Neuroanatomy

Cerebrum and cerebral cortex

The cerebrum makes up most of the brain. It is found in the cranial vault. The cerebrum consists of two cerebral hemispheres (left and right) and five lobes. All lobes (except one) are named according to the cranial bones on which they lean: frontal, parietal, temporal, occipital and insular lobes. The insular lobe is hidden just beneath the frontal, temporal, and parietal lobes. ‘Insula’ means island, which the insular lobe is indeed–an isle of gray matter hidden under the surface of the cerebrum. 

You’ll probably encounter the term limbic lobe as well, but this is not an actual lobe. Rather it’s a functional group of interconnected regions of the brain which together control emotions, memory and spatial perception. The cerebrum together with the hippocampus, amygdala, olfactory bulb, and basal ganglia comprise the telencephalon. 

The most superficial layer of the cerebrum is the cerebral cortex. It is a layer of grey matter which displays numerous folds (sulci and gyri), can be categorization structurally (cortical cytoarchitecture) or functionally (Brodmann areas), and is home to areas such as the primary motor cortex and the primary somatosensory cortex, both of which house a homunculus.

White matter connections extend between the cerebral cortex grey matter to other parts of the same cerebral hemisphere (association fibers); to the opposite hemisphere (commissural fibers); and to structures outside the cortex (projection fibers). Learn more about cerebral cortex and its structure with our study unit:

Subcortical structures

The subcortical structures are a group of diverse structures found deep within the brain. They include the diencephalon (thalamus, epithalamus, subthalamus and hypothalamus), pituitary gland, limbic structures and the basal ganglia. 

These structures show a great variety of functions, for example;

• The hypothalamus and pituitary gland are involved in hormone production and regulation 
• The limbic system (includes; hippocampal formation, fornix, amygdala, insular cortex, and others) is involved in memory, olfaction, emotional behaviour and overall physiological balance of the body (homeostasis)
• The basal ganglia are a functional group of nuclei which together comprise a unit of the extrapyramidal motor system, modifying motor activity

Learn more about the subcortical structures here.

Brainstem

The brainstem is the most caudal part of the brain. It consists of the midbrain (mesencephalon), pons and medulla oblongata. The cerebellum, pons and medulla oblongata are often grouped together under the name hindbrain (rhombencephalon). The importance of the brainstem lies in several of its features, together giving the brainstem the informal definition as our survival center:

• It contains all cranial nerve nuclei (except CN I, CNII), providing somatic and autonomic control of the head and neck. 
• It contains the nuclei of the reticular formation
• It contains sympathetic and parasympathetic nuclei, providing vital centers that control activities like breathing, heart rate and vasomotion. 
• All pathways between the spinal cord and the cerebrum / cerebellum pass through the brainstem. 

Get to know the anatomy of the brainstem with our resources.

Cerebellum

The cerebellum lies between the cerebrum and the medulla of the brainstem. It plays an important role in regulating motor functions; participating in the planning and modulation of motor activity, including the coordination of the body while moving.

Like the cerebrum, the cerebellum has two hemispheres (left and right). They are connected by a midline mass called the vermis. It also has three lobes; anterior, posterior and flocculonodular. The cerebellar cortex forms the outermost layer of cerebellum and is comprised of grey matter. Important structures of the cerebellum include it's many paired nuclei within its deep white matter and the cerebellar peduncles. You can learn more about cerebellum anatomy and function here.

Spinal cord

The spinal cord is found in the vertebral column. It is continuous with the brainstem, extending from the foramen magnum of the occipital bone to the L1/L2 vertebra. This caudal part of the CNS transmits information to and from the periphery by interacting with the peripheral nervous system. However, it is more than just a channel between the brain and the body–it also modifies and integrates the information that passes through it and participates in reflexes.

The spinal cord consists of 5 segmental groups;

• Cervical spine - 8 segments (C1-C8)
• Thoracic spine - 12 segments (T1-T12)
• Lumbar spine - 5 segments (L1-L5)
• Sacral spine - 5 segments (S1-S5)
• Coccygeal spine - 1 segment (Co1)

Contrary to the brain, the spinal cord’s outermost layer is formed of white matter. This is divided into three funiculi (anterior, lateral, and posterior) containing pathways travelling between the brain and the periphery. The central mass of the spinal cord is a butterfly-shaped grey matter which contains neuronal cell bodies.

Master spinal cord anatomy with our study materials.

Meninges

The meninges are the three membranous layers of dura, arachnoid and pia mater that envelope the brain and the spinal cord. 

