Unit 3a - Sensory Systems Flashcards
What is proprioception?
sensory information from skin and muscle/joint receptors
What are simple receptors?
neurons with free nerve endings, and may have myelinated or unmyelinated axons
What are complex neural receptors?
have nerve endings enclosed in connective tissue capsules
What are special sense receptors?
cells that release neurotransmitter onto sensory neurons, initiating an action potential, eg. hair cell
What are chemoreceptors?
respond to chemical ligands that bind to the receptor
What are mechanoreceptors?
respond to various forms of mechanical energy
What are photoreceptors?
respond of light
What are thermoreceptors?
respond to temperature
What does each sensory receptor have?
an adequate stimulus (type of energy to which it responds best)
although sensors are specific for one form of energy, they respond to most other forms if the intensity is high enough
What does the stimulus do to receptors?
stimulus opens or closes ion channels in receptor cell membrane (directly or via second membrane systems)
they mostly open cation channels which results in influx of Na or Ca (depolarization)
sometimes there is an efflux of K (hyperpolarization)
What is receptor potential?
change in sensory receptor membrane potential (graded potential)
What is a receptive field?
a certain physical area which stimuli must full on to activate somatosensory and visual neurons
eg. photoreceptors - light falling on area of retina
are often defined by neurons further up the pathway (sensory input can then be gathered from more than one primary sensory neuron)
What are the two afferent neurons pathways to the brain?
- first order (primary) sensory neuron which directly associates with stimuli
- second order (secondary) neuron which relays information from first neuron
What do many primary neurons converge onto?
a secondary neuron, so the secondary receptive field is very large
- convergence allows summation of multiple stimuli
Does convergence (large receptive fields) have two-point discrimination?
no, the two stimuli fall within same secondary receptive field so only one signal goes to brain and is perceived as a single point
Does smaller receptive fields have better two-point discrimination?
yes, the two stimuli activate separate pathway to the brain, so the two points are perceived as distinct stimuli
Where is sensory information integrated in?
the CNS
1. olfactory pathways from the nose project through the olfactory bulb to the olfactory cortex
2. most sensory pathways project to the thalamus. the thalamus modifies/processes and relays information to appropriate cortex
3. equilibrium pathways project primarily to the cerebellum
summary: smell smth, goes to the bulb, then the thalamus, then the cortex
What are the sensory pathways for visceral sensory information? (neurons that monitor the internal environment and organ systems)
are mostly integrated in brain stem and spinal cord
does not usually reach conscious perception
eg. completely subconscious (blood pressure)
How are sensations decoded and processed in the CNS if all stimuli converted to graded potentials to APs are all identical?
CNS must be able to distinguish 4 properties of a stimulus: type of stimulus (modality), location, intensity, and duration
How is sensory modality determined?
determined by the type of neuron activated and where the pathway terminates in the brain
How is location coded?
according to which receptive fields are activated
eg. touch receptors from a particular part of body project to a specific location in somatosensory cortex
What is lateral inhibition and what does it do?
the ability of an excited neuron to reduce the activity of its neighbours (inhibits neurons farther from stimulus)
it enhances contrast and makes a stimulus easier to perceive
eg. the responses of primary sensory neurons A,B,C are proprotional to the intensity of the stimulus in each receptor field, secondary sensory neuron B inhibits secondary neurons A and C, creating grater contrast between B and its neighbours
How is stimulus intensity coded?
- number of receptors activated (population coding)
- different thresholds for stimulation among group of receptors
- with low intensity stimulus, most sensitive (lowest threshold) receptors recruited first
- as stimulus intensifies, more receptors activated - frequency of APs coming from individual receptor cells
- frequency of APs increases with stimulus intensity, up to maximum that the axon can transmit
How is duration coded?
longer or stronger stimuli release more neurotransmitter
duration of a series of action potentials is proportional to stimulus duration
What are the two types of receptors that adapt (cease to respond) to sustained stimulation?
