Central Nervous System

Central Nervous System
Part II Chapter 13

Protection to the Brain
Nervous tissue is soft and delicate, and the irreplaceable neurons can be injured or destroyed by even slight pressure The brain is protected from injury by… The skull Surrounding membranes called meninges A watery cushion of cerebrospinal fluid The blood-brain barrier

Protection to the Brain
The skull is a self-bracing arrangement of bones that encapsulates the brain It was presented in Chapter 7 and thus will receive only passing reference in this section

Meninges The meninges are three connective tissue membranes that lie just external to the brain and spinal cord

Meninges The meningeal membranes Cover and protect the CNS structures
Protect blood vessels and enclose venous sinuses Contain cerebrospinal fluid Form partitions within the skull

Meninges From external to internal, the meningeal layers are
Dura mater Arachnoid Pia mater

The Dura Mater The leathery dura mater is by far the strongest of the meninges Where it surrounds the brain it is a double layer membrane

The Dura Mater The periosteal layer is the superficial and lines the inner surface (periostium) of the skull The deeper meningeal layer forms the true external covering of the brain

The Dura Mater The brain’s dural layers are fused together except in certain areas where they enclose the blood filled dural sinuses The dural sinuses collect venous blood and direct it into the internal jugular veins of the neck

The Dura Mater In several places the meningeal dura mater extends inward to form flat septa (partitions) that limit movement of the brain within the skull

The Dura Mater The falx cerebri dips into the longitudinal fissure
It attaches to the crista galli of the ethmoid bone

The Dura Mater The falx cerebelli forms a midline partition that runs along the vermis of the cerebellum

The Dura Mater The tentorium cerebelli extends into the transverse fissure between the cerebral hemispheres and the cerebellum

The Arachnoid Mater The middle membrane forms a loose brain covering over the surface of the cerebrum It is separated from the dura mater by a narrow serous cavity, the subdural space Beneath the arachnoid membrane is the wide subarachnoid space

The Arachnoid Mater The subarachnoid space is filled with cerebrospinal fluid and contains the largest blood vessels serving the brain Since the arachnoid is fine and elastic, these blood vessels are rather poorly protected

The Arachnoid Mater Arachnoid villi protrude through the overlying dura mater and into the dural sinuses overlying the superior aspect of the brain Cerebrospinal fluid is absorbed into the venous blood sinuses through these valvelike villi

The Pia Mater The pia mater is a delicate connective tissue that is richly invested with tiny blood vessels It is the only membrane that clings tightly to the brain, following its every convolution

The Pia Mater Meningitis is an inflammation of the meningeal layers that is caused by either a bacterial or viral infection that can spread to the underlying nerve tissue Brain inflammation is called encephalitis

Cerebrospinal Fluid (CSF)
CSF is a watery “ broth”found in and around the brain and spinal cord It forms a liquid cushion that gives buoyancy to the CNS organs With the brain floating, CSF reduces brain weight by 97% and thus prevents the brain from crushing under its own weight CSF also protects the brain and spinal cord from trauma

Cerebrospinal Fluid CSF
CSF also helps to nourish the brain It also helps to remove wastes produced by neurons Finally, it carries chemical signals between different parts of the CNS Although it performs many functions there is ml of fluid (about a half cup) present in the body at any one time

CSF is a similar in composition to blood plasma, from which it arises It contains less protein and more sodium and chloride ions

The figure at the right depicts the sites of CSF production and its circulation Most CSF is made in the choroid plexuses which are membranes on the roofs of the four brain ventricles

Choroid Plexus Choroid plexus hang from the roof of each ventricle
The plexuses are clusters of thin walled capillaries enclosed by a layer of ependymal cells

Choroid Plexus The capillaries of the choroid plexus are fairly permeable and fluid filters continuously from the bloodstream into the ventricles

Choroid Plexus The choroid plexus cells are joined by tight junctions and have ion pumps that allow them to modify this filtrate by actively transporting only certain ions across their membranes into the CSF pool

Choroid Plexus After entering the ventricles, the CSF moves freely through these chambers Some CSF enters the central canal of the spinal cord, but most enters the subarachnoid space through the lateral and median apertures in the walls of the fourth ventricle In the subarachnoid space, the CSF bathes the outer surface of the brain and cord

The Choroid Plexus Cerebrospinal fluid arises from the blood and returns to it at a rate of about 500 ml a day The choroid plexus also helps to cleanse the CSF by removing waste products and other unnecessary solutes