Meninges act to protect CNS structures, form partitions and provide spaces. The dural partitions separate the cerebrum and cerebellum from each other and divide their hemispheres (falx cerebri, tentorium cerebelli, falx cerebelli, diaphragma sellae). The meningeal spaces contain the dural venous sinuses and the cerebrospinal fluid filled subarachnoid cisterns. Study the anatomy of meninges with our resources.

Ventricles and CSF

The ventricles are interconnected cavities located deep within the brain. They are filled with cerebrospinal fluid (CSF) which functions to protect the brain and spinal cord, provide nutrients and remove waste. There are the four ventricles housed within the brain:

• Two lateral ventricles - within the lobes of the cerebrum
• Third ventricle - between the thalami
• Fourth ventricle - located over the pons and medulla oblongata and underneath the cerebellum 

CSF is produced by the cells of the choroid plexuses found within the walls of these ventricles. It then circulates through the ventricles via foramina located between them. From the fourth ventricle CSF accesses a system of subarachnoid cisterns, flowing through the subarachnoid space of the brain and spinal cord until it’s finally absorbed into the venous system of the CNS. Fill your neurons with some quality knowledge about CSF and the ventricular system.

Brain blood supply

The brain's arterial supply is provided by two major sources; the internal carotid and the vertebral arteries. The internal carotid artery forms the anterior circulation of the brain, supplying the anterior and middle parts of the cerebrum. The vertebral arteries supply the posterior cerebrum, brainstem and cerebellum (posterior circulation). These two circulations anastomose at the base of the brain forming a vascular network called the circle of Willis.  We have you covered with everything you need to know about the arterial supply of the brain and spinal cord.

Venous blood from the brain is drained through a system of superficial and deep cerebral veins. Superficial cerebral veins drain the cortex, while deep veins drain deep brain structures. Both groups eventually empty into the dural venous sinuses, enlarged venous channels found within the dura mater. Venous sinuses drain into the internal jugular vein. Want to learn more about the venous drainage of the brain? We’ve got you covered:

Cranial nerves

Cranial nerves originate from nuclei in the brain. They emerge from foramina and fissures of the cranium to provide sensory and motor supply to the head and neck. Only the vagus nerve extends beyond the neck to innervate thoracic and abdominal viscera. Their numerical order (1-12) is determined by their skull exit location (rostral to caudal). 

Learn cranial nerve anatomy with study units:

Spinal nerves

Spinal nerves are the major output of the spinal cord. They arise in pairs from corresponding spinal cord segments. There are 31 pairs of spinal nerves; 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal 

Spinal nerves are sort of a hybrid output for each part of the nervous system, as they are mixed nerves containing sensory, motor and autonomic fibers. They arise from two roots, anterior (motor) and posterior (sensory). The roots unite and the nerve exits from the spinal cord through the corresponding intervertebral foramen to innervate areas of the body. Spinal nerves innervate their target organs directly, or by forming neural networks called plexuses. Notable plexuses are:

• Cervical plexus (C1-C4) - innervates neck structures 
• Brachial plexus (C5-T1) - innervates the upper limb
• Lumbar plexus (L1-L4) - innervates the lower abdominal wall, anterior hip and thigh
• Sacral plexus (L4-S4) - innervates the pelvis and the lower limb

Areas of skin innervated by one spinal nerve are called dermatomes. Groups of muscles innervated by one spinal nerve are called myotomes. Learn the anatomy of spinal nerves with this set of study materials.

Ascending pathways

Ascending pathways transmit sensory information from our internal and external body environment. Sensations of proprioception and fine touch are carried through the dorsal columns which, together with the medial longitudinal fasciculus, forms the dorsal column-medial lemniscus pathway (gracile and cuneate fasciculi).

Sensations of temperature, crude touch and pain travel through the anterior and lateral funiculi of the spinal cord (spinothalamic tracts, also referred to as the anterolateral sensory system). The anterolateral system also includes the spinoreticular pathway (behavioural awareness) and spinotectal (spinomesencephalic) pathway (inhibiting and controlling pain sensations). Dive into our sensory pathways article and find out why we are so sensitive.

Descending pathways

Descending pathways control the movements of the body. They are classified in two large groups; pyramidal and extrapyramidal. The pyramidal motor system (corticonuclear and corticospinal pathways) originates from the motor cortex to control voluntary movements of skeletal muscles.

The extrapyramidal motor system (rubrospinal, tectospinal, reticulospinal, and vestibulospinal tracts) originate from nuclei in the brainstem. By synapsing in the spinal cord this system controls other aspects of locomotor activity besides pure movement, such as coordination, reflexive movements and body posture.

Learn the anatomy of spinal cord descending tracts here.