Brain Anatomy and Major Structures

Cerebral Lobes and Surface Landmarks

The brain is divided into several lobes and regions, each with distinct functions and anatomical landmarks.

• Central Sulcus: Separates the frontal and parietal lobes.

• Lateral Sulcus: Divides the temporal lobe from the frontal and parietal lobes.

• Frontal Lobe: Involved in voluntary movement, planning, and reasoning.

• Parietal Lobe: Processes sensory information such as touch and spatial orientation.

• Temporal Lobe: Responsible for auditory processing and memory.

• Occipital Lobe: Main center for visual processing.

• Insula: Deep within the lateral sulcus; involved in consciousness and emotion.

• Precentral Gyrus: Primary motor cortex; controls voluntary movements.

• Postcentral Gyrus: Primary somatosensory cortex; processes tactile information.

• Longitudinal Fissure: Divides the two cerebral hemispheres.

• Transverse Cerebral Fissure: Separates the cerebrum from the cerebellum.

Pons and Medulla are parts of the brainstem involved in autonomic functions and relaying information between the brain and spinal cord.

Internal Brain Structures (Half Brain Model)

• Septum Pellucidum: Thin membrane separating lateral ventricles.

• Intermediate Mass of Thalamus: Connects the two thalamic bodies.

• Anterior Commissure: Bundle of nerve fibers connecting the two cerebral hemispheres.

• Hypothalamus: Regulates homeostasis, endocrine, and autonomic functions.

• Optic Chiasma: Crossing point of optic nerves.

• Pituitary Gland: Major endocrine gland under hypothalamic control.

• Mammillary Body: Involved in memory processing.

• Arbor Vitae: Tree-like white matter in the cerebellum.

• Pineal Body (Gland): Secretes melatonin, regulates circadian rhythms.

• Thalamus: Relay station for sensory information.

• Choroid Plexus: Produces cerebrospinal fluid (CSF).

• Fornix: Connects hippocampus to hypothalamus.

• Corpus Callosum: Largest commissure connecting hemispheres.

• Corpora Quadrigemina: Four colliculi (superior for vision, inferior for hearing).

• Vermis: Connects cerebellar hemispheres.

• Olfactory Bulb: Receives olfactory (smell) input.

• Cerebral Cortex: Outer gray matter of cerebrum; site of higher functions.

• Superior Sagittal Sinus: Venous sinus in dura mater.

• Arachnoid Villi: Absorb CSF into venous circulation.

• Pia Mater, Arachnoid Mater, Dura Mater: Three protective meninges of the brain.

Ventricular System

The brain's ventricles are interconnected cavities filled with cerebrospinal fluid (CSF), which cushions and nourishes the brain.

• Central Canal: Runs through the spinal cord, continuous with the ventricles.

• Lateral Ventricle: Paired, C-shaped chambers in each hemisphere.

• Anterior, Posterior, Inferior Horns: Extensions of the lateral ventricles.

• Interventricular Foramen: Connects lateral ventricles to the third ventricle.

• Third Ventricle: Midline ventricle between the two thalami.

• Cerebral Aqueduct: Connects third and fourth ventricles.

• Fourth Ventricle: Located between pons and cerebellum.

• Lateral and Median Apertures: Allow CSF to flow into subarachnoid space.

Cranial Nerves

Overview

There are twelve pairs of cranial nerves, each with specific sensory, motor, or mixed functions. They emerge directly from the brain and brainstem.

• Function: Sensory (S), Motor (M), or Both (B).

• Testing: Each nerve can be tested clinically (e.g., smell for CN I, eye movement for CN III, IV, VI).

Example: The Oculomotor nerve (CN III) controls most eye movements and pupil constriction; tested by following a moving object and checking pupil response to light.

Additional info: For a full list, refer to standard tables of cranial nerves, their functions, and clinical tests.

Spinal Cord and Spinal Nerves

Spinal Cord Structure

The spinal cord is a cylindrical structure that transmits neural signals between the brain and the body.

• Posterior, Anterior, Lateral Funiculi: White matter columns containing ascending and descending tracts.

• Dorsal Root Ganglia: Contain sensory neuron cell bodies.

• Dorsal Root: Carries sensory input to the spinal cord.

• Ventral Root: Carries motor output from the spinal cord.

• Posterior Median Sulcus: Shallow groove on dorsal surface.

• Ventral Median Fissure: Deep groove on ventral surface.

• Posterior, Anterior, Lateral Horns: Gray matter regions for sensory and motor processing.

• Gray Matter: Contains neuron cell bodies.

• White Matter: Contains myelinated axons.

• Central Canal: Contains CSF.

