2.5 Sensory systems Flashcards
Sensory homonculus
What the male body would look like if each part grew in proportion to the area of the cerebral cortex of the brain concerned with its sensory perception
Frontal lobe
Anterior half of cortex, motor functions
Parietal lobe
Posterior half of cortex, sensory functions
Stimulus
Change in the external or internal environment that causes a reaction-specifically potentials- in peripheral NS receptors
Examples of stimuli
Light, sound, touch, chemical odor, chemical taste
Sensation (Perception)
Conscious awareness, in the cerebral cortex, of stimuli received by peripheral sensory receptors
1st step of afferent sensory circuit
Stimulation and transduction (peripheral sensory receptor neuron transforms stimulus into electrical signals–> graded potentials)
2nd step of afferent sensory circuit
Generation of action potentials in the 1st order sensory neuron (may be separate cell from sensory receptor)
-Triggers at threshold stimulus level
AP frequency dictated by…
Strength of stimulus and graded potentials
Sensation
APs transmission to the thalamus, then to cerebral cortex where selective attention yields conscious perception of a stimulus
What is the thalamus’ role in sensation
Filters “noise,” decides whether or not it’s worth it to perceive incoming information
Types of peripheral sensory receptors
Exteroreceptors
Interoreceptors
Propioreceptors
Exteroreceptors
Monitor external environment; associated with skin (somatic sensory receptors) or special sense organs of the head
Interoreceptors
AKA visceral sensory receptors
Monitor internal environment
Associated with internal organs and tissues (ex: blood vessels, visceral organs)
3 types of interoreceptors
Chemoreceptors
Thermoreceptors
Stretch receptors
Propioceptors
Monitor body position and movements, muscle length / tension, joint positions
Associated with joints, tendons , muscles and vestibular organ
Receptors classified by modes of stimuli
Chemoreceptors Thermoreceptors Mechanoreceptors Photoreceptors Baroreceptors Nociceptors
Chemoreceptors
Detect chemicals detected in fluid-liquid or airborne
Ex: blood gases, salts, scents in the air, flavors in foods)
Thermoreceptors
Respond to changes in temperature
Mechanoreceptors
Respond to touch, external pressure, vibration and stretch (mostly skin but also gut and bladder)
Photoreceptors
Respond to photons (light)
Baroreceptors
Sense pressure by ‘reading’ deformation of a body structure
Ex: blood vessel stretch for blood pressure regulation
Nociceptors
React to painful, noxious stimuli, can be both extero and intero (but fewer intero)
Stimulation of a sensory receptor causes…
A local dendritic depolarization
Transduction
Transforming physical or chemical stimulus into electrical signal
Generator potential
The first order neuron itself has has the graded potential and will itself fire the the action potential towards the central nervous system
What do generator potentials apply to?
Common, applies to all tactile (mechano) receptors in the skin and visceral organs
Receptor potentials
A dedicated receptor experiences a graded potential response that varies with stimulus intensity–> Stimulates action potentials in adjacent first order neuron
What are receptor potentials used by?
Relatively rare, used by gustatory and photoreceptors
Tactile (touch) receptors
Where generator potentials can start
Often “free nerve endings” (i.e. dendrites) that transduce mechanical stimuli
What do deep pressure receptors sense?
Vibration
What do slowly adapting touch receptors sense?
Rough?
Skin averages ~ ___ tactile receptors/ square inch
400
Where are afferent tactile signals processed / perceived?
Primary somatic sensory cortex
Small receptive fields
Surface area of skin occupied by a single receptor
Small receptive fields
Has many MORE neurons in the primary cortex
(surface area of skin occupied by a single receptor)
-appears larger on “map”
ex: fingers
Large receptive fields
Represented by fewer neurons in the cortex, appear smaller
ex: trunk
Smaller receptive fields allow for what?
