Chapter 10

What is the purpose of sensory systems?

Taking external information (signals) and turning it into chemical/electrical signals for the CNS to interpret

Define transduction

The process of turning external information into chemical/electrical signals

Stimuli are interpreted by _________ ________ ________

Free nerve endings

Describe the process of stimulus transduction in a simple system

-Stimulus interacts with free nerve endings

-Nerve endings produce a signal that travels down the axon after depolarization

- Signal passed on

What complexity of sensory system has specialized sense receptors?

Complex sensory systems

What is a specialized receptor?

A cell that is not a neuron but releases neurotransmitter on to neurons

Why are accessory structures important to specialized sense receptors?

They are often key to function by enhancing the cells ability to gather information

What are the 4 major groups of sensory receptors?

Chemoreceptors

Mechanoreceptors

Photoreceptor

Thermoreceptors

What do chemoreceptors response to?

Chemical ligands

What do mechanoreceptors respond to?

Mechanical energy (pressure)

What kind of receptors mediates olfaction (smell)?

Chemoreceptor

What kind of receptor is responsible for hearing?

Mechanoreceptor

What do photoreceptors respond to?

Light

How can sensory receptors impact cells?

-Opening ion channels

-Releasing secondary messengers

How do sensory receptors change membrane potential of neurons or cells?

-Opening ion channels

-Releasing secondary messengers

What occurs between a change in membrane potential and an action potential?

A graded potential

What is a receptive field?

The part of sensory space where stimuli activate a neuron

Where do primary neurons typically project?

Secondary neurons

Where do secondary neurons typically extend to?

The CNS

All receptive fields in a region will be the same size.

True or False

False

Receptor fields often vary in size

What is the sensitivity of a system is dependent on?

Size of receptor fields

Arrangement of neurons (divergence or convergence)

Describe the relationship:

Receptor field size and neuron arrangement

and

System sensitivity

Small receptor field and less convergence

=

Greater sensitivity

Where is the ascending pathway to the CNS located?

Between the spinal cord or cranial nerves and brain

Where are visceral reflexes integrated?

Brain stem and spinal cord

Where do visceral reflexes often reach?

The thalamus

Almost all sensors project first to the _________ before passing to the ____________

Thalamus

Cerebrum

Which sense does not first pass through the thalamus? Why?

Olfaction (smell) does not pass the thalamus possibly because it is the oldest evolutionary sense

What does the brains interpretation of sensory information depend on?

Where neurons terminate in the brain

Define labeled line coding

1:1 correspondence of receptors with a sensation

How are vision and taste examples of labeled line coding?

transmitted along their own dedicated, insulated neural pathway

How is sensory information coded?

Action potential frequency

What 2 processes are responsible for the strength of an encoded stimulus?

-Action protentional frequency

-Amount of neurotransmitter release

What are the 4 modalities of the somatosensory system?

Touch

Proprioception

Temperature

Nociception

What is proprioception?

Knowing where your body is in space

What can nociception be broken down to?

Pain and itch

What neurons are the primary sensory neurons?

Pseudounipolar neurons

Where are nerve cell bodies of primary sensory neurons located and where do they extend?

Cell bodies: Dorsal root ganglia

Extend: Dorsal horn of spinal cord

Where do primary sensory neurons synapse?

Secondary sensory neurons specific to the type of sense being experienced

All primary sensory neurons synapse at the same place.

True or False

False

Synapse location is dependent on somatic modality (type of sense)

Fine touch and vibration sensory neurons synapse in the ________ before they __________ and projecting onto the ________

Medulla

Decussate

Thalamus

Where do nociception, temperature, and coarse touch receptors project before passing the medulla?

The spinal cord

Where is the primary somatosensory cortex located?

Parietal lobe

Where does perception of sensation occur?

The primary somatosensory cortex

Fine touch, proprioception, and vibration are integrated in where

spinal cord

Nociception, temperature, and coarse touch are integrated on what side of the spinal cord?

Left

If there is damage to the left side of the spinal cord, what senses would be lost, and which side would they be lost on?

Would these senses still occur on the other side?

Nociception, temperature, and coarse touch would be lost ONLY on the right side of the body.

They would still occur on the left side on the body

How do we know that the somatosensory cortex is highly specialized?

Sensory tracts from different parts of the body have corresponding regions of the cortex

What is a sensory field?

The region of the somatosensory cortex associated with specific body parts

Each sensory field contains columns of neurons. What are they associated with on the somatosensory cortex

Sensory modalities

How were researchers able to determine what regions of the somatosensory cortex corresponded to certain body parts? Why could they do this?

By touching different parts of the brain and asking patients what they felt. They could do this because there are no pain receptors in the brain?

Body parts are equally represented in the somatosensory cortex.

True or False

Explain

False

Body parts with greater sensitivity have greater representation

Which body part has greater sensitivity? Why?

Legs or Hands

Hands because they have greater sensitivity

Describe the relationship between use of body part and size in cortex

The more a body part is used, the larger the size occupied in the cortex

How is plasticity observed in the somatosensory cortex?

The more a body part is used, the larger the size occupied in the cortex

If a body part is used less, the size will eventually reduce

What are the most prevalent receptors in the body?

