Neural Networks and the Vertebrate CNS

• Neurons in the nervous system create circuits with specific functions.
• The most complex circuits are in the brain, where billions of neurons form intricate converging and diverging networks, creating infinite pathways.
• Signaling within these pathways leads to thinking, language, feeling, learning, and memory.
• The neuron circuit, not the individual neuron, has been proposed as the nervous system's functional unit.

Emergent Properties of Neural Networks

• Inexplicable properties of a neuron cannot explain the complex responses of neural processing and integration.
• Neuron circuits can maximize an individual neuron's potential.
• Plasticity allows the brain to reorganize from circuit to circuit based on past experiences and sensory input.

The Vertebrate CNS

• It includes the brain and spinal cord
• Brains become more complex as one moves up the phylogenetic tree from fish to humans.
• The CNS contains layers of neural tissue around a fluid-filled central cavity lined with epithelium in vertebrates.

Gray Matter and White Matter

• The central nervous system consists of neurons and glial cells, just like the peripheral nervous system.
• Interneurons are contained entirely within the CNS.
• Afferent (sensory) and efferent neurons connect interneurons to peripheral receptors and effectors.

Gray Matter

• It consists of axonal terminals, nerve cell bodies, and unmyelinated dendrites.
• Cell bodies are arranged in an organized manner in both the spinal cord and the brain.
• It forms layers in some brain regions while clustering into neuron groups with similar functions in others.
• Nuclei are groups of cell bodies in the brain and spinal cord.
• Nuclei are identified by specific names; visual information is processed in the lateral geniculate nucleus.

White Matter

• Mostly myelinated axons with very few cell bodies are included.
• Myelin sheaths around the axons give it its pale color.
• Tracts are bundles of axons connecting different CNS regions.
• Tracts found in the CNS are equivalent to nerves in the peripheral nervous system.

Protection of the CNS

• The brain and spinal cord are soft and jellylike.
• Neural tissue depends on external support because it has a minimal extracellular matrix for protection from harm.
• This support is provided by bone, three layers of connective tissue membrane (meninges), and fluid between the membranes.

Cerebrospinal Fluid

• It's a salty solution produced continuously by the choroid plexus, which is located on the walls of the ventricles.
• The lateral ventricles include the first and second ventricles.
• The third and fourth ventricles extend through the brainstem and connect to the central canal that runs through the spinal cord.
• The choroid plexus cells selectively pump sodium and other solutes from plasma into the ventricles, which creates an osmotic gradient that draws water along with these solutes.
• Cerebrospinal fluid then flows into the subarachnoid space between the arachnoid membrane and the pia mater, surrounding the brain and spinal cord.
• Next, special villi on the arachnoid membrane in the cranium absorb it back into the blood supply.
• The rate of fluid flow is sufficient to refill the entire volume of cerebrospinal fluid about three times per day.

The CSF: Two Purposes

• Physical Protection
• The buoyancy of CSF in the thin fluid layer between the membranes reduces the weight of the brain around 30-fold.
• Lighter weight leads to less pressure exerted on the CNS's blood vessels and nerves.
• CSF also provides protective padding.
• Chemical Protection
• CSF maintains a tightly regulated extracellular environment for neurons.
• The choroid plexus selectively transports the substances that form the CSF, so the composition of CSF differs from that of plasma.
• Compared to plasma, CSF has a lower concentration of [K+], a higher concentration of [H+], and the same concentration of [Na+].
• CSF has no blood cells and very little protein.
• CSF exchanges solutes with the CNS's interstitial fluid and provides a route to remove wastes.

Clinical Significance of CSF

• CSF samples are indicators of the brain's chemical environment.
• A spinal tap (lumbar puncture) involves withdrawing CSF from the subarachnoid area between vertebrae towards the lower end of the spinal cord.
• The presence of proteins or blood cells in CSF suggests an infection.

The Blood-Brain Barrier

• It's a functional barrier that acts as the brain's final line of defense between the interstitial fluid and the blood.
• This barrier isolates the body's main control center (the brain) from potentially harmful blood-borne substances and pathogens (like bacteria).
• Most of the 400 miles of brain capillaries help to achieve this protection through the creation of a functional BBB.
• The highly selective permeability of brain capillaries provides this shelter from toxins and fluctuations in neuroactive substances (i.e., neurotransmitters), ions, and hormones in the blood.
• Brain capillaries are less permeable because the endothelial cells create connections that prevent solute movement between the cells.
• Tight junction formation is induced by paracrine signals from adjacent astrocytes with foot processes that surround the capillary.
• The brain tissue alone creates the blood-brain barrier.
• The blood-brain barrier has selective permeability due to its transport properties.
• Nutrients are moved from blood to the interstitital fluid by membrane carriers and channels found in capillary endothelium.
• Other transporters move wastes from the interstitial fluid into the plamsa.
• Without a carrier, no water-soluble molecule can pass the blood-brain barrier.

Metabolic Needs of Neural Tissue

• Oxygen freely passes across the blood-brain barrier.
• The brain receives 15% of blood pumped by the heart.
• Glucose transporters move glucose from plasma to the brain's interstitial fluid.
• The brain consumes about half of the body's total glucose.
• Confusion, unconsciousness, and death can result from hypoglycemia.

