Nervous System Organization

Organization of the Nervous System

• The nervous system has two main divisions: the Central Nervous System (CNS) and the Peripheral Nervous System (PNS).
• This division arises from functional and property differences in the neurons and systems.
• The PNS includes the Somatic Nervous System, responsible for reacting to the external environment.
• The Autonomic Nervous System (ANS) is also part of the PNS, and regulates the body’s internal environment.
• The ANS has two branches: the Sympathetic Nervous System and the Parasympathetic Nervous System.
• The CNS, comprising the brain and spinal cord, exchanges sensory and motor information with the PNS through spinal and cranial nerves.
• Cranial nerves uniquely transmit special motor and sensory information directly to the brain, bypassing the spinal cord.
• Elements of the PNS transmit information about the environment to the CNS and carry commands from the CNS to the body.
• The somatic nervous system consists of afferent nerves conveying messages from sense organs to the CNS and efferent nerves carrying motor signals from the CNS to muscles.
• The autonomic nervous system provides neural control over internal organs such as the heart and intestines.
• The ANS has central and peripheral components and includes the sympathetic division.
• The sympathetic division expends bodily energy and is mainly involved in the "fight or flight" response.
• The parasympathetic branch of the ANS conserves energy, promoting relaxation, increases the supply of stored energy, and boosts digestive and intestinal functions.
• Most autonomic organs receive both sympathetic and parasympathetic inputs, influenced by their relative activity levels, with some organs experiencing balanced and opposing activities.

Central Nervous System (CNS) Properties

• While the structure and function of neurons differ between the CNS and PNS, the CNS has unique protective and supportive properties
• CNS primarily consists of the brain and spinal cord.
• The CNS is physically protected by the bony cavities of the skull and spine, along with the meninges and cerebrospinal fluid (CSF).
• The brain receives a constant supply of blood and nutrition, aided by backup systems, ensuring adequate nourishment.

Spinal Cord Composition

• The spinal cord, approximately 46 cm long, is an extension of the brain.
• It's protected by bone, meninges, and CSF, housed within the spinal column.
• Spinal cord consists of sensory and motor neurons and relays somatosensory information from the trunk and limbs to the brain and relays simple messages from the brain back to the trunk and limbs.
• The spinal cord is made up of outer white matter and inner gray matter.
• Spinal cord injuries can result in sensory and motor impairments.
• Each area of the spinal cord corresponds to specific body locations, controlling sensation and movement.
• The spinal cord consists of 1 coccyx, 5 sacral, 5 lumbar, 12 thoracic, and 8 cervical levels.

Brain Protection and Sustenance.

• The skull-brain relationship is crucial for understanding brain vulnerability to skull-related injuries and damage.
• The brain is protected by the meninges.
• The meninges are flexible, semi-permeable membranes that act as protective pads around the brain and spinal cord.
• Meningiomas, tumors, may form on the meninges.
• Tumors on the meninges can impact the brain.
• Cerebrospinal fluid (CSF) circulates around the CNS via the ventricular system.
• CSF provides cushioning and physiological protection and it also facilitates the blood-brain barrier.

Skull Structure

• The skull includes the frontal bone, two parietal bones, two temporal bones, the occipital bone, and the sphenoid bone.
• Cerebral lobes are named following the cranial plates.
• The skull features grooves for blood vessels in the calvaria (roof).
• The fossae, found at the base, anchor the brain.
• Also found at the base of the skull are orifices or foramina for nerve and blood cell passage.
• The largest opening, the foramen magnum, found in the occipital bone, accommodates the spinal cord's passage into the brainstem.
• In newborns, cranial plates aren't fully fused, separated by soft membranous tissue.
• Fontanelles are soft openings near the parietal bone corners that may fluctuate with intracranial pressure.
• The anterior fontanelle, the largest, is located atop the head between the frontal and parietal bones and usually closes approximately two years of age.
• The skull protects the brain from external forces, but when rigid, the skull prevents the release of excess fluid, which can be lethal.
• Bony projections within the skull function to hold the brain in place.
• Injuries around bony projections of the frontal and temporal lobes during whiplash can cause the gelatinous brain to reverberate within the skull.
• Thin temporal and sphenoid bones are susceptible to damage from side impacts.
• Fractures at the skull base risk tearing cranial nerves and leakage of CSF, causing bleeding.

