CNS Protection Structures

Structures Protecting the Brain and Spinal Cord

• The brain and spinal cord are surrounded by bones, membranes, and fluid.
• The bones include the skull (cranium) and vertebrae.
• The membranes are called meninges, which consist of three layers: dura, arachnoid, and pia mater.

Meninges

• Dura: the outermost layer, made of fibrous connective tissue, houses blood vessels and nerves, and is attached to the inside of the cranial cavity.
• Arachnoid: a thin, netlike layer, avascular, and located between the dura and pia mater, with a subarachnoid space containing cerebrospinal fluid (CSF).
• Pia mater: a thin layer, closely adhered to the organs of the CNS, houses many nerves and blood vessels that aid in nourishing the brain and spinal cord.

Subarachnoid, Subdural, and Epidural Spaces

• Subarachnoid space: between the arachnoid and pia mater, contains CSF.
• Epidural space: above the dura mater, inside the bony cranium, contains loose connective tissue and fat.
• Subdural space: closest to the brain, above the arachnoid.

Divisions of the Brain

• The brain is composed of many neurons and nerve fibers, allowing communication within the nervous system.
• Divided into three major divisions: cerebrum, cerebellum, and brainstem.
• Cerebrum: the largest portion, provides sensory and motor function, and is associated with higher mental functions.
• Cerebellum: assists in balance, coordinated, and smooth muscle movements.
• Brainstem: connects the nervous system to higher centers, contains vital centers controlling visceral activities.

Structure and Function of the Cerebrum

• The cerebrum consists of two large masses: cerebral hemispheres.
• The hemispheres are connected by the corpus callosum.
• The cerebral cortex is a "veil" of gray matter, containing 75% of the neuron's cell bodies in the nervous system.
• The corpus callosum allows communication between the two hemispheres and their various functions.

Cerebral Hemispheres

• The hemispheres control the opposite side of the body due to the projection fibers crossing over at the level of the medulla oblongata.
• Most people have left hemisphere dominance, which is associated with language, mathematical abilities, and logic.

Cerebral Tracts

• Beneath the cerebral cortex are masses of white matter, classified into three types: commissural, association, and projection.
• Commisural tracts connect corresponding areas in the right and left hemispheres.
• Association tracts transmit impulses within a single hemisphere.
• Projection tracts tie the cortex to the rest of the nervous system.

Basal Nuclei

• Located deep within the cerebral white matter, a group of gray matter nuclei.
• Involved in regulating voluntary motor activity, serving as a relay station for motor impulses.
• Dopamine is produced in the basal nuclei, assisting in controlling neural circuits.

Cerebellum

• A large, cauliflower-shaped mass of brain tissue, posterior and inferior to the cerebrum.
• Communicates with the rest of the CNS via three pairs of tracts, providing precise timing for skeletal muscle activity.
• Fibers reach the cerebellum from equilibrium receptors, eye, and proprioceptors of skeletal muscles and tendons.

Brainstem

• Located beneath the cerebrum and anterior to the cerebellum, consisting of midbrain, pons, and medulla oblongata.
• The brainstem connects the nervous system to higher centers.

Thalamus, Hypothalamus, and Medulla Oblongata

• Thalamus: a relay station for ascending (sensory) impulses, connecting other parts of the nervous system to the cerebral cortex.
• Hypothalamus: regulates many visceral activities, playing a key role in homeostasis.
• Medulla oblongata: the location where tracts cross over, containing the vasomotor, cardiac, and respiratory centers.

Reticular Activating System (RAS)

• A complex network of nerve fibers and gray matter scattered throughout the brain.
• Activates the cerebral cortex into a state of wakefulness, essential for wakefulness, attention, and concentration.

Limbic System

• A complex network of fiber tracts and gray matter found interconnecting parts of the cerebral cortex.
• Monitors emotional experiences and expressions, interacting with higher cerebral cortical areas.
• Plays a role in memory storage, with distinct feelings and emotions.

Spinal Cord

• A nerve column, posteriorly located, with a central canal housing CSF.
• The anterior horns house the large cell bodies and give rise to motor fibers.
• The white matter surrounds the gray matter, divided into anterior, lateral, and posterior columns.
• Functions include conducting nerve impulses, providing a two-way communication between the brain and outside the nervous system, and serving as a center for spinal reflexes.

Cerebrospinal Fluid (CSF)

• A clear liquid, secreted by the choroid plexus within the roof of the ventricles.
• Circulates in the subarachnoid space, supporting and protecting the CNS.
• Amount remains constant, with a little over ½ cup (150 ml) present at any time.
• Composition is similar to blood plasma, with major solutes including glucose, proteins, and NaCl.
• Plays a role in maintaining a stable ionic concentration, carrying wastes, and providing a pathway for waste products.

Tracts

• Pyramidal tracts: descending motor tracts, originating in the primary motor area (cortex) and crossing at the level of the medulla.
• Extrapyramidal tracts: all other motor tracts, relaying to numerous other sites within the brain.

Ascending and Descending Tracts

• Ascending tracts: carry sensory information from the periphery to the brain.
• Descending tracts: carry motor impulses from the brain to the effector.