SENSES Flashcards
Sensory receptor organs
neurons specialized to detect a certain stimulus
Adequate stimulus
An adequate stimulus is the type of stimulus to which a sensory organ is particularly adapted. An example is photic (light) energy for the eye
Basic properties of all sensory systems
1) Sensory systems have a restricted range of responsiveness. Example: the frequency range for hearing, which varies with species.
2) Each sensory system has specialized neurons that respond to only that specific stimulus
3) Despite the fact that the neurons are specialized, the sensory stimulus is always turned in to action potential for communication. The term to describe this conversion of a sensory stimulus to an electrical signal is sensory transduction
Touch Adequate Stimuli
contact with or deformation of body surface
Pain Adequate Stimuli
Tissue damage
Hearing Adequate Stimuli
sound vibrations in air or water
Function/Shape of Sensory Neurons
- The primary sensory neurons aka the afferents that detect touch are located in the peripheral nervous system, with the cell bodies located in the dorsal root ganglion outside of the spinal cord
Touch Sensory Neuron
- The primary sensory neuron for touch is pseudounipolar, in that it has one axon and no dendrites. One axonal end is in the skin and the other in the spinal cord
- The axons are medium diameter with myelination, called AB fibers
- The area on skin which can be differentiated from an adjacent area by touch is defined as a receptive field. - The two-point discrimination test demonstrates how large/small receptive fields can be. The size of the receptive field is based on the density of neuron innervation
What is a dermatome?
A single dorsal root ganglion, which contains many sensory neurons, innervates a region of the skin called a dermatome
What are the 3 layers of the skin?
Epidermis, Dermis, Hypodermis
Epidermis
Epidermis—outermost layer, thinnest
Dermis
Dermis—middle layer, contains nerve fibers
Hypodermis
Hypodermis anchors muscles and helps shape body
Pacinian Afferents
a. Vibration- activated, fast-adapting
b. located deep in dermis
c. Essential for skilled use of objects
Meissner’s Afferents
a. tips made from schwann cells; enriched in finger tips,
b. touch, fast-adapting, essential for vibration (when objects moved across skin)
Merkel’s Afferents
a. are enriched in finger tips
b. touch, slow-adapting and
c. ideal for form and texture
Ruffini’s Afferents
a. located in subcutaneous layers
b. stretch, slow-adapting
c. important for stretching of digits
What happens when the appropriate stimulus is detected and what type of axons does somatosensation use?
- When the appropriate stimulus is detected in the specialized neuron, ions flow in to the neuron and cause a depolarization.
- The neuron can then fire an action potential and cause neurotransmitter release in the spinal cord
- The speed at which the generated action potential travels to the spinal cord varies on the type of axon each sensory neuron has
- Somatosensation uses myelinated, medium sized axons
Neuron Pathway for Touch Sensation
There is a three neuron sequence: (1) skin sensory neuron synapses on neuron in the (2) medulla. The neuron in medulla synapses on a neuron in the (3) thalamus. The neuron in the thalamus synapses onto the primary somatosensory cortex.
When the first neuron (the skin sensory neuron) goes into the spinal cord where does it go?
- When the first neuron (the skin sensory neuron) goes into the spinal cord, it travels up in what is called the dorsal columns.
- These columns are just the axons of the sensory neurons bundled together and running up the spinal cord
What 2 things are the dorsal columns split into?
The dorsal column is split into the fasciculus gracilis for lower body axons and fasciculus cuneatus for upper body axons
After the medulla where do neurons synapse onto?
- These medulla neurons then synapse onto the ventral posterior lateral (VPL) nucleus in the thalamus
- Sensory processing happens in the cortex (third neuron from thalamus synapses in cortex)
How is touch represented in the brain?
- The more neurons that innervate a specific region, the more cortical representation that region has
- somatotopic map
Secondary Association Areas
- Once the stimulus gets initially “processed” in the primary areas, the neurons within the primary areas go to secondary association areas in the lobe
1) These secondary areas can then communicate with additional areas in the brain:- Limbic Areas, such as the amygdala and hippocampus
- Premotor areas in frontal lobe
2) These secondary areas also send their axons to the secondary areas in the opposite hemisphere
Damage to somatosensory region/cortex
- Damage to a somatosensory region, for example a digit amputation, will lead to cortical reorganization, with adjacent cortical regions “taking over the neurons that were dedicated to the now amputated limb
- The ability of the cortex to be plastic and dynamic allows for recovery after injury and an ability to alter neuronal processing as needed
What is Synesthesia?
- Synesthesia is a condition in which a stimulus in one sensation creates a perception of another sensation
1) So synesthete may show activation in visual area when presented with a tone
2) Synesthesia runs in families, so it is possible there is a genetic basis for the “cross-wiring”
What is Proprioception?
