2. Somatosensory Pathways & Thalamus Flashcards
In what two types of nerves (broadly speaking) is sensory information carried to the CNS
- Spinal Nerves
- Cranial Nerves
(Afferent Fibres in Each Case)
What are the two subdivisions of the sensory system?
- Somatosensory System
2. Special Senses
How is somatosensory information carried to the CNS?
Peripheral information carried from trunk/limbs in spinal nerves, or from the head in one cranial nerve (trigeminal, V).
What is the somatosensory system
System comprising peripheral receptors and processing centres to produce the somatic sensory modalities. System reacts to diverse stimuli through the receptors, causing impulses to flow via sensory (afferent) nerves, through tracts in the spinal cord, and into the brain for processing
List the somatosensory modalities and any sub-types.
- Touch (Discriminative & Non-Discriminative)
- Temperature
- Proprioception (Conscious & Unconscious)
- Nociception
Differentiate between discriminative and non-discriminative touch?
i. Discriminative (density of receptors increase, able to discriminate between single point or multiple point touch. E.g. finger v shoulder)
ii. Non-Discriminative Compass points on skin of finger tips v skin on shoulder.
What is proprioception?
Sense of the relative position of neighbouring parts of the body and strength of effort being employed in movement. It is provided by proprioceptors in muscle spindles of skeletal striated muscles and in joints. Essentially, a computation of the degree of contraction of muscle fibres allow this awareness.
Differentiate between conscious and unconscious proprioception?
Conscious proprioception(CP): Sensory information from from receptors in limbs and trunk is carried by peripheral nerves then the spinal cord (via faciculus cuneatus and faciculus gracillus) to the brainstem where it crosses over to the contralateral cerebral cortex; here precise information about the position and orientation of limbs and trunk is perceived and organized. (for schematic of this, see above) This information is utilized to facilitate complex motor activity, e.g. running/catching a frisbee, fighting Defects anywhere along the pathway are manifest as stumbling, knuckling; however, the routine gait and posture may be normal (depending on status of UCP...see below)
Unconcious Proprioception(UCP):
Sensory information from receptors in the limbs and trunk are transmitted via peripheral nerves to the spinocerebellar tracts where the information terminates on the ipsolateral cerebellum. This information is utilized to coordinate basic posturing during sitting, standing and simple gait activities
Defects anywhere along the pathway may be manifest as postural deficits, ataxia
What is nociception?
Sensing of harmful stimuli such as intense chemical, mechanical or thermal stimulation.
Nociception utilises nociceptors or “pain receptors
Outline the Organisation of Sensory Pathways
1st Order Neurons
• Pressure, vibration, tickling (from pacinian corpuscle, via large myelinated)
• Free nerve endings = pain, temperature (via small myelinated)
2nd Order Neurons
• Cell body lies in grey matter of the spinal (spinal nerve) or the brainstem (trigeminal).
• Axons pass through the ascending tracts of the cord and cross over (decussate).
• Most axons then project to the terminate in thalamus (aka gateway to cerebral cortex)
3rd Order Neurons/Thalamus
• Third order neurone projects from here to cerebral cortex (primary somatosensory cortex)
• Parietal lobe strip the primary destination for most of these axons.
• Right & left thalami control flow of sensory information to cortex.
• Joined across midline (interthalamic adhesion)
• Form lateral walls of third ventricle
• Largest part of diencephalon (between brainstem/cerebral hemispheres)
• All sensory information to brain synapses in the thalamus before going to the cortex (smell is a partial exception)
• Cranial nerve II (optic) connects directly to the thalamus
• Other cranial nerves send fibres to the thalamus from lower (brainstem) nuclei
Describe the gross anatomy of the thalamus.
- Internal capsule (white matter made up of axons squeezing through the grey matter of the thalamus)
- Thalamic Nuclei
What are the two types of spinal roots?
Dorsal
Ventral
Where does the the spinal roots join? What properties does this structure have?
Join at spinal nerve trunk which is a mixed nerve
What type of fibers are contained in the anterior and posterior rami and from what structure are they derived?
The anterior and posterior rami contain mixed fibres derived from the the spinal nerve trunk, which in turn is derived from the dorsal horn (sensory) and ventral horn (motor).
Which is thicker the anterior or posterior rami? Why?
Anterior ramus thicker as carries more info from ant trunk and whole of limbs
What is the white matter of the spinal cord made up of?
Consists of longitudinally running nerve fibres
How are the nerve fibers of the white matter of the spinal cord organised?
Organised into a series of Ascending and Descending Tracts
Name the 3 ascending pathways (tracts) of the spinal cord?
– Dorsal Columns
– Spinothalamic Tract
– Spinocerebellar Tract
What sensory information is transmitted through the Dorsal Column Pathway?
Discriminative (fine) touch
Vibratory sense
Conscious proprioception
Describe the location and course of the first order neurones of the Dorsal Column Pathway?
Enter the cord & ascend ipsilaterally in the gracile fasciculus (lower limb, lower trunk) and the cuneate fasciculus (upper limb, upper trunk) to medulla oblongata and synapse (nucleus gracilis or nucleus cuneatus)
(The cell bodies of somatic sensory afferent fibers lie in ganglia throughout the spine)
Describe the location and course of the second order neurones of the Dorsal Column Pathway/Medial Lemniscus?
Second order axons cross and ascend as contralateral medial lemniscus (or trigeminal lemniscus) to the thalamus (VPL, VPM nucleus)
Describe the location and course of the third order neurones of the Dorsal Column Pathway/Medial Lemniscus?
Third order neurones project from thalamus to postcentral gyrus ‘somatotopically’
What are the possible effects of lesions affecting the Dorsal Column Pathway?
Lesions result in loss of fine tactile sensation and interference with proprioception
e.g. Sensory ataxia (ataxia = loss of co-ordination)
Where is the primary somatosensory area of the sensory cortex located?
In the Post-central Gyrus of the parietal lobe
