Ascending Sensory Pathways Flashcards
What are the somatosensory pathways responsible for?
- Consist of the chain of neurons, from receptor organ to cerebral cortex
- Start with the peripheral first-order (1°) afferent
- End in the cerebral cortex
- Responsible for the perception of bodily senses
What are the somatosensory pathways for the body?
- Spinothalmic pathway: Pain, temperature, crude touch, pressure
- Dorsal column medial lemniscus pathway: Fine touch, vibration, pressure and position
Which of the following corresponds to the primary somatosensory cortex? Where is it located?
Red (where all the somatosensory pathways end) –> post central gyrus
What information is received in the primary somatosensory cortex?
Pain, temperature and touch sensation
Which area is responsible for perceiving visual information?
Blue (occipital lobe)
Which area is responsible for perceiving auditory information?
Green - The auditory cortex is the part of the temporal lobe
What is another word for somatosensory?
Somaesthetic
The general sensory (somaesthetic) pathways carry information to the brain from where? What are examples of these modalities?
- Skin
- Mucous membranes
- Joints and muscles
Modalities: vibration, touch, pressure, pain, temperature, proprioception
What is proprioception?
Our awareness of where our body parts are in space
What is each modality detected by (in the general somaesthetic pathways)?
Each modality is detected by specific sensory receptors / specialised nerve endings
e.g. specific receptor for vibration
These receptors then send their axons all the way via the spinal peripheral nerves to enter the spinal cord.
How does sensory info enter the spinal cord?
Dorsally
Sensory pathways travel from peripheral nerves into the CNS and ascend through spinal cord/brainstem to the brain.
Which nerves transmit sensory info from:
- the body?
- the head, face and neck?
In the body: info transmitted by spinal nerves
In head, face and neck: info transmitted by cranial nerves.
What is a dermatome?
A specific region of skin of the body innervated by the sensory fibres of a single segmental spinal nerve (spinal cord segment):
- E.g. Any sensation at level of nipple is transmitted by the T4 segmental spinal nerve
- E.g. Any sensation at level of umbilicus is transmitted by the T10 segmental spinal nerve
Similar to dermatomes, areas of skin of the face supplied by a single cranial nerve can be identified and mapped. BUT this is not called dermatome.
What is the cranial nerve that supplies sensation to areas of the face?
Branches of CN V trigeminal
The herpes varicella zoster virus causes chickenpox. After you recover from chicken pox, the virus can enter your nervous system and lie dormant for years.
- Where does the varicella zoster virus lie dormant?
- What can it reactivate as?
- Which dermatome is affected during reactivation?
- Lies dormant in the sensory (dorsal) roots of specific spinal or cranial nerves
- Shingles
- Cause of reactivation is not well known (but could be linked to dip in immunity)
- Only affects at level of sensory spinal nerves where virus remained dormant
- E.g. in photo, eruption of vesicles is localised to the area of skin supplied by the T4 spinal nerve
What are the 2 groups of somaesthetic modalities?
- Modalities that are essential to survival
- Pain, temperature
- Some touch and pressure
- Modalities that increase detail
- Discriminative touch: 2-point discrimination and pressure vibration
- Proprioception
How are modalities that are essential to survival carried? What is the conduction rate?
- Carried via thin, poorly myelinated or unmyelinated fibres -
- Conduction relatively slow
How are modalities that increase detail carried? What is the conduction rate?
- Carried via large diameter, heavily myelinated fibres
- Fast conduction
Review of Spinal Cord - Transverse Section
- Grey matter confined to inside and subdivided into regions:
- Dorsal grey horn
- Receives sensory fibres
- Many synapses occur here
- Lateral grey horn
- Contains preganglionic visceral neurons
- Ventral grey horn:
- Contains motor somatic neurons
- Dorsal grey horn
- White matter on outside –> this is where we will find the sensory and motor pathway
- Dorsal white column
- Lateral white column
- Ventral white column
- Ventral white commissure
- Allows fibres to cross from either side
Are the sensory and motor pathways found in grey or white matter?
White matter
Functionally, what can the ascending/somatosensory tracts be divided into?
- Conscious tracts: comprised of the dorsal column-medial lemniscal pathway and the anterolateral system.