Meninges

• Meninges contain thin membranes covering the brain and spinal cord, providing support and acting as a protective buffer.
• The CNS meninges consist of three layers: the pia mater, arachnoid mater, and dura mater.
• From inner to outer, the layers are pia mater, arachnoid membrane, and dura mater.
• Pia Mater adheres to the CNS surface, following its contours.
• Arachnoid Mater: A spider web-like membrane overlies the subarachnoid space and contains CSF.
• Dura Mater is a dense, inelastic, double-layered membrane adhering to the skull's inner surface.
• Meningeal veins protect the dura.
• The epidural space lies between two dural layers.
• The subdural space is found between the dura and arachnoid mater.
• Cerebral veins crossing the subdural space are fragile and prone to injury due to minimal support.

Function and Ventricular System

• While meninges provide a protective covering, they do not engage in cognitive processes, playing instead a role in injury to the meninges which can result in inflammation (meningitis) resulting from bacterial and viral infections.
• The brain contains four interconnected, fluid-filled ventricles.
• There is one lateral ventricle in each hemisphere.
• Also, the third and fourth ventricles are part of the ventricular system.
• The lateral ventricles, occupying significant space in both hemispheres, connect via the interventricular foramen (or foramen of Monro).
• The third ventricle is located between the two lateral ventricles at the thalamus and hypothalamus level.
• The third ventricle connects to the fourth ventricle.
• The connection to the fourth ventricle passes through the midbrain and expands into the fourth ventricle which lies in the brainstem, just beneath and anterior to the cerebellum by way of the cerebral aqueduct.
• The choroid plexus produces CSF, which flows through the ventricles and around the brain and spinal cord.
• Approximately 450 ml of CSF is produced daily, recirculating every 6-7 hours.
• The ventricular system buffers the brain and spinal cord.
• The CSF helps maintain buoyancy and cushions the brain against internal and external forces and disposing of waste.
• Ventricles have no direct cognitive function.
• Abnormal intracranial ventricular pressure may cause cognitive deficits.
• Fluid-enlarged ventricles can lead to brain swelling, raised intracranial pressure, and behavioral/cognitive changes.
• Increased intracranial pressure leading to hydrocephalus, may cause skull swelling.
• If hydrocephalus develops in childhood, it can cause enlargement.
• Normal pressure hydrocephalus (NPII) is a variant that occurs in adults and the elderly.

Vascular System

• Proper brain function requires sufficient oxygen and glucose from the bloodstream
• Obstructed blood vessels may cause brain cells to start to die because the blood vessels cannot receive oxygen and nutrients and then may start to die
• This affects organ functioning controlled by specific brain regions.
• Major cerebral structures and meninges receive blood from four major arteries.
• Four major arteries include: the right and left internal carotid arteries and two vertebral arteries.
• These arteries originate from the aortic arch, which comes from the heart’s left ventricle.
• The two vertebral arteries connect at the brainstem and form the basal artery, which divides into posterior or cerebral arteries on both sides.
• This system provides roughly 20% of total cerebral blood flow, contributing ~20% of cerebrovascular accidents (e.g., stroke).
• Cross-brain connections from both internal carotid and basilar systems create the circle of Willis.
• Circle of Willis sits near the brain’s base and is a vascular structure designed to connecting the brain's blood supply.

Lower Brainstem

• The lower brainstem includes the medulla oblongata, pons, midbrain, cranial nerves, and the reticular activation system and is the center for neural communication by ferrying information from the telencephalon and spinal cord.
• The lower brainstem is one of the first structures to develop.
• Contains cranial nerves, specialized nuclei, and the reticular activation system.
• The medulla is superior to the spinal cord and forms a bridge between elementary and more complex neuronal configurations from the spinal column to the brain structures.
• The medulla contains myelinated tracts carrying the motor and sensory information.
• In the medulla, tracts decussate, crossing information from one side of the body to the contralateral brain hemisphere.
• The pons, superior and anterior to the medulla (and inferior to the midbrain), resembles two bulbs forming a bridge.
• The cerebellum connects to the posterior of the pons.
• Info is funneled to the pons via cerebellar peduncles.

Midbrain

• The midbrain lies between the cerebrum and pons
• The midbrain is the smallest brainstem portion, and can be divided into the tectum (roof) and tegmentum (covering).
• The tectum has four elevations: inferior colliculi (auditory processing) and superior colliculi (visual processing and reflexes).
• The tegmentum surrounds the cerebral aqueduct.
• Contains nuclear groups, cranial nerves, pathways connecting the spinal cord and cerebellum and telencephalon.
• The reticular formation is the RAS, or the reticular activation system.
• The lower brainstem is the origin of 10 out of 12 pairs of cranial nerves.
• Cranial nerves are directly connected to the brain and transmit information to and from the brain.
• Cranial nerves, named by Galen, originate from different nuclei in the brainstem.
• Nerves I to IV originate from the midbrain and forebrain, while nerves V to XII originate from the medulla and the pons to the hindbrain.
• Cranial nerves integrate sensory information and motor output.
• Reticular formation is an old system in the nervous system and is retained in humans.
• It contains a diffuse arrangement of ascending and descending neurons.
• It interacts with major brain neural tracks and consists of a collection of nerve cells.
• The RAS provides nonspecific arousal processes for cortical activities and regulates sleep/wake cycles.
• Deficits to the reticular system leads to: changes in consciousness (coma) and medulla damage that impairs respiratory/cardiovascular functions.
• Reticular formation plays a role in selective attention, which can be disrupted with damage to the RAS.
• Disruption in the brainstem can affect arousal, orientation, and awareness.

