W10 Spinal Reflexes and Somatosensory reflexes

Question 1

Somatic/spinal reflexes

Answer 1

involving the somatic nervous system.

Question 2

What is a Reflex?

Answer 2

An involuntary, rapid, stereotypes and coordinated response to a sensory stimulus. Involves muscle contraction. Can be learned: Pavlovian.

Question 3

What is the stretch (myotatic) Reflex?

Answer 3

Knee jerk (not response to pain), tap on knee stretches the thigh extensor muscle and associated tendon and sets in motion a process to correct the stretching. Key in keeping body posture (part of proprioceptive system).

Question 5

Muscle Spindle

Answer 5

Special sensory receptor (proprioceptors) that detects muscle stretch.
Innervated by Ia Sensory fibers, provide feedback to motor neurons innervating the surrounding muscle (alpha motor neurons) on the amount of muscle stretch that is occurring. Found in most striated muscle and abundant in muscles involved in fine motor control (hand).

Question 6

Does the muscel spindle have muscles

Answer 6

Yes. They are innervated by axons from gamma () motor neurons. These stimulate the intrafusal (muscle) fibres to adjust the tension in the spindle as the extrafusal (muscle) fibres of the surrounding muscle contract.

Question 7

Proprioception

Answer 7

Sense of the position of parts of his body relative to other parts of the body.

Question 8

Kinaesthesia

Answer 8

Sense of your body moving in space

Question 9

What is the Golgi tendon organ?

Answer 9

Another kind of proprioceptor, it detects muscle tension due to muscle contraction, not muscle strength. Activation of GTO sensory (Ib) afferents lead to activation of inhibitory interneurons which inhibit  motor neurons that innervate the same muscle.

Question 10

Parallel after-discharge circuit

Answer 10

Assume time taken to cross each synapse is the same.
Therefore, stimulus initiated by A will take different times to reach output neuron Z.
Result is that initial signal is sustained over extended period.

Question 11

Polysunaptic reflex

Answer 11

activation of multiple excitatory interneurons sustains the response. As in the stretch reflex, inhibitory interneurons are also activated to relax extensor muscle (reciprocal inhibition). Because rapid withdrawal of limb may lead to imbalance. Often include a contralateral element.

Question 12

Crossed Extensor reflex

Answer 12

provides postural support udring limb withdrawal

Question 13

Central pattern generator

Answer 13

Local circuits that can generate the pattern of alternating flexion/extension.

Question 14

Cellular basis of pattern generators has been worked out in simple organisms

Answer 14

e.g. lamprey alternates contracts and relaxes along body. Wave of excitation moves down and up, similar to the cat

Question 15

Nematode worm ( proprioception at molecular level)

Answer 15

Mutagenesis identified a mutant with ‘loopy’ swimming.
Due to loss of TRP-4 ‘stretch’ receptor in DVA neuron, which detects body bending directly using the TRP-4 mechanosensory. DVA appears to inhibit motor neurons, allowing alternating muscle contraction.

Question 16

TRP

Answer 16

Transient Receptor Potential proteins
A molecule that can directly detect body movement is a critical part of proprioception.

Question 17

Reflexes involve what?

Answer 17

Reflexes may involve a single synapse between sensory and motor neuron (ie monosynaptic, e.g. stretch reflex), or may involve interneurons (ie polysynaptic, e.g. flexor reflex).

Question 18

Reflexes - Pain

Answer 18

Reflexes are also important in responses to noxious (harmful stimuli) and help the organism avoid pain. Such reflexes involve nociceptors (e.g. flexor withdrawal).

Question 19

Two major input components of the representation of the body in the brain

Answer 19

I. Mechanical stimuli (touch, vibration, pressure and cutaneous tension)

II. Painful stimuli and temperature

Question 20

What does the input and interpretation of the components enable us to do?

Answer 20

To identify the shape and textures of objects.
To monitor the internal and external forces acting on the body.
To detect potentially harmful circumstances.

And, therefore, to have a sense of ourselves within our environment and so plan our actions accordingly. Focus on mechanosensory subsystem.

