6.0 Afferent pathways: Somatosensory system

What is the pathway from stimulus to response in the nervous system?

1. Stimulus (input)

2. Sensory division (PNS) → carries signals from receptors to CNS

3. CNS (brain & spinal cord) → integration and processing

4. Motor division (PNS) → sends signals to effectors

5. Response (output)

👉 Key idea:
Input → Integration → Output

What are the main divisions of the nervous system and their functions?

• Central Nervous System (CNS)

• Peripheral Nervous System (PNS)

• Motor (efferent)

Motor (efferent) division

• Carries signals from CNS to body

• Somatic nervous system:
  → Controls skeletal muscle (voluntary)

• Autonomic nervous system:
  → Controls smooth muscle, cardiac muscle, glands (involuntary)

Peripheral Nervous System (PNS)

• Carries information to CNS from receptors

Central Nervous System (CNS)

• Brain + spinal cord

• Function: processing, integration, decision-making

The information of different sensations, is captured by 

• Sensory receptors (PNS) → sent to diffeerent processing information areas in CNS

Sensory processing begins when

• information reaches CNS & sensory information is encoded as electrical signals

The CNS must be able to identify four characteristics of sensory information:

• Type/nature of the stimulus

• Location

• Intensity

• Duration

Type/nature of the stimulus:

• There is a receptor-stimulus association (e.g., photoreceptor, mechanoreceptor, etc) determines the interpretation of sensory information, regardless of the type of stimulus that caused the stimulation of the receptor.

• Example: “seing stars” when rubbing eyes.

Location

• encoded by the receptive fields

Intensity

• is determined by the frequency of action potentials

Duration

• is encoded by how long the receptor’s is activated, tonic receptors keep firing as long as the stimulus persists.

SENSORY RECEPTORS (types)

• Mechanoreceptors

• Thermoreceptors

• Nociceptors

• Photoreceptors

• Chemoreceptors

• Osmoreceptors

Osmoreceptors

•  changes in the blood’s osmolarity

Chemoreceptors

• changes in the chemical molecules (mouth, nose, & internal fluids).

Photoreceptors

• light (in the retina)

Nociceptors

• painful stimuli (tissue damage)

Thermoreceptors

• changes in temperature

Mechanoreceptors

• mechanical stimuli: touch, pressure, hearing, balance.

Location of the stimulus

• Exteroceptive

• Proprioceptive

• Interoceptive

Interoceptive receptors

• They are in visceral organs, blood vessels

• Provide information regarding the internal body environment (O2, osmotic pressure...)

Proprioceptive receptors

• They generate conscious information about body position & movement (kinesthesia)

• They are in the muscles, tendons & joints

• They respond to stretch, tension & movement in these structures.

Exteroceptive receptors

• Located in the body surface, sensitive to stimuli from the external environment

• Mechanoreceptors, nociceptors, thermoreceptors, photoreceptors, 

Transduction

• Receptors encode the stimulus into an electrical signal.

The phases of transfduction

• Stimulus activates the receptor → Change in membrane permeability to ions (graded pot.)→ threshold reached → AP triggered in sensory neuron.

Stimulus quality.

• Encoded by the type & location of the receptor.

Stimulus intensity

• Encoded by the frequency (rate) of the action potentials.( I I I or IIIIIIIII) & by the number of receptors activated (a stronger stimulus may activate a larger receptor area).

Stimulus intensity

Increased frequency

Activation of more receptors

Tonic (slowly adapting) receptors

• They respond to the onset of the stimulus & keep sending signals as long as the stimulus exists.

• Static property

Static property

• They provide spatial & temporal information about the stimulus (size, shape, duration).

Phasic (rapidly adapting) receptors

• Rapid discharge at stimulus onset & then silence even if the stimulus persists “adaptation”.

• Dynamic property

Dynamic property

• They inform about changes in stimulation, not steady states.

Topic receptor + phasix receptors

Tonic or slowly-adapting receptors:

• Continuously transmit signals as long as the stimulus is present (where, how long).

• Steady and sustained responses throughout the duration of the stimulus.

• Allow continuous monitoring & adjustment of bodily functions.

Examples of Tonic or slowly-adapting receptors

• Baroreceptors, in the walls of certain blood vessels (e.g., carotid), they sense blood pressure.

• Nociceptors, pain.

• Proprioceptors, body position.

Phasic or rapidly adapting receptors

• They respond to changes in the stimulus intensity or quality.

• Adapt to a constant stimulus, by decreasing their response over the time.

• This allows these receptors to sense new or changes in the stimuli.

•  They filter out continuous stimulus & focus in detecting relevant changes in the environment.

Examples of Phasic or rapidly adapting receptors:

• Olfactory receptors.

• Pacinian receptors, involved in touch & vibration.

In order for the cerebral cortex to exercise its sensory functions

• impulses must first be conducted to the sensory & association areas.

SOMATOSENSORY PATHWAYS

• PNS (1st order neuron) → Medulla or spinal cord (2nd order neuron) → Thalamus (3rd neuron) → Sensory cortex

1st order neuron

• Dendrites: Sensory receptors

• Soma: Dorsal root ganglion

• Synapse with the 2nd order neuron in the medulla oblongata

2nd order neuron

• Soma in the medulla oblongata (gracile nucleus for lower body, cuneate nucleus for upper body)

• Decussation (= opposite side crossing)

• Synapse with 3rd orden neuron: thalamus

3rd order neuron

• Soma: into the thalamus

• Goes up to the 1st somatosensory cortical layer (also associative cortex).

POSTERIOR /DORSAL COLUMN TRACT

• Fine touch, vibration & proprioception

SPINOTHALAMIC TRACT

• Pain, temperature (lateral), coarse touch, & itching and pain (anterior)

TRIGEMINOTHALAMIC TRACT

• Mechanosensitive information of the face