Upper Brainstem: Diencephalon

• The diencephalon (also known as the "interbrain") is located at the top of the brainstem.
• The diencephalon connects the cortex with the lower brain structures and consists of the thalamus and hypothalamus.
• The thalamus consists of ancient nuclei that may have facilitated reflexive responses to pleasant and unpleasant stimuli prior to the evolution of the cerebral cortex.
• The hypothalamus is instrumental helping to control the autonomic nervous system and regulate emotional responses and other functions such as: thirst, appetite, digestion, sleep, temperature, and smooth muscles of internal organs.
• The hypothalamus forms the floor and part of the lateral wall of the third ventricle.
• The mass of the hypothalamus is usually usually about 4 grams but is packed with small and complex nuclei located at the junction of the midbrain and the thalamus.
• Within the hypothalamus contains a dozen identifiable cellular bodies known as hypothalamic nuclei.
• Hypthalamus has three longitudinal zones: lateral, medial and periventricular regions.
• It is tightly connected with: the thalamus at connections, the efferent and afferent connections to and from regions outside the hypothalamus and regulating peripheral processes.
• The hypothalamus is connected to the pituitary gland (master gland) and regulates the endocrine activity of the pituitary, both directly and indirectly.
• Axonal bundles constitute of the pituitary stalk which is a funnel-shaped structure that connects the brain's hypothalamus to the pituitary gland.
• The hypothalamus’s direct control over releasing antidiuretic hormones.
• Antidiuretic hormones are involved with homeostatic regulation while oxytocin influences contractions at birth and lactation.
• The hypothalamus regulates homeostatis, physical growth, and sexual development.
• Hypothalmus also influences digestion, arousal, and thirst.
• The thalamus provides critical processing for all sensory systems except the olfactory system, serving as a pathway for sensory and motor impulses.
• Thalamus provides a function for preliminary sensory and motor classifying which it sends to the cortex for further processing to the same hemisphere via the ipsilateral route.
• Thalamic lesions and tumors can result in deficits for: sensory and motor functioning or defects in spacial ability, facial recognition and perception of music.

Cerebellum

• The cerebellum contains 50% of the brain’s neurons representing ~10% of the brain’s total weight.
• The cerebellum aids with coordinating movement, postural adjustments, general motor behavior, and maintains posture by controlling muscles that maintain the body-upright.
• Lesions to specific areas of the cerebellum result in: disorientation, jerky movements, intention tremor, static tremor, balance, and gait.

Telencephalon

• Also known as the end brain, the telencephalon is comprised of two cerebral hemispheres which are connected by the corpus callosum with cell bodies (gray matter) that affect voluntary behavior such as motor behavior, memory, and thinking.
• Each hemisphere contains a lateral ventricle and a collection of nuclei known as basal ganglia which supports motor neurons.
• The basal forebrain serves as a subdivision containing strongly interconnected limbic structures.
• Basal ganglia are also called as the basal nuclei, supports deep nuclei of telencephalon, communicates with the with the cortex, and the thalamus.
• Possesses efferent outputs and afferent puts that are associated with higher movement.
• It is positioned inside the cortex and is usually symmetrical to subcritical gray matter structures.
• Enclosed by: the cerebral cortex and surrounds the thalamus and cerebral white which allows for sensory projections to assist with nervous system out flow.
• Includes caudate nucleus, substantia nigra, and sub caudate nucleus, which help perform motor functions. Structures include: caudata nucleus, putamen and globus palidus that connect with the coretex along with various reticular formations though usually not with spinal cords.
• Known as the striatum and its associated the dopaminergic nigrostriatal system, it provides key functions with motor behavior. Regulates voluntary movement especially for initiating it, is associated with extrapyramidal system, and helps with integrative function for visual centers.
• It may also play a role in language and can aid with attention.
• It is part of the limbic system (coined by Paul Broca) and is assumed to be associated with olfaction, learning, and memory as well as the rhinencephalon structure.
• Primary lobe structures include: cingulate gyrus, parahippocampal gyrus, hippocampal formation, forniz, hypothalamus, amygalda, and the septum and plays a role in the papez circuit.