Ascending pathways Flashcards
What do Hair follicle receptors sense?
Hair follicles = touch
What do Merkel endings sense?
Merkel endings = pressure and low frequency vibration
What do Meissner corpuscles sense?
Meissner corpuscles = light touch
What do Pacinian corpuscles sense?
Pacinian corpuscles = vibration and JPS
What do Nociceptors sense?
Nociceptors = Pain (can be chemical, mechanical, heat, cold - hence why don’t have same mechanical ending as others)
What do Golgi tendon organs sense?
Golgi tendon organs = JPS
What do Muscle spindles sense?
Muscle spindles = muscle length, proprioception
What do Ruffini endings sense?
Ruffini endings = Skin stretch, pressure, JPS
What are the different nerve receptors that you can find in the conscious and unconscious ascending pathways and their modalities?
Each nerve receptor specialises in one or more sensations.
Receptors;
Hair follicle receptors - sense touch
Merkel endings - sense pressure and low frequency vibration
Meissner corpuscles - sense light touch
Pacinian corpuscles - sense vibration and JPS
Ruffini endings - Skin stretch, pressure, JPS
Nociceptors - Pain (can be chemical, mechanical, heat, cold - hence why don’t have same mechanical ending as others)
Muscle spindles - muscle length, proprioception
Golgi tendon organs - JPS
Most of these apart from pain use stetch receptors to pass on signal
Why are dermatomes important?
As they tell up what nerve supplies regions of skin and where a lesion may be!
How do the different sensory cutaneous receptors vary?
Obviously function is the main variation but the different receptor types also detect sensory input over a variety of receptive fields;
E.g Meissner’s corpuscles have smaller receptive fields when they are in the tips of the fingers than when they are on the palms of the hands and Pacinian corpuscles have generally large receptive fields each of which has a smaller more sensitive area within it
This allows for better discrimination in fingertips than palms, important in fingers not palms
Why do the receptor fields overlap?
Each sensory neuron has a receptive field where dendrites are dense in the centre of it and diffuse at the periphery. Stimuli at the centre of the field will activate more dendrites than at the periphery so stronger stimulation will cause a faster firing rate of the action potential.
Receptive fields overlap and if you provide a stimulus in middle of A you get faster AP from A than B and allows the brain know the stimulus is closer to the centre of receptor field A . Its getting no reespponse from receptive field c so can roughly locate the stimulis
What is the concept of lateral inhibition?
Lateral inhibition relies on reciprocal inhibition between two adjacent neurons where the extent of inhibition from each one is linked to the stimulus point on the neuronal receptive field overlap.
Example;
When red is firing fast it will cause inhibition to the green receptor field and the green will slightly inhibit the red one back so we get a slight reduction in the area with the stimulus (red) but we have a much better idea of the area of where the stimulation is coming from and allows us to locate. Side paths of fields are inhibited and centre is being left alone
How do different receptors vary in their axons?
Different receptors types have specific axon types.
This affects the speed of transmission of the information to the CNS.
Some modalities therefore are slower at being transmitted to the brain than others.
Axon from muscles types;
Group 1; 13-29 µm diameter, 270mph conduction rate, its receptors are proprioceptors of skeletal muscle (Aa axons from skin)
Group 2; 6-12 µm diameter, 167mph conduction rate, its receptors are mechanoreceptors of the skin (Aß axons from skin)
Group 3; 1-5 µm diameter, 67mph conduction rate, its receptors are for pain and temperature (A∂ axons from skin) (supposed to be delta)
Group 4; 0.2-1.5 µm diameter, 5 mph conduction rate, its receptors are for itch, pain and temperature (C axons from skin) - unmylinated
These all use saltatory conduction except from last as isn’t myelinated!
How does somatic sensory input work?
The signal comes in from the spinal nerve to the dorsal root ganglion where it synapses and passes on to the dorsal root (sensory) into the dorsal horn of the spinal cord
How does the spinal cord adapt to be able to carry out it neurological functions?
The structure of the spinal cord changes along its length
Where additional neurones are needed (to deal with lower limbs and then upper limbs) the spinal cord swells
As sensory information is added, the internal structure changes to accommodate this
How is the white matter of the spinal cord divided ?
Its divided up into tracts of axons carrying typified information in the spinal cord
E.g;
- Fasciculus gracilis which carries information from the lower body extremities
- Fasciculus cuneatus which carries information from upper body extremities
- Dorsal and Ventral spinocerebellar tracts carry proprioceptive information from the muscle spindles (dorsal) and Golgi organs (ventral)
- Spinothalmic tracts which carry pain and temperature transmission etc. - Also known as anterolateral as found in 2 places, anterior and lateral
How is the primary somatosensory cortex arranged topographically ?
Laterally (closer to lateral sulcus) it has all the sensitive parts you would find more so in the face and more superiorly it is for upper extremities and medially it is for waist down (closer to longitudinal sulcus)
What is the layout of the general somatosensory system?
Most conscious sensory tracts follow this pattern
The primary afferent neuron is a first order neuron which picks up the impulse from the sensory nerve fibres and carries it into the spinal cord or brain stem where it synapses with a second order neuron which then projects it into the thalamus where another synapse occurs and it is projected by a 3rd order neuron up into the somatosensory cortex.
Where does all sensory information go before it goes into the cortex?
Through the thalamus!
What is the thalamus?
A reciprocal relay station between the periphery and the cortex (kinda like a train station that organises impulses and sends them to the right area of brain).
How does feedback benefit the thalamus?
The thalamus also receives feedback and can modular cortical activity.
This gives it the option to either upregulate or downregulate signals to and from the brain!
What are the three principal ascending tract systems in the spinal cord
1). Dorsal column-medial lemniscus pathways
2). Spinothalamic pathways
3). Spinocerebellar pathways
What is the Dorsal column-medial lemniscus pathway for?
Its for mechanical stimuli;
- Conscious proprioception
- Discriminative touch
- Vibration
- Pressure
