Neuroanatomy: Nervous System Layout and Divisions

2: Neuroanatomy

General Layout of the Nervous System

• The nervous system operates as a system of twos.

Divisions

• The Central Nervous System (CNS) includes the brain and spinal cord.
• The Peripheral Nervous System (PNS) consists of the somatic nervous system (SNS) and the autonomic nervous system (ANS).
• The SNS contains the sympathetic nervous system and the parasympathetic nervous system.

Peripheral Nervous System

• Located outside the skull and spine, with most nerves projecting from the spinal cord.
• Contains 12 pairs of cranial nerves projecting from the brain.
• Cranial nerves include purely sensory, sensory and motor, and autonomic motor fibers.
• Cranial nerves are numbered in sequence from front to back.
• Olfactory (I) and Optic nerves (II) are purely sensory.
• Vagus nerves (X) are sensory and motor, running to and from the gut.
• Autonomic motor fibers are parasympathetic.
• Neurologists assess cranial nerve functions and locations to diagnose brain lesions.
• Receives information from sensory nerves.

Somatic Nervous System (SNS)

• Facilitates interaction with the external environment through afferent and efferent nerves.
• Afferent nerves carry sensory signals from senses to the CNS.
• Efferent nerves carry motor signals from the CNS to skeletal muscles.

Autonomic Nervous System (ANS)

• Regulates the body's internal environment with afferent and efferent nerves responding to internal stimuli, such as those from organs.
• Efferent nerves include sympathetic and parasympathetic systems with two types of efferent nerves.
• The sympathetic nervous system is associated with "fight or flight or freeze" responses.
• Sympathetic nerves project from the CNS, synapsing on second-stage neurons at a distance from target organs.
• The parasympathetic nervous system is associated with "rest and digest" functions.
• Parasympathetic nerves project from the CNS, synapsing near target organs.
• Autonomic target organs receive input from both sympathetic and parasympathetic systems, controlled by relative activity levels.
• Changes reflect psychological arousal or relaxation.

Meninges (CNS)

• The brain and spinal cord are protected by bone and three protective membranes called meninges.
• The dura mater is the tough, outer meninx.
• The arachnoid membrane is the fine meninx inside the dura mater.
• The subarachnoid space beneath contains large blood vessels and cerebrospinal fluid.
• The pia mater is the delicate, innermost meninx adhering to the surface of the CNS.

Ventricles and Cerebrospinal Fluid

• Ventricles contain cerebrospinal fluid (CSF).
• CSF protects the CNS by supporting and cushioning and prevents buoyancy.
• CSF supplies nutrients and removes waste.
• CSF fills the subarachnoid space, central canal, and cerebral ventricles.
• CSF is produced by choroid plexuses (capillaries protruding into ventricles).
• Excess CSF is absorbed into dural sinuses and drains into jugular veins.
• Hydrocephalus is the buildup of fluid in ventricles due to obstructions, causing brain expansion.
• The central canal is a small channel running the length of the spinal cord.
• Cerebral ventricles include four large internal chambers.
• Two lateral ventricles.
• The third ventricle.
• The fourth ventricle.
• The cerebral aqueduct connects the 3rd and 4th ventricles.
• The subarachnoid space, central canal, and cerebral ventricles are interconnected, forming a single reservoir.

Blood-Brain Barrier

• Brain is a finely tuned electrochemical organ, and it possesses a mechanism that impedes the passage of toxic substances from the blood.
• Brain cells of blood vessel walls are tightly packed, forming a barrier against large molecules, such as proteins
• Critical molecules like glucose are actively transported across cerebral blood vessel walls.
• Some brain areas allow certain large molecules to pass.
• The degree to which drugs influence brain activity depends on their ability to penetrate the blood-brain barrier.
• Impairment of the blood-brain barrier is associated with many CNS disorders.
• The area postrema is a region in the medulla with a weak barrier.
• Allows toxins to enter, triggering chemoreceptor trigger zones and vomiting.

Cells of the Nervous System

• Specialized cells receive, conduct, and transmit electrochemical signals.

Neuron Cell Membrane

• The neuron cell membrane contains a lipid bilayer (two layers of fat molecules).
• Protein molecules, crucial for membrane functions, are embedded in the lipid bilayer.
• Channel proteins allow certain molecules to pass.
• Signal proteins transfer signals inside the neuron when specific molecules bind outside.

Classes of Neurons

Multipolar neuron

• Multipolar neurons have more than two processes extending from the cell body and are the most common type.

Bipolar neuron

• Bipolar neurons have two processes.

Unipolar neuron

• Unipolar neurons have one process.

Interneurons

• Interneurons have short or no axons.
• They integrate neural activity within a brain structure rather than conducting signals between structures.

Neurons and Neuroanatomical Structure

• Gross neural structures include those of cell bodies and those of axons.

Terms of the central nervous system (CNS)

• A cluster of cell bodies is called a nucleus.
• A bundle of axons is called a tract.

Terms of the peripheral nervous system (PNS)

• A cluster of cell bodies is called a ganglion.
• A bundle of axons is called a nerve.

Glial Cells/Glia

• Glial cells roughly outnumber neurons in the brain, with approximately two glial cells for every three neurons.
• Glia supports neurons by supplying nutrition, clearing waste, and forming a physical matrix.
• Types of glial cells include oligodendrocytes, Schwann cells, microglia, and astrocytes.

