brainstem and cerebellum (lec3)

Central Nervous System (CNS)

  • The Central Nervous System (CNS) is composed of the brain and the spinal cord (bits contained/protected within a bony structure) - protected by blood-brain barrier 

  • It serves as the main processing center for all information received from the sensory organs and for sending out commands to the muscles and glands.

  • Distinction between CNS and Peripheral Nervous System (PNS):

    • The CNS primarily interprets and integrates information.

    • The PNS (Peripheral Nervous System) consists of all neural structures outside the brain and spinal cord. This includes cranial nerves (12 pairs originating from the brain), spinal nerves (31 pairs originating from the spinal cord), ganglia (clusters of neuron cell bodies), and sensory receptors. The PNS acts as a communication link between the CNS and the rest of the body.

Brain Structure

  • The brain is broadly divided into three major regions: the forebrain, midbrain, and hindbrain.

    • Forebrain: Develops into the cerebrum (involved in higher functions like thought and voluntary action), thalamus (relay station for sensory information), and hypothalamus (regulates primitive brain functions).

    • Midbrain: Connects the forebrain and hindbrain; involved in motor movement, particularly movements of the eye, and in auditory and visual processing.

    • Hindbrain: Includes the cerebellum (coordination, balance), pons (sleep, respiration), and medulla oblongata (heart rate, breathing).

  • Hemispheres: The cerebrum is divided into two cerebral hemispheres (left and right), which exhibit bilateral symmetry but often have specialized, non-homologous functions (lateralization).

  • Folding of the brain: The outer surface of the cerebrum, the cerebral cortex, is characterized by convolutions consisting of ridges (gyri) and grooves (sulci). This folding significantly increases the surface area of the cortex within the confines of the skull, allowing for a greater number of neurons and more complex processing capabilities.

  • Grey and White Matter: The brain consists of grey matter (neuron cell bodies, dendrites, unmyelinated axons, glial cells) primarily found in the cortex and deep nuclei, and white matter (myelinated axons) which forms tracts connecting different brain regions.

Brain Hemispheres Functions

  • While both hemispheres work together, some functions are predominantly localized:

    • Left Brain: Typically associated with analytical, logical, and sequential tasks. It excels in language functions (speech, writing, comprehension), organization, numerical calculations, scientific reasoning, and processing details.

    • Right Brain: Generally associated with holistic, intuitive, and spatial processing. It is involved in emotional expression and interpretation, creativity (art, music), recognizing faces, non-verbal communication, big picture thinking, and processing abstract concepts.

Brain Lobes

  • The cerebrum is further divided into five main lobes:

    • Frontal Lobe: Located at the front of the brain, responsible for executive functions such as decision-making, planning, problem-solving, voluntary motor control, personality, and expressive language. The primary motor cortex (Brodmann area 4) is located here, initiating voluntary movements.

    • Parietal Lobe: Situated behind the frontal lobe, it processes sensory information from the body (touch, temperature, pain, pressure) and integrates it, contributing to spatial awareness and navigation. The primary somatosensory cortex (Brodmann areas 3, 1, 2) receives sensory input.

    • Temporal Lobe: Located below the parietal and frontal lobes, it is crucial for auditory processing, memory formation, object recognition, and language comprehension. The primary auditory cortex is here.

    • Occipital Lobe: Positioned at the back of the brain, it is primarily responsible for processing visual information, including color, form, and motion. The primary visual cortex is located here.

    • Insula: Often considered the "fifth lobe," the insula is a hidden cortical region located deep within the lateral sulcus (Sylvian fissure), separating the temporal from the parietal and frontal lobes. It plays roles in interoception (awareness of internal body states), emotion (disgust, pain), taste, olfaction, and integrating sensory and limbic information.

  • Major Sulci:

    • Central Sulcus: A prominent groove that separates the frontal lobe from the parietal lobe.

    • Sylvian Fissure (Lateral Fissure): A deep groove that separates the temporal lobe from the frontal and parietal lobes.

Brodmann's Areas

  • Brodmann's areas are a system of numbering specific brain regions based on their cytoarchitecture (cellular structure and organization of neurons) observed histologically.

  • These areas are functionally associated with different brain regions, identified by their distinct cellular compositions and often linked to specific functions:

    • Broca's Area (Brodmann areas 44 and 45): Located in the inferior frontal gyrus (typically left hemisphere), it is critical for language production and articulation (motor speech). Damage here can lead to Broca's aphasia, characterized by difficulty producing coherent speech.