- Unconscious tracts: comprised of the spinocerebellar tracts.
DCML Pathway:
- What is it?
- Location?
- What information does it carry?
- Where do fibres decussate?
- What: Ascending sensory pathway
- Location: Dorsal column (white matter) of spinal cord
- Information:
- Fine touch
- Vibration
- Joint position sense (proprioception)
- Decussation: Level of medulla
What side of the body does the DCML pathway convey sensory info from? (i.e. ipsilateral or opposite)
Ipsilateral: If the lesion occurs in the spinal cord (which is most common), the sensory loss will be ipsilateral –> decussates at level of brainstem
How would a lesion of the DCML pathway present?
A loss of proprioception and fine touch. However, a small number of tactile fibres travel within the anterolateral system, and so the patient is still able to perform tasks requiring tactile information processing.
What does the dorsal/posterior column refer to?
the posterior spinal cord, which contains ascending sensory pathways
What forms the dorsal column?
Formed by two large fasciculi (bundles of nerve fibers –> white matter) running through the posterior spinal cord:
- fasciculus gracilis
- fasciculus cuneatus.
These fasciculi gather sensory information from our body’s periphery via skin and joint receptors, and send this information to superior cerebral structures.
What 2 parts does the DCML pathway consist of?
- Dorsal column: Nerve tracts in the white matter of the dorsal columns carry info from spinal cord to medulla
- Medial lemniscus: Continuation of the dorsal column, this pathway starts within the brainstem and sends sensory input to the thalamus and postcentral gyrus
Where is the DCML pathway located in the cord?
Dorsally
Spinothalamic Pathway:
- What?
- Location?
- What info does it carry?
- What is it also referred to as?
- Location of decussation of fibres?
- Ascending pathway consisting of 2 separate tracts; anterior spinothalamic and lateral spinothalamic tract
- Location: Anterior and lateral column (white matter) of spinal cord
- Information:
- Anterior spinothalamic –> crude touch and pressure
- Lateral spinothalamic –> pain and temperature
- Also referred to as the ventrolateral (anterolateral) system
- Decussation –> level of spinal cord
Where does the spinothalamic tract decussate? What does this then mean?
Decussates at the level of the spinal cord, rather than at the brainstem –> left spinothalamic tract conveys sensory info of the right
Where is the spinothalamic tract located in the cord?
Anterior (and lateral) part of white matter i.e. anterolateral system
What are the corticospinal tracts (lateral and ventral)?
Descending motor pathways
The general sensory pathways typically consist of a three neuron chain. Describe the 1st neuron in this chain:
- shape?
- location?
- location of cell body: body and head/neck?
Receptor sends info via axon to cell body
- Primary neuron is the sensory receptor in the periphery that detects sensory stimuli
- Pseudounipolar: one branch going to PNS and other to CNS
- If sensation in body: Cell body is housed in the dorsal root ganglion of a spinal nerve
- If sensation is head or neck: Cell body is housed in the ganglia of the trigeminal or cranial nerves
Describe a pseudounipolar neuron
A type of neuron which has one extension from its cell body. This type of neuron contains an axon that has split into two branches; one branch travels to the peripheral nervous system and the other to the central nervous system.
The axon of the 1ary neuron in the 3 neuron chain of the general sensory pathway then enters the spinal cord via the dorsal root of the spinal nerve to synapse with the 2ary neuron.
- Describe the pathway of the 2ary neuron.
- Where does it start?
- Where does it ascend to?
- The secondary neuron acts as a relay and is located in either the spinal cord or the brainstem.
- 2ary neuron is in the ipsilateral grey matter of the CNS
- Axon of 2ary neuron crosses the midline and ascend up the spinal cord to the brain
- Ascends to the ventral posterior thalamus
For info coming from the body, where does the 2ary neuron ascend to?
Ventral posterior thalamus –> to the ventroposterolateral (VPL) nucleus
For info coming from the face, where does the 2ary neuron ascend to?
Ventral posterior thalamus –> VPM (ventroposterior medial nucleus)
Once entering the ventral posterior thalamus, what does the 2ary neuron do?
Synapses with the 3ary neuron (or, some terminate here)
Regarding the 3ary neuron of the 3 neuron chain:
- Where is its cell body?