CSF Circulation The motion of the CSF is aided by the long microvilli of the ependymal cells lining the ventricles

Blood-Brain Barrier The brain has a rich supply of capillaries that provide its nervous tissues with nutrients, oxygen, and all other vital molecules However, some blood-borne molecules that can cross other capillaries of the body cannot cross the brain capillaries

Blood-Brain Barrier Blood-borne toxins, such are urea, mild toxins from food, bacterial toxins, are prevented from entering brain tissue by the blood-brain barrier The barrier is a protective mechanism that helps maintain a stable internal environment for the brain

Blood-Brain Barrier The brain is very dependent on a constant internal environment Fluctuations in the concentration of ions, hormones, or amino acids, would alter the brain’s function Hormones and amino acids can influence neurotransmitters Ions (K+) can affect neuron thresholds

Blood-Brain Barrier Blood-borne substances within the brain’s capillaries are separated from the extra- cellular space and neurons by Continuous endothelium of the capillary walls Relatively thick basal lamina surrounding the external face of the capillary To a limited extend the “feet” of the astrocytes that cling to the capillaries

Blood-Brain Barrier Basal lamina (cut) The capillary endothelial cells are joined almost seamlessly by tight junctions They are the least permeable capillaries in the body The relative impermeability of brain capillaries accounts for most of the blood brain barrier

Blood-Brain Barrier The blood-brain barrier is a selective, rather than absolute barrier Nutrients, such as glucose, essential amino acids, and some electrolytes, move passively by facilitated diffusion through the endothelial cell membranes

Blood-Brain Barrier The barrier is ineffective against fats, fatty acids, oxygen, and carbon dioxide, and other fat-soluble molecules that diffuse easily through all plasma membranes This explains why blood-borne alcohol, nicotine, and anesthetics can affect the brain The barrier is not completely uniform and not completely developed in infants

The Spinal Cord The spinal cord runs through the vertebral canal of the vertebral column from the foramen magnum superiorly to the level of vertebra L1 or L2 inferiorly

The Spinal Cord The functions of the spinal cord include:
Through the nerves that attach to it, the spinal cord is involved in the sensory and motor innervation of the entire body inferior to the head It provides a two-way conduction pathway for signals between the body and the brain It is a major center for reflexes

The Spinal Cord The spinal cord is protected by bone, cerebrospinal fluid, and meninges Dura mater, arachnoid, pia mater

The Spinal Cord Between the bony vertebrae and the spinal dural sheath is a large epidural space filled with a soft padding of fat and a network of veins Cerebrospinal fluid fills the subarachnoid space

The Spinal Cord Inferiorly, the dural and subarachnoid membranes extend to the level of S2 while the spinal cord ends at L1 Subarachnoid space beyond L1 is an ideal site for a spinal tap

The Spinal Cord The spinal cord does not extend the full length of the vertebral column, ending in the superior lumbar region The spinal cord does not run all the way to the coccyx because it grows slower caudally than the spinal column

The Spinal Cord At 3 months after conception it extends to the coccyx
At the time of birth it ends at L3 During childhood it attains the adult position, terminating at the level of the intervertebral disc between L1 and L2 But it does vary among people, ranging from T12 to the superior margin of L3

The Spinal Cord The spinal cord terminates in a tapering cone shaped structure called the conus medullaris The cone tapers into a long filament of connective tissue, the filum terminale, which is covered with pia mater and attaches to the coccyx inferiorly

The Spinal Cord There are 31 pairs of spinal nerves (PNS) that arise from the spinal cord by paired roots and exit from the vertebral column via the intervertebral formina

The Spinal Cord Each segment of the spinal cord is defined by a pair of spinal nerves that lie just superior to their corresponding vertebra 8 cervical 12 thoracic 5 lumbar 5 sacral 1 coccygeal

The Spinal Cord The segments of the spinal cord all lie superior to their corresponding vertebrae because of the rostral shift of the spinal cord during development

The Spinal Cord The spinal cord has obvious enlargements where the nerves serving the upper and lower limb arise Cervical enlargement Lumbar enlargement

The Spinal Cord Because the cord does not reach the end of the vertebral column, the lumbar and sacral spinal nerve roots angle sharply downward and travel inferiorly before reaching their intervertebral foramina This collection of nerve roots at the inferior end of the vertebral canal is called the cauda equina

The SpinalCord The arrangement of the cauda equina reflects the fact that vertebral column growth proceeds more rapidly than does the growth of the spinal cord

