Premed · Premed · Anatomy Physiology 1
Lecture 17: The Brain II — Cerebrum and Higher Functions
Anatomy and Physiology I
Learning Objectives
By the end of this lecture, students will be able to:
- Describe the gross anatomy of the cerebrum including lobes, sulci, and gyri
- Identify the major functional areas of the cerebral cortex and their roles
- Explain the organization and function of cerebral white matter
- Describe the basal nuclei and their role in motor control
- Describe the components and functions of the limbic system
- Explain lateralization of cortical function
- Define the types and stages of memory and identify the brain regions involved
- Describe the electroencephalogram and stages of sleep
Lecture Content
I. Gross Anatomy of the Cerebrum
The cerebrum is the largest part of the brain, accounting for approximately 83 percent of total brain mass. It is divided into two cerebral hemispheres by the longitudinal fissure, and the hemispheres are connected by the corpus callosum, the largest commissure — a white matter tract that connects corresponding areas of the two hemispheres. The cerebral surface is marked by elevated ridges called gyri (singular: gyrus), shallow grooves called sulci (singular: sulcus), and deep grooves called fissures. This folding dramatically increases the surface area of the cortex to approximately 2,500 square centimeters.
Lobes of the Cerebrum
Each hemisphere is divided into five lobes. The frontal lobe lies anterior to the central sulcus and superior to the lateral sulcus and is responsible for motor functions, executive functions, personality, and speech production. The parietal lobe lies posterior to the central sulcus and superior to the lateral sulcus and handles somatosensory processing and spatial awareness. The temporal lobe lies inferior to the lateral sulcus and is involved in auditory processing, language comprehension, and memory. The occipital lobe is the most posterior lobe, dedicated to visual processing. The insula is a hidden lobe deep within the lateral sulcus, involved in visceral sensation, taste, and emotional awareness.
Key Sulci and Gyri
The central sulcus (of Rolando) separates the frontal and parietal lobes. The lateral sulcus (of Sylvius) separates the temporal lobe from the frontal and parietal lobes. The parieto-occipital sulcus separates the parietal and occipital lobes and is more visible on the medial surface. The precentral gyrus, immediately anterior to the central sulcus, is the primary motor cortex. The postcentral gyrus, immediately posterior to the central sulcus, is the primary somatosensory cortex.
II. Cerebral Cortex — Functional Areas
The cerebral cortex is 2 to 4 millimeters of thick gray matter containing approximately 20 to 25 billion neurons, organized into six histological layers (neocortex). Functional areas are classified as motor areas, sensory areas, and association areas.
Motor Areas (Frontal Lobe)
The primary motor cortex (M1) occupies the precentral gyrus (Brodmann area 4) and directly controls voluntary movements of skeletal muscles. It is organized somatotopically as a motor homunculus, in which the body is mapped across the cortex with the size of each region proportional to the precision of control (large areas are devoted to the hands, face, and tongue). Upper motor neurons here send axons through the corticospinal tract to synapse on lower motor neurons in the spinal cord.
The premotor cortex lies anterior to M1 (Brodmann area 6) and plans and programs complex movement sequences, coordinating movements that involve multiple muscle groups. The supplementary motor area (SMA) is on the medial surface of the frontal lobe, anterior to M1, and plans and sequences complex movements; it is active during mental rehearsal of movements. Broca area (motor speech area) is located in the inferior frontal gyrus of the left hemisphere in most people (Brodmann areas 44 and 45). It controls the muscles of speech production (lips, tongue, larynx, and pharynx), and damage produces Broca aphasia — nonfluent, effortful speech with relatively preserved comprehension. The frontal eye field lies anterior to the premotor cortex (Brodmann area 8) and controls voluntary eye movements (saccades).
Sensory Areas
The primary somatosensory cortex (S1) occupies the postcentral gyrus (Brodmann areas 3, 1, and 2) in the parietal lobe. It receives sensory input — touch, pressure, pain, temperature, and proprioception — from the contralateral body and is organized somatotopically as a sensory homunculus, with areas devoted to each body region proportional to the density of sensory receptors (large areas for fingers, lips, and tongue). The somatosensory association cortex lies posterior to S1 (Brodmann areas 5 and 7) and integrates and interprets sensory input, enabling recognition of objects by touch (stereognosis).
