Afferent Nervous System

Photoreceptors

Sensory neurons for vision

Mechanoreceptors

Big category for both somatic and visceral systems

Somatic: Muscle and joint stretch; special sense: hearing and balance via distortion/bending of hairs on the cell

Visceral: Stretch of hollow organs

Proprioceptors

Type of mecahnoreceptor that gives information of position of muscles, joints, and tendons.

Thermoreceptors

Heat and cold

Osmoreceptors

Detects solute concentration by examining concentration of various components

Chemoreceptors

Detect chemical in the body. Taste and smel, respiration O2 and CO2, glucose, pH

Nociceptors

Pain, sensitive to tissue damage

Interoceptors

Transduce signals from inside of the body e.g., pH , blood pressure

Exteroceptors

Transduce signals from outside the body e.g., touch, somatic vision, pain, hearing

Chemical signal receptor transduction process

1. Ligand binds to receptor

2. Ca2+ channels open inside and outside of the cell

3. Calcium causes vesicles to fuse with membrane

4. NT releases proportional to the amount of stimulation of receptor

5. Receptor potentials are only graded potentials in receptors

Only graded potentials

Primary Sensory Pathway

Pathway of signal to cortex

Primary sensory neuron

Receptor to spinal cord

Secondary sensory neuron

Spinal cord to thalamus (interneuron)

Tertiary sensory pathway

Thalamus to sensory cortex

Secondary sensory pathway

Transition of signal to subcortical regions, e.g., hyporthalamus for endocrine responses

Convergence of sensory neurons

Ascending pathways for somatosensory information often converge.

Primary sensory neurons converge on a single secondary sensory neuron. The secondary sensory neuron then has a receptive field made up of the primary sensory neurons that synapse on it

Receptive field

The area around a receptor in which the receptor can detect stimuli

Perception of sensory information

Based on several factors:

1. Sensory acuity: Stimulus location, type and intensity accurately

2. Stimulus intensity: Frequency and population code

3. Duration

Receptive field

Region of sensory surface sensed by a neuron. Area around a receptor in which the receptor can detect stimuli

How are sensory acuity and convergence related?

Sensory acuity decreases the more primary sensory neurons are converging on a secondary sensory neuron

Lateral Inhibition

1. Stimulus detected

2. Primary neuron response is proportional to stimulus strength

3. Pathway closest to stimulus inhibits neighbors

4. Inhibition of lateral neurons enhances perception of stimulus

Creates high contrast between signals between adjacent areas to make it easier to accurately locate the stimulus

Tonic receptor

Slow adapting or not adapting at all

1. Respond during the entirety of stimulus

2. Indicate presence of stimlus

Nociceptors, proprioceptors

Phasic receptors

Fast adapting. Responds to beginning and ending of stimulus

Indicate change in stimulus

e.g., pressure mechanoreceptors

Types of nociceptors

Mechanical: Crushing, cutting, pinching

Thermal: Temperature extremes

Multimodal: Activated by all mechanical and temperatire stimuli as well as chemicals released during tissue damage and inflammation

Pain pathways

Fast: Reflex movement away from painful stimulus

Slow: Behavioral response

Fast pain

Detected by A-delta fibers

Fast signal transmission

Releases NT glutamate

Primary function is to trigger reflex arc but also travels to brain ( two secondary sensory neuron pathways)

Easily localized

One modality per primary sensory neuron

Slow Pain

Detected by C-Fibers

Slow signal transmission

NT released is substance p

NO reflex arc

Primary and secondary pathways are to brain

Polyodal nociception

Difficult to localize

Long lasing

Role of prostaglandins in pain

Hormone-like substances

Increase response to painful stimuli (More APs)

Made by enzyme - COX

NSAIDS (non steroidal anti inflammatory drugs e.g., ibuprofen) inhibit COX

Slow Pain Pathway

Both primary and secondary pathways

Secondary pain pathway rerouted to subcortical area directly to elicit behavioral responses like emotional or motivational

C fibers can directly stimulate secondary pain fiber or indirectly stimulate them through the inhibition of gate interneurons

Gate interneuron

Provides pre-synaptic inhibition to C fibers (involved in slow pain pathway). Tonically active - continually represses slow pain signals

C-fibers (slow pain) inhibits the inhibitory gate interneuron

Anaglesic effect of non-painful touch

A-beta fibers stimulated by non-painful touches stimulate the gate interneuron to strengthen the inhibition of c-fibers