Lec 6 - General Properties of Sensory System and Somatic Senses

All sensory systems: what do they do (outline)?

Detect a specific type of stimulus, convert it into neural signals (transduction), encode intensity, duration, and location, then transmit along labeled lines to the brain, where perception occurs.

What are the somatic senses topics in lecture 6?

Pathways from receptors to the cortex; pain and pain modulation; gate control model.

Special senses?

Vision, hearing, taste, smell, equilibrium.

Somatic senses?

Touch, temperature, pain, itch, proprioception (proprioceptors).

What does a sensory receptor respond to?

Stimulus as physical energy; the receptor acts as a transducer.

Intracellular signal from a receptor?

Usually a change in membrane potential.

Sensory signaling chain?

Stimulus -> threshold -> action potential to CNS.

Where is sensory information integrated?

In the CNS: cerebral cortex, or acted on subconsciously.

Table 10.1: stimulus processing usually conscious (special senses)?

Vision, hearing, taste, smell, equilibrium.

Table 10.1: stimulus processing usually conscious (somatic senses)?

Touch, temperature, pain, itch, proprioception.

Table 10.1: usually subconscious somatic stimuli?

Muscle length and tension; proprioception.

Table 10.1: usually subconscious visceral stimuli?

Blood pressure; distension of GI tract; blood glucose concentration; internal body temperature; osmolarity of body fluids; lung inflation; pH of cerebrospinal fluid; pH and oxygen content of blood.

Neural receptors types?

Naked ("free") nerves; complex neural receptors encased in connective tissue capsules.

Nonneural receptors?

For four special senses; highly specialized cells associated with sensory neurons.

Accessory structures?

Enhance the sensory system (e.g., somatosensory receptors).

Fig 10.1a: simple receptors?

Neurons with free nerve endings; may have myelinated or unmyelinated axons.

Fig 10.1b: complex neural receptors?

Nerve endings enclosed in connective tissue capsules; e.g., Pacinian corpuscle, which senses touch.

Fig 10.1c: special sense receptors?

Cells that release neurotransmitter onto sensory neurons, initiating an action potential; e.g., hair cell in the ear.

Table 10.2: chemoreceptors?

Stimuli: oxygen, pH, various organic molecules such as glucose.

Table 10.2: mechanoreceptors?

Stimuli: pressure (baroreceptors), cell stretch (osmoreceptors), vibration, acceleration, sound.

Table 10.2: photoreceptors?

Photons of light.

Table 10.2: thermoreceptors?

Varying degrees of heat.

Receptive field?

The physical area where a stimulus activates a neuron.

Which neurons have receptive fields?

Primary sensory neurons and secondary sensory neurons.

Receptive fields frequently?

Overlap.

Neighboring fields may exhibit?

Convergence.

What determines sensitivity?

Receptive field size.

Sensitive areas vs less sensitive areas?

Sensitive areas have smaller receptive fields; less sensitive areas have larger receptive fields.

Test of receptive field size?

Two-point discrimination test.

Fig 10.2a: convergence?

Creates large receptive fields; receptive fields of three primary neurons overlap to form one large secondary receptive field.

Fig 10.2a: what does convergence of primary neurons allow?

Simultaneous subthreshold stimuli to sum at the secondary sensory neuron and initiate an action potential.

Fig 10.2a: two stimuli in the same secondary receptive field?

Perceived as a single point because only one signal goes to the brain; no two-point discrimination.

Fig 10.2b: small receptive fields?

Found in more sensitive areas; less convergence of primary neurons; secondary receptive fields much smaller.

Fig 10.2b: two stimuli in separate pathways?

Activate separate pathways to the brain; perceived as distinct, so there is two-point discrimination (compass points 20 mm apart).

How does sensory information reach the brain?

Spinal cord to brain by ascending pathways; directly to brain stem via cranial nerves.

Visceral reflexes integrated in brain stem or spinal cord?

Usually do not reach conscious perception.

Perceptual threshold?

Level of stimulus necessary to be aware of a particular sensation.

Habituation?

Decreased perception through inhibitory modulation; falls below perceptual threshold.

Brain processing of sensory information?

Each major division of the brain processes one or more types of sensory information.

Fig 10.3: most sensory pathways pass through?

The thalamus on the way to the cerebral cortex; the thalamus modifies and relays information to cortical centers.

Fig 10.3: olfactory pathways?

From the nose through the olfactory bulb to the olfactory cortex.

Fig 10.3: equilibrium pathways?

Project primarily to the cerebellum.

Fig 10.3: cortical areas labeled?

Primary somatic sensory, gustatory, olfactory, auditory, visual cortex.

Sensory modality coding?

Which sensory neurons are activated and where neurons terminate in the brain.

Labeled line coding?

1:1 association of receptor with sensation.

Location of the stimulus coding?

Which receptive fields are activated; lateral inhibition and population coding.

Intensity of the stimulus coding?

Number of receptors activated and frequency coding.

Duration of the stimulus coding?

Duration of action potentials; receptors adapt (tonic vs phasic receptors).

Fig 10.4: how does the brain localize sound?

Using timing differences; sound takes longer to reach the far ear; signals from the near side reach the brain first.

Fig 10.5: lateral inhibition?