The primary visual cortex (V1) is in the occipital lobe along the calcarine sulcus (Brodmann area 17) and receives visual input from the lateral geniculate nucleus of the thalamus. Visual association areas surround V1 (Brodmann areas 18 and 19) and interpret visual information including color, form, motion, object recognition, and face recognition. The primary auditory cortex (A1) is in the superior temporal gyrus (Brodmann areas 41 and 42) and receives auditory input from the medial geniculate nucleus of the thalamus. The auditory association area surrounds A1 and interprets sounds, including speech recognition, music, and sound localization.
Wernicke area occupies the posterior part of the superior temporal gyrus and adjacent parietal cortex, typically in the left hemisphere (Brodmann area 22 and surroundings). It is responsible for language comprehension (both spoken and written), and damage produces Wernicke aphasia — fluent but nonsensical speech with severely impaired comprehension.
The primary gustatory cortex is located in the insula and frontal operculum. The primary olfactory cortex is in the medial temporal lobe (piriform cortex) and is unique in receiving direct input without thalamic relay. The primary vestibular cortex is in the posterior insula and parietal operculum.
Association Areas
Association areas make up the majority of the cortical surface and integrate information from multiple sensory and motor areas to produce higher-order cognition. The prefrontal cortex in the anterior frontal lobe manages executive functions including planning, decision-making, judgment, working memory, personality, social behavior, and impulse control. It is responsible for the most complex integrative functions and is the last region of the cortex to fully mature (not reaching full development until the mid-20s). The general (posterior) association area, where the parietal, temporal, and occipital lobes meet, handles multimodal integration, spatial awareness, and reading comprehension. The limbic association area on the medial surfaces of the cerebral hemispheres is involved in emotion, memory, and motivation.
<image>A lateral view of the left cerebral hemisphere with the five lobes color-coded (frontal in blue, parietal in yellow, temporal in green, occipital in red, insula shown in a pulled-away inset). Major sulci are labeled: central sulcus, lateral sulcus, parieto-occipital sulcus. Functional areas are mapped onto the cortex: primary motor cortex on the precentral gyrus, premotor cortex anterior to it, Broca area on the inferior frontal gyrus, prefrontal cortex in the anterior frontal lobe, primary somatosensory cortex on the postcentral gyrus, somatosensory association area behind it, Wernicke area on the posterior superior temporal gyrus, primary auditory cortex on the superior temporal gyrus, primary visual cortex on the occipital pole, and visual association areas surrounding it. Motor and sensory homunculi are shown as insets on the precentral and postcentral gyri respectively, depicting the somatotopic organization with disproportionately large hands, face, and tongue regions.</image>
III. Cerebral White Matter
The cerebral white matter is located deep to the cortex and consists of myelinated axons organized into three types of fiber tracts. Commissural fibers connect corresponding regions of the two hemispheres; the corpus callosum is the largest commissure, connecting most cortical areas, while the anterior commissure connects olfactory and temporal lobe regions, and the posterior commissure connects midbrain regions. Association fibers connect different cortical areas within the SAME hemisphere; short (arcuate) fibers connect adjacent gyri, while long fibers connect distant cortical regions, including the arcuate fasciculus (connecting Broca and Wernicke areas), the superior longitudinal fasciculus, and the cingulum. Projection fibers connect the cortex with lower brain regions and the spinal cord; they run vertically, fanning out as the corona radiata and converging as the internal capsule between the thalamus and basal nuclei, and include corticospinal, thalamocortical, and corticopontine fibers.
IV. Basal Nuclei (Basal Ganglia)
The basal nuclei are deep masses of gray matter within each cerebral hemisphere. Their components include the caudate nucleus, which is C-shaped and arches over the thalamus; the putamen, lateral to the caudate; and the globus pallidus (internal and external segments), medial to the putamen. The caudate plus putamen together form the striatum (the receiving component), while the putamen plus globus pallidus form the lentiform nucleus (an anatomical grouping). Functionally associated structures include the subthalamic nucleus (in the diencephalon) and the substantia nigra (in the midbrain).
