Nervous System

The nervous system is structurally divided into the Central Nervous System (CNS), comprising the brain and spinal cord, and the Peripheral Nervous System (PNS), which includes cranial nerves, spinal nerves, autonomic nerves, and the enteric nervous system.

During the third week of development, the ectoderm thickens to form a neural plate. This plate develops a neural groove, flanked by neural folds, which fuse during neurulation to form the neural tube. The neural tube gives rise to the brain vesicles and surrounding neural structures.

The PNS originates from nerve fibers extending from the CNS and from migratory neural crest cells, which also arise from the surface ectoderm.

Directional terms like ventral, dorsal, rostral, and caudal have varying meanings along the CNS due to an obligatory bend at the midbrain-diencephalic junction. Anterior, posterior, superior, and inferior are constant directional references.

The nervous system is studied using three planes of section: coronal (divides into anterior and posterior), sagittal (divides into left and right), and horizontal/axial/transverse (divides into superior and inferior).

The primary cellular components of the nervous system are neurons and glial cells. Neurons transmit signals, while glial cells provide support and insulation.

A typical neuron consists of a cell body (soma) containing the nucleus, dendrites for receiving input, and an axon for conducting signals away from the cell body.

Glial cells form a myelin sheath around axons to prevent signal loss. Exposed segments of the axon, called nodes of Ranvier, facilitate rapid signal conduction through saltatory conduction.

Functionally, the nervous system is divided into the somatic nervous system (conscious sensation and voluntary muscle control) and the visceral nervous system (homeostatic regulation).

The cerebral cortex, the outer surface of the brain, is gray matter (cell bodies). Internally, the cerebral hemispheres contain white matter (myelinated axons). This arrangement is reversed in the spinal cord.

The surface of the cerebral hemispheres features gyri (elevations) and sulci (infoldings), which significantly increase the brain's surface area.

Each cerebral hemisphere is divided into four major lobes: frontal, parietal, occipital, and temporal, demarcated by specific sulci.

The insula is a region of cortex concealed laterally by portions of the frontal, parietal, and temporal lobes, collectively known as the operculum.

The corona radiata is an expansion of white matter beneath the cortical gray matter. It condenses to form the internal capsule, a crucial pathway for axons traversing to and from cortical and deep structures.

The corpus callosum is a large bundle of myelinated axons that horizontally links the two cerebral hemispheres, divided into rostrum, genu, body, and splenium.

The ventricular system, derived from the neural tube's lumen, consists of cavities within the brain that contain cerebrospinal fluid (CSF). It includes the lateral ventricles, third ventricle, and fourth ventricle.

The two C-shaped lateral ventricles are the most rostral cavities of the ventricular system, located within the cerebral hemispheres. They have five parts: anterior horn, body, posterior horn, inferior horn, and atrium/trigone.

Cerebrospinal fluid (CSF) is produced by the choroid plexus, a series of modified ependymal cells lining most of the ventricles.

CSF flows from the lateral ventricles through the interventricular foramina to the third ventricle, then via the cerebral aqueduct to the fourth ventricle, and finally exits into the subarachnoid space through the foramina of Luschka and Magendie.

The CNS is protected by three concentric connective tissue coverings called meninges: the dura mater, arachnoid mater, and pia mater.

The dura mater is a tough, fibrous sheet with two layers: the outer periosteal layer and the inner meningeal layer. It forms dural reflections (septa) like the falx cerebri and tentorium cerebelli.

The arachnoid mater is a delicate membrane beneath the dura, separated by the subdural space. The pia mater is a thin, vascular layer that closely adheres to the brain's surface, separated from the arachnoid by the subarachnoid space containing CSF.

Dural venous sinuses are spaces formed between the layers of the dura mater that receive cerebral venous drainage. Examples include the superior sagittal sinus and cavernous sinus.

The brain's vascular supply is divided into anterior circulation (from internal carotid arteries) and posterior circulation (from vertebral arteries).

The ACA, arising from the internal carotid artery, perfuses the medial aspect of the brain from the frontal lobe to the anterior parietal lobe, including branches like the callosomarginal and pericallosal arteries.

The MCA, branching laterally from the internal carotid artery, supplies the lateral surface of the cerebral hemispheres, including the lateral frontal, temporal, and anterolateral parietal lobes.

The PCA, arising from the vertebrobasilar system, perfuses the inferior and medial temporal and occipital lobes.

The Circle of Willis is an arterial anastomosis on the inferior surface of the brain that connects the anterior and posterior circulations, providing collateral blood supply.

Venous drainage involves deep veins, superficial veins, and dural venous sinuses, which ultimately drain into the internal jugular vein. Vessels in this network lack valves.

The spinal cord is continuous with the medulla oblongata and extends to the L1/L2 vertebral level in adults. It contains ascending and descending tracts for sensory and motor information.

Similar to cranial meninges, the spinal cord is covered by dura mater, arachnoid mater, and pia mater. The subarachnoid space extends to S2, providing a safe site for CSF withdrawal.

The spinal cord has an anterior median fissure and a posterior median sulcus. Enlargements, the cervical and lumbar enlargements, accommodate neurons innervating the extremities.

A cross-section reveals H-shaped gray matter (neuronal cell bodies) and outer white matter (myelinated axons). The anterior horns contain motor neurons, and the posterior horns receive sensory information.

The anterolateral pathways (spinothalamic, spinoreticular, spinomesencephalic) convey pain, temperature, and crude touch sensations. They involve three neurons and cross to the contralateral side.

