ANA 534: CNS Overview I & II

• ANA 534 covers Gross Anatomy, Embryology, and Neuroanatomy.
• The lecture focuses on CNS Overview I & II, taught by Richard C. Grondin.

CNS Components

• The CNS comprises the brain, brainstem, and spinal cord.
• The brain includes two cerebral hemispheres, each with several lobes.
• The brainstem consists of the medulla, pons, and midbrain, with the midbrain connecting to the thalamus.
• The spinal cord extends from the brainstem (medulla) at the foramen magnum level, ending at the L1/L2 vertebral level in adults.

Terms of Direction

• Cephalic flexure occurs at the midbrain-thalamus junction, with an approx. 80° bend between the brainstem and forebrain axes.

Brain Hemispheres and Structures

• The brain divides into two cerebral hemispheres via the longitudinal fissure.
• The cerebral hemispheres connect through commissural fibers within the corpus callosum.
• Sulci are the grooves, and gyri are the bumps on the surface of the brain, each with specific functions.

Brain Lobes and Sulci

• The brain lobes include the frontal, parietal, temporal, and occipital lobes.
• The central sulcus separates the frontal cortex from the parietal cortex, starting in the longitudinal fissure and stopping short of the lateral sulcus.
• The lateral sulcus (Sylvian fissure) separates the temporal cortex from the parietal and frontal cortices.
• The parieto-occipital sulcus on the medial surface separates the parietal cortex from the occipital cortex.

Primary Brain Cortices

• Primary motor cortex is located in the precentral gyrus of the frontal lobe.
• Conscious awareness of general somatic senses lies in the primary somatosensory cortex of the postcentral gyrus in the parietal lobe.
• The primary auditory cortex is located in the transverse gyrus of Heschl of the temporal lobe.
• The primary visual cortex in the occipital lobe receives visual information from the retina.
• The transverse gyri of Heschl, or primary auditory cortex, are on the superior aspect of the temporal lobe, within the lateral sulcus.

Motor and Sensory Homunculus

• The motor outputs map from the primary motor cortex in the frontal lobe to motor neurons in the brainstem or spinal cord, in an upside-down organization
• Body mapping is upside-down in each hemisphere.
• Sensory inputs map from the periphery to the primary somatosensory cortex in the parietal lobe in an upside-down organization.

Cranial Cavity

• The brain is housed in the cranial cavity and protected by the frontal, ethmoid, temporal, parietal, sphenoid, and occipital bones.
• The anterior cranial fossa holds the frontal lobe.
• The middle cranial fossa holds the temporal lobe.
• The posterior cranial fossa holds the occipital lobe, cerebellum, and brainstem.

Meninges

• The brain is protected by three connective tissue layers: dura mater, arachnoid mater, and pia mater.
• Dura mater, the outermost layer, is double-layered with periosteal and meningeal layers.
• Pia mater is the innermost layer that closely covers the brain's surface.
• Arachnoid mater is on the deep side of the dura mater and connects to the pia mater via trabeculae, with the subarachnoid space filled with CSF.
• Arachnoid granulations extend from the arachnoid membrane through the meningeal layer of the dura mater into the superior sagittal sinus.
• Arachnoid granulations allow one-way flow re-absorption of CSF from the subarachnoid space into the blood.

Ventricular System

• During brain formation, the neural tube's hollow space forms an interconnected system of chambers, the ventricular system.
• The ventricular system contains two lateral ventricles, one in each hemisphere, a third ventricle in the middle between the two thalami, and a fourth ventricle between the pons and cerebellum.
• The ventricular system is filled with clear cerebrospinal fluid (CSF).
• The choroid plexus, derived from the pia mater within all ventricles, produces CSF at about 500 ml/day and is composed of vascular secretory epithelial tissue.
• CSF flows from the lateral ventricles to the third ventricle via the interventricular foramen, then to the fourth ventricle via the cerebral aqueduct.
• CSF flows from the fourth ventricle to the cisterna magna/subarachnoid space via one medial and two lateral openings in the fourth ventricle.
• CSF supports a human brain weighing 1,500 grams, reducing its net weight in CSF to 25-50 grams.
• CSF offers protection (fluid cushion) against external forces and nourishment (glucose, proteins, lipids, and electrolytes) and helps remove waste material.
• CSF buildup in the ventricular system causes the ventricles to expand, known as hydrocephaly.
• Hydrocephaly can be congenital (present at birth) or acquired (in adults) with causes including genetic abnormality, infection in the uterus, tumors/cysts, and head injury.
• Clinical signs of hydrocephaly include nausea, vomiting, headaches, visual impairments, lethargy, and an enlarged head. Communicating hydrocephalus leads to CSF overproduction or decreased venous blood absorbtion. Non-communication hydrocephalus is a CSF flow interruption.