Function
The basal nuclei regulate the initiation and execution of voluntary movement through circuits with the cerebral cortex and thalamus. They do NOT initiate movement directly but rather filter and modulate motor commands from the cortex. The direct pathway facilitates desired movements by producing an excitatory net effect on the cortex via thalamic relay, while the indirect pathway inhibits unwanted movements by producing an inhibitory net effect. The basal nuclei are also involved in procedural learning, habit formation, eye movements, and cognitive and emotional functions.
Clinical Correlations
Parkinson disease results from the loss of dopaminergic neurons in the substantia nigra, leading to reduced facilitation of voluntary movement and producing resting tremor, rigidity, bradykinesia, and postural instability. Huntington disease involves degeneration of the caudate nucleus and putamen, causing excessive involuntary movements (chorea), cognitive decline, and psychiatric symptoms; it is inherited in an autosomal dominant pattern.
V. The Limbic System
The limbic system is a group of cortical and subcortical structures on the medial aspect of the cerebral hemispheres and diencephalon. It encircles the upper brainstem and is often called the "emotional brain." Key components include the hippocampus in the medial temporal lobe, which is essential for forming new explicit (declarative) memories and for converting short-term memory to long-term memory; the amygdala, anterior to the hippocampus, which processes emotions (especially fear and aggression) and attaches emotional significance to memories; the cingulate gyrus, which arches over the corpus callosum and is involved in emotional and autonomic responses, attention, and pain perception; the fornix, an arching white matter tract from the hippocampus to the mammillary bodies; the mammillary bodies, hypothalamic relay nuclei for memory circuits; the septal nuclei, which function as a pleasure and reward center; and the olfactory cortex, whose strong connections to the limbic system explain why smells evoke strong emotional memories.
<image>A medial view of the right cerebral hemisphere with the limbic system structures highlighted. The cingulate gyrus is shown arching over the corpus callosum. The hippocampus is depicted in the medial temporal lobe (shown partially exposed with a cut-away). The amygdala is anterior to the hippocampus. The fornix is shown as a curved fiber tract from the hippocampus to the mammillary bodies of the hypothalamus. The thalamus (anterior nucleus) is shown as a relay point. Arrows indicate the Papez circuit: hippocampus to fornix to mammillary bodies to anterior thalamus to cingulate gyrus and back to hippocampus. The septal nuclei and olfactory bulb connections are also indicated. A second panel shows a coronal section through the cerebral hemispheres, labeling the basal nuclei (caudate nucleus, putamen, globus pallidus) lateral to the thalamus, the internal capsule between them, and the corpus callosum at the top.</image>
VI. Lateralization of Cortical Function
Although the two hemispheres appear symmetrical, they are functionally specialized, a phenomenon known as cerebral lateralization (dominance). The left hemisphere is dominant in approximately 95 percent of right-handed and 70 percent of left-handed individuals and is specialized for language (speech production in Broca area, comprehension in Wernicke area, reading, and writing), mathematical calculation, and logical and analytical reasoning. The right hemisphere is specialized for spatial perception and visuospatial skills, face recognition (damage causes prosopagnosia), musical and artistic abilities, emotional processing (recognizing emotions in faces and tone of voice), and holistic, intuitive processing.
The two hemispheres communicate via the corpus callosum. Split-brain studies, in which the corpus callosum was severed to treat epilepsy, demonstrate that each hemisphere can function independently, providing powerful evidence for lateralization.
VII. Memory
Types of Memory
Short-term (working) memory provides temporary storage with a limited capacity of approximately seven items and is maintained by rehearsal. It depends on the prefrontal cortex and thalamus. Long-term memory has potentially unlimited capacity and is stored in widely distributed areas of the cerebral cortex. Long-term memory is further divided into two categories. Explicit (declarative) memory involves conscious recall and includes episodic memory (personal experiences and events) and semantic memory (facts and general knowledge). Formation of explicit memories depends on the hippocampus and diencephalon, while storage involves distributed cortical areas. Implicit (nondeclarative) memory is unconscious and automatic, and includes procedural memory (motor skills and habits such as riding a bike), which depends on the basal nuclei and cerebellum, and emotional conditioning, which depends on the amygdala.
