Central Nervous System: Brain Structures

Central Nervous System (CNS)

• The CNS consists of the brain and spinal cord.
• The CNS is protected by the blood-brain barrier, cerebrospinal fluid, meninges, and bone (skull and vertebral column).

Brain

• The adult brain weighs approximately 3 pounds (~ 1.3 – 1.4 kg).
• The brain is divided into four major regions: cerebrum, diencephalon, brainstem, and cerebellum.
• The cerebrum is the largest and most superior part of the brain, consisting of paired highly-convoluted cerebral hemispheres.

Cerebrum

• The cerebral hemispheres are separated by the longitudinal fissure.
• The cerebrum surface exhibits elevated ridges of tissue (gyri) separated by shallow grooves (sulci) or deeper grooves (fissures).
• The lobes are named for the cranial bones that lie over them.
• Each cerebral hemisphere has three basic regions: cerebral cortex, cerebral white matter, and basal nuclei.

Cerebral Cortex

• The cerebral cortex is responsible for speech, memory, logical and emotional responses, consciousness, interpretation of sensation, and voluntary movement.
• The postcentral gyrus (primary somatic sensory cortex) receives and interprets impulses from the body's sensory receptors (except special senses).
• The precentral gyrus (primary motor cortex) sends impulses to the body's skeletal muscles.

Cerebral White Matter

• The cerebral white matter is composed of myelinated nerve fibers (tracts) that carry impulses to and from the cortex.
• Commissural fibers (corpus callosum) connect the cerebral hemispheres.
• Association fibers connect areas within a cerebral hemisphere.
• Projection fibers connect the cerebrum with lower CNS centers (e.g., brainstem).

Diencephalon

• The diencephalon is located superior to the brainstem.
• Major structures of the diencephalon include the thalamus, hypothalamus, and epithalamus.
• The epithalamus contains the pineal gland, which produces the hormone melatonin, and the choroid plexus of the third ventricle, which forms the cerebrospinal fluid (CSF).

Brainstem

• The brainstem is located superior to the spinal cord.
• Major structures of the brainstem include the midbrain, pons, and medulla oblongata.
• The brainstem provides a pathway for ascending and descending tracts and controls vital activities.
• The brainstem is the site of cranial nerves' cell bodies.

Reticular Formation

• The reticular formation is a diffuse mass of gray matter that extends the entire length of the brainstem.
• The reticular formation is a pathway for ascending and descending tracts and contains the reticular activating system (RAS), which filters out weak and repetitive sensory signals.

Cerebellum

• The cerebellum is divided into two hemispheres, with a convoluted surface and outer cortex of gray matter and inner region of white matter.
• The cerebellum coordinates muscle movements, maintains posture, and regulates balance.

Spinal Cord

• The spinal cord is a cylindrical structure, approximately 17 inches (42 cm) long, and is a continuation of the brainstem.
• The spinal cord is enclosed within the vertebral/spinal column (spine) and is located in the dorsal spinal cavity.
• The spinal cord is a two-way conduction pathway to and from the brain and is a major reflex center.
• 31 pairs of spinal nerves arise from the spinal cord and exit from the spine to serve close body areas.

Gray Matter in the Spinal Cord

• The gray matter in the spinal cord is a central butterfly-shaped or H-shaped area.
• The gray matter has two posterior projections (dorsal/posterior horns), two anterior projections (ventral/anterior horns), and two lateral projections (lateral horns).
• The gray matter surrounds the central canal, where CSF circulates.
• Interneurons are found in the dorsal horns, and cell bodies of sensory neurons are found in the dorsal root ganglion.
• Cell bodies of somatic (voluntary) motor neurons are found in the ventral horns, and nerve fibers of somatic (voluntary) motor neurons exit the spinal cord through the ventral root.

White Matter in the Spinal Cord

• The white matter in the spinal cord is composed of myelinated nerve fiber tracts.
• The tracts conduct nerve impulses to the brain (sensory/afferent), from the brain (motor/efferent), or from one side of the spinal cord to the other.
• There are two dorsal columns, two ventral columns, and two lateral columns.
• The dorsal columns contain ascending (sensory) tracts, and the lateral columns and ventral columns contain both ascending (sensory) and descending (motor) tracts.

Protection of the CNS

• The CNS is protected by bone (skull and vertebral column), meninges, cerebrospinal fluid, and the blood-brain barrier.
• The meninges are three connective tissue membranes covering the CNS structures: dura mater, arachnoid mater, and pia mater.

Meninges

• The dura mater is the outermost meningeal layer, a double-layered membrane.
• The arachnoid mater is the middle meningeal layer, with threadlike extensions that span the subarachnoid space.
• The pia mater is the innermost meningeal layer, a delicate membrane that clings tightly to the surface of the brain and spinal cord.

Cerebrospinal Fluid (CSF)

• The CSF is produced continually by the choroid plexuses in each of the brain ventricles.
• The CSF is a watery cushion that protects the fragile CNS structures from trauma.
• The CSF circulates continuously in and around the brain and spinal cord.
• The CSF is continually replaced, and CSF forms and drains at a constant rate to maintain normal volume and pressure.

Blood-Brain Barrier (BBB)

• The BBB is composed of the least permeable capillaries in the body.
• The BBB is formed by tight junctions between endothelial cells in the blood vessel wall and astrocytes' feet.
• The BBB keeps neurons separated from bloodborne substances.
• Lipophilic substances can pass through the capillary walls into the brain tissue.