Question 21

Meissner Corpuscle

Answer 21

Location : dermal papillae of skin (palms, eyelids, lips, tongue)
Modality: light touch, texture (movement)
Sesitive to 30-50Hz and rapidly adapting

Question 22

Pacinia (or lamellated) Corpuscle

Answer 22

Location : dermis, joint capsules, viscera
Modality: deep pressure, stretch, tickle, vibration
Sesitive to 250-350Hz and rapidly adapting

Question 23

Encapsulated Nrve endings

Answer 23

Meissner Corpuscle, Pacinian corpuscles adn Ruffini Corpuscles

Question 24

Ruffini corpuscles

Answer 24

Location : dermis, subcutaneous tissue, joint capsules
Modality: heavy touch, pressure, skin stretch, joint movements
Slowly adapting

Question 25

Merkel or tactile discs

Answer 25

Location : superficial skin
Modality: light touch, texture, edges, shapes
Slowly adapting

Question 26

Free nerve endings

Answer 26

Location : widespread in epithelia and connective tissues
Modality: pain, heat, cold

Question 27

Unencapsulated Nerve endings

Answer 27

Merkle discs adn free nerve endings

Question 28

Hari follicles

Answer 28

Location: widespread in epithelia
Modality: varied according to type both rapid and slowly adpating subtypes

Question 29

Sensory receptors - sensory neurons

Answer 29

Sensory receptors sometimes are actually sensory neurons themselves. However sensory receptors can be specialist cell types. For example: photoreceptors, auditory and vestibular hair cells and taste receptors. In these cases, the specialist cell type synapses with a primary afferent neuron to relay the sensory information to the CNS.

Question 30

Phasic receptors

Answer 30

(Pacinian (Lamellated) Corpuscles): in dermis, joint capsules, viscera. Deep pressure stretch, tickle, vibration. Sensitive to 250-350Hz. Rapidly adapting, or phasic receptors give information about changes in the stimulus.

Question 31

Tonic Receptors

Answer 31

(e.g. Ruffini Corpuscle): in dermis, subcutaneous tissue, joint capsules. Heavy touch, pressure, skin stretch, join movements. Slowly adapting or tonic receptors, continue to respond as long as stimulus is present

Question 32

Afferent axon subtypes

Answer 32

Classified according to conduction velocity, reflects dimater.

Question 33

Subtypes for axons coming from the muscle

Answer 33

Axons coming from the msucle designated by roman numerals, I largest, IV smallest. I group futher broken down Ia, Ib.
Pain fibres are slower than proprioceptors

Question 34

How is sensory informatio organised?

Answer 34

Sensory inputs are organized, starting with layers in the dorsal horn of the spinal cord dorsal horn.
The cell bodies of different classes of sensory neurons are grouped in the DRG and their projections are organized to different layers of the dorsal horn.

Question 35

Subtypes for axons coming form the skin

Answer 35

Axon coming from the skin are designated by letter (A (faster/larger) - C (slowest/smallest) Group A broken down in greek charcters

Question 36

What are the two main routes for sensory information to get to the brain?

Answer 36

(1) Medial Lemniscal tracts carry mechanoreceptive and proprioceptive signals to the thalamus.

(2) The Spinothalamic tract carries pain and temperature signals to the thalamus.

Question 37

3 neurons to reach higher centres are ? - sensory information

Answer 37

First-order neurons detect the stimulus and transmit to spinal cord.

Second-order neurons relay the signal to the thalamus, the gateway to the cortex.
2nd order axons cross the midline, are commissural.

Third order carry the signal from the thalamus to the cortex.

Question 38

How is the Medial Lemniscal System organised?

Answer 38

Axons of the medial lemniscal pathway are topologically organised. 3rd orders of axons.

Question 39

Medial Lemniscal System - 1st Order axon

Answer 39

1st order axon from the upper body follows the lateral (red) pathway and

1st order axons from the lower body (below vertebra T6) follow the more medial (purple) pathway and synapse on neurons in the gracile nucleus.

Question 40

Medial Lemniscal System - 2nd Order axon

Answer 40

2nd order axons cross the midline and ascend in the medial lemniscus. In doing so, their topology is reversed, relative to the midline, so that lower body axons are more lateral on reaching the thalamus.

synapse on 2nd order neurons in the cuneate nucleus.

