Coverings of the CNS
• 1) Bone – Cranium, Vertebrae
• 2) Meninges – Three connective tissue membranes
covering the brain and spinal cord
•
a) Dura Mater – outermost , composed of tough
fibrous connective tissue. Vascular. Attached to
cranium but not to the vertebrae. Epidural Space
exists between the vertebra and Dura Mater.
Composed of fat.
•
b) Arachnoid Mater – middle layer. Thin, web-like
layer
•
c) Pia Mater – innermost layer. Very thin,
vascular. Clings to the surface of the brain. Aids in
nourishing underlying brain cells
• Subarachnoid Space – exists between
the Arachnoid and Pia Mater. Filled with
cerebrospinal fluid.
The Brain
• Composed of 100 Billion Neurons – Divided into
4 major regions
• 1) Cerebrum – largest region. Surface
has elevated ridges called Gyri, separated by
shallow grooves called Sulci and less
numerous but deeper grooves called Fissures.
• Sulci and fissures divide the cerebrum into
lobes named for the cranial bones above them.
• Divided into hemispheres by the Longitudinal
Cerebral Fissure.
• Hemispheres connected by a bridge of nerve
fibers called the Corpus Callosum
• Impulses cross over to the other side of
the body in the brainstem. Impulses from
the right side of the brain control muscles
on the left side of the body.
• Hemisphere Dominance – both
hemispheres participate in basic functions.
In most people one side acts as a
dominate hemisphere for other functions.
90% of people are left hemisphere
dominant
• Within cerebral hemispheres and
brainstem are interconnected cavities
called Ventricles
• The ventricles are continuous with the
central canal of the spinal cord and are
filled with Cerebrospinal Fluid
• Cerebrospinal fluid (CSF) completely
surrounds the brain and functions to:
• 1) support and protect
• 2) Maintain ion concentration of the CNS
• 3) Remove wastes
Functions of the Cerebrum
• 1) Interpret sensory impulses
• 2) Initiate voluntary muscle
movement
• 3) Store Information
• 4) Reasoning
• 5) Personality, Intelligence
• 2) Diencephalon – Sits atop the
brainstem and consists of 3 regions
•
Thalamus – relay station for sensory
impulses
•
Hypothalamus – Plays a role in the
regulation of body temperature, water
balance and metabolism. Also thirst,
appetite, pain and pleasure centers are in
the hypothalamus.
•
Epithalamus – tissues lining the
epithalamus form cerebrospinal fluid
• 3) Brain Stem – Bundle of nerve
tissue that connects the cerebrum to the
spinal cord. It also has many areas of
gray matter that controls vital activities.
3 Main Regions
•
a) Midbrain – Center for auditory and
visual reflexes
•
b) Pons – Relay impulses from the
medulla to the cerebrum
•
c) Medulla Oblongata – Regulates
blood pressure, heart rate, breathing and
certain reflexes
• 4) Cerebellum – Acts as a control
center in the coordination of skeletal
muscle movements
Cranial Nerves
• 12 pairs of nerves arise from the underside
of the brain that mostly serve the head and
neck
• Numbered in order, front to back
• Most are mixed nerves, but three are
sensory only
• Olfactory – Sense of smell
• Optic – Sense of Vision
• Vestibulocochlear – Hearing and balance
• Vagus – Sensations and movements of
Visceral organs.
Spinal Cord
• Extends from the foramen magnum to the
disk between the 1st and 2nd Lumbar
vertebrae
• It is surrounded and protected by the
meninges
• The meninges extends below the end of
the cord and provide for safe sampling of
CSF below L3 (spinal tap)
• Spinal cord gives rise to 31 pairs of spinal
nerves that exit the vertebrae and serve
the body close by.
• Nerves exiting below the end of the cord travel
through the vertebral canal and form the Cauda
Equina (horse’s tail)
• Cross section of the cord reveals a core of gray
matter surrounded by white matter. Pattern of
gray matter resembles a butterfly
• Neurons of the gray matter are interneurons.
Neurons in the white matter are nerve tracts
• Central canal filled with Cerebrospinal Fluid
• Divided into right and left halves much like the
brain
Spinal Cord Functions
• 1) Reflex Center (gray matter)
• 2) Conduct impulses to and from the brain
(white matter)
Peripheral Nervous System
• Nerves that branch out of the CNS to
different parts of the body
• 2 Divisions – Somatic and Autonomic
• Somatic Nerves – Nerves that lead to the
skin and skeletal muscles involved in
conscious activities
• Autonomic Nerves – Nerves that lead to
the visceral organs involved in unconcious
activities
Autonomic Nervous System
• Further subdivided into the Sympathetic
and Parasympathetic divisions
• Impulses from one set of nerves activate
an organ. Impulses from the other nerves
inhibit the organ
• Sympathetic division is concerned with
preparing the body for energy expending,
stressful or emergency situations
• 31 pairs of spinal nerves originate from the
spinal cord
• All are mixed nerves. Not named but
numbered
• 8 pairs of Cervical nerves
• 12 pairs of Thoracic nerves
• 5 pairs of Lumbar nerves
• 5 pairs of Sacral nerves
• 1 pair of Coccygeal nerves
• Each spinal nerve emerges from the spinal
cord as two short branches or “roots”
• The dorsal root is composed of sensory
fibers and the ventral root is composed of
motor fibers
• The dorsal and ventral root unite to form a
spinal nerve which passes outward from
the vertebral canal through the
intervertebral foramen
• After emerging from the vertebral canal,
main portions of the spinal nerves
combine to form complex networks called
plexuses
Plexuses
• 1) Cervical Plexus – C1-C4 – muscles of the
neck, diaphragm
• 2) Brachial Plexus – C5-T1 – arm, forearm,
hand
• 3) Lumbosacral Plexus – T12-S5 – Lower
abdomen, legs