Unit 3A Sensory Systems

special senses

vision, hearing, taste, smell, equilibirum

somatic senses

touch, temp, pain, itch, proprioception

somatic stimuli

muscle length and tension, proprioception

sensory pathways

stimulus → receptor transduces stimulus into intracellular signal → aps travel along afferent neuron -< info reaches subcortial integrating/relay centres → info reaches appropriate regions in cortex

receptors for special senses

usually cells that release neurotransmitter onto sensory neurons

receptors with nerve endings enclosed in

connective tissue capsules

chemoreceptor

oxygen, pH, various organic molecules such. as glucose

mechanoreceptors

pressure (baroreceptors), cell stretch (osmoreceptors), vibration, acceleration, sound

photoreceptors

photons of light the

thermoreceptors

varying degrees of heat

each sensory receptor has an

adequate stimulus, type of energy to which it responds to best

mecahnoreceptors respond best to

deformations of membrane that open ion channels

stimulus opens or closes ion channels in receptor cell membrane

directly or via second messenger systems

graded potential mechanism

mostly: open cation channels → influx of Na/Ca → depolarization

sometimes: efflux of K+, → hyperpolarization

receptor potential

change in membrane potential

somatosensory neurons and visual neurons are activated by

stimuli that fall wihtin a certain physical area

cutaneous receptors

patch of skin

photoreceptors receptive field

light falling on area of retina

two afferent neurons in pathway to brain

1. first order primary sensory neuron, directly associated with sitmuli

2. second order, secondary neuron, relays info from first neuron

receptive filed often defined by neurons further up the pathway

sensory input can then be gathered from more than one primary sensory neuron

receptive fields for primary sensory neurons often

overlap

the primary sensory neurons

converge on one secondary sensory neuron

convergence allows summation of multiple stimuli

creating larger receptive fields

the receptive fields of three primary sensory neurons overlap to form

one large secondary receptive field

smaller receptive fields → better two-point discrimination

the two stimuli activate separate pathways to the, the two points are perceived as distinct stimuli

somatic senses, hearing, vision, taste to appropriate cortex AFTER

processing in thalamus

olfactory goes

directly to brain, olfactory bulb → olfactory cortex

equilibrium pathways project primarily to

cerebellum, minor input to thalamus

integration of visceral sensory info

mostly integrated in brain stem and spinal cord, does not usually reach conscious perception

• completely subconscious → blood pressure

• can reach consciousness → fullness (pressure), pain

sensations are decoded and procesesed in the CNS

all stimuli converted to graded potentials → APS

• all aps are identical

how are diff sensations distinguished

cns must be able to decode:

• type of stimulus → modality

• location

• intensity

• duration

sensory modality is determined by

type of neuron activated and where pathway terminates in brain

labelled line coding

adequate stimulus for that receptor type, brain associated info from that receptor type w/ that modality, e.g touch receptors → percieved as touch

location is coded according to

which receptive fields are activated

touch receptors from a particular part of body project to

a specific location in somatosensory cortex

hair cells in ear respond to diff frequencies but no

receptive fields relating to lcation of sound source, brain uses various strategies to localize origin of sounds, including differences in time of arrival and level in each ear

stimulus intensity coded by

1. number of receptors activated (population coding)

   • diff thresholds for stimulation among group of receptors

   • with low intensity stimulus, most sensitive (lowest threshold) receptors recruited first

   • as stimulus intensifies more receptors activated

2. frequency of aps coming from individual receptor cells

   • frequency of APs increase with stimulus intensity, up to max that the axon can transmit

coding for stimulus intensity and duration

1. receptor potential strength and duration vary with stimulus

2. receptor potential is integrated at the trigger zone

3. frequency of aps is proportional to stimulus intensity, duration of a series of aps is proportional to stimulus duration

4. neurotransmitter release varies with pattern of APs arriving at axon terminal

tonic receptors

slowly adapting, responding throughout stimulus

phasic receptors

rapidly adapt to a constant stimulus and turn off

superficial location and small receptive field

Merkel’s Disks and Meissner’s Corpuscles

Deep location and Large receptive field

Pacinian Corpuscles and Ruffini’s Corpuscles

Rapid adaptation

Meissner and Pacinian Corpuscles

Slow adaptation

Merkel’s disk & Ruffini’s Corpuscle

function of Meissner’s Corpuscle

beginning and end of fine touch/pressure f

function of Merkel’s disk

sustained touch/pressure, texture

Function of pacinian corpuscle

beginning and end of crude touch/vibration

function of Ruffini’s corpuscle

sustained crude touch/ vibration/ stretch

function of free nerve endings in cutaneous sensory receptors

pain, temp, hair movement

why are nociceptions not called pain receptors

“pain” is a sensation processed/percieved in brain rather than a stimulus

nociception is mediated by free nerve endings expressing

ion channels that respond to a variety of strong stimuli, chemical/mechanical/thermal

