intro / sensory systems (I) Flashcards
Our mind is a pattern of _____ running on a special kind of machine: our brain
information processing
Central nervous system (CNS) components
- Brain
- Spinal cord
Peripheral nervous system (PNS) components
Everything else that connects to the CNS…
- Afferent fibers (sensory neurons)
- Efferent fibers (motor neurons)
- Autonomic fibers
- Enteric nervous system
What is the ultimate purpose of the nervous system?
The nervous system uses sensory data and stored knowledge of the structure of the world to produce motor responses that yield the highest possible inclusive fitness for the organism.
→there is a continuous flow of sensory information that allows us to complete various actions (continuous interaction)
chart slide 7
Main functions of sensory information
- Perception
- Control of movement
- Regulation of the function of internal organs
- Maintenance of arousal
Advantages of sensory perception
- Enables extraction of information about the world to then use it to guide behaviour
- Gives us conscious sensations
Somatic sensory system
The somatic sensory system encodes sensory information about the skin surface (e.g. touch, vibration, temperature, pain) and sensory information from inside the body (e.g. visceral pain).
Common properties of sensations
- Modality
- Intensity
- Duration
- Location
Modality
Quality of the sensation
→i.e. somatic sensation (vision, hearing,…)
Submodality
A subcategory within the larger modality as a result of underlying neurological explanations
→i.e. warmth, fine touch, heat
How is somatic sensory information conveyed to the brain from the body?
Through two parallel pathways,
- Dorsal column-medial lemniscal pathway
- Anterolateral pathway
Dorsal column-medial lemniscal pathway
- fine touch & proprioception
- Aa & Ab fibers
→The name dorsal-column medial lemniscus comes from the two structures that carry the sensory information: the dorsal columns of the spinal cord, and the medial lemniscus in the brainstem.
Anterolateral pathway
- pain & temperature
- Ad & C fibers
→The pathway crosses over at the level of the spinal cord, rather than in the brainstem
Proprioception
The body’s ability to sense movement, action, and location.
→sensory information coming from muscles and joints (feedback from limbs), which enables you to have good control over your limbs since you are aware of where they are in space
Primary somatic sensory afferents
Detect stimuli on the skin surface and convey somatic sensory information to the CNS.
→first in the sensory pathway
→different fibers have different physiological/anatomical/ morphological properties
→different endings transduce different sensations
Primary somatic sensory afferent types
- Aa
- Ab
- Ad
- C
Draw the dorsal column-medial lemniscal pathway
☆slide 18☆
Draw the anterolateral pathway
☆slide 18☆
T or F: Primary somatic sensory afferents have the same morphology as other neurons.
False.
They have an unusual morphology.
Draw + explain the morphology of a typical neuron
- dendrites, cell body, axon, presynaptic terminal
- dendrites are where the synapses are, where the neuron receives input from other neurons
- based on that input from all the synapses,
a decision to fire an AP is made - the AP will propagate down the axon and to the presynaptic terminal
- the AP is then a message that will be sent to other parts of the nervous system
Draw + explain the morphology of a primary somatic sensory afferent
- no dendrites
- consist of one long axon (one end in the skin, the other going up to the CNS)
- cell body lies outside of the spinal cord
- presynaptic terminal in the CNS
Sensory transduction
The first step in sensory processing which transforms sensory information from the external (or internal) environment into the opening/closing of ion channels in receptor cells.
→essentially, transforming input into electrical signal
Sensory transduction example: Stretch-activated ion channels
Indentation of the skin causes opening of ion channels in low-threshold stretch-activated mechanoreceptors.
Sensory transduction example: Primary sensory fiber
Pressure on the skin surface is encoded as action
potentials in primary sensory fiber.
→more stretching, higher frequency, ending more depolarized
→less stretching, lower frequency, weaker receptor potential
How can neurons alter the way they convey information?
By changing (1) the pattern & (2) frequency of APs.
NOT by changing the size & duration of APs
Sensory receptors
Sensory receptors (e.g. the endings of primary somatic sensory fibers) act as filters, extracting specific forms of sensory information
and ignoring others.
T or F: All fibers have the same sensory receptors.
False!