This pathway conveys discriminative touch, vibration, and conscious proprioception. It ascends ipsilaterally in the posterior columns to the medulla, then crosses to form the medial lemniscus.

The lateral motor system includes the lateral corticospinal tract (voluntary limb movement) and rubrospinal tract (limb movement, uncertain in humans). They primarily control contralateral movements.

The medial motor system includes the anterior corticospinal tract and vestibulospinal, reticulospinal, and tectospinal tracts. They regulate axial muscles, posture, balance, and head/eye movements, often with bilateral projections.

The basal nuclei are deep gray matter structures involved in controlling posture, voluntary movement, and limbic system functions.

The corpus striatum comprises the caudate nucleus and lentiform nucleus (globus pallidus and putamen). It is named for the striated appearance caused by fibers interconnecting the caudate and putamen.

The direct pathway through the basal nuclei increases motor activity, while the indirect pathway, involving the subthalamic nucleus, decreases motor activity.

The cerebellum, the largest part of the hindbrain, is crucial for maintaining balance, posture, and coordinating voluntary muscle movements.

The cerebellum is divided into anterior, posterior, and flocculonodular lobes by fissures such as the primary and posterolateral fissures.

Deep within the cerebellum are four masses of cerebellar nuclei: dentate, emboliform, globose (collectively interposed), and fastigial. Output from the cerebellum primarily originates from these nuclei.

The cerebellar cortex is functionally divided into the vermis (trunk movements), intermediate zone (distal limb movements), and lateral zone (motor planning).

The cerebellum receives input from the cerebral cortex (via pontine nuclei), spinal cord (spinocerebellar tracts), and brainstem vestibular nuclei and reticular formation.

Output from the cerebellum projects to the thalamus, red nucleus, vestibular nuclei, and reticular formation, influencing motor control, posture, and balance.

Light enters the eye through the cornea, passes through the pupil and lens, and is projected onto the retina. The retina contains photoreceptors (rods and cones) that transduce light into electrical signals.

The retina has several neural layers, including photoreceptors (rods for dim light, cones for color and acuity), bipolar cells, and ganglion cells. Horizontal and amacrine cells modulate neural activity.

Axons from retinal ganglion cells form the optic nerve, converge at the optic chiasm (where nasal fibers cross), and continue as the optic tract to the lateral geniculate nucleus of the thalamus.

Optic radiations project from the lateral geniculate nucleus to the primary visual cortex in the occipital lobe, maintaining a retinotopic map of the visual field.

Sound waves are collected by the outer ear, transmitted through the middle ear ossicles, and converted into pressure waves in the fluid-filled cochlea of the inner ear.

Within the cochlea, the organ of Corti converts pressure waves into electrical signals via hair cells. These synapse with sensory neurons whose axons form the cochlear nerve.

Auditory information ascends from the cochlear nuclei through the superior olivary nucleus, lateral lemniscus, inferior colliculus, and medial geniculate nucleus of the thalamus to the primary auditory cortex.

The vestibular nerve transmits sensory information about head movement and position from the semicircular ducts, utricle, and saccule of the inner ear.

Vestibular axons terminate in the four vestibular nuclei (superior, inferior, medial, lateral) in the brainstem. Pathways connect to visual motor, descending motor, and cerebellar systems for balance and posture.

The hypothalamus is a neuroendocrine organ regulating survival functions like feeding, temperature control, sleep-wake cycles, growth, and reproduction.

Located in the ventral diencephalon, the hypothalamus is bordered by the lamina terminalis, hypothalamic sulcus, midbrain tegmentum, substantia innominata, and the posterior limb of the internal capsule.

The hypothalamus connects to the pituitary gland via the infundibulum. It regulates hormone synthesis and release in both the anterior (adenohypophysis) and posterior (neurohypophysis) pituitary.

The hypothalamus is divided into medial and lateral zones, and further into preoptic, periventricular, supraoptic, tuberal, and mammillary regions, each containing specific nuclei with distinct functions.

Olfactory receptor neurons in the nasal epithelium transmit signals through the olfactory nerve to the olfactory bulb. Unlike other sensory pathways, there is no thalamic relay before reaching the primary olfactory cortex.

Axons from the olfactory bulb form the olfactory tract, which divides into medial and lateral olfactory striae, projecting to areas like the septal area, amygdala, and piriform cortex.

The limbic system includes cortical structures (cingulate gyrus, parahippocampal gyrus) and subcortical nuclei (amygdala, hippocampal formation, thalamic nuclei, septal nuclei, nucleus accumbens).

The amygdala is associated with processing emotions, particularly fear, and influences autonomic and neuroendocrine pathways. It connects to cortical areas, septal nuclei, and the hypothalamus via the uncinate fasciculus, stria terminalis, and ventral amygdalofugal pathway.

The nucleus accumbens in the ventral forebrain is considered a gratification center and plays a role in behaviors related to addiction. It receives input from the amygdala and hippocampus and projects to the hypothalamus and globus pallidus.

The septal region is involved in pleasurable behaviors and emotional regulation. It receives input from the amygdala, hippocampus, and olfactory tract and connects to the habenular nuclei.

The hippocampal formation (hippocampus, dentate gyrus, subiculum) is crucial for memory processes, including episodic, short-term, and working memory, and memory consolidation.

Input to the hippocampus primarily comes from the entorhinal cortex via the perforant and alvear pathways. Output exits via the fornix, connecting to various limbic and hypothalamic structures.

The Papez circuit describes a pathway linking the subiculum, mammillary nuclei, anterior thalamus, cingulate gyrus, and parahippocampal gyrus, playing a role in emotion and memory.