Sensations from the Head and Face

Sensations from the Head and Face

• Somatic sensations from the head are carried mainly by the Trigeminal nerve, which consists of three divisions that carry cutaneous sensations from the face, forehead, and anterior part of the scalp, conjunctiva, cornea, and nasal mucosa.
• The Trigeminal nerve also carries proprioceptive sensations from the mandibular muscles.

Headache

• Headache is pain felt in the head region, referred to the surface of the head from deep structures.
• There are two types of headache:
  • Extracranial headache: resulting from the stimulation of pain receptors located in extracranial structures, such as eyes, nose, oral causes, and psychogenic headache.
  • Intracranial headache: resulting from the stimulation of pain receptors in intracranial pain-sensitive structures, such as meninges, cerebral and dural blood vessels, venous sinuses, and nerves V, IX, X.
• The site of reference of intracranial headache depends on the location of the pain:
  • Pain arising from structures above the tentorium cerebelli is felt in the front of the ears (frontal headache).
  • Pain arising from structures below the tentorium cerebelli is felt in the posterior part of the head (occipital headache).

Causes of Intracranial Headache

• Meningeal causes: inflammation of meninges (meningitis), trauma or injury, and irritation of the meninges as in cases of excess alcohol drinking or constipation.
• Brain tissue causes: brain tumors, which cause pressure, traction, and irritation of the intracranial pain-sensitive structures.
• CSF causes: decrease in CSF pressure, especially if the person is erect, which can cause traction on the meninges.
• Vascular headache: resulting from the overstretching of the walls of the intracranial blood vessels, especially in conditions such as hypertension, fever, and migraine.

Somatic Sensory Cortex

• The somatic sensory cortex is the part of the cerebral cortex concerned with the perception and interpretation of sensory information.
• It is divided into three areas:
  • Primary sensory cortex (3, 1 & 2): located in the parietal cortex, consists of 6 layers, and is concerned with the perception of various somatic sensory senses.
  • Secondary somatic sensory cortex (area 40): located behind and lateral to the face representation area, receives direct input from VPN of thalamus and primary somatic sensory cortex, and is involved in learning based on tactile discrimination.
  • Sensory "Somatic" association cortex (5 & 7): located behind the uppermost part of SMI, is concerned with building up and disclosing the meanings of various types of sensations, and transmitting pre-analyzed kinesthetic sensory information to brain motor centers.

Somatic Motor System

• The somatic motor system consists of higher motor centers located in the cerebral cortex and brain stem, and their axons form descending motor tracts that descend to and synapse with the neurons of lower motor centers in the brain stem and spinal cord.
• The somatic motor system is composed of two sets of neurons:
  • Upper motor neurons (U.M.N.): whose cell bodies lie in the higher motor centers and their axons constitute the descending motor tracts.
  • Lower motor neurons (L.M.N.): whose cell bodies lie in the spinal ventral horns or the corresponding cranial nuclei, and their axons constitute the efferent motor fibers in the peripheral somatic nerves.

Cortical Motor Areas

• The motor cortex is located in the frontal lobe and comprises:
  • Primary motor area (area 4): located in the precentral gyrus, concerned with the initiation and performance of voluntary movements, and discharges motor impulses that produce voluntary movements done by the distal limb muscles.
  • Premotor area (area 6): located anterior to the primary motor area, concerned with planning of complex movements, initiating gross movements, and controlling posture and muscle tone.
  • Supplemental motor area: located in the upper medial side of the frontal lobe, concerned with evoking complex movements, planning and programming of complex movements, and providing suitable background for the performance of fine skilled movements.