Central Nervous System

The spinal cord, spinal nerves, and somatic reflexes

Functions of the Spinal Cord
Conduction—nerve fibers conduct sensory and motor information up and down the spinal cord
Neural integration—spinal neurons receive input from
multiple sources, integrate it, and execute appropriate output (for example, bladder control)
Locomotion—spinal cord contains central pattern generators:
groups of neurons that coordinate repetitive sequences of contractions for walking
Reflexes—involuntary responses to stimuli that are vital to posture, coordination and protection

Meninges of the Spinal Cord
Meninges—three fibrous membranes that enclose the brain and spinal cord

  • They separate soft tissue of central nervous system from bones of cranium and vertebral canal

  • From superficial to deep: dura mater, arachnoid mater, and pia mater

  • Dural sheath surrounds spinal cord and is separated from vertebrae by epidural space

  • Arachnoid membrane adheres to dura and is separated from pia by fibers spanning the subarachnoid space that is filled with cerebrospinal fluid (CSF)

  • Lumbar puncture (spinal tap) takes sample of CSF

  • Pia is delicate membrane that follows contours of spinal cord and continues inferiorly as fibrous terminal filum that fuses with dura to form coccygeal ligament

Central area of gray matter shaped like a butterfly and surrounded by white matter
Gray matter—neuron cell bodies with little myelin

  • Site of information processing, synaptic integration

White matter—abundantly myelinated axons

  • Carry signals from one part of the CNS to another


Spinal Tracts
Fibers in a given tract have similar origin, destination and function
Ascending tracts—carry sensory information up
Descending tracts—carry motor information down
Decussation—crossing of the midline that occurs in many tracts so that brain senses and controls contralateral side of body


General Anatomy of Nerves and Ganglia
Spinal cord communicates with the rest of the body by way of spinal nerves
Nerve—a cord-like organ composed of numerous nerve fibers (axons) bound together by connective tissue

  • Mixed nerves contain both afferent (sensory) and efferent (motor) fibers

General Anatomy of Nerves and Ganglia 4
Ganglion—cluster of neurosomas outside the CNS


Spinal Cord Trauma
In United States, 10,000 to 12,000 people paralyzed each year by spinal cord trauma
• Usually by vertebral fractures
• Highest risk group: males 16 to 30 years old
55% occur in automobile or motorcycle accidents

Chapter 14
The Brain and Cranial Nerves

Major Landmarks
Directional terms:
• Rostral—toward the forehead
• Caudal—toward the spinal cord
Important anatomical features:
• Longitudinal fissure—deep groove that separates cerebral hemispheres
• Gyri—thick folds on brain surface
• Sulci—shallow grooves between gyri
• Corpus callosum—thick nerve bundle at bottom of longitudinal fissure that connects hemispheres


Three major portions of the brain:

  • Cerebrum (cerebral hemispheres)

    • 83% of brain volume

    • Important features and landmarks: gyri and sulci, longitudinal cerebral fissure, corpus callosum

  • Cerebellum

    • Second largest brain region

    • Contains 50% of the neurons; about 10% of brain volume

    • Located in posterior cranial fossa

    • Separated from cerebrum by transverse cerebral fissure

  • Brainstem

    • Includes diencephalon, midbrain, pons, medulla oblongata

Gray and White Matter
Gray matter
• Contains cell bodies, dendrites, and synapses
• Darker color due to very little myelin present in tissue
• Forms surface layer (cortex) over cerebrum and cerebellum
• Forms nuclei deep within brain
White matter
• Bundles of axons
• White color from lots of myelin
• Called “tracts” in the CNS, “nerves” in the PNS
• Deep to cortical gray matter in brain
• Superficial to gray matter in spinal cord

Meninges
Meninges—three membranes surrounding brain and spinal
cord

  • Lie between the nervous tissue and bone

  • Protect the brain and provide structural framework for its
    arteries and veins

  • From outermost to innermost:

    • Dura mater

    • Arachnoid mater

    • Pia mater

Dura mater presses closely against cranial bones

  • No epidural space (unlike spinal cord)

  • Not directly attached to bone except around foramen magnum, sella turcica, crista galli, and sutures of the skull

Folds of dura mater extend inward, separate some brain
regions

  • Falx cerebri separates the two cerebral hemispheres

  • Tentorium cerebelli separates cerebrum from cerebellum

  • Falx cerebelli separates right and left halves of cerebellum

Arachnoid mater
• Transparent membrane over brain surface
• Subarachnoid space separates it from pia mater below
Pia mater
• Very thin membrane, not usually visible without
microscope
• Follows all contours of brain
• Follows arteries as they penetrate into cerebrum


Meningitis
Meningitis—inflammation of the meninges
• Serious disease of infancy and childhood
• Especially between 3 months and 2 years of age
• Bacterial or viral invasion of CNS through nose and throat
• Pia mater and arachnoid are most often affected
• Can cause swelling of the brain, enlargment of the ventricles,
and hemorrhage
• Signs include high fever, stiff neck, drowsiness, intense
headache
• May progress to coma, then death within hours of onset
• Diagnosed by examining CSF obtained by lumbar puncture
(spinal tap)

Ventricles and Cerebrospinal Fluid
Ventricles—four fluid-filled chambers within brain
• Two lateral ventricles
• One in each cerebral hemisphere
• Third ventricle
• Narrow medial space beneath corpus callosum
• Fourth ventricle
• Small triangular chamber between pons and cerebellum

Cerebrospinal fluid (CSF)
• Clear, colorless liquid
• Fills the ventricles and canals of CNS
• Surrounds brain and bathes its external surface


Production of CSF begins with filtration of blood plasma through
capillaries of the brain
• Choroid plexus—spongy mass of blood capillaries on the floor of each
ventricle
• Ependyma—type of neuroglia that lines ventricles and covers choroid
plexus
• Ependymal cells modify the filtrate. Compared to plasma, CSF has more sodium and chloride, less potassium, calcium, glucose, and very little protein

Functions of CSF
• Buoyancy
• Allows brain to attain considerable size without being impaired by
its own weight
• If brain rested heavily on floor of cranium, pressure would kill the
nervous tissue
• Protection
• Protects brain from striking cranium when head is jolted
• Shaken child syndrome and concussions still occur from severe
jolting
• Chemical stability
• Flow of CSF rinses away metabolic wastes from nervous tissue and homeostatically regulates chemical environment

Blood Supply and the Brain Barrier System
Brain only contributes 2% of adult body weight but receives 15% of the blood
• 750 mL/min
• Neurons have high demand for ATP (therefore oxygen and glucose) so constant supply of blood critical
• 10-second interruption—loss of consciousness
• 1- to 2-minute interruption—significant impairment of neural function
• 4-minute interruption—irreversible brain damage

The brain barrier system regulates what substances can get from bloodstream into tissue fluid of the brain
• Although blood is crucial, it can also contain harmful agents
• The brain barrier system is selectively permeable
• Highly permeable to water, glucose, and lipid-soluble substances such as oxygen, carbon dioxide, alcohol, caffeine, nicotine, anesthetics
• Slightly permeable to sodium, potassium, chloride, waste products urea and creatinine

Two points of entry must be guarded:
• Blood capillaries throughout the brain tissue
• Capillaries of the choroid plexus