Case 20- Physiology and Anatomy Flashcards
Decorticate and Decerebrate posturing
- Decorticate- damage to motor pathways above the level of the red nucleus: midbrain, internal capsule, cerebral hemisphere
- Decerebrate- damage at or below the red nucleus: brainstem, cerebellum, pons
Corticospinal pathway
Starts in the motor cortex decussates at the pyramids in the medulla and crosses over there.
Tectospinal pathway
Control head movement in response to visual stimuli i.e. tracking. From the superior colliculi, it crosses over (contralateral) and terminates in the cervical area
Rubrospinal pathway (minor in humans)
Excites flexor activity and inhibits extensor activity. Most input is via the cerebellum and so may regulate learned movement.
Reticulospinal pathway
- Originates from the brain stem
- Controls coordinate movement in muscles- maintenance of balance and muscle tone
- Pontine and medullary reticulospinal tracts- originates in the oral and caudal pontine reticular nuclei (ipsilateral). Gigantocellular nucleus in the medulla (bilateral)
- Many reticulospinal fibres shift from ventral to lateral funiculi when descending
Vestibulospinal tracts
- Originate from the vestibular nucleus
- Uncrossed
- Sometimes split into lateral vestibulospinal tract and medial vestibulospinal tract
- Lateral VST- regulating tone of muscles in posture and balance. Excites extensors, inhibits flexors- muscle tone
- Medial VST- contact cervical motor neurones, head position, fixation of gaze (travels in medial longitudinal fasciculus)
Decorticate posturing
- Involuntary flexion of upper extremities in response to external stimuli (excess red nucleus influence- RN only influences upper extremities)
- The arms are flexed or bent inwards in the chest
- The hands are clenched into fists
- The legs extended and feet turned inwards
Decerebrate posturing
- Involuntary extension of upper extremities in response to external stimuli- excess influence from the vestibulospinal tract through the pontine reticular nucleus
- The head is arched back
- The arms are extended by the sides and rotated internally
- The legs are extended and rotated internally
- The patient is rigid with their teeth clenched
Abnormal posturing
Input from cortex and cerebellum modulates the activities of these pathways. Lesions such as those caused by stroke, upset the balance of activity of these pathways. Lesions above the red nucleus cause decorticate posturing and excites flexors and inhibits extensors. Lesions between the red nucleus and vestibular nucleus cause decerebrate posturing as it looses influence from the red nucleus. Increased tone and rigidity over excitation of extensors and inhibition of flexors
Divisions of the skull
The skull is made up of 22 bones joined by sutures. It can be split into two parts:
• Neurocranium- the bony case covering the brain. Has a roof (calvaria) and a floor/base (basicranium)
•Viscerocranium- the facial skeleton
The Bones forming the neurocranium
- Parietal bones (paired)
- Temporal bones (paired)
- Occipital bone (unpaired)
- Parts of the frontal bone (unpaired)
- Parts of the sphenoid bone (unpaired)
Structures of the vascerocranium (facial skeleton)- paired/unpaired
- Nasal bones (paired)
- Maxilla (paired)
- Zygomatic bones (paired)
- Lacrimal (paired)
- Vomer (unpaired)
- Palatine bones (paired)
- Inferior nasal conachae (paired)
The ethmoid bone
The ethmoid bone is sometimes considered part of the neurocranium and in other parts the viscerocranium. It makes a minor contribution to the neurocranium but also forms part of the medial wall of the orbits and nasal cavity.
Important features on the ethmoid bone
- Crista galli- is the attachment for the falx cerebri
- Ethmoidal air cell- sinuses
- Superior and middle nasal concha- extends into the nasal cavity
- Perpendicular plate- forms the superior two thirds of the nasal septum
- Cribiform plate- a sieve like structure which allows the olfactory nerve fibres to pass
The Zygomatic bones
The zygomatic bones form the check bones as they articulate with the temporal bones. When you are palpating the check bones you are touching the zygomatic arches.
The internal surface of the cranial base has 3 large depressions
- Anterior cranial fossa- formed of the frontal, ethmoid and sphenoid bones
- Middle cranial fossa- formed from parts of the sphenoid and temporal bones
- Posterior cranial fossa- consists of parts of the temporal and occipital bones with small contribution from the sphenoid and parietal bones
Sutures
Sutures are a form of fibrous joint that are only found in the skull. They are immovable in adults and completely fuse by 20 years. The main sutures:
• Coronal suture- between the frontal bone and the two parietal bones
• Sagittal suture- between the parietal bones
• Lamboid suture- between the occipital bone and the parietal bones
Additional sutures
Take the name from the bones they are found between: • Squamous suture • Occipitomastoid suture • Sphenosquamous suture • Sphenofrontal suture • Sphenoparietal suture
Craniometric points
Used to compare and describe the shape of the cranium and note variations:
• Pterion- junction of the sphenoid, temporal, frontal and parietal bones
• Lambda- junction of lamboid and sagittal suture. Site of the posterior fontanelle in neonates (typically closes at 2 months)
• Bregma- junction of coronal and sagittal suture. Site of the anterior fontanelle in neonates. (closes at approx. 18 months)
The pterion
Overlies the middle meningeal artery, a fracture in this area can injure the vessel. If the vessel is injured can accumulate between the skull and the dura mater forming an extradural haematoma