The Spinal Cord The spinal cord is wider laterally than from an anterior/posterior perspective Two deep grooves, the posterior median sulcus and the anterior median fissure run the length of the cord and divide it into right and left halves

Gray Matter of the Spinal Cord
The spinal cord consists of an outer region of white matter and an inner region of gray matter As in other parts of the CNS, the gray matter of the spinal cord consists of a mixture of neuron cell bodies, short unmyelinated axons and dendrites and neuroglia

Gray Matter and Spinal Roots
The gray matter consists of a mixture of neuron cell bodies, their unmyelinated processes, and neuroglia (support cells)

The white matter is composed of myelinated and unmyelinated nerve fibers that represent ascending, descending and transverse pathways

In cross section the gray matter is shaped like an H The gray commissure contains the narrow central cavity of the spinal cord, the central canal

The two posterior arms of the H are the posterior horns, whereas the two anterior arms are the anterior horns

In three dimensions these arms run the entire length of the spinal cord and are called the dorsal and ventral columns

Additionally, small lateral columns called lateral horns are present in the thoracic and superior lumbar segments of the spinal cord

This illustration depicts the basic organization of the spinal gray matter The posterior horns are almost entirely comprised of interneurons

These interneurons receive information from sensory neurons whose cell bodies lie outside the spinal cord in dorsal root ganglia, and whose axons reach the cord in dorsal roots

The anterior (and lateral horns) contain cell bodies of motor neurons that send their axons out of the cord in ventral roots to supply muscles and glands

Interneurons also occur in the anterior horns, but they are not emphasized in this picture The size of the anterior motor horns varies along the length of the spinal cord reflecting the amount of skeletal musculature innervated

The anterior horns are the largest in the cervical and lumbar regions of the cord, which innervate the upper and lower limbs respectively

The gray matter can be further divided according to the innervation of the somatic and visceral regions of the body

This scheme recognizes four zones of spinal cord gray matter; somatic sensory (ss), visceral sensory (vs); Visceral motor (vm), and somatic motor (sm)

White Matter and Spinal Cord
The white matter of the spinal cord is composed of mylinated and unmylinated axons

Communication within the white matter of the spinal cord occurs between different parts of the spinal cord and between the spinal cord and the bran These fibers are of three types Ascending Descending Commissural

Ascending fibers in the spinal cord carry sensory information from the sensory neurons of the body to the brain Descending fibers carry motor instructions from the brain to the spinal cord, to stimulate contraction of the body’s muscles and secretion of its glands Commissural fibers cross from one side of the cord to the other

The ascending and descending tracts make up most of the white matter of the spinal cord Ascending tracts are shown in blue and labeled at left Descending tracts are shown in red and labeled at right

The white matter on each side of the spinal cord is divided into three white columns, or funiculi, named according to their positions in the cord, posterior, anterior, and lateral funiculi

Posterior funiculi - also called dorsal white column Anterior funiculi -adjacent the anterior median fissure Lateral funiculi - adjacent the lateral horn

The three funiculi contain many fiber tracts, each of which consists of axons with similar destinations and functions For the most part these spinal tracts are named according to their origin and destination

The ascending and descending tracts make up most of the white matter of the spinal cord Ascending tracts are shown in blue and labeled at left Descending tracts are shown in red and labeled at right

Sensory and Motor Pathways
All major spinal tracts are segments of multi-neuron pathways that connect the brain to the body Sensory information to the brain Instructions to effectors to the body

Sensory and Motor Pathways
Generalizations about spinal pathways Most pathways cross over from one side of the CNS to the other at some point Most consist of a chain of two or three neurons that contribute to successive tracts Most exhibit somatotopy, a precise spatial relationship among the tract fibers that reflects the orderly mapping of the body All pathways and tracts are paired (right and left) with a member of the pair on each side of the spinal cord or brain

Ascending (Sensory) Pathways
Foot

Neuron Pathways Axons of sensory (1st order) neurons enter dorsal root of spinal cord Synapse with 2nd order neurons in medial lemniscal tract and ascend to Thalamus Synapse with 3rd order neurons which transmit to somatosensory cortex

Ascending (Sensory) Tracts
The ascending pathways conduct sensory impulses upward, typically through chains of three successive neurons (first-, second, and third-order neurons) to various regions of the brain Most of the incoming information results from stimulation of General sensory receptors Touch / pressure / temperature / pain Stimulation of proprioceptors Muscle stretch / tendon / joint

