Lecture 3 - peripheral and central afferent pathways

Somatosensory afferent neurons

mecahnoreceptors are specialized cells that project to an afferent neuron or are actually part of the afferent neuron

Factors that determine how fast the afferent information reaches the CNS:

1. Amount of insulation around the axon - insulation prevents ions from leaking out of the axons and forces ions to move along the axon

2. The diameter of the axon of the afferent neuron - A larger diameter allows ions to flow unobstructed along the axon (faster conduction)

Receptor type: Cutaneous mechanoreceptor 

Afferent fiber classification: Group II, A-beta

receptor type: Muscle spindles

Afferent fiber type: Group Ia/A-alpha

Receptor type: Dynamic Static

Afferent fiber: Group II/A-beta

receptor type: Golgi tendon organs

Afferent fiber type: Group Ib/A-alpha

receptor type: joint receptors

afferent fiber type: group II/A-beta

receptor type: free nerve endings

Afferent fiber type: Group III and Group IV/A-delta and C fibers, which respond to pain, temperature, and crude touch.

Afferent projections to the CNS

travel to gray matter regions in the CNS specific to that sensory system

• sensory info from mult systems converge in association areas to generate a unified percept or action

Divergence

the same input separates/disperses to multiple locations

• 1 input —> many outputs

Convergence

multiple inputs project to a common location

• many inputs —> 1 output

Topographic mapping

ordered projection of a sensory surface, like the retina or the skin, within the nuclei (neurons) in the CNS

topography is found throughout all levels of the CNS

Ipsilateral spatial mapping

belonging to or occurring from the same side of the body

Contralateral spatial mapping

belonging to or occuring for the opposite of the body. 

Geniculostriate visual system

90% of fibers

• primary visual pathway

  • retina —> lateral geniculate (LGN) thalamic nucleus —> Primary visual cortex

Tectopulvinar visual system

10% of fibers

• the secondary visual pathway

  • retina —> superior colliculus —> pulvinar nucleus of the thalamus —> visual areas of the cortex

Retinotopic organization

is the orderly mapping of the visual field onto the visual cortex such that adjacent areas in the retina correspond to adjacent areas in the cortex.

how would your perception change if: you had a lesion of the right optic nerve

loss of peripheral aspect of the right visual field

how would your perception change if: you had a lesion of the optic chiasm

loss of peripheral vision in both sides —> tunnel vision, loose all lateral portions of vision

how would your perception change if: you suffered a lesion to the right optic tract

loss of the left visual field, affecting both peripheral and central vision in that area. 3

visual field

the part of the visual environment that can be detected by both eyes

Hemifield

the left or right half of the visual field

optic chiasm

the point in the brain where the optic nerves cross, allowing visual information from both eyes to be integrated.

Left hemifield

right thalamus/V1

right hemifield

left thalamus/V1

Dorsal stream

A pathway in the brain that relates the visual environment (spatial location, motion) to the body and guides actions related to spatial awareness.

• The “where” or “how” pathway

Ventral stream

A pathway in the brain responsible for processing visual information related to object recognition and form.

• It is often referred to as the "what" pathway.

prosopagnosia

A neurological condition characterized by the inability to recognize faces, often due to damage in the fusiform gyrus.

what cranial nerve carries vestibular (balance) information to the brain?

Vestibulocochlear nerve (CNVIII)

Do cranial nerves run through the spinal cord?

No, they go directly to the brain stem

Where do vestibular axons project directly to in the brain?

the ipsilateral vestibular nuclei (pons and medulla)

the ipsilateral cerebellum

how many nuclei make up the vestibular nuclear complex?

four nuclei

where do somatosensory nerves enter the spinal cord?

dorsal horn

what do somatosensory nerve do once they enter the spinal cord?

Ascending pathways are formed by layered neurons with divergent projections for reflexes and local processing.

posterior column pathway

A major somatosensory pathway that transmits fine touch, proprioception, and vibratory sensations from the lower body and trunk to the brain.

third order neuron

Neuron that relays information from the thalamus to the sensory cortex.

second order neuron

neuron that crosses the body midline in the medulla and projects to the thalamic nucleus

first order neuron

axon that projects from the receptor to the medulla

Spinothalamic somatosensory pathways

1. anterior spinothalamic tract

   1. more anterior-medial part of the spinothalamic tract

   2. crude, poorly localized information about touch

2. lateral spinothalamic tract

   1. more lateral part of the spinothalamic tract

   2. info abt painful stimuli and temperature

Dorsal spinocerebellar tract (DSCT)

• projects to the inferior peduncle of the cerebellum

• proprioceptive information from the lower limbs (spindle fibers via Ia afferent neurons)

Rostral spinocerebellar tract (RSCT)

• projects to the inferior peduncle of the cerebellum

• proprioceptive information from the upper limbs

Ventral spinocerebellar tract (VSCT)

• Projects to the superior peduncle of the cerebellum

• carries proprioceptive information from the lower limbs, integrating sensory feedback for motor coordination (golgi tendon organs via Ib afferent neurons).