Chapter 3 - Anatomy of the Nervous System

Central Nervous System

Division of the nervous system located within the skull and spine

-Composed of two parts; Brain and Spinal Cord

Peripheral Nervous System

Division located outside the skull and spine

-Composed of two parts: Somatic Nervous System (SNS) and the Autonomic Nervous System (ANS)

Somatic Nervous System

Part of the PNS that interacts with the external environment

-Composed of Afferent Nerves; nerves that carry sensatory signals from the skin, skeletal muscles, joints, eyes, ears to the CNS

-Also composed of efferent nerves; neves that carry motor signals from the CNS to the skeletal muscles

Autonomic Nervous System

Regulates the body’s internal environment (digestion, heart rate, etc)

-Composed of afferent nerves that carry sensory signals from internal organs to the CNS

-Composed of efferent nerves that carry motor signals from the CNS to internal organs

Efferent nerves are further divided into the Sympathetic and Parasympathetic Nerves

Sympathetic Efferent Nerves

Autonomic motor nerves that projects from the CNS in the lumbar (small of the back) and Thoracic (chest area) regions of the spinal cord

-Project from the CNS synapse on second-stage neurons at substantial distance from their target organs (2nd stage neurons are longer, so must travel longer distance)

-neurons leaving spinal cord are 1st stage, then interacts with 2nd stage neurons

Parasympathetic Efferent Nerves

Autonomic motor nerves that project from the brain and sacral (lower back) region of the spinal cord

-Project from the CNS synapse near their target organs on very short second-stage neurons

Functions of Parasympathetic and Sympathetic Nerves

1. Sympathetic nerves stimulate, organize, and mobilize energy resources in threatening stages while parasympathetic nerves conserve energy

2. Each autonomic target organ receives opposing sympathetic and parasympathetic input, and its activity is thus controlled by relative levels of sympathetic/parasympathetic activity

3. Sympathetic changes are indicative of psychological arousal, while parasympathetic changes are indicative of psychological relaxation

Cranial Nerves

Most nerves of the PNS project from the spinal cord, the exception being Cranial Nerves

-12 pairs of nerves that project from the brain, numbered with roman numerals

-Cranial Nerves are either sensory (provide sensory information to the brain) or motor function (provide motor functions to the body), some can perform both

-Motor Cranial Nerves are Parasympathetic
(I) - Olfactory nerve

(II) - Optic nerve

(X) - Motor and sensory fibers traveling to and from the gut (longest cranial nerve)

-Functions of cranial nerves are assessed for neural diagnosis

Protective Membrane of the CNS

-Brain and spinal cord protected by skull and vertebrae column (physical)

Brain and spinal cord also protected by three protective membranes called meninges

1. Dura Mater - outer most layer, tough membrane

2. Arachnoid Membrane/Mater - Immediately under the dura mater, fine membrane

• Directly beneath is the subarachnoid space, contains large blood vessels arnd cerebrospinal fluid

3. Pia Mater - Inner most meninx (layer), adheres to the surface of the CNS

Cerebrospinal Fluid (CSF)

Fills the subarachnoid space, central canal of the spinal cord, and the cerebral ventricles of the brain

-Supports and cushions the brain

-Produced by the Choroid Plexus, network of capillaries that protrude into ventricles from the Pia Mater

-Excess CSF is absorbed from the subarachnoid space then to the Dural Sinuses and drains into the jugular vein

Central Canal and Cerebral Ventricles

Central Canal - Small central channel that runs the length of the spinal cord

Cerebral Ventricles - Four large internal chambers of the brain; the two lateral ventricles, the third ventricles and the fourth ventricle

Cerebral Aqueduct - connects the third and fourth ventricles

-The subarachnoid space, central canal and cerebral ventricles are connected via series of openings and form a single reservoir

-Ventricles also provide support/protection for the brain

Blood-Brain Barrier

Impedes the passage of toxic substances from the blood into the brain

-Consequence of special structure of cerebral blood vessels

-Cells of these vessels are tightly packed rather than being loosely packed (like the rest of the body), forming a barrier to the passage of most molecules (particularly proteins/largem molecules)

-Some parts of the barrier may also for unimpeded movement of molecules

-Some large molecules (glucose, etc) are actively transported

Neuron and Neuron Cell Membrane

Cells that are specialized for reception, conduction and trasmission of electrochemical signals