A. Proprioception is the ability to detect position of limbs and other body parts in space
B. There are numerous mechanical afferents that are involved in proprioception, and they are located in and around skeletal muscles
C. Within each striated muscle, which is muscle you can voluntary control, there is a muscle spindle
Proprioception: Muscle Spindle
1) This spindle is a specialized structure that has intrafusal muscle that has sensory mechanical afferents coiled around it
2) These afferents detect stretch through mechanically gated ion channels and fire action potentials
Proprioception Golgi Tendon Organs
- In addition to spindles, you also have golgi tendon organs (GTO) that are located on muscle tendons (think the muscles that allow joints to move)
- The GTOs are innervated by mechanically responsive afferents, but they provide information about muscle tension
WHat is the touch pathway called?
the dorsal column system
What are nociceptors?
- The neurons responsible for sensing noxious stimuli from the external and internal environment are called nociceptors
- These neurons are pseudounipolar, with one axon in the organ and the other projecting to the spinal cord
Characteristics of nociceptors that help detect noxious stimuli:
1) Have free nerve endings that innervate target
2) Broken into two main groups: Aδ and C fibers.
3) Express channels and receptors that help mechanical, thermal, chemical stimuli
Aδ FIbers Properties
Aδ axons are myelinated and as such, transmit pain signals quickly from the skin—1st pain, fast and intense
C-Fiber Properties
C-fibers are unmyelinated and as such, transmit pain signals slower from the skin—2nd pain, throbbing and chronic
How do Thermal/Mechanical stimuli activate nociceptors?
1) Thermal/mechanical: primarily detected through a family of ion channels called “TRP channels”
1) Influx of Ca and/or Na
when activated
2) How does this help
action potentials
How does Chemical Stimuli activate nociceptors?
Chemical stimuli: TRP channels, other ion channels and GPCRs
The Ascending Pain Pathways
A. Ascending pathways alert your CNS to presence of noxious stimuli. The anterolateral system is the key ascending pathway that conveys pain to cortex
1) The axons of nociceptors synapse onto neurons in the dorsal horn of the spinal cord
2) These spinal cord neurons can then go to various structures as part of the anterolateral system
2 components of the Anterolateral System
pathway for sensory discrimination and pathway for affective (emotional) composition
Anterolateral: Sensory Discriminative Pathway
Nociceptor to spinal cord to ventral posterior thalamus to cortex
Anterolateral: Affective (emotional) Pathway
a. Nociceptor to spinal cord/brainstem
b. Nociceptor to spinal cord to midline thalamus to anterior cingulate cortex and insular cortex
The Descending Pathways
Descending pathways arise in the cortex and project axons down to the spinal cord to inhibit ascending pain pathways. The goal of these pathways are to provide analgesia
1) The neurons in the somatosensory cortex send axons down to a variety of cortical (amygdala and hypothalamus) and brainstem (periaqueductal gray) structures. The neurons in these areas then send their axons down to the dorsal horn of the spinal cord, where ascending pain pathways originate
Pain Descending PAthway: Where do neurons go after come into the dorsal horn?
- they synapse onto inhibitory local circuit neurons.
- The inhibitory neurons release substances called opoids onto nociceptor axon terminals. The binding of this substances causes hyperpolarization of the nociceptor, preventing activation of the ascending pathway
- The silencing of the nociceptor prevents pain signal transmission
Hyperglasia
Hyperalgesia is an increased sensitivity to pain to an already painful stimulus
Allodynia
Allodynia is when a normally non-painful stimulus becomes painful
Sensitization of nociceptors
1) When a nociceptor becomes easier to “excite” or starts responding to innocuous stimuli, the nociceptor has become sensitized
2) The substances released from damaged skin or the recruitment of inflammatory cells is what causes sensitization
3) Receptors, such a TRPV1, become easier to fire or are expressed in greater numbers. This facilitates further nociceptive signaling
4) Peripheral sensitization can lead to central sensitization, where neurons in the spinal cord or brainstem become hyper responsive
What is the difference between somatic and visceral pain?
- We feel pain mostly from our skin, known as somatic pain
- Most internal organs, also referred to as visceral organs, have very limited pain detecting capabilities. Some organs, like the brain, have no pain neurons/receptors at all
Somatic Pain
1) Dense innervation of the skin by nociceptors
2) Variety of receptors: thermal, chemical, mechanical nociceptors
3) Localized pain sensation, which means you can pinpoint where on your skin something hurts
Visceral Pain
1) Sparser innervation of the organs by nociceptors, with some organs having no nociceptors
2) Mainly mechanical nociceptors, so less variety. This is why pain in the internal organs often feels “crampy” and not sharp like a paper cut
3) Diffuse pain sensation, which means you feel a general pain in an area that is hard to localize
4) Referred pain
Nociceptive Pain
- noxious stimuli act on intact nociceptors to produce pain
- intense, sharp, localized pain
- no neuron damage
- protective
Neuropathic Pain
- damage to neural structure itself leads to pain
- burning pins and needles
- neuron damage
- not protective
Outer Ear
- collect sound waves along with the pinna and ear canal
1) The shape of the external ear transforms sound energy.