Very fine spatial resolution of sensations.. impacts two-point discrimination
If receptors are numerous and close together, leads to
Small receptor fields
For generator potentials in a tactile receptor, transduction leads to this type of membrane potential in the dendrite of the first order sensory neuron
Graded potential
THERE ARE NOT ACTION POTENTIALS IN DENDRITES
What are nociceptors stimulated by?
Thermal, mechanical or noxious (potentially damaging) stimuli
Chemical stimuli of nociceptors
Injuries locally release prostaglandins and other substances that stimulate nociceptors
Mechanical stimuli of nociceptors
Severe distention (gut, joints), skeletal muscle spasms or a “charlie horse”
What do the stimuli of nociceptors initiate?
Generator potentials in free nerve endings in skin and visceral organs
What happens after there is a generator potential in nociceptors?
AP delivered to peripheral NS nerves, ultimately to CNS tracts in DORSAL HORN and GANGLION of the spinal cord
The more painful the stimulus…
The faster the action potentials fire from nociceptors
What do nociceptors express that facilitate Ca2+ release at their presynaptic junctions?
μ-opioid receptors
Ca2+ release in nociceptors (presynaptic junction) is essential for…
Substance P neurotransmitter release
What does morphine do in pre-synaptic neuron?
It blocks the Ca2+ release in nociceptors in presynaptic junction which stops AP propogation
POSTsynaptic sensory neurons in the dorsal horn also express…
μ-opioid receptors
What do μ-opioid receptors normally do in the post-synaptic sensory neurons?
Depolarize the cell..
What does morphine do in the post-synaptic neuron?
Blocks the μ-opioid receptors and hyper polarizes the post-synaptic dendrite
What natural substance can work in a similar fashion to morphine?
Endogenous endorphins (enkephalins and dynorphins)
Propioreceptors provide information about…
- The position and rate of movement of body parts
- The weight of held objects (golgi tendon organs)
- The range of movement of a joint
Propioreceptors are located in
- Deep skin, joints and muscles (muscle spindles and Golgi tendon organs)
- Vestibular organ (ears)
Where is propioreceptive info typically processed?
- First in the cerebellum (balance, coordination)
- Ultimately perceived as your body sense in the cerebral cortex
Propioreceptors are (tonic / phasic)
Tonic
Never fatigue
Nociceptors are (tonic / phasic)
Phasic
Fatigue
Trigenimothalamic tract
Conveys pain, tickle itch and temperature from face to contralateral cortex (neck up)
The Trigenimothalamic is a _____ neuron system
3
Trigenimothalamic tract: primary
Trigeminal (fifth) nerve from face, nose, mouth to brainstem
Trigenimothalamic tract: secondary
Crosses to opposite side (decussates) in brainstem, enters trigenimothalamic tract, ascends to thalamus
Trigenimothalamic tract: tertiary
Thalamus to somatosensory cortex
Spinothalamic tract
Conveys pain, tickle, itch and temperature (from body to contralateral cortex)
Neck down
The spinothalamic tract is a ____ neuron system
3
Spinothalamic tract: primary (1st order)
From body periphery to posterior (dorsal) horn of spinal cord
Spinothalamic tract: secondary (2nd order)
Crosses to opposite side (decussates) in spinal cord, enters spinothalamic tract, ascends to thalamus
Spinothalamic tract: tertiary (3rd order)
Thalamus to somatosensory cortex
Spinocerebellar system
- Carries tonic propioreceptive information to cerebellum (unconscious)
- Coordinates movement of major motor units (cerebrum) effecting major muscle groups
The spinocerebellar is a ____ neuron pathway
2
Spinocerebellar system: steps
- Primary neuron to spinal cord
- Secondary neuron to ipsilateral cerebellum
Does the spinocerebellar system decussate?
NO
: The vestibular organs in your inner ear contain numerous ___________ that communicate information about body position & balance to the
___________
Proprioceptors / cerebellum
Where are tactile inputs processed?
Primary somatic sensory cortex
