Ascending neural pathways Flashcards
Describe the basic neural pathway of the somatosensory system
- Primary afferent is a first order neurone and terminates in the spinal cord or brain stem
- Second order neurone projects to the thalamus
- Third order neurone projects to the brain
Where does the sensory information enter the spinal cord?
Through the dorsal roots of the spinal nerves
After the sensory information has entered the spinal cord, how can it get to the brain?
Either:
• The dorsal column-medial lemniscal system
• The anterolateral system
Where do the two systems (Dorsal column-medial lemniscal and anterolateral) come partially back together?
The thalamus
What are the key features of the Dorsal column-medial lemniscal pathway?
- Large myelinated fibres
- 30-110m/s
- Discrete types of mechanoreceptive sensation
- High degree of spatial orientation of the nerve fibres with respect to their origin
What are the key features of the Anterolateral system?
- Small myelinated fibres
- up to 40m/s
- Broad spectrum of sensory modalities
- Less spatial orientation with respect to their origin
Which sensations are related to the Dorsal column-medial lemniscal system?
- Touch sensations requiring a high degree of localisation of the stimulus
- Touch sensations requiting transmission of fine gradations of intensity
- Phasic sensations e.g. vibration
- Sensations that signal movement against the skin
- Position sensations from the joints
- Pressure sensations related to fine degrees of judgement of pressure intensity
What sensations are associated with the anterolateral pain system?
- Pain
- Thermal sensations, both warm and cold
- Crude touch and pressure sensations capable of only crude localising ability on the surface of the body
- tickle and itch sensations
- Sexual sensations
Describe the route in the Dorsal column-medial lemniscal system
- Carries signals up to the medulla in the dorsal columns of the spinal cord
- Synapse
- Cross to the opposite site of the medulla
- Up through the brainstem to the thalamus via the medial lemniscus
Describe the route in the Anterolateral system
- Enter the spinal cord via dorsal spinal root nerves
- Synapse in dorsal horns of spinal grey matter
- Cross to the opposite site of the cord
- Ascend through the anterior and lateral white columns of cord
- Terminate at all levels of lower brain stem and in the thalamus
In the dorsal column-medial lemniscal pathway, where do the nerves pass uninterrupted to?
• The Dorsal medulla
Where do the neurones synapse in the dorsal column-medial lemniscal pathway?
Dosal column nuclei (cuneate and gracile nuclei)
In the dorsal column-medial lemniscal pathway, what happens immediately after the neurones have synapsed?
The second order neurones cross immediately to the opposite side of the Brian stem and continue up thorough the medial lemnisci to the thalamus
In the Dorsal column-medial lemniscal pathway, where do the medial lemniscal fibres terminate?
- thalamic sensory relay area
* Called the ventrobasal complex
In the Dorsal column-medial lemniscal pathway, where do the third order nerve fibres project to?
Mainly to the post-central gyrus of the cerebral cortex e.g. somatic sensory area 1
In the dorsal column-medial lemniscal system, where in the dorsal column do neurones from the lower parts of the body lie?
Towards the centre of the cord
Describe the location of the thalamus where the tail end and the head/face end are represented (dorsal column-medial lemniscal system)
- Tail end= most lateral portion of the ventrobasal complex
- Head and face: Medial areas of the complex
- Left side of the body = right side of the thalamus and vice versa
Where is the somatic sensory cortex?
In a strip posterior to the post central sulcus of the Brian
Why is the right side of the body represented by the left side of the thalamus in the dorsal column-medial lemniscal system?
Because of the crossing of the medial lemnsici in the medulla
What will areas of higher discrimination have?
• Higher proportion of area
What are the somatosensory areas 1 and 2?
separarte sensory areas in anterior pariteal lobe
Compare the somatosensory areas in terms of their localisation
- Area 1= high degree of localisation of different body parts
- Area 2: poor localisation
Which areas of the body are control are controlled by the somatosensory area 2?
- Leg
- Arm
- Face
Where is the head area located in the somatosensory area 1?
The most lateral area
Describe the characteristics as the anterolateral pathway
- Velocities 8-40m/s
- Degree of spatial localisation of signals is poor
- Gradations of intestates are not very accurate; 10-20 gradations of strength (100 in DCS)
- Ability to transmit rapidly changing or repeating stimuli is poor
Describe fast pain
- Felt within 0.1s
- Sharp, pricking, acute, electric pain
- e.g. when a needle is stuck in the skin, when ski is acutely burned, electric shock
- Not felt in the deep tissues of the body
Describe slow pain
- Felt after 1s and increases slowly over seconds or minutes
- Slow burning, chronic, aching, throbbing, nauseous pain
- usually associated with tissue destruction
- Can lead to prolonged almost unbearable suffering
- Occurs in both the skin and in almost any deep tissue or organ
What are pain receptors?
• Free nerve endings widespread in the superficial layers of the skin and in some internal tissues
What are pain receptors excited by?
- Mechanical stimuli (fast and slow)
- Thermal stimuli (fast and slow)
- Chemical stimuli (slow)
When would there be an increase in sensitivity of the pain receptor?
hyperalgesia
Which fibres transmit fast pain?
A delta
Which fibres transmit slow pain?
C fibres
Describe the fast sharp pain pathway
- Signal elicited by mechanical or thermal pain
* Transmitted via small A delta fibres at 6-30m/s
Describe the slow chronic pain pathway
- Signals mainly elicited by mechanical pain
* Transmitted via C fibres at 0.5-2m/s
What is double pain sensation?
- Sharp pain transmitted by A delta fibres followed by slow pain transmitted by C fibres
- Fast pain allows for immediate reaction and slow pain tends to get greater over time and stimulates the person to keep trying to relieve the cause
What is the fast sharp pain pathway carried by?
Neospinothalamic tract
Where do the neurones the fast pain pathway terminate?
- Lamina 1 lamina marginalis) of the dorsal horns
* Excite 2nd order neurones
In the neospinothalamic tract, what do the 2nd order neurones give rise to?
Long fibres that cross immediately to opposite sides of the cord through anterior commissure then turn upward passing into the Brian in the anterolateral columns
What is the slow chronic pain pathway known as?
Paleospinothalamic pathway
Where do the peripheral fibres of the paleospinothalamic pathway terminate?
- In the spinal cord
* Almost entirely in laminae II and III of dorsal horns
In the paleopinothalamic pathway, what do the neurones in lamina V in the dorsal horn give his hot?
Long axons that mostly join the fibres fast pain pathway, passing first through the anterior commissure to the opposite side of the cord then upward into the brain in the anterolateral pathway
What is referred pain?
Pain in a location remote from the tissue causing pain
Describe the mechanism of referred pain
- Branches of the visceral pain fibres synapse in the spinal cord on the same 2nd order neurones that receive signals from the skin
- When the visceral pain receptors and stimulated, pain signals from the viscera are conducted through some of the same neurones causing pain signals form the skin so feels like the sensations are originating from the skin
Where would someone feel referred pain from the oesophagus?
- Neck
* Middle of the trunk (especially left side)
Where would a person feel referred pain from the heart?
- Neck
- Back of head
- Left forearm (medial side)
- Left breast
Where would a person feel referred pain form the bladder?
Around the genitalia and bum
Where would a person feel referred pain from the left ureter?
Left abs and lower left back
Where would a person feel referred pain from the right prostate?
- Right abs and mid lower back
- Right calf and foot
- Right inner thigh
