Structure and function

The brain is composed of neurons, neuroglia, and blood vessels. Each neuron is composed of a cell body, dendrites, which are short non-myelinated processes, and one or more axons whose length varies from a few millimetres to over 1 m. Neurons may be unipolar, bipolar or multipolar; the first two are primarily afferent and convey sensory information from receptor endings to the central nervous system (CNS). The majority of neurons in the CNS are of the multipolar type. In the peripheral nervous system (PNS), axons are ensheathed by neurilemmal cells which form myelin in myelinated axons, although unmyelinated axons have a sheath but no myelin. The myelinated axon has regular gaps in the myelin called nodes of Ranvier. In the CNS, axons may be myelinated or unmyelinated, and some neurons, such as those in the anterior horn cell of the spinal cord, have very long axons.

In both the CNS and autonomic nervous system, axons make contact with a neuron, a dendrite or another axon through a synapse. At most synapses a nervous impulse is chemically mediated and is due to the release of a specific transmitting substance stored in the axonal ending and thus transmission is unidirectional. Synapses may be excitatory

Posterior clinoid process Frontal bone

Lesser wing of sphenoid

Greater wing of sphenoid

Foramen spinosum Petrous temporal bone Jugular foramen

Groove for sigmoid sinus

Parietal bone

Groove for petrosal sinus

Cribriform plate of ethmoid

Optic canal

Posterior clinoid process Frontal bone

Cribriform plate of ethmoid

Lesser wing of sphenoid

Greater wing of sphenoid

Foramen spinosum Petrous temporal bone Jugular foramen

Groove for sigmoid sinus

Parietal bone

Optic canal

Anterior clinoid process Foramen ovale Foramen lacerum

Internal acoustic meatus

Hypoglossal canal

Groove for transverse sinus

Groove for petrosal sinus

Internal occipital protuberance

Anterior clinoid process Foramen ovale Foramen lacerum

Internal acoustic meatus

Hypoglossal canal

Groove for transverse sinus

Internal occipital protuberance

Figure 21.1. Internal surface of the skull base.

Table 21.1. Foramina of the skull.

Foramen

Structures passing through

Optic

Superior orbital fissure

Foramen rotundum

Foramen ovale

Foramen spinosum

Foramen lacerum

Carotid canal Stylomastoid Internal acoustic meatus

Jugular foramen

Foramen magnum

Optic nerve, ophthalmic artery Occulomotor nerve, trochlear nerve, abducens nerve, trigeminal nerve (ophthalmic division V1) Trigeminal nerve (maxillary division, V2) Trigeminal nerve (mandibular division, V2), lesser petrosal nerve Middle meningeal artery, meningeal branch of mandibular nerve Carotid artery enters into side above closed inferior portion Carotid artery, sympathetic plexus Facial nerve (exit)

Facial nerve, cochlear nerve, superior and inferior vestibular nerves, labyrinthine artery and vein Glossopharyngeal nerves, vagus nerve, accessory nerve, sigmoid sinus, inferior petrosal sinus Spinal cord, hypoglossal nerve, vertebral arteries, spinal arteries, cervical accessory nerve or inhibitory. There are many synaptic substances within the brain (central transmitters), the best known being dopa-mine, noradrenaline, adrenaline, serotonin, acetylcholine, and gamma amino-butyric acid (GABA). Excitation of a neuron gives rise to a propagated action potential which travels along the axon by a wave of depolarization at constant speed. In myelinated fibres, conduction is faster since depolarization jumps from one node of Ranvier to the next (saltatory conduction). All this depends on the permeability of the cell membrane to sodium and potassium, and the sodium-potassium pump.

Neuroglia are cells which neither form synapses nor conduct impulses. Oligodendrocytes predominate in the white matter and play the same role as the neurilemmal cells in the PNS. Astrocytes are larger and stellate in form and though extremely numerous much remains to be learnt regarding their influence on neuronal activity. Microglia are derived from mononuclear cells which migrate from blood vessels and can become macrophagic in response to brain injury or disease. Ependymal cells are ciliated and line the ventricles and the central canal of the spinal cord. The neurilemma are described above.

Aggregations of neurons are called nuclei and collections or bundles of axons, fibre tracts.

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