Neurons & the LGB

Central nervous system = brain & spinal cord (cervical, thoracic, lumbar, sacral etc)

Peripheral nervous system = spinal nerves away from the centre

 

Neurons have an axon, axon hillock (where action potential is formed), a cell body (soma), dendrites & can be myelinated (axon is surrounded by layers of Schwann cells & gaps in between these called nodes of ranvier) or unmyelinated (no Schwann cells). Myelinated axons are contained in the white matter of the spinal column), whereas the cell bodies/soma are found in the grey matter. Neurons can be structurally classified into unipolar (one axon), bipolar (two axons) or multipolar (most common type & widespread).

Neurons can be functionally classified into afferent, interneurons & efferent. Afferent neurons collect sensory information & deliver info into the CNS from the environment, they are usually unipolar & lie largely in the PNS. Interneurons are almost all located withing the CNS & are a link between the sensory input & the motor output. There are approx 20 billion of these. Efferent neurons take information away from the CNS into the PNS, The soma is located within the CNS(whereas for afferent soma is in PNS) & it is a motor neuron. Axons are covered with endoneurium which is connective tissue. A bundle of axons (called a fascicle) is covered with perineurium & a bundle of fascicles are covered with epineurium (which makes a nerve).

The optic chiasma is covered with pia mater & is the junction where the optic nerves of each eye cross. The optic tracts emerge from the posterolateral angles of the optic chiasma, most of the nerve fibres in the optic tract terminate in the LGB; the lateral root of the optic tract is concerned with conscious visual sensation, the smaller medial root contains fibres of unknown function. Nasal fibres of the eye travel contralaterally, temporal fibres of the eye travel ipsilaterally.

The lateral geniculate body is located on the posterior surface of the thalamus & has six layers – 3 of which are: magnocellular, parvocellular, koniocellular. The layers are separated by white bands of optic nerve fibres, which are axons of the ganglion cell layer of the retina & come from the temporal half of the ipsilateral eye & the nasal half of the contralateral eye which cross the midline in the optic chiasma. The nerve fibres that cross the midline terminate in layers 1, 4 & 6, whereas the nerve fibres that do not cross terminate in layers 2, 3 & 5.

Optic radiations (aka geniculocalcarine tracts) are located in the white matter of the cerebral hemispheres & travels to the occipital lobe where the primary visual cortex is located.

The cerebral cortex contains the primary sensory areas (sensation, vision, hearing etc.), motor areas are also present & the rest is referred to as the association cortex.

The primary visual cortex surrounds the calcarine sulcus on the medial surface of the occipital lobe & extends posteriorly to occipital pole. It receives input from the LGN. It’s 2mm wide & organised into many horizontal layers. It has a characteristic presence of a white line/stria in the grey matter, which is formed it in fourth layer of the cortex by presence of myelinated fibres from the optic radiations.