Touch receptors

What do touch receptors respond to macroscopically?

Physical contact

What do touch receptors respond to microscopically?

Stretching of the plasma membrane to change the membrane potential of the neurons

What 2 things cause different touch receptors to have different properties in how they detect stimuli?

Adaptation to stimuli

Location in the skin

Where are Pacinian corpuscles found and what do they respond to?

Found in deep layers of skin and respond to vibrations

Describe the structure of a Pacinian corpuscle

Free nerve ending surrounded by a capsule of tissue

What kind of receptor is located on superficial layers of skin and responds to steady pressure

Merkle receptors

Where are Meissner's corpuscles located and what do they respond to?

They are located in the superficial layer of skin and respond to flutter and stroking movements

What receptors are located deepest in skin and are sensitive to stretch?

Ruffini corpuscle

What do free nerve endings of touch receptors typically respond to?

Noxious stimuli

Subset wrapped around hair bulb and respond to hair movement

Where are temperature receptors located?

Subcutaneous nerve endings

What are TRP channels?

Transient receptor potential channels: Ion channels that open in response to differences in temperature relative to body temperature

When do the 2 subsets of TRP (transient receptor potential) channels open?

Warm receptors: Open when temp > 37 C

Cold receptors: Open when temp < 37 C

What kind of pain do nociceptors respond to?

Chemical

Thermal

Mechanical

Where are nociceptors located?

Subcutaneous layer of skin

Joints

Muscles

Bones

Internal organs

Where are nociceptors notably NOT located?

CNS

What are the 3 classes of fibers (axons) used by somatosensory neurons?

A beta

A delta

C

Describe the fiber (axon) characteristics of A beta, A delta, and C axons and how this effects their speed

A beta: Large, myelinated --> Fastest (50m/s)

A delta: Small, myelinated --> Middle (20m/s)

C: Small, unmyelinated --> Slowest (1 m/s)

What form of pain/temperature is each axon (fiber) type associated with?

A beta: Mechanical

A delta: Cold, fast pain, mechanical

C: Small, unmyelinated --> Slow pain, heat, cold, mechanical stimuli

Describe fast and slow pain using the example of a stubbed toe

Fast pain: Initial pain of stubbing toe

Slow pain: Throbbing felt after initial pain

Pain from internal organs can be felt far from the source of the pain.

True or False

Explain.

True

Visceral organ pain is poorly localized because it is uncommon

Why are patients told to look for pain off of the left arm during heart attacks?

Because internal organ pain is poorly localized

Where is pain often felt if the liver and gallbladder are hurt?

Shoulder/neck area

Why may the brain have a hard time distinguishing pain from visceral organs?

-Uncommon stimulus: less free nerve endings in those regions

-Overlap between uncommon (organs) and common stimuli at secondary sensory neurons in the spinal cord may confuse the brain

What is sound?

The alternative compression and rarefaction of air

How are the key characteristics of sound measured?

Frequency --> Wavelength measured in Hz

Intensity --> Loudness in dB

How is frequency related to wavelength

Frequency is inverse to wavelength

Larger frequency = Skinnier wave (smaller wavelength)

Define wavelength as it relates to sound

The distance between 2 peaks of a sound wave

What part of the external ear helps direct sound into the ear canal?

Pinna

Describe the flow of a sound wave through the ear

-Wave enters though the ear canal

-Ear canal leads to tympanic membrane (eardrum) causing it to vibrate

-Vibrations are then amplified though the middle ear bones

-Amplified vibration passes through oval window

-Oval window movement causes pressure waves in vestibular duct of cochlea

-Vibration of vestibular duct fluid causes basilar membrane to vibrate

-Vibrations cause pressure waves which go out thorough the round window

What structures make up the vestibular system?

Semicircular canals

Utricle

Saccule

What is the fluid in the vestibular duct called?

Perilymph

What are the vestibular apparatus and cochlea important for?

Vestibular apparatus: Balance

Cochlea: Sound

Where is endolymph found?

Cochlear duct

Where can perilymph be found?

Vestibular and tympanic ducts

When does transduction of sound waves to neural signals occur?

When sound waves pass though the basilar membrane of the cochlea

Where does sound enter the cochlea?

Oval window

What are the 3 fluid filled ducts of the cochlea?

Vestibular

Cochlear

Tympanic

Where are the vestibular and tympanic duct connected?

Helicotrema

Where does coding of sound information begin?

Organ of Corti and Basilar membrane

Describe the structure of the basilar membrane near the round window and near the helicotrema

Why do they differ?

Oval window: Stiff, narrow region used for high pitch (frequency) detection

Helicotrema: Flexible, wide region used for low pitch (frequency) detection

What allows the inner ear to translate the frequency of sound for the CNS

The structure of the basilar membrane and where it vibrates most

What kind of non-neuronal cells sit atop the basilar membrane?

Hair cells

What is the function of hair cells on the basilar membrane below the tectorial membrane?

Signal transducers for movement caused by sound vibrations

How do hair cells relay information from the basilar membrane to the tectorial membrane?

As the basilar membrane moves, cilia on hair cells bends against the tectorial membrane

What is the specific name for the cilia on hair cells on the basilar membrane?

Stereocilia