The the skull has many neurovascular tunnels including formamen to connect blood vessels and nerves inside the skull with the sensory organs, vital systems and muscles outside of the skull. Upright posture predisposes the brainstem and certain cranial nerves and blood vessels to compression within the neurovascular outlets and subsequent neurological signs and symptoms.
The most vulnerable of the openings is the large hole in the base of the skull, called the foramen magnum (FM), through which the brainstem connects to the cord. When the brainstem sinks or gets pushed down, it gets squeezed into the FM, which compresses it.
Descent of the brainstem into the foramen magnum also compresses the vertebral arteries which supply the brainstem, and the emissary veins that connect the dural sinus drainage system of the brain to the vertebral veins of the spinal canal. Lastly, it compresses the subarachnoid space of the cord which contains cerebrospinal fluid (CSF) on its way back to the brain from the lower cord. For further information see the section on Chiari malformations.
The other vulnerable openings are the optic canal (opthalmic artery and optic nerve), the foramen rotundum (maxillary division of trigeminal nerve), the foramen ovale (accessory meningeal artery and mandibular division of trigeminal nerve), the foramen spinosum (middle meningeal artery and meningeal branch of mandibular branch of trigeminal nerve),the internal accoustic meatus (labyrinthine artery,facial nerve and vestibulochoclear nerve, the jugular foramen (inferior petrosal sinus, sigmoid sinus, glossopharyngeal nerve, vagus nerve and accessory nerve),and the hypoglossal canal (hypoglossal nerve).
Compression of these tunnels can cause neurovascular signs and symptoms. For further information see the appropriate sections on optic neuritis, trigeminal neuralgia, Bell’s Palsy (facial muscle paralysis), tinnitis (rigning in the ears), dizziness, glossitis (burning tongue), vegetative/autonomic signs of organs and circulatory systems, such as sweating, high blood pressure, etc.,, as well as weakness and atrophy of the trapezius and sternocleidomastoid (SCM) neck muscles.