W10 Spinal Reflexes and Somatosensory reflexes Flashcards
Somatic/spinal reflexes
involving the somatic nervous system.
What is a Reflex?
An involuntary, rapid, stereotypes and coordinated response to a sensory stimulus. Involves muscle contraction. Can be learned: Pavlovian.
What is the stretch (myotatic) Reflex?
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).
Muscle Spindle
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).
Does the muscel spindle have muscles
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.
Proprioception
Sense of the position of parts of his body relative to other parts of the body.
Kinaesthesia
Sense of your body moving in space
What is the Golgi tendon organ?
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.
Parallel after-discharge circuit
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.
Polysunaptic reflex
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.
Crossed Extensor reflex
provides postural support udring limb withdrawal
Central pattern generator
Local circuits that can generate the pattern of alternating flexion/extension.
Cellular basis of pattern generators has been worked out in simple organisms
e.g. lamprey alternates contracts and relaxes along body. Wave of excitation moves down and up, similar to the cat
Nematode worm ( proprioception at molecular level)
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.
TRP
Transient Receptor Potential proteins
A molecule that can directly detect body movement is a critical part of proprioception.
Reflexes involve what?
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).
Reflexes - Pain
Reflexes are also important in responses to noxious (harmful stimuli) and help the organism avoid pain. Such reflexes involve nociceptors (e.g. flexor withdrawal).
Two major input components of the representation of the body in the brain
I. Mechanical stimuli (touch, vibration, pressure and cutaneous tension)
II. Painful stimuli and temperature
What does the input and interpretation of the components enable us to do?
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.
Meissner Corpuscle
Location : dermal papillae of skin (palms, eyelids, lips, tongue)
Modality: light touch, texture (movement)
Sesitive to 30-50Hz and rapidly adapting
Pacinia (or lamellated) Corpuscle
Location : dermis, joint capsules, viscera
Modality: deep pressure, stretch, tickle, vibration
Sesitive to 250-350Hz and rapidly adapting
Encapsulated Nrve endings
Meissner Corpuscle, Pacinian corpuscles adn Ruffini Corpuscles
Ruffini corpuscles
Location : dermis, subcutaneous tissue, joint capsules
Modality: heavy touch, pressure, skin stretch, joint movements
Slowly adapting
Merkel or tactile discs
Location : superficial skin
Modality: light touch, texture, edges, shapes
Slowly adapting
Free nerve endings
Location : widespread in epithelia and connective tissues
Modality: pain, heat, cold
Unencapsulated Nerve endings
Merkle discs adn free nerve endings
Hari follicles
Location: widespread in epithelia
Modality: varied according to type both rapid and slowly adpating subtypes
Sensory receptors - sensory neurons
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.
Phasic receptors
(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.
Tonic Receptors
(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
Afferent axon subtypes
Classified according to conduction velocity, reflects dimater.
Subtypes for axons coming from the muscle
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
How is sensory informatio organised?
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.
Subtypes for axons coming form the skin
Axon coming from the skin are designated by letter (A (faster/larger) - C (slowest/smallest) Group A broken down in greek charcters
What are the two main routes for sensory information to get to the brain?
(1) Medial Lemniscal tracts carry mechanoreceptive and proprioceptive signals to the thalamus.
(2) The Spinothalamic tract carries pain and temperature signals to the thalamus.
3 neurons to reach higher centres are ? - sensory information
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.
How is the Medial Lemniscal System organised?
Axons of the medial lemniscal pathway are topologically organised. 3rd orders of axons.
Medial Lemniscal System - 1st Order axon
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.
Medial Lemniscal System - 2nd Order axon
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.
Medial Lemniscal System - 3rd Order axon
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.
Dorsal column nuclei
Cuneate nucleus and Gracile nucleus.
Topological organization
the spatial arrangement of the objects relative to one another.
This topological organization is preserved in the spinal cord and the somatosensory projections.
Somatotopic order in the brain
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.
Dermatome
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.
Somite
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.
Topographic map
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
Topological
the way in which constituent parts are interrelated or arranged.
Topographical
relating to or representing the physical distributions of parts of features on the surface of or within on organ or organism.
What determines cortical representation?
Behavioral significance.
While the representation of the hand is large inhuman cortex in rodents it’s the fields corresponding to whiskers that dominate.
Sensory modality is alos represented where?
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.
Cortical map Plasticity - Loss of stimulation
Results in a disappearance of the area normally devoted to the stimualted digigts and increase in that devoted to neighbouring digits
Cortical Map Plasiticity - Increase in stimulation
Results in a increase in the area dovoted to the stimulated digits at the expense of neighbouring unstimulated areas.
Receptive fields
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.
How can we measure a reeptive field
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.
Varaition in receptive field size
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.
Where are located first/second and third order neurons
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.
How are areas of cortex dedicated to each area of the body
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
Somatosensory sysem process information
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).