Memory Consolidation
Memory consolidation is the process of converting short-term memory into long-term memory. The hippocampus is critical during consolidation but is NOT the final storage site. Long-term potentiation (LTP) is a sustained increase in synaptic strength after repeated stimulation and is the proposed cellular basis for learning and memory, involving NMDA receptors and increased AMPA receptor expression. Sleep plays a critical role in memory consolidation.
VIII. Consciousness, Sleep, and EEG
Electroencephalogram (EEG)
The EEG records the electrical activity of the cerebral cortex via scalp electrodes. Brain waves are classified by frequency: alpha waves (8-13 Hz) are characteristic of a relaxed, awake state with eyes closed; beta waves (14-30 Hz) indicate an alert, concentrated state with active mental processing; theta waves (4-7 Hz) are seen during drowsiness, light sleep, and in children; and delta waves (0.5-4 Hz) characterize deep sleep and are abnormal in awake adults.
Stages of Sleep
Non-REM (NREM) sleep consists of progressively deeper stages. Stage N1 is light sleep with theta waves, easy arousal, and brief duration (1 to 7 minutes). Stage N2 is slightly deeper, characterized by sleep spindles and K-complexes on EEG, and accounts for approximately 45 to 55 percent of total sleep. Stage N3 is deep (slow-wave) sleep with predominant delta waves; it is the most restorative stage, is difficult to arouse from, and is when growth hormone is secreted and immune function is enhanced.
REM (rapid eye movement) sleep features brain activity resembling the awake state (beta-like waves), rapid eye movements, vivid dreaming, and muscle atonia (voluntary muscles are paralyzed except the eye and diaphragm muscles). REM sleep is important for memory consolidation, emotional processing, and learning, and accounts for approximately 20 to 25 percent of total sleep in adults.
Sleep cycles approximately every 90 minutes, cycling through NREM and REM stages 4 to 6 times per night. REM periods lengthen as the night progresses.
<image>A two-panel figure on sleep and EEG. Panel A shows a sample EEG recording with four rows of brain waves: alpha waves (8-13 Hz, moderate amplitude, regular), beta waves (14-30 Hz, low amplitude, irregular), theta waves (4-7 Hz, moderate amplitude), and delta waves (0.5-4 Hz, high amplitude, slow). Each wave type is labeled with its associated brain state. Panel B shows a hypnogram plotting sleep stage (y-axis: Awake, REM, N1, N2, N3) versus time of night over approximately 8 hours (x-axis). The graph shows 4-6 sleep cycles, with deep N3 sleep predominating early in the night and REM episodes becoming longer and more frequent toward morning. Key features of each stage are annotated: N2 with sleep spindles, N3 as restorative slow-wave sleep, and REM with dreaming and muscle atonia.</image>
IX. Clinical Correlations
Aphasia
Broca aphasia results from damage to Broca area; the patient understands language but cannot produce fluent speech, which is halting and effortful. Wernicke aphasia results from damage to Wernicke area; the patient produces fluent but meaningless speech and cannot comprehend language. Global aphasia results from damage to both areas and involves severe loss of all language ability. Conduction aphasia results from damage to the arcuate fasciculus connecting Broca and Wernicke areas; the patient can comprehend and produce speech but cannot repeat words.
Alzheimer Disease
Alzheimer disease is a progressive neurodegenerative disease and the most common cause of dementia. It is characterized by neurofibrillary tangles (hyperphosphorylated tau protein) and amyloid plaques (beta-amyloid protein deposits). It initially affects the hippocampus, causing memory loss, then spreads to the cortex, producing cognitive decline and personality changes.
Cerebrovascular Accident (Stroke)
A stroke is an interruption of blood supply to a brain region, either ischemic or hemorrhagic. Symptoms depend on the area affected and may include contralateral motor or sensory deficits, aphasia, visual field loss, and neglect syndromes.