Oligodendrocytes

• Oligodendrocytes are glial cells in the CNS that wrap extensions around axons, creating myelin sheaths.
• Extensions are rich in myelin, a fatty insulating substance.
• Myelin sheaths enhance the speed of axonal conduction.
• Schwann cells are PNS equivalent and guide axonal regeneration.
• Axonal regeneration is restricted to the PNS in mammalian nervous systems.
• Oligodendrocytes create several myelin segments, while Schwann cells create one.

Microglia

• Smallest glial cells respond to injury or disease, multiplying and engulfing cellular debris or cells.
• Trigger inflammatory responses.

Astrocytes

• Largest, star-shaped glial cells cover blood vessels in the brain, contacting neurons.
• Allow passage of some chemicals into CNS neurons while blocking others.
• Ability to contract or relax blood vessels affects blood flow based on brain demands.

Neuroanatomical Techniques and Directions

Neuroanatomical Techniques

• The Golgi stain was created by Camillo Golgi in the early 1870s and stains meninges.
• Shows silhouette.
• The Nissl stain was created by Franz Nissl in the 1880s, using cresyl violet dye.
• Can estimate the number of cell bodies.
• Electron Microscopy is a neuroanatomical technique providing info about the details of neural structure.
• Coat thin slices of neural tissue with an electron-absorbing substance, then pass a beam of electrons through the tissue onto a photographic film→ electron micrograph.
• Scanning electron microscope provides electron micrgraphs in 3D.

Neuroanatomical Tracing Techniques

• Neuroanatomical tracing techniques include anterograde (forward) and retrograde (backward) tracing methods.

Anterograde tracing

paths of axons projecting away from cell bodies.

• Injecting chemicals taken up by cell bodies and transported forward along axons to terminal buttons leading brain to be removed and sliced, slices treated to see location

Retrograde Tracing

• Chemical injected into the brain is taken up by terminal buttons- transported up axon to cell body; reverse of anterograde

Directions in the Vertebrate

Alternative Names

• Anterior = rostral. Posterior = caudal (means tail). Used instead of dorsal and ventral which changes direction at the primate head.
• Superior to Above
• Inferior to Below

Sections

Frontal Sections = coronal sections

• Midsagittal: a section cut down between hemispheres.
• Cross Section an angle to any long structures

Spinal Cord

• Divided into four regions named cervical, thoracic, lumbar, and sacral.
• Consists of an inner H-shaped core of gray matter which is surrounded by white matter.
• The gray matter is composed of cell bodies and unmyelinated interneurons.
• The dorsal horns and Ventral horns are apart of gray matter
• White matter is composed of myelinated axons.
• Attached to the spinal cord are 62 spinal nerves, in 31 pairs, at different levels.
• Each divide; divided axons connecting via one of two roots.
• Each division is a ventral or dorsal Root
• Dorsal root axons, whether somatic or autonomic, are sensory with cell bodies grouped outside the cord.
• Form the dorsal root ganglia
• motor and and found terminally in dorsal horns
• Found terminally in ventral horns and include SNS and ANS project to skeletal muscles

5 Major divisions of the Brain

• Forms early on during mammalian development
• starts as small-fluid filled tube
• Early Development has 3-swellings anteriorly which become forebrain, midbrain and hindbrain
• Brain eventually has 5-swellings as fore and hindbrain split
• Encephalon = inside the brain

Top of the head and alphabetical order

1. Telencephalon
2. Diencephalon
3. Mesencephalon (midbrain)
4. Metencephalon
5. Myelencephalon

• Medullar is apart of myelencephalon

Medulla

• posterior part of the brain

• largely composed of tracts moving signals in and out of the brain

• Vital area for breathing and heart rate- found just high and above spinal cord

• All parts of the structure are complex and run stem to back/front of brain ( sleep, reflexes, and other movements)

• Metencephalon

  • Houses many ascending and descending
  • Pons is anterior most division which maintains breathing & autnomic function
  • Cerebellum is convolution like- affects movements & cognitive behaviour
  • Cerebellum- allows for correct movement and cognition ( Not just Sensorimotor only can produce new neurons Both include dorsal surface of Midbrain- sensory component
  • Mid brain has
    • two part = ( tectum = roofs part and
    • Tegmentum- central part

• Diencephalon- two main parts & thalamus & hypothalamus All signals flow to this for communication - Thalamus contains info processing.

• Includes hypo affects the effects and and body and body hormones

Telencephalon or the head end of the brain

• The largest part of the brain

Limbic System

Includes structure-

• regulation for Moe behaviours

Amyda- in the amterior end of the head

• hypo is always posterior cingulate - surrounds front lobes - conscious emotion response can influence hypo Formix
• Main tract from
• Ends in septum areas

Basal ganglia - integration system

Include caudate and putamen- receives inputs to neo cerbra cortex Caudate Striatum

• all have out from cerbra cortex plays role from performance for volant reactions

Diencepalon is also associated with basal ganglia which has 4 regions

These are often associated and can result of the trauma impacts the areas

• The ventricles Both have effects like continuous bleeding

This lesson covers the general layout of the nervous system, emphasizing its paired operation. It details the divisions, including the Central Nervous System (CNS) and Peripheral Nervous System (PNS), further breaking down the PNS into the somatic and autonomic systems. Cranial nerves and their functions are also explored.