Enhances contrast and makes a stimulus easier to perceive.

Fig 10.5: primary neuron response?

Proportional to stimulus strength.

Fig 10.5: how does lateral inhibition work?

Secondary neuron B (closest to stimulus) inhibits neighbors A and C, creating greater contrast; inhibition of lateral neurons enhances perception.

Fig 10.6 Step 1: coding intensity and duration?

Receptor potential strength and duration vary with the stimulus.

Fig 10.6 Step 2?

Receptor potential is integrated at the trigger zone.

Fig 10.6 Step 3?

Frequency of action potentials is proportional to stimulus intensity; duration of the series of action potentials is proportional to stimulus duration.

Fig 10.6 Step 4?

Neurotransmitter release varies with the pattern of action potentials arriving at the axon terminal.

Fig 10.6: longer or stronger stimuli?

Release more neurotransmitter.

Fig 10.7: receptor adaptation?

Receptors adapt to a sustained stimulus.

Tonic receptors?

Slowly adapting receptors that respond for the duration of a stimulus.

Phasic receptors?

Rapidly adapt to a constant stimulus and turn off.

Somatic senses list?

Touch, proprioception, temperature, nociception (pain and itch).

Where do somatic perception pathways project?

To the cortex and cerebellum.

Where are somatic receptors located?

In skin and viscera.

Primary sensory neurons?

Synapse in the CNS with secondary sensory neurons.

Secondary sensory neurons?

Interneurons in the CNS; synapse with tertiary sensory neurons in the thalamus.

Tertiary sensory neurons?

Project to the somatosensory cortex; many project to the cerebellum.

Dorsal column system: sensations?

Fine touch and proprioception (from right side of body in figure).

Dorsal column system: structures?

Dorsal root ganglion (first-order neuron), fasciculus gracilis and cuneatus, nucleus gracilis and cuneatus, medial lemniscus, thalamus (third-order neuron), postcentral gyrus.

Spinothalamic tract: sensations?

Pain and temperature.

Spinothalamic tract: structures?

Dorsal root ganglion (first-order neuron), spinal cord, lateral spinothalamic tract (second-order neuron), thalamus (third-order neuron), postcentral gyrus.

Other tracts listed with the spinothalamic tract?

Spinoreticular, spinomesencephalic, spinohypothalamic tracts.

Fig 10.8 Step 1?

Pain, temperature, and coarse touch cross the midline in the spinal cord.

Fig 10.8 Step 2?

Fine touch, vibration, and proprioception pathways cross the midline in the medulla.

Fig 10.8 Step 3?

Sensory pathways synapse in the thalamus.

Fig 10.8 Step 4?

Sensations are perceived in the primary somatic sensory cortex.

Fine touch, proprioception, vibration: primary, secondary, tertiary?

Primary neuron synapses in the medulla; secondary neuron crosses midline in medulla; synapses with tertiary neuron in thalamus; tertiary neuron terminates in somatosensory cortex.

Irritants, temperature, coarse touch: primary, secondary, tertiary?

Primary neuron synapses in dorsal horn of spinal cord; secondary neuron crosses midline in spinal cord; synapses with tertiary neuron in thalamus; tertiary neuron terminates in somatosensory cortex.

Fig 10.9: somatosensory cortex mapping?

Each body part is represented next to the area of sensory cortex that processes its stimuli; called a homunculus (little man).

Who created the homunculus mapping and when?

Neurosurgeons W. Penfield and T. Rasmussen, 1950 (Wilder Graves Penfield).

Somatosensory cortex space per body part?

Proportional to the sensitivity of that part.

Touch receptors?

Most common receptors in the body.

Touch receptors are stimulated by?

Stretch, steady pressure, fluttering or stroking movements, vibrations, texture.

Where are touch receptors found?

In the skin or deeper regions.

Touch receptor examples?

Pacinian corpuscles, Merkel receptors.

Skin temperature receptors?

Free nerve endings that terminate in subcutaneous layers.

Cold receptors?

Respond to temperatures lower than body temperature.

Warm receptors?

Respond above body temperature to about 45°C.

Pain receptors and temperature?

Activated above 45°C.

What channels do thermoreceptors use?

Cation channels called transient receptor potential (TRP) channels.

Table 10.3: Abeta fiber?

Large, myelinated; 30-70 m/sec; mechanical stimuli.

Table 10.3: Adelta fiber?

Small, myelinated; 12-30 m/sec; cold, fast pain, mechanical stimuli.

Table 10.3: C fiber?

Small, unmyelinated; 0.5-2 m/sec; slow pain, heat, cold, mechanical stimuli.

Nociceptors?

Neurons with free nerve endings that respond to strong noxious stimuli that may damage tissue.

Where are nociceptors found?

Skin, joints, muscles, bones, and viscera; not found in the CNS.

Nociceptor activation?

A reflexive protective response integrated in the spinal cord.

Two types of primary sensory fibers for nociceptors?

Adelta fibers and C fibers.

Pain is?

A subjective perception.

Fast pain?

Sharp and localized; by Adelta fibers.

Slow pain?

Duller and more diffuse; by C fibers.

Itch?

From skin nociceptors; histamine (and other substances) activate C fibers, causing itch.