Spinal Cord

• It is a major information pathway between the body/skin/joints/muscles and the brain.
• It contains neural networks for locomotion.
• Spinal cord injuries can lead to loss of sensation from skin and muscles, as well as paralysis and the inability to control muscles voluntarily.
• It is divided into four regions: cervical, thoracic, lumbar, and sacral, which are named after the vertebrae next to them.
• The spinal cord has two enlargements and ends in the lumbar area, each spinal area is subdivided into segments.
• Spinal nerves are part of the PNS carrying both sensory and motor neuron fibers.
• Each segment in a spinal region gives rise to a bilateral spinal nerve pair.
• Sensory (afferent) information is carried to the CNS by the dorsal root of the spinal cord.
• Information to muscles and glands (efferent) is carried by the ventral root.

Spinal Cord Anatomy

• Gray matter found in the spinal cord: sensory and motor nuclei and cell bodies.
• The dorsal horn has somatic sensory nuclei and visceral sensory nuclei.
• Motor nuclei include autonomic efferent and somatic motor.
• In the spinal cord, white matter consists of axons transmitting information to and from the brain.
• Ascending tracts carry sensory information to the brain.
• Commands to motor neurons are carried by descending tracts.
• Propriospinal tracts stay within the spinal cord.

Spinal Cord Integrating Center

• Simple reflexes can be integrated by the spinal cord without input from the brain.
• Spinal interneurons can route sensory information to the brain through ascending tracts or bring commands from the brain to motor neurons.
• A spinal reflex initiates a response without input from the brain.

Brain Composition

• An adult human brain has a mass of about 1400 g and 10^12 neurons.
• Each neuron can have as many as 200,000 synapses to create a mind boggling number of possible connections.
• Synapses continuously change and are not fixed.
• Multiple brain regions are involved with spinal cord even with simple functions.
• One brain region is involved in several functions at the same time.

Brain Stem

• The most primitive brain region
• Ascending and descending tracts pass through it.
• It is divided into three regions: the medulla oblongota, the pons, and the midbrain.
• Each region has nuclei along with cranial nerves and regulates autonomic function.
• Reticular formation is associated with many nuclei with different functions, consists in white fibers interconnecting different brain area.
• Medulla Oblongata- It connects to spinal cord, controls involuntary funtions.

Reticular Formation

• It's a diffuse collection of neurons that extends throughout the brain stem and discrete nerve cell groups are included.
• It consists of clusters of (white matter) neurons that core the brainstem.
• It keeps the brain alter, influences arousal, and regulates sleep.
• It directs visceral activity (vomiting, heart rate).
• An important motor tract goes down the spinal cord and involved in movement.

Cranial Nerves

• 11 of 12 originate along with the brain stem
• Olfactory is the first one entering the forebrain
• It carries motor and sensory information for the head and neck.

Cerebellum

• The second largest structure in brain
• Most of the brain's nerve cells are located in the cerebellum.
• Coordinates the execution of movement based on receptors for equilibrium and balance in the inner ear, and somatic in the periphery of the body.
• It processes sensory information and motor input from the cerebrum; it smoothes and coordinates movement.

Diencephalon

• Lies between the cerebrum and brainstem
• The Thalamus, and the Hypothalamus, and two endocrine structures being the pituitary and pineal glands, are its components.
• The integrating/relay station: Thalamus
• The homeostasis center + controls behavior relates to homeostasis: Hypothalamus
• Pineal + Pituitary glands are the two endocrine structures.

Hypothalamus

• It focuses on specific functions with many nuclei.
• Regulates body temperature and controls body osmolarity.
• Has sympathetic nervous system activation.
• Controls reproductive functions and food intake.
• Has interactions with limbic system influences emotions and behavior.
• Influences the cardiovascular control center in medulla oblongata.

Cerebrum

• Higher brain functions occur there.
• A distinct corpus callosum connects the two hemispheres to form the brain.
• The two hemispheres cooperate and communicate through this important connection.
• Each cerebral hemisphere divides into four lobes (parietal, temporal, occipital, and frontal), named after the skull bones under which they are located.
• The two hemispheres are connected by the white matter of the corpus callosum.

Gray Matter of the Cerebrum

• Higher brain functioning happens in the cerebral cortex, which has 30 layers.
• Processing level is directly related to surface area.
• Movement is controlled in the basal ganglia -- neural calculators.

Limbic System

• Governs emotion, memory, learning, and visceral responses
• Higher cognitive functions link to primitive emotional ones.
• Includes sensory regions, basal ganglia, and the diencephalon (hypothalamus) to relate state of mind to physiological functions.

Brain Function

• Signals and data is generated by the brain in the absence of external input.
• Motor output is influenced by sensory information, cognition, and behavior and doesn't necessarily need sensory input.

Functional Areas of the Cerebral Cortex

• Areas are localized to direct movement, perceive sensation, and integrate information.
• Taste, smell, hearing, and spatial visualization have highly specialized and distinct areas.

Cerebral Lateralization

• Lobe functions are not equally shared from each side.
• Left hemisphere dominance is exhibited by right-handed people.

Neurons create circuits with specific functions. The most complex circuits are in the brain. Signaling within these pathways leads to thinking, language, feeling, learning, and memory. The neuron circuit is the nervous system's functional unit.