• Pia Mater, Arachnoid Mater, Dura Mater: Protective coverings (meninges).

Spinal Nerves and Plexuses

Spinal nerves are classified by the region of the vertebral column from which they emerge. Plexuses are networks of nerves that supply specific body regions.

• Cervical Nerves: C1–C8

• Thoracic Nerves: T1–T12

• Lumbar Nerves: L1–L5

• Sacral Nerves: S1–S5

• Conus Medullaris: Tapered end of the spinal cord.

• Cauda Equina: Bundle of spinal nerves below the conus medullaris.

• Filum Terminale: Fibrous extension anchoring the spinal cord.

Cervical Plexus

• Phrenic Nerve: Innervates the diaphragm (essential for breathing).

• Supraclavicular Nerve: Supplies skin over the clavicle and shoulder.

Brachial Plexus

• Axillary Nerve: Deltoid muscle.

• Musculocutaneous Nerve: Flexor muscles in anterior arm (biceps brachii, brachialis).

• Radial Nerve: Posterior muscles of arm and forearm (triceps brachii, supinator, brachioradialis, extensors).

• Median Nerve: Flexor group of anterior forearm (palmaris longus, flexor carpi radialis, flexor digitorum superficialis/profundus, pronators).

• Ulnar Nerve: Flexor carpi ulnaris.

Lumbar Plexus

• Iliohypogastric Nerve: Internal oblique and transversus abdominis.

• Ilioinguinal Nerve: Skin of external genitalia, inferior abdominal muscles.

• Femoral Nerve: Quadriceps, sartorius.

• Obturator Nerve: Adductor longus, gracilis.

• Lateral Femoral Cutaneous Nerve: Skin of lateral thigh.

Sacral Plexus

• Superior Gluteal Nerve: Gluteus medius, tensor fasciae latae.

• Inferior Gluteal Nerve: Gluteus maximus.

• Sciatic Nerve: Largest nerve; splits into tibial (hamstrings, flexor digitorum longus) and common fibular (biceps femoris, tibialis anterior, extensor digitorum longus, fibularis).

Special Senses

Eye Anatomy

The eye is a complex organ responsible for vision, composed of several layers and structures.

• Cornea: Transparent anterior part; refracts light.

• Lacrimal Gland/Sac: Produces and drains tears.

• Ciliary Body: Controls lens shape, produces aqueous humor.

• Iris: Colored part; regulates pupil size.

• Pupil: Opening for light entry.

• Anterior Segment: Contains aqueous humor.

• Lens: Focuses light on retina.

• Posterior Segment: Contains vitreous humor.

• Sclera: White, fibrous outer layer.

• Suspensory Ligament: Holds lens in place.

• Optic Disc (Blind Spot): Where optic nerve exits; no photoreceptors.

• Choroid: Vascular layer; nourishes retina.

• Retina: Contains photoreceptors (rods and cones).

• Fovea Centralis: Area of sharpest vision.

• Optic Nerve: Transmits visual information to brain.

• Photoreceptors: Rods (dim light), cones (color vision).

• Bipolar Cells: Relay signals from photoreceptors to ganglion cells.

• Ganglion Cells: Axons form the optic nerve.

Extrinsic Eye Muscles: Six muscles control eye movement; each is innervated by specific cranial nerves (III, IV, VI).

Tongue Papillae

The tongue contains specialized structures called papillae, which house taste buds.

• Fungiform Papillae: Mushroom-shaped; scattered across tongue.

• Foliate Papillae: Leaf-like; on sides of tongue.

• Circumvallate Papillae: Large, arranged in a V-shape at back of tongue.

Ear Anatomy

The ear is divided into external, middle, and inner regions, each with specialized structures for hearing and balance.

• External Ear: Auricle (pinna), external acoustic meatus (ear canal).

• Middle Ear: Tympanic membrane (eardrum), auditory ossicles (malleus, incus, stapes).

• Inner Ear:

  • Semicircular Canals: Anterior, posterior, lateral; detect rotational movement; ampullae are enlarged ends.

  • Cochlea: Contains scala vestibuli (perilymph), scala tympani (perilymph), scala media (endolymph), spiral organ of Corti (hearing receptor), tectorial membrane, hair cells.

  • Vestibule: Central part; involved in balance.

• Auditory Tube: Connects middle ear to pharynx; equalizes pressure.

• Round Window and Oval Window: Openings between middle and inner ear.

Selective Nerves and Their Muscle Innervation

The following table summarizes key peripheral nerves and the muscles they innervate:

Additional info: For clinical testing of nerves and muscles, refer to standard neurological examination protocols.