b223 general senses 15

afferent sensory pathways

sensory information comes into CNS from the sensory receptors via peripheral nerves

synapse onto sensory processing interneurons in posterior horn of spinal cord or cranial nerve nuclei in brainstem

only small percentage of sensory input reaches conscious awareness

sensory response occurs only if receptors exist that are sensitive to the stimuli

• e.g. humans can not “see” ultraviolet light

sensory receptor

specialized cell or cell processes that respond to specific stimuli

• dendrite of sensory neuron

• specialized cell that synapses onto dendrite of sensory neuron

translate stimuli into bioelectrical activity of the nervous system

transduction

• stimulus changes membrane ion permeability producing the receptor potential (graded potential)

• receptor potential controls depolarization at AP initiation site in sensory neuron dendrite

• AP frequency (#/time) provides the CNS information on stimulus intensity

receptor specificity

each receptor respond only to certain types of stimuli

receptive field

area monitored by a single sensory receptor

• the larger the receptive field, the less precise the localization of the stimulus

transduction

stimulus changes membrane ion permeability producing the receptor potential (graded potential)

receptor potential controls depolarization at AP initiation site in sensory neuron dendrite

AP frequency (#/time) provides the CNS information on stimulus intensity

sensation

information arriving from a stimulus

perception

conscious awareness of a sensation

labeled line

identifies type (modality) of stimulus and body location of receptor

• projects to brain processing centers that are organized somatotopically

• most sensory information crosses over (decusate) to contralateral areas of brain

sensory coding

pattern of APs arriving convey information on strength, duration, and variation of the stimulus

adaptation

reduction in sensitivity in the presence of a constant stimulus due to changes in receptor response or central processing

tonic receptors

always active if stimulation is present—rate of AP changes with changes in level of stimulation

phasic receptors

produce AP only in response to changes in level of stimulation

fast-adapting receptors

tonic receptors (photo)

examples: nociceptors (pain), light touch, thermoreceptors (will get slightly less sensitive)

• free nerve endings

are always active. action potentials are generated at a frequency that reflects the background level of stimulation. when the stimulus increases or decreases, the rate of action potential generation changes accordingly

phasic receptors (photo)

examples: chemoreceptors (O2, CO2, pH), joint position/muscle length, corpuscles (touch)→deep pressure)

are normally inactive. action potentials are generated only for a short time in response to a change in the conditions they are monitoring

peripheral adaptation

reduces amount of information that reaches the CNS (phasic receptors)

central adaptation

inhibition along a sensory pathway within CNS

restricts amount of information that reaches the cortex and conscious awareness

reticular activating system

reduces or increases awareness of arriving sensations

pain

nociceptors

temperature

thermoreceptors

physical distortion

mechanoreceptors

chemical detection

chemoreceptors

general senses

receptors throughout the body

pain → nocireceptors

temperature → thermoreceptors

physical distortion → mechanoreceptors

chemical distortion → chemoreceptors

present in all spinal nerves

present in CN V (trigeminal nerve) and CN X (vagus nerve)

nocireceptors

pain

free nerve endings - sensory neuron dendrites sensitive to various stimuli

• extremes of temperature

• mechanical damage

• chemicals released by damaged cells

  • converted to prostaglandins

abundant in superficial skin, joint capsules, bone periosteum, blood vessel walls

few in deep tissues and visceral organs

large receptive fields → poor localization

referred pain

felt pain in a body region not necessarily damaged

fast pain

prickling pain

carried by myelinated type A fibers

trigger somatic muscle reflexes

relayed to cortex for conscious awareness

can be localized to within a few inches

slow pain

burning, aching pain

carried by unmyelinated type C fibers

can be localized only to large body area or referred pain

adaptation to pain

little peripheral adaptation

• nociceptors are tonic receptors → respond to prostaglandins

central adaptation via inhibition in pain processing pathways

• excitatory neurotransmitters of pain pathway: glutamate and substance P

• inhibitory neurotransmitters: endorphins and other ”natural opiates”

pain management

anesthetics - block all sensations

• local - block AP propagation → block voltage-gated Na+ channels

• general - suppress consciousness

analgesics that reduce pain stimulus

• inhibit prostaglandin synthesis by blocking cyclo-oxygenase (COX) enzymes

• non-steroidal anti-inflammatory drugs (NSAIDs) block both COX-1 and COX-2 enzymes

• COX 2 inhibitors - selective blockers

analgesics that reduce transmission of information about pain in CNS

• opiates - agonists of endorphins

thermoreceptors

free nerve endings sensitive to change in temperature

• more cold sensitive than warm sensitive receptors

abundant in dermis, skeletal muscle, liver, and hypothalamus

very large receptive fields

rapidly adapting receptors (tonic, but will lose sensitivity slowly over time)

warm receptors

sensitive to temperatures above 25 C (77 F)

unresponsive to temperature above 45 C (113 F)

cold receptors

sensitive to temperature between 10 C (50 F) and 20 C (68 F)

pain receptors (nociceptors)

respond to temperatures below 10 C

respond to temperatures above 45 C

chemoreceptors

sensitive to change in pH, CO2, and O2 levels of body fluids

• cerebral spinal fluid - brainstem respiratory centers

• arterial blood - carotid and aortic bodies

rapidly adapting receptors (phasic)

reflex control of respiration and cardiovascular system

no pathways to cortex for conscious awareness

chemoreceptors (in and near respiratory centers of medulla oblongata)

sensitive to changes in pH and CO2 in cerebrospinal fluid

chemoreceptors (carotid bodies)

sensitive to changes in pH, CO2, and O2 in blood

chemoreceptors (aortic bodies)

sensitive to changes in pH, CO2, and O2 in blood

mechanoreceptors

sensitive to distortion of the cell membrane of the dendrites

• mechanically gated ion channels open in response to stretching, compression, etc.

1. baroreceptors - pressure in blood vessels and hollow organs

2. proprioceptors - position of joints, muscles

3. tactile receptors - tough, pressure, vibration

baroreceptors

pressure in blood vessels and hollow organs

proprioceptors

position of joints, muscles

tactile receptors

touch, pressure, vibration

baroreceptors (carotid sinus and aortic sinus)

provide information on blood pressure to cardiovascular and respiratory control centers

baroreceptors (lung)

provide information on lung expansion to respiratory rhythmicity centers for control of respiratory rate

baroreceptors (digestive tract)

provide information on volume of tract segments, trigger reflex movement of materials along tract

baroreceptors (colon)

provide information on volume of fecal material in colon, trigger defecation reflex

baroreceptors (bladder wall)

provide information on volume of urinary bladder, trigger urination reflex

sensory pathway

photo sensory pathway

proprioceptors (photo)

photo proprioceptors

frontal lobe

primary motor cortex (precentral gyrus)

somatic motor association area (premotor cortex)

parietal lobe

primary sensory cortex (postcentral gyrus) - feel

somatic sensory association area - interpret memory

occipital lobe

visual association area

visual cortex

temporal lobe

auditory association area

auditory cortex

olfactory cortex

somatosensory cortex (photo)

photo somatosensory cortex

processing of sensory information

somatotopically organized = orderly representation of body regions

amount of space in cortex and pathways devoted to a particular body region is proportional to the number of sensory receptors it contains, not to the body region’s absolute size