2) Only mammals have pinnae, and animals like bats that have exceptional hearing can move the pinnae with great dexterity
3) The folds in the pinnae help amplify and suppress certain sounds ranges
The Middle Ear
1) The middle ear concentrates sound energies. It consists of the ossicles and the tympanic membrane.
2) Three ossicles—malleus, incus, and stapes—connect the tympanic membrane (eardrum) to the oval window.
3) Sound comes in to the pinnae and causes the tympanic membrane to vibrate, which in turn gets transferred to the ossicles. The ossicles vibrate and transfer that to the oval window
Three ossicles of the middle ear
malleus, incus, and stapes
2 muscles that attach to the ossicles in the middle ear
tensor tympani muscle
stapedius muscle
- When you talk or cough, the reason you don’t blow your own ears out if because these two muscle stiffen right before you start
Inner Ear
Inner ear structures convert sound into neural activity.
1) Mammals have a fluid-filled cochlea
2) Within the cochlea, there is the organ of Corti, which is what coverts the sound from the outer/middle ear into a neural signal.
3) The basilar membrane within the organ of Corti vibrates in response to sounds and the Hair cells in this region turn that into a neural signal
The organ of Corti has two sets of hair cells:
a. Inner hair cells (IHCs)
b. Outer hair cells (OHCs)
c. Stereocilia, or hairs, protrude from each hair cell.
How do the hair cells transmit sound signals to the innervating nerve?
a. The stereocilia actually move in response to the vibration of sound
b. The ends of the stereocilia have links between them, called tip links, which are bound to ion channels.
Hearing 1st Neuron
- When the stereocilia move, tension is created on the tip links that physically open up an ion channel
- The opening of ion channels allows cations to go into the hair cell and depolarize the cell.
- Voltage-gated Calcium channels open in response to this depolarization and cause vesicles to fuse with the hair cell membrane and release neurotransmitter onto the neuron
Hearing 2nd Neuron
- The nerves then take this electrical signal that results from a sound via the vestibulocochlear nerve .
- From there the nerve goes to superior olivary nucleus in the brainstem (2nd neuron).
Hearing 3rd Neuron
Next, the neuron goes to the midbrain (inferior colliculi) (3rd neuron). Finally, that neuron goes to the thalamus, from which the final neuron goes to the primary auditory cortex (4th neuron).
Conduction Deafness
Conduction deafness—disorders of the outer or middle ear that prevent sounds from reaching the cochlea
Sensorineural Deafness
Sensorineural deafness originates from cochlear or auditory nerve lesions
Central Deafness
Central deafness—hearing loss caused by brain lesions (such as stroke), with complex results
Vestibular System
- The vestibular system is key for balance and position of your body in space
- The vestibular system is part of the inner ear and connects to the cochlear structure
- The components of the vestibular system are the three fluid-filled semicircular canals
- There are structures at the base of the semicircular canals that have hair cells, just like with the auditory system.
- When you move your head in space, the fluid moves and pushes on the hair cell cilia. The cilia then depolarize and release transmitter onto neurons.
What causes motion sickness?
Sensory conflict theory—motion sickness is due to conflict in visual and vestibular information
False Climb Illusion
Pilots can experience vestibular “hallucinations.” Your vestibular organs transmit linear movement and upward head tilt the same way. When pilots are in a low visibility situation, they perceive an upward tilt of the plane even when they are going straight. This can cause pilots to dive the plane to correct for what they think is an upward climb of the plane. This is called the false-climb illusion
Taste receptor cells
- Taste receptor cells are located within taste buds on papillae on the tongue.
- Each taste cell can detect only one type of basic taste
- Taste cells extend cilia into the taste pore to contact tastants.
What tastes are detected on ionotropic receptors?
Salty—sodium (Na+)
Sour—all acids taste sour because they release hydrogen ions (H+)
What tastes are detected on metabotropic receptors?
Sweet, Bitter, Umami
What cells make up the olfactory epithelium?
Three types of cells in the epithelium: receptor neurons, supporting cells, and basal cells
Which neurons can be regenerated?
If the olfactory epithelium is damaged, it can be regenerated (including the neurons) and will properly reconnect to the olfactory bulb.