-Come in a variety of shapes and sizes

Cell membrane is composed of a lipid bilayer, embedded with carbohydrates, fats and proteins (channel proteins, signal proteins)

External Features of Neurons

Dendrite - Receive most synaptic contracts from Neurons

Axon Hillock - Region at the junction between axon and cell body

Axon - Projects signals from the cell body

Myelin - Fatty insulation around many axons

-Nodes of Ranvier - Gaps between sections of myelin

Axon Buttons - Buttonlike endings of axon branches, release chemicals into synapses

Synapses - gaps between adjacent neurons, chemical signals are transmitted

Internal Features of Neurons

Nucleus - Houses DNA

Endoplasmic Reticulum - Folded membranes, plays a role in synthesis of membrane (rough E R) while the smooth ER plays a role in synthesis of fats

Cytoplasm - Internal fluid

Ribosomes - Located in cytoplasm, synthesize proteins

Golgi Complex - Membranes that package molecules into vesicles

Mitochondria - Site of energy production/release

Microtubules - Rapid transport of molecules throughout the neuron

Synaptic Vesicles - membrane packages storing neurotransmitters ready to be released into synapse

Classes of Neurons

Multipolar Neuron - Neuron with more than two processes extending from its cell body

Unipolar Neuron - Neuron with one process extending from its cell body

Bipolar Neuron - Neuron with two processes extending from its cell body

Interneuron - Neurons with a short axon or no axon at all, function is to integrate neural activity within a single brain structure (not to conduct signals from one structure to another)

Neural Structures Naming

Two types of structures, those composed primarily of cell bodies and those composed primarily of axons

-In the CNS, clusters of cell bodies are called “nuclei” while in the PNS they are called ganglia

-In the CNS, bundles of axons are called “tracts” while in the PNS they are called “nerves”

Glial Cells

Several classes of nonneural cells of the nervous system

-Previously believed to only be involved in supporting neurons (nutrition, clearing waste, providing physical structure)

But Astrocytes now shown to also exchange chemical signals with neurons and each other, controlling the establishment/maintenance of synapses between neurons, and forming functional networks

-Said to have 10x more glial cells than neurons in the body

Four classes: Oligodendrocytes, Schwann Cells, Microglia and Astrocytes

Oligodendrocytes

Glial cells with extensions that wrap around the axons of certain neurons in the CNS, these extensions are rich in myelin forming myelin sheaths

-Fatty insulating substances that increase speed of axonal conduction

Schwann Cells

Glial cells that perform a similar function to Oligodendrocytes in the PNS

Microglia

Smallest glial cells, respond to injury or disease by multiplying, engulfing debris or other cells and triggering inflammatory responses

Astrocytes

Largest glial cells (star-shaped), extensions of astrocytes cover outer surface of blood vessels in the brain

-Part of the blood-brain barrier

-play a role in allowing the passage of some chemicals from the blood into CNS neurons

-ability to contract or relax blood vessels based on blood flow demands of brain regions

Oligodendrocytes vs Schwann Cells

Oligodendrocytes are found in the CNS, provides several myelin segments often on more than one axon

Schwann cells are found in the PNS, constitutes only one myelin segment

-Schwann cells can only guide axonal regeneration (regrowth) after damage (hence why axonal regeneration is restricted to the PNS)

Golgi Stain

Neural tissue exposed to Potassium Dichromate and Silver Nitrate were turned entirely black (as a result of the Silver Chromate from the reaction)

-Allows the structure of individual neurons to be observed

-But provides no indication of the number of neurons in an area

Nissl Stain

Cresyl violet and other Nissl dyes penetrate cells and bind to molecules (DNA, RNA) allowing for the number of cell bodies in an area to be identified by cointing the Nissl-stained dots

Electron Microscopy

Provides the 3D details of neuronal structure, allowing for general aspects of neuroanatomical structure to be visualized

Neuroanatomic Tracing Techniques

Anterograde Tracing Methods - Used to trace the paths of axons projecting away from cell bodies in a particular area

-Injecting a chemical into the cell body which is then transported forward along the axon to their terminal buttons, brain is then sliced and treated to reveal locations of the chemical

Retrograde Tracing Methods - Used to trace the paths of axons projecting into a particular area