pain is mediated via release of local chemicals

K+, histamine, prostaglandins, serotonin, substance P can either directly activate nociceptors or sensitize them (inflammatory pain)

nociception mediated by

transient receptor potential (TRP) channels

28 different

trp channels across the animal kingdom

trp expressed on membranes of many diff cell types and mediates

variety of sensations including pain, heat/warmth, cold, some tastes, pressure, vision, osmotic pressure, stretch

trp are RELATIVELY

non selective cation channels (Na, Ca, Mg)

TRPV (vanilloid receptors) - 6 types in humans

receptors in this group respond to

1. temp (receptor subtypes have diff ranges)

2. pepper, allicin (garlic), clove oil, thyme, oregano, wasabi,

3. menthol, peppermint

TRPV1 responds to

heat, capsaicin, mustard, wasabi, H+

information from nociceptors can follow several pathways

1. spinal reflexes

2. ascending pathways to cerebral cortex

   • info sent to limbic system & hypothalamus

   • emotional rxns

   • autonomic responses (nausea, voniting, sweating)

different types of pain travel on

different fibre types

visceral pain; refereed pain often

poorly localized, percieved to be in distant parts of body or on surface

proprioceptors

receptors that sense changes in join movements, muscle length and tension, and send info to cns

depending on appropriate response, cns activates motor neurons to make

motor units contract, or activates inhibitory interneurons to inhibit motoro neurons → muscles relax

muscle spnidles

monitor muscle strength

golgi tendon organs

monitor muscle tension

join receptors

mechanical distortion as bones are repositioned

each spindle consists of

3-12 intrafusal muscle fibres arranged in parallel to extrafusal fibres (almsot) 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

muscle spindles mediate stretch reflexes

induces contraction when muscle stretched and tends to maintain muscle at a constant length

muscle spindles unloaded when muscle shortens

unless “tighetened up” by contraction of its intrafusal fibres (innervated by gamma motor neurons) alpha-gamma coactivation

golgi tendon organ located

between muscle fibres and tendon “in series” with msucle fibres

whether isotonic or isometric, contraction of muscle causes tendon and

GTO to stretch, most sensitive to isometric contraction

gto relatively insensitive to muscle strentch, changes in length

monitors tension → force of contraction

sensory info from gtos combines with info from

spindles and joint receptors in cns integrating centres, monitoring/control of posture and movement

pain, temp, and crude touch cross midline in spinal cord

ascend via spinothalamic tracts

fine touch, vibration and proprioception ascend in dorsal columns and

cross midline in medula

somatosensory pathways

synapse in the thalamus

smooth and cardiac muscle, glands, adipose tissue controlled

by autonomic neurons

classification of neural reflexes

1. according to effector

2. integrating centre

3. number of neurons in pathway

monosynaptic (only afferent and efferent neurons)

somatic motor reflexes only

polysynaptic

autonomic reflexes, reflexes involving interneurons

some autonomic reflexes are integrated in spinal cord

• can often be modulated by signals from higher centres

• inhibition by higher centres can be a learned response

skeletal muscle reflex - proprioception monitor

position of limbs in space, relative position of body parts, effort exerted in liftingm holding objects - muscle length/stretch, muscle tension (tone), joint angles

skeletal muscle reflex - proprioception integrating centre

CNS, via networks of excitatory and inhibitory neurons, integrated within spine and/or higher brain regions

skeletal muscle reflex - proprioception efferent pathway

somatic motor neurons (alpha motor neurons)

skeletal muscle reflex - proprioception effectors

contractile skeletal muscle fibres (extrafusal msucle fibres)

body must continuously adjust its position to compensate for

differences between intended movement and actual one

muscles CANNOT communicate with each other

all coordination mediated via cns

muscle reflex primarily driven by external stimuli

mostly handled at level of spinal cord or brain stem with modulation by higher centres

voluntary movement is most complex, integrated in cerebral cortex

learned movements can improve with practice - become subconscious

parkinsons disease results from

death of dopamine-secreting neurons in a particular region of the basal ganglia

motor symptoms of parkinsons disease include

tremor at rest, slowness of movement, rigidity (increase in muscle tone) and many non-motor effects as well

main treatment for parkinson’s disease

replacing dopamine with L-dopa (precursor that crosses blood-brain barrier)