Anatomy of the Brain and Function Localization

Anatomical Reference Planes

• The brain is oriented with superior, anterior (rostral), and posterior reference points.
• The spinal cord is oriented with ventral (anterior) and dorsal (posterior) reference points.

Brain Lobes and Sulci

• The cerebrum is divided into lobes: frontal, parietal, temporal, and occipital.
• Key sulci divide the lobes: central sulcus, lateral sulcus, paraoccipital sulcus, cingulate sulcus, calcarine sulcus, and collateral sulcus.

Localisation of Function

• The cerebral cortex is organized into lobes, each with specific functions.
• The left cerebral hemisphere controls the right side of the body, and vice versa.

Frontal Lobe

• Contains the primary motor cortex, premotor area, supplementary motor area, Broca's area, and frontal eye fields.
• The primary motor cortex is located in the precentral gyrus and controls voluntary movement.
• The premotor area and supplementary motor area are involved in planning and coordinating movements.
• Broca's area is responsible for speech production.
• The frontal eye fields control voluntary eye movements.

Parietal Lobe

• Contains the primary somatosensory cortex, somatosensory association cortex, and Wernicke's area.
• The primary somatosensory cortex is located in the postcentral gyrus and receives sensory information from the body.
• The somatosensory association cortex processes sensory information and is also involved in body awareness.
• Wernicke's area is involved in language comprehension.

Temporal Lobe

• Contains the primary auditory cortex, auditory association cortex, and parts of Wernicke's area.
• The primary auditory cortex receives auditory information from the ears.
• The auditory association cortex processes auditory information and is also involved in speech recognition.

Occipital Lobe

• Contains the primary visual cortex.
• The primary visual cortex receives visual information from the eyes.
• The calcarine sulcus divides the primary visual cortex into two parts, each responsible for a different half of the visual field.

Limbic Lobe

• Includes parts of the frontal, parietal, and temporal lobes.
• Contains the hippocampus, amygdala, olfactory bulb, and cingulate cortex.
• The hippocampus plays a key role in memory formation.
• The amygdala is involved in emotions, particularly fear and anxiety.
• The olfactory bulb is responsible for smell.
• The cingulate cortex is involved in emotion regulation and motivation.

Spinal Cord

• Extends from the foramen magnum to the L1 vertebra and is encased in the dural sac.
• Contains 31 segments: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal.
• Has a ventral horn, dorsal horn, and intermediolateral cell column (lateral horn).
• The ventral horn contains motor neurons, the dorsal horn contains sensory neurons, and the intermediolateral cell column contains preganglionic sympathetic neurons.
• The spinal cord terminates at the conus medullaris at the level of L1-L2.
• The cauda equina, a bundle of nerve roots, extends from the conus medullaris.

Meninges

• The spinal cord is covered by three layers of meninges: dura mater, arachnoid mater, and pia mater.
• The dura mater is the outermost layer and forms a sac around the spinal cord.
• The arachnoid mater is a delicate membrane located between the dura mater and the pia mater.
• The pia mater is the innermost layer and adheres directly to the spinal cord.
• The subarachnoid space, filled with CSF, lies between the arachnoid and pia mater.
• The epidural space, filled with fat, veins, and loose connective tissue, lies between the dura mater and the wall of the vertebral canal.

CSF Distribution

• CSF circulates in the subarachnoid space and is also found in the ventricles of the brain.
• The dura mater is continuous with the dura mater of the brain.
• Denticulate ligaments, triangular extensions of the pia mater, help to anchor the spinal cord within the dural sac.

Lumbar Puncture

• A lumbar puncture is performed to collect CSF for diagnostic purposes.
• It is performed below the level of L1-L2, where the spinal cord terminates and the nerve roots are mobile.
• This reduces the risk of injury to the spinal cord.

Spinal Cord and Brainstem

• Houses cells directly communicating with the body and external environment.
• Motor neurons originate in the spinal cord and innervate skeletal and smooth muscle.
• Sensory input returns to the CNS via the nervous system, except for olfaction and vision.

Spinal Cord Organization

• Similar organization throughout due to embryonic development.
• Consistent positioning of neuron classes, from dorsal to ventral.
• 31 segments: Cervical, Thoracic, Lumbar, Sacral, Coccygeal, corresponding to the vertebral column.
• Each spinal cord segment receives information from and controls specific body regions.

Dermatome and Myotome

• Dermatome: Area of skin supplied by a specific spinal cord segment.
• Myotome: Muscle mass supplied by a single spinal cord segment.
• Myotomes develop from somites, which also contribute to the axial skeleton and other structures.

Cervical and Lumbar Enlargements

• Present due to increased cell numbers needed for limb function and sensory input.

Autonomic Nervous System

• Sympathetic: Located from thoracic spine to L1/2, innervates visceral smooth muscle.
• Parasympathetic: Located in the brainstem and sacral region, also innervates visceral smooth muscle.

Spinal Cord Cross-Section

• Ventral Side: Contains motor neurons innervating striated skeletal muscle.
• Lateral Horn: Houses autonomic motor neurons, projecting to ganglia in the periphery.
• Dorsal Side: Receives sensory information via the dorsal root.
• Dorsal Root Ganglia: Contain cell bodies of sensory neurons, sending axons to the dorsal aspect of the spinal cord.