-Injecting a chemical into the brain which is then taken up by terminal buttons and transported backwards along the axons to their cell bodies, brain is sliced and treated to reveal location of the chemical

Directions in the Vertebrate NS

Anterior - Towards the nose end

Posterior - Towards the tail end

Dorsal - Towards the surface of the back and/or top of the head

Ventral - Towards the surface of the chest or bottom of the head

Medial - Towards the midline of the body (inwards)

Lateral - Away from the midline (outwards)

Proximal - Closer to the CNS

Distal - Farther from the CNS

Sections

Horizontal Sections - Cut in a plane parallel to the top of the brain

Frontal/Coronal Section - Cut in a plane parallel to the face

Sagittal Sections - Cut in a plane that is parallel to the side of the brain

-Section down the center of the brain (between the two hemispheres) is a midsagittal section

Cross Section - Cut at a right angle to any long, narrow structure (spinal cord, nerve, etc)

Spinal Cord Anatomy

Spinal cord is within the spinal canal of the vertebrate column

-5 groups of 31 spinal levels (first 8 pairs are cervical, then 12 Thoracic nerve pairs, 5 pairs of Lumbar and 5 pairs of Sacral nerves, and 1 pair of Coccygeal nerves)

Gray Matter - Inner H-shaped core, composed of cell bodies and unmyelinated interneurons

White Matter - Outer, composed largely of myelinated axons

Dorsal Horns - Two dorsal arms of the spinal gray matter, has multiple sensory neurons (afferent neurons)

Ventral Horns - Two ventral arms of the spinal gray matter, has motor neurons (efferent neurons)

-Pairs of spinal nerves are attached to the spinal cord, its axons are joined to one of two roots: Dorsal root or Ventral Root

Dorsal Root Ganglion - Grouping of all dorsal root axons just outside the cord

Brain Development

Initial three swellings in the neural tube become five; Telencephalon, Diencephalon, Mesencephalon, Metencephalon, and the Myelencephalon

-All except the Telencephalon make up the Brain Stem (stem on which the cerebral hemispheres sit, critical in reflexive activities for survival)

Forebrain, Midbrain, Hindbrain

Forebrain - Telencephalon and Diencephalon

Midbrain - Mesencephalon

Hindbrain - Metencephalon and Myelencephalon

Myelencephalon (or Medulla) - Hind Brain

Most posterior division, composed largely of tracts carrying signals between the brain and the body

-Contains the Reticular Formation - Network of 100 tiny nuclei occupying the central core of the brain stem

Medulla is involved in sleep, attention, movement, maintenance of muscle tone, cardiac/circulatory/respiratory reflexes

Metencephalon - Hind Bain

Includes ascending and descending tracts and part of the reticular formation, and two key structures:

Pons - Structure that creates a bulge on the ventral surface of the brain stem

Cerebellum - “little brain”, sensorimotor structure involved in sensation/precise movement and movement adaption, also involved in decision making/language

Mesencephalon - Midbrain

Divided into two classes: Tectum and Tegmentum

Tectum - Dorsal surface of the mid brain, composed of two pairs of bumps (Colliculi)

• Inferior Colliculi - Posterior pair, auditory function

• Superior Colliculi - Anterior pair, have visual-motor function (body orientation direction to/away from visual stimuli)

Tegmentum - Contains reticular formations, tracts of passage, and three colorful structures

• Periaqueductal Gray - gray matter situated around the Cerebral Aqueduct (duct connecting 3rd/4th ventricles), mediates pain-reducing effects of opioid drugs

• Substantia Nigra - Also involved in motor movement

• Red Nucleus - Involved in motor movement

-Substantia Nigra and Red Nucleus are involved in the sensory-motor system

Diencephalon

Composed of two structures: Thalamus and the Hypothalamus

Thalamus - Large, two-lobed structure sitting on the third ventricle with the lobes joined by the Mass Intermedia

-Contains white lamina on the surface, composed of myelinated axons

Hypothalamus - Below the anterior thalamus, regulates eating, sleep and sexual behaviour, regulates the hormones from the pituitary gland

Thalamic Nuclei

Sensory Relay Nuclei - Receive signals from sensory receptors, process and transmit to appropriate areas of sensory cortex