There are four main ascending pathways The dorsal column (fasciculus gracilis and fasciculus cuneatus) and spinothalamic (lateral and anterior) pathways transmit sensory impulses to the primary somatosensory cortex for interpretation The posterior and anterior spinocerebellar pathways convey information on proprioception to the cerebellum which uses this information to coordinate body movements

In general, sensory information is conveyed along these main pathways on each side of the spinal cord Four transmit impulses to the sensory cortex for conscious interpretation Fasciculi cuneatus Fasciculi gracilis Lateral spinothalamic tract Anterior spinothalamic tract Two transmit impulses to the cerebellum to coordinate muscle activity Anterior spinocerebellur tract Posterior spinocerebellur tract

Posterior funiculi (dorsal white column) Fasciculi cuneatus Fasciculi gracilis Transmit information from the fine touch and pressure receptors and joint proprioceptors These tracts comprise what is referred to as discriminative touch and conscious proprioception

Lateral and anterior funiculi Lateral spinothalamic tract Anterior spinothalamic tract Convey information on pain, temperature, deep pressure and course touch (undiscriminated)

Anterior and posterior funiculi Anterior spinocerebellar tract Posterior spinocerebellar tract Convey information from proprioceptors (muscle and tendon stretch) to the cerebellum which uses this information to coordinate skeletal muscle activity

Since the spinocerebellar tracts do not terminate in the cortex, these pathways do not contribute to conscious sensation The spinocerebellar tracts do not decussate and thus contribute to ipsilateral innervation

Descending (Motor) Tracts
The descending motor tracts that deliver impulses from the brain to the spinal cord are divided into two groups Pyramidal tracts All others

Motor pathways involve two neurons, referred to as upper and lower motor neurons The pyramidal cells of the motor cortex, as well as the neurons in subcortical motor nuclei that give rise to other descending motor pathways, are called upper motor neurons The anterior horn motor neurons, which actually innervate the skeletal muscles are called lower motor neurons

The lateral (pyramdial) and anterior corticospinal tracts are the major motor pathways concerned with voluntary movement, particularly precise or skilled movement

The pyramdial tracts are also called the direct pathways because their axons descend without synapsing from the pyramidal cells of the primary motor cortex all the way to the spinal cord

Pyramidal tracts synapse primarily with interneurons, but also directly with anterior horn motor neurons, principally those controlling limb muscles The anterior horn motor neurons activate the skeletal muscles with which they are associated

The remaining descending tracts include: Rubrospinal Anterior reticulospinal Lateral reticulospinal Vestibulospinal Tectospinal

The remaining tracts originate in different subcortical motor nuclei of the brain stem These tracts were formerly lumped together as the extrapyramidal tracts The current term is to label them indirect pathways or just the names of the individual pathway

Although the cerebellum coordinates voluntary muscle activity, no motor efferents descend directly from the cerebellum to the spinal cord The cerebellum influences motor activity by acting through relays on the motor cortex

Spinal Cord Trauma Damage to the spinal cord is associated with some form of loss of function Paralysis / loss of function Paresthesis / sensory loss Flaccid paralysis / motor loss Spastic paralysis / upper motor neuron loss Body regions below lesion Quadriplegia / spinal cord injury - 4 limbs Paraplegia / spinal cord injury - 2 limbs Hemiplegia / brain injury - one side of body

Developmental Aspects of CNS
Fetal alcohol syndrome Cerebral palsy Anencephaly (without brain) Spina bifida (forked spine)

Embryonic Development
The spinal cord develops from the caudal portion of the embryonic neural tube By the end of the 6th week each side of the developing cord has two clusters of neuroblasts that have migrated outwarded from the neural tube

The two clusters are the dorsal alar plate and a ventral basal plate Alar plate neurons become interneurons The basal plate neurons become motor neurons that sprout axons that grow out to the effector organs

Axons that emerge from alar plate cells form the external white matter of the cord by growing outward along the length of the CNS The alar plates expand dorsally and the basal plates expand vertically to become the H-shaped mass of gray matter

Neural crest cells that come to lie alongside the cord form the dorsal root ganglia containing sensory nerve cell bodies, which send their axons to the dorsal aspect of the brain Neural crest cells

The lateral horn neurons are autonomic (sympathetic) motor neurons that serve the visceral organs Their axons also leave the cord via the ventral root

Afferent fibers carrying impulses from peripheral sensory receptors form the dorsal roots of the spinal cord

The cell bodies of the associated sensory neurons are found in an enlarged region of the dorsal root called the dorsal root ganglion or spinal ganglion

After entering the cord, the axons take a number of routes Some enter the posterior white matter of the cord or brain, others synapse with interneurons

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