- Where does it relay info to?
- Cell body in the thalamus
- Projects to the postcentral gyrus (primary sensory cortex) of the parietal lobe
Summary of 3 neuron chain of general sensory pathways:
Comprise 3 neurons (3 neuron chain): first order, second order and third order
- First order neurons
- From PNS to CNS. Cell bodies in peripheral ganglia (eg. Dorsal root ganglion)
- Pseudounipolar.
- Second order neurons
- Cell bodies of 2nd order neurons reside in CNS ipsilateral grey matter
- Axons cross midline.
- Ascend to Ventroposterolateral (VPL) nucleus of thalamus
- Third order neurons
- Cell bodies of 3rd order neurons reside in thalamus and axons project to somato-sensory cortex
It is important to remember that synaptic transmission from 1st, 2nd and 3rd neurons isn’t a simple relay. Can be modified by other inputs: eg. descending pathways.
How are sensory pathways organised?
Somatotopically organised
What is the spinothalamic pathway responsible for?
pain, temperature, crude touch, some pressure
Describe pathway of spinothalamic pathway from receptor to brain
1ary neuron:
- Receptor located in the periphery and fibres are sent using the dorsal aspect of spinal nerve
- Cell body is found in the dorsal root ganglion (pseudounipolar neuron)
- Fiibres enter into spinal cord and pass briefly through Lissauer’s tract to enter the dorsal horn
- Synapses with 2ary neuron either:
- at same level of entry
- or ascends 1-2 segments to synapse with 2ary neuron
2ary neuron:
- Sends axons that cross the midline in the ventral white commissure
- Axon then enters the spinothalamic tract (ventral white matter)
- Axon then ascends contralaterally in the anterolateral section of the spinal cord to the opposite thalamus
Where is a huge collection of cell bodies of 2ary neurons found?
In dorsal grey horn
Regarding the spinothalamic pathway: After the 1ary neuron enters the dorsal grey horn and synapses with the 2ary neuron, where does the 2ary neuron then send its axon?
- Sends axons that cross the midline in the ventral white commissure
- Axon then enters the spinothalamic tract
What does the spinothalamic tract consist of?
collection of axons that have crossed the midline
The spinothalamic pathway retains somatotopic organisation. Describe the order of fibres from the arm, trunk and leg.
- Any fibre from the arm travels more medially in the tract
- Any fibres from the trunk travels in the middle of the tract
- Any fibres from the leg travels more laterally in the tract
(ATL - more medial to lateral)
In which lamina are the synapses from the spinothalamic tract found?
Lamina I, II, III and IV
i.e. any fibres carrying these modalities will synapse in these few lamina
What is substantia gelatinosa? What lamina does it contain?
- The substantia gelatinosa is a grey gelatinous mass of neuroglia and nerve cells that is located in a cap like formation at the apex of the spinal cord posterior grey matter.
- It extends the entire length of the spinal cord to the medulla.
- It contains Rexed lamina II.
In which lamina do fibres carrying crude touch synapse?
In either lamina I, III or IV (marginal zone or nucleus proprius)
Example: pain DUE TO crude touch e.g. pinch
Pain & crude touch –> spinothalamic
- BOTH fibres are stimulated
- Pain and crude touch pathway is activated
- Both reach brain at same time
- Brain perceives pressure as source of pain
Spinothalamic pathway described completely:
- 1ary neuron carrying sensation from pain, temperature,
- some touch and pressure receptors
- Contacts 2ary neuron either:
- At level of entry if pain or temperature
- Ascends first if crude touch or pressure
- 2ary neuron axon crosses midline and enters the contralateral spinothalamic tract
- Spinothalamic tract bring info all the way up contralaterally (through brainstem) to the thalamus
- N.B. after passing through the medulla, the spinothalamic tract CHANGES NAME to the spinal lemniscus
- Reaches contralateral thalamus (the VPL) where it synapsing with 3ary neuron
- 3ary neuron then leaves the thalamus to reach the postcentral gyrus
What is the sensory homunuculs?
- The map of sensory space
- Fibres (e.g. pain) coming from hand then these fibres end in the hand region of the postcentral gyrus
- Fibres coming from trunk end more medially
- Fibres from leg end even more medially
What makes up the dorsal column?