Different fibers tend to have specialized endings, which allows them to extract different kinds of somatic sensory information.
Sensory receptor types
- Merkel cells (SA1)
- Ruffini endings (SA2)
- Meissner corpuscle (RA1)
- Pacinian corpuscle (RA2)
- Free nerve ending
What do SA & RA refer to?
They refer to the fiber.
→SA = slow-adapting
→RA = rapid-adapting
Which sensory receptors are found in the dermis?
- Meissner corpuscle
- Pacinian corpuscle
Which sensory receptors are found in the epidermis?
- Merkel cells
- Ruffini endings
- Free nerve endings
Which sensory receptors allow for the detection of touch?
- Merkel cells
- Ruffini endings
- Meissner corpuscle
- Pacinian corpuscle
Role of Merkel cells & Ruffini endings
Good at detecting constant pressure throughout the pulse/stimulus (they will eventually desensitize, but it takes awhile).
→ filters out info about vibration
Role of Meissner corpuscle & Pacinian corpuscle
Only fire at the beginning of the pressure pulse and the end, good at detecting c changes in pressure on the skin surface (i.e., vibrations).
→ filters out info about constant pressure
Role of free nerve endings
- no specialized ending associated with them
- responsible for detection of temperature and pain
Labeled line coding
- each submodality is mediated by a specific receptor/fiber type and a specific labeled line.
- labeled line coding refers to the fact that the brain perceives information arriving from a specific neuronal tract as the adequate stimulus of the primary afferent sensory receptor, even if a different stimulus activates it.
- it doesn’t matter how it is activated, but once a labeled line is activated, then you will feel it
☆go read examples on slide 24☆
Combinatorial processing
Perceived sensations (e.g., wetness, color perception, the smell and taste of food) are typically caused by activation of multiple receptor types and integration of multiple parallel channels by the brain.
→there is no labeled line for wetness, but the sensation can be felt due to the activation of multiple labeled lines and it interprets the relative activation of those different labeled lines
→sensation felt due to the brain’s interpretation based on relative activation of the different receptors
Afferent
Incoming information.
→EX: primary somatic sensory afferents
Efferent
Outgoing information.
→EX: efferent motor neurons
Ascending
Heading up to higher levels of the nervous system.
→from spinal cord to cortex
→EX: ascending sensory afferents
Descending
Heading down from higher levels of the nervous system
→EX: descending corticospinal projections
Ipsilateral
On the same side of the body.
Contralateral
On the opposite side of the body.
Which characteristics define sensory fibers?
- Size of diameter
- Myelination of axon
Which fibers are the fastest?
Those with the largest diameter and most myelination.
→Aa
Large diameter & well-myelinated fibers
- Aa
- Ab
→correspond to specialized sensory endings
→responsible for fine touch and proprioception
Small diameter, thinly myelinated or unmyelinated fibers
- Ad
- C
→correspond to free nerve endings
→responsible for pain and temperature sensation
Aa fiber role
Muscle spindles, Golgi tendon organs
Ab fiber role
Low threshold mechanoreceptors
Ad fiber role
Free nerve endings
C fiber role
Free nerve endings
Explain what occurs at the level of sensory fibers when you touch something hot.
When you touch something hot, the fast pain (jolting) occurs quickly by Ad fibers, but then a slower and throbbing pain occurs, which is caused by C fibers
Receptive fields
The primary somatic sensory neurons innervate a specific region of the body surface. A stimulus within this receptive field will excite the cell.
→each nerve ending has its own receptive field
→this concept applies to all sensory modalities
→receptive fields are combined together and integrated
Size of receptive fields
- High acuity regions → small receptive field
→more sensory neurons innervating
→EX: fingertips - Low acuity regions → large receptive field
→less sensory neurons innervating
→EX: torso
Give an example on how precision is not synonymous with acuity.
☆read p.29☆
+ review in visual system section :)
Medial
Towards the middle/center
Lateral
To the side of, or away from, the middle of the body
Dorsal
Refers to the back portion of the body
Ventral
Refers to the front part of the body
Rostral/Anterior
Refers to a structure found toward the front of the body (situated toward the oral or nasal region)
Caudal/Posterior
Situated in or directed toward the hind part of the body (i.e. tail)