- Tonic receptors (slowly adapting, respond throughout stimulus)
- Phasic receptors (rapidly adapt to a constant stimulus and turn off)
What are the types of sensory modalities?
touch, temperature, nociception (pain/itch), proprioception (sensory information from skin and muscle/joint receptors)
Where does pain, temperature, and coarse touch cross the midline?
spinal cord
Where does fine touch, vibration, and proprioception cross the midline?
in the medulla
What is the function of the Merkel’s disk receptor?
sensing sustained touch/pressure, texture in the skin
What is the function of the Meissner’s corpuscle receptor?
sensing beginning and end of fine touch/pressure in the skin
What is the function of the Ruffini’s corpuscle receptor?
sensing sustained touch/vibration/stretch in the skin
What is the function of the Pacinian corpuscle receptor?
sensing beginning and end of touch/vibration in the skin
What is the function of the free nerve ending receptors?
sensing pain, temperature, and hair movement in the skin
What is nociception?
pain, and is found in many tissues, not just skin
What is pain?
a sensation rather than a stimulus
How is nociception mediated?
by free nerve endings expressing ion channels that respond to a variety of strong stimuli (causes that have the potential to cause tissue damage)
How is pain mediated?
is mediated via release of local chemicals (K+, histamine, prostaglandins, serotonin, substance P)
- can either directly activate nociceptors or sensitize them
What are TRP (Transient Receptor Potential) Channels?
mediates a variety of sensations including pain, heat/warmth, cold, some tastes, pressure, vision, osmotic pressure, stretch, etc
relatively non-selective cation channels (Na, Ca, Mg)
What is somatic pain?
information from nociceptors can follow several pathways: spinal reflexes or ascending pathways to cerebral cortex (info is also sent to limbic system and hypothalamus which results in emotional reactions and autonomic responses like nausea, vomiting, sweating)
different types of pain travel on different fibre types
What is visceral/referred pain?
pain in internal organs that is often sensed on the surface of the body
- often poorly localized, and perceived to be in distance parts of body, or on surface
How do you classify neural reflexes?
- according to effector
- skeletal muscle (controlled by somatic motor neurons)
- smooth and cardiac muscle, glands, adipose tissue (controlled by autonomic neurons) - according to integrating centre
- spinal reflexes (integrated in spinal cord)
- cranial reflexes (integrated in the brain) - number of neurons in pathway
- monosynaptic reflex (somatic motor reflexes only)
- polysynaptic reflex (autonomic reflexes)
What are autonomic (visceral) reflexes?
polysynaptic, with at least one synapse in the CNS and another in the autonomic ganglion
- some are spinal reflexes and some are integrated in brain
- can often be modulated by signals from higher centres
- inhibition by higher centres can be a learning response (holding pee)
- emotional stimuli can be converted into visceral responses (so nervous you want to pee)
What is the skeletal muscle reflex monitor?
proprioception (position of limbs in space, relative position of body parts)
- effort exerted in lifting and holding objects
What is the efferent pathway of skeletal muscle reflexes?
somatic motor neurons
What are proprioceptors?
receptors that sense changes in joint movements, muscle length, muscle tension, and send info to CNS
depending on appropriate response, CNS activates motor neurons to make motor units contract, or activate inhibitory interneurons to inhibit motor neurons then muscles relax
What does muscle spindles do (a type of proprioceptor)?
monitor muscle stretch, and sends information about muscle stretch to the CNS
- they are buried among the extrafusal fibers of the muscle
- each spindle consists of 3-12 intrafusal muscle fibres arranged in parallel to extrafusal fibres
- almost every muscle in body has spindles
- most sensitive to muscle stretch (increased muscle length)
-tonically active, sending steady stream of APs even at resting length (CNS is informed about muscle tone) - mediate stretch reflexes (induces contraction when muscle stretched and tends to maintain muscle at a constant length)
- unloaded when muscle shortens unless ‘tightened up’ by contraction of its intrafusal fibres
What does Golgi tendon organs do (a type of proprioceptor)?
monitor muscle tension (force of contraction), found between muscle fibres and tendon
- whether isotonic or isometric, contraction of muscle causes tendon and GTO to stretch (most sensitive to isometric contraction)
- relatively insensitive to muscle stretch (changes in length)
- sensory info from GTOs combines with info from spindles and joint receptors in CNS integrating centres (monitors/controls of posture and movement)
What is withdrawal (flexion) and crossed extensor reflex?
a reflex that helps maintain balance when one foot is lifted from the ground
1. painful stimulus activates nociceptor
2. primary sensory neuron enters spinal cord and diverges
3 cc to brain to inform on sensation (pain) and postural adjustment
4. flexation and relaxation of appropriate muscles move foot away from pain
5. crossed extensor reflex supports body as weight shifts away from painful stimulus