Question 41

Medial Lemniscal System - 3rd Order axon

Answer 41

3rd Order axons again reverse the topology so that lower body axons synapse on more medial cortical neurons, where’s upper body axons map to the lateral cortex.

Question 42

Dorsal column nuclei

Answer 42

Cuneate nucleus and Gracile nucleus.

Question 43

Topological organization

Answer 43

the spatial arrangement of the objects relative to one another.

This topological organization is preserved in the spinal cord and the somatosensory projections.

Question 44

Somatotopic order in the brain

Answer 44

The result of this topological projection is a map of the body in the cortex. This map is a very fine, not simply lower verses upper body. Thid reflects the fact that each DRG innervates a specific domain of the body called a dermatome.

Question 45

Dermatome

Answer 45

Each sensory ganglion innervates a specific region of skin. These regions arise because the dermis of each region is derived from a specific embryonic structure called the somite.

Question 46

Somite

Answer 46

are iterated structures that give rise to the underlying musculature and skeleton.
In the embryo, each sensory ganglion (DRG) is associated with a specific somite and subsequently innervates the tissues arising from the somite.

Question 47

Topographic map

Answer 47

The area occupied by the different regions in the cortex is not proportional to their physical size. Logically there must also be a map in the thalamus, the waystation between the 2nd and 3rd order neurons, and in the dorsal column nuclei.T

Question 48

Topological

Answer 48

the way in which constituent parts are interrelated or arranged.

Question 49

Topographical

Answer 49

relating to or representing the physical distributions of parts of features on the surface of or within on organ or organism.

Question 50

What determines cortical representation?

Answer 50

Behavioral significance.
While the representation of the hand is large inhuman cortex in rodents it’s the fields corresponding to whiskers that dominate.

Question 51

Sensory modality is alos represented where?

Answer 51

The cortex.

The somatotopic map is preserved in the coronal plane in the postcentral gyrus. Throughout the postcentral gyrus, different sensory modalities are localized along the sagittal axis. Ie these different regions will receive inputs from the different types of sensory receptors. These are called Brodmann areas.

Question 52

Cortical map Plasticity - Loss of stimulation

Answer 52

Results in a disappearance of the area normally devoted to the stimualted digigts and increase in that devoted to neighbouring digits

Question 53

Cortical Map Plasiticity - Increase in stimulation

Answer 53

Results in a increase in the area dovoted to the stimulated digits at the expense of neighbouring unstimulated areas.

Question 54

Receptive fields

Answer 54

Each sensory neuron has a receptive field. The size of the receptive field for any particular neuron will vary depending on where it is in the body, some regions having denser innervation than others.

Question 55

How can we measure a reeptive field

Answer 55

The size of the receptive field can be measured by assessing the ability to discriminate two sharp points set apart at different distances. If the subject feels two pinpoints then the distance between the points is larger than the receptive field. Using this method, can determine the sizes of receptive fields in different parts of the body.

Question 56

Varaition in receptive field size

Answer 56

Where receptive fields are large, discrimination is low (legs/arms), whereas where receptive fields are small, discrimination is high (fingers).
This explains why more cortex is dedicated to regions where receptive fields are small: The number of receptors per unit area may be similar. The number of endings from different neurons is higher, therefore the amount of derived information is higher. Which result in larger number of axons per unit area being represented in the cortex. The number of sensory neurons innervating a particular area is related to the behavioral significance of the area.

Question 57

Where are located first/second and third order neurons

Answer 57

First-order neurons are located in the dorsal root ganglia, second-order neurons in the dorsal column nuclei, and third order are found in the thalamus.

Question 58

How are areas of cortex dedicated to each area of the body

Answer 58

The area of cortex dedicated to each area of the body is proportional to the density of innervation in that area (and hence its behavioral significance) not to the physical size of the area

Question 59

Somatosensory sysem process information

Answer 59

The components of the somatosensory system process information conveyed by mechanical stimuli that impinge upon the body surface or that are generated within the body itself (proprioception).