Proprioception and Pain/Temperature Pathways

• Proprioceptive Fibers: Travel to the brainstem, through the dorsal columns (gracile and cuneate fasciculi).
• Pain/Temperature Fibers: Cross the spinal cord and relay on the contralateral side, ascending through the anterolateral system.

Anterior and Posterior Rami

• Anterior Rami: Innervate hypomere-derived structures (limbs).
• Posterior Rami: Innervate epimere-derived structures (back).

Corticospinal Tract

• Lateral Corticospinal Tract: Primarily contralateral, responsible for fine motor control, particularly of the hands and arms.
• Anterior Corticospinal Tract: Can be ipsilateral or contralateral, involved in posture and trunk movements.

Brainstem and Corticospinal Tract

• Midbrain: Houses cerebral peduncles, which transmit corticospinal tract fibers.
• Pons: A significant portion of corticospinal fibers synapse on pontine nuclei, relaying information to the cerebellum.
• Medulla: Contains the medullary pyramids housing the corticospinal tract.
• Pyramidal Decussation: At the caudal aspect of the medulla, most corticospinal fibers cross over to the contralateral side, forming the lateral corticospinal tract.

Dorsal Column-Medial Lemniscus Pathway (Touch and Proprioception)

• Sensory afferents from the periphery synapse in the gracile and cuneate nuclei in the medulla.
• Fibers cross the brainstem and ascend as the medial lemniscus to the thalamus (VPL and VPM) and then to the postcentral gyrus.

Spinothalamic Pathway (Pain and Temperature)

• Sensory afferents synapse in the dorsal horn, axons cross to the contralateral side and ascend in the anterolateral system.
• Terminate in the thalamus (VPL and VPM) and project to the postcentral gyrus.

Periaqueductal Gray

• Involved in modulating pain and temperature signals.
• Opioids act at this site.

Dorsal Horn Organization

• Contains different laminae (layers) with distinct functions.
• Proprioceptive fibers synapse in the upper layers, pain/temperature fibers in the deeper layers.

Altered Reflexes

• Lower Motor Neurons: Located in the ventral horn, directly innervate skeletal muscle.
• Upper Motor Neurons: Located in the primary motor cortex, project through the corticospinal tract to lower motor neurons.
• Transverse Spinal Cord Section: Damage to the spinal cord can interrupt communication between upper and lower motor neurons, affecting reflexes.

Brainstem Topography

• Midbrain: Cerebral peduncles are located on the anterior surface.
• Pons: Contains the brachium pons, connecting the cerebellum to the other side of the brainstem.
• Medulla: Contains the medullary pyramids and the pyramidal decussation.

Sensory Pathway Topography

• Dorsal Column: Gracile and cuneate fasciculi are visible on the posterior surface of the brainstem.
• Spinothalamic Pathway: Fibers travel in the anterolateral system.

Motor Homunculus

• Shows the representation of body parts in the motor cortex.
• Hands and arms have a large representation, reflecting fine motor control.

Sensory Afferent Input to Lower Motor Neurons

• Sensory afferents, including proprioceptive and pain/temperature receptors, send collateral branches directly to lower motor neurons (LMNs).
• This direct input allows for rapid reflex responses, like quickly pulling away from a painful stimulus.
• These collaterals also travel up and down the spinal cord, influencing other neurons.

Inhibitory Interneurons and Muscle Control

• Inhibitory interneurons in the ventral horn receive the majority of cortical input, and contribute to controlling muscle activity.
• Muscles are not simply "turned on" by the brain; ongoing sensory input contributes to their "baseline" activity.
• Cortical output can be decreased to contract muscles.

Upper Motor Neuron Injuries and Reflexes

• Injuries to upper motor neurons (UMNs) disrupt the balance of excitatory and inhibitory input to LMNs, leading to exaggerated reflexes.
• Exaggerated reflexes, such as a brisk patellar reflex or a positive Babinski sign, are indicative of UMN lesions.
• Diminished reflexes suggest damage to the afferent or efferent pathway to the muscle.

Testing Different Reflexes

• Specific reflexes can be used to test the integrity of different nerve pathways at specific spinal cord levels.
• Biceps jerk reflex tests C6, brachioradialis reflex tests C6, triceps reflex tests C7/C8, patellar reflex tests L2-4, and ankle jerk reflex tests S1/2.

Complex Reflexes

• Complex reflexes, like the Babinski reflex, change with age due to the development of cortical influence.
• The Babinski reflex is abnormal in adults, indicating a lack of upper motor neuron inhibition.
• Other complex reflexes include the Hoffman sign, glabella reflex, palmamental reflex, and rooting reflex.

Clonus and Upper Motor Neuron Lesions

• Clonus, a repetitive muscle contraction following a stretch, is another sign of upper motor neuron lesion.
• It results from disinhibition of LMNs due to the loss of inhibitory input from UMNs.
• Upper motor neuron disorders, like multiple sclerosis, can present with exaggerated reflexes and clonus due to demyelination and disruption of electrical activity.

Explore the anatomical reference planes, brain lobes, and their sulci in this quiz. Understand the localization of function within the cerebral cortex, including the specific roles of the frontal lobe and its components. Test your knowledge of neuroanatomy and how different brain regions interact.