-Lateral Geniculate Nuclei (relay station in the visual system), Medial geniculate nuclei (auditory relay station), Ventral posterior nuclei (somatosensory relay station)

Parts of the Hypothalamus

Pituitary gland

Optic Chiasm - Point where optic nerves from each eye comes together and decussate (cross over to the other side of the brain)

-Decussating fibers are contralateral, projecting from one side of the body to the other

-Non-decussating fibers are ipsilateral (staying on the same side of the body)

Mamillary Bodies - Pair of spherical nuclei located on the inferior surface,

Telencephalon

Largest division of the human brain, mediates the brain’s complex functions

-initiates voluntary movement, interprets sensory input, cognitive processes, etc

Cerebral Cortex - Layer covering cerebral hemispheres, composed mainly of unmyelinated neurons (gray matter) whilst the layer below is myelinated (white matter)

-Deeply convoluted (furrowed), increasing amount of cerebral cortex without volume of the brain

-Most mammals are lissencephaly (smooth-brained)

Furrow Types

Fissures - Large furrows in the convoluted cortex

-Longitudinal Fissure - largest of the fissures, separates the cerebral hemisphere

-Lateral Fissure and Central Fissure

Sulci - Small fissures in the convoluted cortex

Gyri - Ridges between fissures and sulci

-Precentral Gyri - Primarily motor function

-Postcentral Gyri - Primarily somatosensory

-Superior Temporal Gyri - Location of the auditory cortex

Cerebral Commissures and Corpus Callosum

Cerebral Commissures - Tracts spanning the longitudinal fissures

Corpus Callosum - The largest cerebral commissure

Four Lobes

Occipital Lobe - Analysis of visual input

Parietal Lobe - Contains the postcentral gyrus (analyzes sensations from the body) while the posterior part perceives location of both objects and bodies in directing our attention

Temporal Lobe - Superior temporal gyrus is involved in hearing/language, inferior temporal cortex identifies complex visual patterns, medial portion of temporal cortex is involved in memory

Frontal Lobe - Precentral gyrus and adjacent frontal cortex have motor functions, frontal cortex anterior to mortor cortx performs cognitive functions (planning, potential outcomes of behaviour, etc)

Neocortex (Isocortex)

90% of the human cortex, six-layered cerebral cortex

-Has Columnar Organization - Neurons in a given vertical column form a mini-circuit that performs a single function

-Thickness can vary from area to area (ex; Layer IV is extra thick in areas of sensory cortex because stellate cells are specialized in receiving sensory signals from the thalamus)

I - Axons and dendrites, few cell bodies

II - Densely packed stellate cells, few small pyramidal cells

III - Loosely packed stellate cells; intermediate sized pyramidal cells

IV - Bands of densely packed stellate cells (NO PYRAMIDAL CELLS)

V - Large pyramid cells, loosely packed stellate cells

VI - Pyramidal cells of various sizes, loosely packed stellate cells

Pyramidal and Stellate Cells

Pyramidal Cells - large, multipolar neurons with pyramid-shaped cell bodies, large dendrites (apical dendrite) that extends towards the cortex surface and a very long axon

Stellate Cells - Small, star-shaped interneurons (neurons with short axon/no axon)

Hippcampus

Located at the medial edge of the cerebral cortex

Contains three major layers

-Plays a major role in some kinds of memory

Limbic System

Collection of interconnected nuclei and tracts that ring the thalamus

-Involved in regulation of motivated behaviours (fleeing, feeding, fighting and sexual behaviour)

-Includes the Mamillary bodies, hippocampus, amygdala, fornix, cingulate cortex and the septum

Amygdala

Structure in the anterior temporal lobe (anterior to the hippocampus), plays a role in emotion (particularly fear)

Cingulate Cortex and Gyrus

Cingulate cortex is on the medial surface of cerebral hemispheres, encircles the dorsal thalamus

Fornix

Major tract of the limbic system, connects the hippocampus with the septum and mammillary bodies

Septum

Midline nucleus of the limbic system, located near the anterior tip of the cingulate cortex

Basal Ganglia

Collection of subcortical nuclei

Long tail (the Caudate) and Putamen receive inputs from the neocortex, together known as the Striatum

Globus Pallidus - Striatum’s major output, medial to the putamen (between putamen and the thalamus)

-Plays a role in performance of voluntary motor responses and decision making