Lecture 11: Sensory Neurons

3 Broad Nervous System Function

  1. Sensory Input

  2. Integration

  3. Motor output

The Peripheral Nervous System Sensory Afferent Information is Received by Sensory Receptors.

Sensory Receptors Can be classified by:

  • Location

  • Structure

  • Function

Sensory Receptor Stimulation initiates a localized depolarization- aka- Receptor Potential

The Receptor Potential can generate an action potential if it reaches threshold

The exception are the Rods and Cones of the Retina

Sensory receptors can also be divided into two classes based on how they adapt to continuous stimulation

  1. Tonic Receptors- Slowly adapting R that Continue to fire as long as the stimulus is present

    1. ex) Baroreceptors and Proprioceptors

  2. Phasic Receptors- Rapidly adapting R that fire when they first receive the stimulus, but cease firing if the strength of the stimulus remains constant. Once a stimulus reaches a steady intensity, these receptors adapt and turn off

    1. Ex) Olfactory Receptors

Sensory Receptors by Function

  1. Mechanoreceptors

    1. Skin Receptors- Specialized R for Light Touch, Deep Touch, Texture, Stretch… etc

    2. Proprioceptors- Found in Muscles and Tendons- Help the Cerebellum determine where our body is in space

    3. Baroreceptors – Sense changes in Arterial Pressure- Found in the Carotid Sinus and Aortic Arch

  1. Thermoreceptors

    1. Cold Receptors- More of these in the Mammal Body

    2. Warm Receptors

  1. Chemoreceptors

    1. Taste- Gustatory Epithelial Cells- aka Taste Buds

    2. Smell- Olfactory Sensory Neurons- Olfactory Epithelium

    3. Osmolality- Detect Osmolarity Changes in the blood

    4. Carbon Dioxide Receptors- Detect Carbon Dioxide concentration changes- located in the Medulla and the Aortic/Carotid bodies

  1. Photoreceptors

    1. Light R- Rods and Cones in the Retina (eye)

  2. Nociceptors

    1. Noxious Stimuli R- Detect Pain and Itch- Free Nerve Endings in the Skin, Muscle, Joints and Organs. Are classified by the type of Nerve Fiber

Sensory Receptors located in the Skin Mechanoreceptors, Thermoreceptors and Nociceptors

Mechanoreceptors

  • Meissner’s Corpuscle: Touch

  • Pacinian Corpuscle: Vibration

  • Ruffini Organ: Stretch

  • Merkel Disks: Steady Pressure and Texture

  • Free Nerve Endings: Pain, Itch, Cold and Warm

Thermoreceptors

Receptor Structure: Free Nerve Endings

These use cation channels called: Transient Receptor Vanilloid 1 Channels (TRPV1)

There are distinct receptors for Cold and Warm → One receptor can NOT detect both

Locations are:

  1. Peripheral (skin)- Sense surface temperatures; Highest concentration in the face and ears

  1. Central (Viscera, Spinal Cord and Hypothalamus) Monitor the body’s core Temp Cold Receptors are found in higher concentrations in the skin than Warm R- This is why we are more sensitive to cold

Osmoreceptors

Classified as either Central or Peripheral based on location

Central Osmoreceptors → Primarily found in the Anterior Hypothalamus

  • This specialized area of the hypothalamus is a No Blood-Brain-Barrier Zone to allow communication about Osmolality status

These Osmoreceptors contain 2 specialized channels:

  1. An osmotically activated ion channel called “Transient Receptor Potential Vanilloid Channel” or TRPV1

  2. A receptor that is sensitive to Angiotensin II called “Angiotensin Receptor Type 1” or AT1R

When Serum Osmolality increases above the threshold limit, the TRPV1 channel stimulates the thirst response of the hypothalamus.

Variations in blood volume as a result of increased osmolarity triggers the kidneys to release Renin (which increases Angiotensin II levels) which regulates ADH release from the Hypothalamus-Posterior Pituitary axis. The AT1R channel is stimulated by an increase in Angiotensin level

Carbon Dioxide Chemoreceptors

1) Central Chemoreceptors-

  • Located mainly in the Medulla but also other areas of the brain that contain respiratory nuclei

Detect rising CO2 levels indirectly, by detecting increases in H+ ions in the CSF

CO2 crosses the Blood-Brain-Barrier rapidly, causing a fall in pH of the CSF

(WHY?)→ Carbonic Anhydrase Pathway that causes a release of H+ ions

It is the H+ ions in the CSF that directly stimulate the central chemoreceptors and increases ventilation rates

They are responsible for most of the Respiratory response to change in CO2

2) Peripheral Chemoreceptors-

Located in the Aortic Arch and Carotid Bodies

Respond to levels of Arterial Oxygen

They can also respond to high CO2 and low pH, but to a lesser extent

The carotid bodies respond to arterial hypoxia by sending signals to the medulla to increase breathing rate and depth

The carotid bodies are more sensitive to changes in CO2 and pH than the aortic bodies

The aortic bodies are connected to the cardiovascular centers of the medulla and when they detect hypoxia (PO2 < 60 mmHg) they signal the medulla to increase breathing rate and depth

Specialized Peripheral Chemoreceptors

1. Olfactory ChemoR

These R respond to a Ligand called an Odorant

These are GPCR that are found on the cell membranes of the CILIA of the Olfactory R Neurons

The Signal from the Odorant molecule is transferred to an electrical signal by the opening of Chemically gated Na/Ca channels

Transduction of olfactory information occurs in the olfactory epithelium

Binding of the Odorant activates the GPCR that contains a special Alpha Subunit called G alpha “olf”

Olfaction has 2 Main Components

  1. Odorant- Induced signal Transduction Pathway

  2. Olfactory Adaptation or Odor Fatigue

Ca2+ in this pathway: Know what calcium and calmodulin are doing in these pathways!!

Gustatory Pathway

Each taste cell is specialized to detect only one type of Ligand.

Taste Receptor Cells (TRC) are present in oval clusters

On the tongue called Taste Buds

Different TRCs are shaded in different colors

Taste ligands create Ca 2+ signals that release Serotonin or ATP

Sodium Taste Receptor Cell

Type III Taste Receptor Cell

Type II Receptor Cell

Sweet, Umami or Bitter Taste Receptor Cell

These all fall under the classification of a Type II Receptor Cell

What differentiates them is the G-Protein Coupled Receptor that is present.

  • Sweet TRC have T1R1 GPCR

  • Umami TRC have T1R1 GPCR

  • Bitter TRC have T2R GPCR

All three signals work by activating cAMP and PLC- which opens VR K Channels and causes IP3 to trigger Ca release- Ca release then triggers ATP release which activates the Neuron underneath

Phototransduction and the Retina

The Retina is the sensory organ of the eye

Neurons of the Retina are organized into 5 layers

1) Photoreceptors

2) Bipolar Cells

3) Ganglion Cells

4) Amacrine Cells

5) Horizontal Cells

Sensory information passes from the photoreceptors to

the bipolar neurons then the ganglion cells whose axons

form the Optic Nerve

Photoreceptors Rods and Cones of the Retina

Photoreceptors are the neurons that convert light energy into electrical signals

Rods

Function well in low light

Used in Night Vision

Black and White Vision

Cones

High-acuity Vision

Color Vision