from the visual pathway.

Auditory cortex 9-6An area in the temporal lobe that receives information from the ear.

Nonspecific ascending pathways 9-7

Activated by sensory units of several different types. Neurons in these pathways are called polymodal neurons.

   

 KEY WORDS MEANING

ASSOCIATION CORTEX AND PERCEPTUAL PROCESSING

 

Cortical association areas

9-8

Lie outside the primary sensory areas of the cortex.

Are not considered part of the sensory pathways.

Association areas play a role in analysis of incoming information and in subsequent behavior.

Some association areas integrate several types of sensory input (ie. position sense and visual input).

(You still know a tree is vertical when you see it, even if your head is tilted).

Factors that distort perception 1. Adaptation

2. Emotions, personality, experience.    

3. Filtering or inhibition of sensory input so it does not become a conscious sensation. If all sensory input reached consciousness this would be overwhelming.

4. We do not have receptors for certain energy forms (ie. x-rays, radio waves).

5. Damaged nerve pathways alter sensory input.

6. Drugs alter perception.

Perception Requires:

1. Transduction of stimulus to action potentials.

2. Transmission of action potentials to the cerebral cortex.

3. Interpretation of this input through association areas. 

   

  KEY WORDS MEANING

PRIMARY SENSORY CODING

 

Modality Another term for stimulus type.

ie. Heat, cold, pressure, sound, taste.

A given receptor type is sensitive to one stimulus modality. 

Stimulus intensity

9-9

Increased stimulus intensity means a larger receptor potential and a higher frequency of action potential firing.  

Recruitment A type of spatial summation in nerve receptors. For example, applying pressure over a larger surface area will activate more receptors.

Stimulus location

9-10, 9-11        

9-12

The precision in locating a stimulus depends on the (1) amount of convergence, (2) the size of the receptive field, and (3) the amount of overlap with nearby receptive fields. Smaller receptive fields, more overlap and less convergence result in more precision in locating a stimulus (ie, fingertip versus the back). 

Receptive field overlap provides a means for locating stimulus because an afferent fiber stimulated in the middle of a receptive field will fire at a higher frequency than the fiber stimulated at the edge of a receptive field. This simultaneous firing at different frequencies helps locate a stimulus.

Lateral inhibition

9-13, 9-14

Information for the edge of a receptive field is inhibited while information at the center is enhanced. The ascending neurons activate inhibitory neurons that suppress activity from adjacent neurons. This helps locate a stimulus.

Rapidly adapting receptors

9-15

Or phasic receptors. Their firing frequency is rapidly decreased even if stimulus strength is maintained.

Slowly adapting receptors

9-15

Maintain firing frequency no matter the duration of the stimulus. An example of their use is in static position sense.

Central control of afferent information

9-16

Ascending information is filtered or inhibited by:

1. Lateral inhibition.

2. Descending inhibitory pathways from the cortex and reticular formation.

   

 KEY WORDS MEANING

SOMATIC SENSATION                

9-17    

9-18        

9-18    

9-18    

9-19

Sensation from the skin, body walls, muscles, bones, tendons and joints.  These are initiated by a variety of somatic receptors. 

 Modalities for these include: -mechanical stimulation of skin, hairs, tissue -temperature -chemical changes

Each sensation is associated with a specific receptor type (9-17).

After entering the CNS the primary afferents (first order neuron) synapse on specific ascending pathways that send a second order neuron into the brainstem and synapse in the thalamus.

They also synapse on interneurons in nonspecifc pathways.

The specific ascending pathways end by sending a third order neuronfrom the thalamus into the precentral gyrus of the somatosensory cortex.

The somatosensory cortex for the right side of the body is on the left side of the brain (and vice versa) because the ascending pathway crosses over in the spinal cord or brainstem.

The specific body locations in the precentral gyrus is sometimes called a homunculus.

Touch-pressure

9-17

 

There are a variety of mechanoreceptors in the skin:

Receptor potentials in these nerves appear to be generated by stretch-activated ion channels.

Type of skin mechanoreceptors:

Ruffini endings, Merkle's discs: pressure intensity (slowly adapting)

Meissner's corpuscles: touch (moderately rapidly adapting)

Pacinian corpuscles: movement, vibration (rapidly adapting)

Free nerve endings: pain, temperature, crude touch (slowly to non-adapting) 

Sense of posture and movement

Muscle spindle stretch receptors: Respond to magnitude and rate of muscle stretch. 

Mechanoreceptors in joints, tendons, ligaments and skin also play a role in kinesthesia, the sense of joint movement. 

Temperature Warmth receptors: respond to temperatures between 30-43 degrees Celsius.

Cold receptors: respond between 20-35 degrees Celsius.

Noxious heat and cold: fire pain receptors

   

PAIN

   

A stimulus that causes or is about to cause tissue damage elicits the sensation of pain. Such stimuli are called noxious and the primary afferent receptors that are stimulated are called nociceptors.

Pain can be defined as "an unpleasant sensory and emotional experience with actual or potential tissue damage, or described in terms of such damage." 

(H. Merskey, Journal Pain, 1986).

Nociceptors The receptors whose stimulation gives rise to pain. These receptors are finely branched endings (free nerve endings) of unmyelinated or small myelinated fibers. Their conduction velocity is about 0.5 to 20 m/s depending on the size of the axon.

These receptors have high mechanical thresholds and respond to intense mechanical and/or thermal stimulation or to irritant chemicals. Those that respond to a variety of inputs (ie heat and mechanical) are called polymodal. The mechanical input may stimulate stretch-sensitive ion channels. 

Chemicals released from damaged or inflamed tissue can depolarize nociceptor nerve endings. These chemicals include H+, K+, histamine, bradykinin, cytokines and prostaglandins.

One action of aspirin is to block prostaglandin synthesis.

Hyperalgesia These chemicals can decrease the nociceptor threshold and increase their firing rate to painful stimuli. This sensitization is called hyperalgesia.

Sunburn and a painful swollen ankle are examples of hyperalgesic states.

Anterolateral system

9-18a and b

Three ascending pathways in the CNS that lead to the perception and localization of pain.

Referred pain The sensation of pain experienced at a site other than the injured site. ie: A heart attack can result in chest tightness or left arm pain. Afferents from both areas converge on the same interneurons in the spinal cord.

Stimulation-produced analgesia

Stimulation of descending inhibitory pathways that release or are sensitive to endogenous opioids. 

TENS, acupuncture and manipulation stimulate afferent fibers that may activate these inhibitory pathways in the spinal cord.