- Gracile fascile
- Found either side of midline
- Runs whole length of spinal cord
- Carries info from below T6 (lower half of body)
- Cuneate fascicle
- Found lateral to gracile fascile
- Found above T6 only
- Carries info from above T6 (up to C1)
The gracile and cuneate fascicles ascend to reach the dorsal medulla. What happens here?
An expansion due to a nucleus (cluster or neurons where axons synapse) –> cuneate and gracile tubercles
Where do the 1ary neurons travelling in the cuneate fascicle (DCML pathway) synapse?
Synapse with the 2ary neurons in the cuneate nucleus
Where do the first order neurons of the DCML pathway carry sensory information regarding touch, proprioception or vibration from/to?
From: peripheral receptors
To: medulla oblongata
The medial lemniscus is somatotopically organised. Fibres arriving from the arm, trunk and leg all have their own sections:
Transverse section of closed medulla (white matter appears darker than grey):
- Central canal
- Pyramids x2 anteriorly
- Dorsal column posteriorly: gracile & cuneate nucleus
- Fibres crossing midline to join contralateral medial lemniscus
DCML pathway summary:
Below T6:
- 1ary sensory fibre (e.g. from leg) enters gracile fascicle without synapsing
- Travels in gracile fascicle to closed medulla
Above T6:
- 1ary sensory fibre (e.g. from arm) enters cuneate fascicle without synapsing
- Travels in cuneate fascicle to closed medulla
- The fibres reach the gracicle/cuneate nucleus where they synapse with the 2ary neuron
- 2ary fibres then send axons that cross midline to enter the medial lemniscus
- Fibres then travel contralaterally to the VPL of the thalamus
- 2ary neurons then synapse with 3ary neurons which then sends fibres to postcentral gyrus
Somatotopic map on post-central gyrus:
Is this cross section above or below T6? Why? Where is this section taken from?
- Above - has a cuneate fasicle as well as gracile fascicle
- Thoracic section:
- Lateral grey horn is present
- Closed medulla
- Cervical cord
- Thoracic cord (above T6)
- Lumbar cord
Difference between them explained:
- Lumbar cord
- Only 1 dorsal column each side (gracile fascicle): cuneate fascicle NOT present
- Ventral grey matter very prominent –> lots of neurons needed to innervate lower limbs
- Thoracic cord (above T6)
- 2 dorsal columns each side: gracile and cuneate fascicles
- Small ventral grey matter
- Lateral grey horn present
- Cervical cord:
- 2 dorsal columns each side: gracile and cuneate fascicles
- Large ventral grey matter –> lots of neurons needed to innervate upper limbs
- Closed medulla:
- Cuneate tubercle with cuneate nucleus
- Gracile tubercle with gracile nucleus
Spinothalamic tract:
DCML pathway:
Synaptic transmission from 1 st, 2nd and 3rd neurons isn’t a simple relay. It can be modified by other input such as convergence/divergence or descending pathways.
- What is a convergence pathway?
- What is a divergence pathway?
- Convergence: allows a neuron to receive input from many neurons in a network.
- Divergence: allows one neuron to communicate with many other neurons in a network
What is the internal capsule? What passes through here?
- A white matter structure situated in the inferomedial part of each cerebral hemisphere of the brain located between the brain and lower centres of CNS.
- ALL the ascending sensory axons travel in the posterior limb of the internal capulse on their way to the postcentral gyrus
General somatic sensation from the head is carried in which cranial nerves? Which area of the head does each innervate?
- Trigeminal CN V –> major carrier of general sensory info from face, nose, scalp and dura
- Facial CN VII –> external ear
- Glossopharyngeal CN IX –> posterior 1/3 tongue, pharynx, middle ear
- Vagus CN X –> auditory canal, larynx, pharynx, oesophagus
All information from these nerves travel in the same central pathway: trigeminal sensory pathway which then carries info to the postcentral gyrus.
Location of trigeminal nuclei (anterior view) in brainstem.
Notice the motor nuclei of the trigeminal lies medial to the sensory nuclei due to large sensory function.
How many sensory/motor nuclei does the trigeminal nerve have?
