lecture 6 pns- afferent divison

afferent division

sensory receptors at peripheral endings of afferent neurons 

• bring about graded potentials (receptor potentials) in receptor

sensory receptors

specialized cells that generate receptor potentials in response to a stimulus

mechanoreceptors

responsive to pressure, touch or stretch (physical distortion to plasma membrane) 

thermoreceptors 

sensitive to heat and cold 

photoreceptors

responsive to visible light

chemoreceptors

sensitive to specific chemicals

nociceptors (pain receptors)

sensitive to tissue damage (cutting or burning)

osmoreceptors

detect changes in solute concentrations in ECF (body water balance)

sensory transduction

when a stimulus is converted into an electrical signal. The stimulus opens Na⁺ channels, causing Na⁺ influx and depolarization (receptor potential). A stronger stimulus causes a larger potential. Since there’s no refractory period, signals can summate, and if strong enough, trigger an action potential.

stimulus intensity distinguished by 

1. frequency of AP’s generated in afferent neuron

2. number of receptors and afferent neurons activated within area

frequency of AP’s generated in afferent neuron

• a stronger stimulus produces a larger receptor potential, which makes the sensory neuron fire action potentials more often

• more frequent firing releases more neurotransmitters, signaling greater intensity to the brain

number of receptors and afferent neurons activated

• a stronger stimulus affects a larger area of the receptor field 

• this activates more sensory receptors and therefore more afferent (sensory) neurons 

• when many neurons send signals at once, the brain interprets it as a stronger or more widespread sensation 

adaptation

when a sensory receptor is exposed to a constant, unchanging stimulus for a while, it becomes less sensitive to it. 

• the receptor “gets used to” the stimulus 

• as a result, it fires fewer APs over time. 

tonic receptors 

always active - they continuously send signals to the brain (steady signal)

• they do not adapt or adapt very slowly to a constant stimulus 

• they provide ongoing information about the state of the body

• the frequency of APs reflects the strength of the stimulus 

phasic receptors 

normally inactive (on/off response)

• respond only to changes in stimulus (onset, offset, movement)

• adapt quickly to constant stimulation 

• show a “off response” when stimulus is removed 

• AP frequency reflect change in intensity 

acuity

how clearly you can locate or tell apart a stimulus

receptive field

smaller receptive fields → more precise sensation → greater acuity 

• ex: fingertips can tell 2 points apart easily; your back cannot

lateral inhibition 

strongly activated neurons inhibit (weaker) nearby ones, sharpening the signal and helping locate the exact spot

• ex: when you press one point on your skin, the center feels strongest while the edges feel weaker - your brain can pinpoint the touch 

two-point discrimination 

the ability to tell whether you’re being touched in one spot or two nearby spots at the same time

• depends on receptive field size and receptor density 

lips or fingertips

they have many sensory receptors with small receptive fields 

• when 2 points touch the skin, they activate 2 different receptors, so you can feel 2 separate touches 

back area 

have fewer receptors with large, widely spaced receptive fields

• two nearby touches may fall within one receptive fields, so your brain feels it as one touch instead of two 

first- order sensory neuron

the afferent neuron that directly detects the stimulus (touch, pain, temp) 

• its receptor is located in the skin, muscle, or organ 

• it carries the signal into the spinal cord and synapses with the second-order neuron in the posterior gray horn 

second-order sensory neuron 

located in the posterior gray horn of the spinal cord 

• acts as an interneuron that receives the signal from the first-order neuron

• it then crosses to the opposite side of the CNS ( called decussation) and ascends to the thalamus- the brains sensory relay center 

third- order sensory neuron

found in the thalamus

• carries the signal from the thalamus to the primary sensory cortex in the parietal lobe of the brain 

• once the signal reaches the cortex, the sensation becomes conscious - you actually feel it

a-delta fibers (fast pain)

myelinated → fast signals 

• carry sharp, stabbing pain 

• easy localized, occurs first, short-lasting 

ex: feeling a quick, sharp pain from a needle prick or paper cut 

c fibers (slow pain)

unmyelinated → slow signals

• carry dull, burning, or aching pain 

• poor localized, occurs second, long lasting (chronic) 

ex: the throbbing, burning ache that follows after the initial sharp pain