Blood Supply to the Brain

• The brain is supplied with blood from the internal carotid and vertebral arteries.
• Internal carotid arteries give rise to the anterior cerebral arteries (ACA) and middle cerebral arteries (MCA).
• The vertebral arteries give rise to the posterior cerebral arteries (PCA).
• MCA supplies the lateral aspects of the frontal lobe, including most of the precentral gyrus, the parietal lobe, including most of the postcentral gyrus, and as well as the temporal lobe.
• ACA supplies the medial aspect of the frontal and parietal lobes.
• PCA supplies the occipital lobe and the medial aspect of the temporal lobe.

Brainstem

• The brainstem is the bridge between the brain and spinal cord and includes the midbrain, pons, and medulla.
• The midbrain contains cerebral peduncles/crus cerebri, midbrain tegmentum, tectum that leads to superior colliculi, and inferior colliculi.
• The pons contains basilar pons and pontine tegmentum.
• The medulla contains pyramids and medullary tegmentum.
• The brainstem comprises cranial nerve nuclei and associated cranial nerves (CN III – CN XII).
• The descending tracts in the brainstem carry motor signals from the brain to the cerebellum, cranial nerve nuclei, and spinal cord.
• The ascending tracts carry sensory signals from cranial nerve nuclei and spinal cord to the brain.
• Vertebral arteries supply the midline medulla.
• The posterior inferior cerebellar artery (PICA) supplies the lateral medulla.
• The basilar artery supplies a midline pons.
• The anterior inferior cerebellar artery supplies a lateral pons.
• The superior cerebellar artery supplies a midbrain tectum.
• The posterior cerebral artery supplies a ventral midbrain (Crus cerebri).

Spinal Cord Anatomy

• The spinal cord is continuous with the medulla, beginning at the foramen magnum.
• There are 31 segments, each giving rise to a pair of spinal nerves: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal.
• The spinal cord has two enlargements: cervical, innervating upper limb muscles, and lumbar, innervating lower limb muscles.
• The spinal cord ends between vertebral levels L1/L2 and terminates at the conus medullaris tapering end.
• The conus medullaris anchors to the coccyx via the filum terminale of the pia mater.
• Spinal nerve roots supplying regions below L2 extend downward through the dural sheath in the lumbar cistern as the cauda equina.
• The spinal cord consists of white matter and gray matter, with the gray matter in a butterfly shape surrounded by white matter.
• The gray matter contains dorsal horns for sensory neuron cell bodies and ventral horns for motor neuron cell bodies.
• Dorsal horns are the location of sensory neurons. Ventral horns are the location of lower motor neurons for limb and trunk skeletal muscle innervation.
• White matter consists of ascending and descending myelinated fiber tracts.
• The myelin gives white matter its lighter color
• Anterior funiculi contain descending tracts.
• Lateral funiculi contain both ascending and descending tracts.
• Posterior funiculi contain ascending tracts.
• The anterior spinal artery supplies the anterior 2/3 of the cord, while the posterior spinal arteries supply the posterior 1/3 of the cord.

Spinal Nerves

• Spinal nerves are mixed nerves that carry sensory and motor fibers.
• Sensory fibers transmit from spinal ganglia to the CNS via dorsal roots.
• Motor fibers transmit from the spinal cord’s ventral horn to muscles via ventral roots.