The trigeminal nerve originates from three sensory nuclei (mesencephalic, chief sensory, spinal nuclei of trigeminal nerve) and one motor nuclei (motor nucleus of the trigeminal nerve).
Where does the trigeminal nerve arise from in the brainstem?
- At the level of the pons (lateral aspect)
- Has 3 divisions
- Each division uses a different foramina to enter the cranium
Where are the cell bodies of the 1ary sensory neurons of the trigeminal nerve located?
In the trigeminal ganglion (for all 3 divisions)
How does each division of CN V enter the cranium?
V1: superior orbital fissure
V2: foramen rotundum
V3: foramen ovale
Distribution of sensory innervation of CN V
This lady is suffering from a reactivation of the Varicella Zoster herpes virus. Where has the virus reactivated?
Infection of the sensory roots of the maxillary division (V2) of the trigeminal nerve –> eruption of vesicles localised to the area of skin supplied by this maxillary division.
Where is each division of the sensory nucleus of CN V located? Where do they converge?
- Mesencephalic nucleus: midbrain
- Pontine/chief nucleus: pons
- Spinal nucleus: medulla
At the level of the pons, the sensory nuclei merge to form a sensory root. The motor nucleus continues to form a motor root.
The 3 different divisions of the sensory nucleus of the trigeminal nerve receives fibres of different modalities. What does each division receive?
- Mesencephalic nucleus –> receives proprioception
- Pontine/chief nucleus –> discriminative touch
- Spinal nucleus:
- More rostral parts –> receives simple touch and pressure
- More caudal part –> receives pain and temperature
Pathway for pain and temperature of head and neck explained:
- 1ary neurons travel from the periphery with cell bodies in the trigeminal ganglion
- Enter pons and 1ary neurons run caudally in the spinal tract of the trigeminal –> fibres descending to reach the caudal spinal nucleus of the trigeminal
- Once they reach the caudal spinal nucleus, they synapse with 2ary neurons
- 2ary neuron then sends an axon that crosses the midline and ascends in the trigeminothalamic tract to the VPM of the thalamus (then postcentral gyrus)
Pathway for crude touch and pressure from the head/face/neck:
- 1ary neuron has receptor in periphery and cell body in the trigeminal ganglion
- Axon enters pons and fibres descend in the spinal tract of trigeminal to reach the rostral part of the spinal nucleus
- Here, the 1ary neurons synapse with the 2ary neurons
- Axons of 2ary neurons then cross the midline and ascend in the trigeminothalamic tract to reach the VPM of the thalamus
Somatotopic map on post-central gyrus: where do sensory fibres from the head go?
More laterally
What would the the sensory consequences of a lesion affecting the lateral medulla on:
- sensation from face?
- sensation from body?
- Face:
- Pain, temperature and crude touch from right side of face affected
- Body:
- Analgesia and thermoanaesthesia (loss of pain and temperature) on left side of body
Cross section through closed medulla showing location of the different tracts
Pathway for discriminatory touch from head/face/neck:
- 1ary sensory neuron with receptor in periphery and cell body in the trigeminal ganglion
- Synapse with 2ary neuron now occurs in the pontine/chief nucleus
- Axon of 2ary neuron crosses the midline and ascends in the trigeminothalamic tract to reach the VPM of the thalamus (and then to the postcentral gyrus)
Pathway for proprioception from head/face/neck:
N.B. this pathway is slightly different
- 1ary neuron has receptor in periphery BUT does not have cell body in the trigeminal ganglion (only exception) –> instead are located in the mesencephalic nucleus
- 1ary neuron then synapses with the 2ary neuron alongside the mesencephalic nucleus
- 2ary neuron then sends axon which crosses the midline and ascends in the trigeminothalamic tract to the VPM of the thalamus
Describe the pathway of the jawjerk reflex
This is proprioception:
- 1ary neuron has cell body in mesencephalic nucleus
- Then, using a small collateral fibre, information is sent to the motor nucleus of V in the pons (trigeminal motor nucleus)
- No time to go to brain as is reflex!
- Motor nuceleus of V then sends motor fibres to muscles of mastication via V3 –> jaw jerk
Summary of trigeminal sensory pathways