    • Wernicke's Area (Brodmann area 22): Located in the posterior part of the superior temporal gyrus (typically left hemisphere), it is essential for language comprehension. Damage here results in Wernicke's aphasia, where individuals can speak fluently but their speech lacks meaning, and they have difficulty understanding language.

Major Brain Structures

  • Corpus Callosum: A large, C-shaped nerve fiber bundle (white matter tract) located beneath the cerebral cortex. It connects the left and right cerebral hemispheres, facilitating interhemispheric communication and allowing information to be shared and integrated between them.

  • Basal Ganglia: A group of subcortical nuclei (including the caudate nucleus, putamen, globus pallidus, substantia nigra, and subthalamic nucleus) deep within the brain. They are primarily involved in the control of voluntary motor movements, procedural learning, habit formation, eye movements, and cognitive functions. Dysfunction is implicated in movement disorders such as Parkinson's Disease (due to degeneration of dopaminergic neurons in the substantia nigra) and Huntington's Disease.

  • Hypothalamus: A small but vital region located below the thalamus. It is a key regulator of primitive brain functions related to maintaining homeostasis, including body temperature, hunger, thirst, sleep-wake cycles (circadian rhythms), and emotional responses. It also controls the release of hormones from the pituitary gland, linking the nervous and endocrine systems.

Brainstem Functions

  • The brainstem connects the cerebrum and cerebellum to the spinal cord and is composed of three main parts, from superior to inferior:

    • Midbrain: Contains nuclei involved in vision, hearing, motor control, sleep and wakefulness, arousal, and temperature regulation.

      • Cerebral aqueduct à continuation of cerebral spinal fluid, flows through aqueduct towards spinal cord,

        Red nucleus à cluster of grey matter (red tinge under microscope), original of rubrospinal tract (skilled voluntary movement)

        Periaqueductal gray à involved in pain regulation

    • Pons: Acts as a bridge connecting the cerebellum to the cerebrum and plays a crucial role in regulating breathing, sleep cycles, and relaying sensory information.

    • Medulla Oblongata: Forms the lowest part of the brainstem, continuous with the spinal cord. It houses vital autonomic centers that regulate essential life-sustaining functions such as heartbeat, blood pressure, respiration, and reflexes like swallowing, coughing, and sneezing. It also contains tracts connecting the cerebrum to the spinal cord.

      • Motor learning: ability to make corrections when we make mistakes

      • Medullary pyramid  – corticospinal tract (decussate or cross at level of medulla)

        Dorsal column nuclei – receive somatosensory input from spinal tracts (dorsal columns) and deliver to thalamus (via medial lemniscus)

    • Thalamus: important for sensor control (clustered with brainstem in this class because most vertebrae species have them) 

  • Many cranial nerves originate from the brainstem, controlling sensory and motor functions of the head and neck.

Cerebellum Functionality

  • The cerebellum, a large structure located dorsally to the pons and medulla, is critical for motor control, coordination, and equilibrium. It fine-tunes motor activity, ensuring smooth, precise movements, and plays a role in motor learning and maintaining posture.

  • Divisions of the Cerebellum:

    • Cerebellum “little brain” à a lot of folds

      Separate divisions of functions:

      - Vestibulocerebellum (purple)

      -Spinocerebellum (orangey) à control of muscle tone, coordination (peripheral limbs)

      -Cerebrocerebellum (pink)

    • Cerebellar Peduncles: Three pairs of nerve fiber bundles (white matter tracts) that connect the cerebellum to the brainstem, allowing for the transmission of information:

      • Superior Cerebellar Peduncle: Primarily efferent (output) pathways from the cerebellum to the midbrain and thalamus.

      • Middle Cerebellar Peduncle: Primarily afferent (input) pathways from the pons to the cerebellum, carrying information about voluntary movements from the cerebral cortex.

      • Inferior Cerebellar Peduncle: Both afferent and efferent pathways, receiving sensory information from the spinal cord and brainstem, and sending output to vestibular nuclei and reticular formation.

    • Deep cerebellar nuclei –  receive output from the vermis and hemispheres of the cerebellum is via these nuclei (clusters of neurons)

      Vestibular nuclei – routput from the flocculonodular lobe is to these nuclei located in the medulla