Introduction to Brain Anatomy
The brain is a complex organ. The brain is generally separated into different distinct areas, mostly based on the locations and the differentiation during embryonic development. The imaginary line that separates the brain into different areas is called neuraxis. There are two lobes of the brain: the left and the right. The two lobes are connected by a structure called the corpus callosum.
Layers
Before we get to the brain, let’s talk about the different layers that work to protect the brain from damage. Going from the most outer layer to the inner layer: Scalp –> Skull –> Dura mater –> Arachnoid –> Pia mater –> Cerebral cortex
- Dura mater, Arachnoid, and Pia mater are known collectively as the meninges
- The Arachnoid has a sublayer called subarachnoid space that is filled with cerebrospinal fluid (CSF)
- Pia mater is a thin layer that lays right on top of nerve tissue
- CSF is formed in the choroid plexus, and it fills cerebral ventricles, such as the subarachnoid space, and the channel in the spinal cord
Brain development
During development, the central nervous system begins as a neural tube. Between days 35-40, the neural tube starts to differentiate into the forebrain, midbrain, hindbrain, and spinal cord. These also differentiate even further into several areas during day 50.
- Forebrain –> Telencephalon and Diencephalon
- Midbrain doesn’t differentiate any further, but it is also known Mesencephalon
- Hindbrain –> Metencephalon and Myelencephalon
The development does not end there. By day 100, the brain developed further into the brain areas that we recognize in adults.
- Telencephalon –> Neocortex, Basal ganglia, and Limbic
- Diencephalon –> Thalamus and Hypothalamus
- Midbrain remains as Midbrain
- Metencephalon –> Cerebellum and Pons
- Myelencephalon –> Medulla Oblongata
Telencephalon – cerebral cortex, basal ganglia, and limbic
- Cerebral cortex is further divided into the frontal lobe, temporal lobe, occipital lobe, and parietal lobe
- Each lobe of the cortex has a primary cortex, which is responsible for the initial processing
- For example, the frontal lobe primary cortex is responsible for voluntary muscle movement
- Each lobe also has a secondary cortex, which is responsible for analyzing information, perception, and memory.
- For example, the front lobe secondary cortex is responsible for muscle memory
- Each lobe also has a tertiary cortex, which is responsible for complex memory that involves multiple systems
- Each lobe of the cortex has a primary cortex, which is responsible for the initial processing
- Basal ganglia have a role in motor control
- Limbic system plays a part in multiple systems, such as learning, motivated behavior, and emotional response. It is further divided into two primary areas: The Hippocampus and Amygdala
- Hippocampus – Spacial memory and new long-term memory
- Amygdala – Emotional responses
Diencephalon – thalamus and hypothalamus
- Thalamus plays a role in passing sensory and motor input to the cortex. This enables the cortex to focus on important inputs.
- Hypothalamus has a lot of roles in the body. The primary function is to maintain homeostasis by working on internal input with the limbic system. These internal inputs include body temperature, salt-water balance, hunger, thirst, metabolism, reproductive, and emotional inputs.
- The hypothalamus also helps control the autonomic and endocrine systems. It has axons extending to both the posterior and anterior pituitary.
Mesencephalon – Midbrain
- Midbrain is one of the parts of the brainstem
- Tectum plays a role in the visual and auditory systems
- Tegmentum plays a role in psychopharmacology. It is separated into three other divisions, also known as “nuclei”
- Periaqueductal gray (PAG) – Modulation of pain. It has a lot of opioid receptors.
- Substantia nigra – Move initiate and modulate movement. It has a long axon that innervates basal ganglia. Dopamine is the primary neurotransmitter.
- Because substantial nigra plays a big role in the body’s movement. Cell deaths in this area are known to cause Parkinson’s disease.
- Ventral Tegmental Area (VTA) – These nuclei use dopamine as their neurotransmitter. It is also part of the mesolimbic tract and mesocortical tract.
Metencephalon – cerebellum, and pons
- Cerebellum is known as the “sensory-motor.” Its primary function is to receive inputs, such as visual, auditory, balance, and somatosensory, from the vestibular system. Cerebellum works to combine the sensory information with motor information from the cortex.
- The axon bundles that connect the cerebellum and the pons are called cerebellar peduncles
- Pons is one of the three parts of the brainstem. It contains several different nuclei:
- Reticular formation, which begins within the pons and extends to the midbrain and medulla. It has some influences on arousal, attention, sleep, and muscle tone.
- Locus coeroleus is a primary nuclei that works with norepinephrine. Its axons extend to areas in the forebrain
- Dorsal and median raphe nuclei use serotonin (5-HT) as the primary neurotransmitter. The axons extend into the forebrains and have parts in sleep regulation, impulsiveness, and emotion.
Myelencephalon – Medulla Oblongata
- Medulla oblongata is the last of the three parts of the brainstem. It plays a role in regulating heart rate, digestion, blood pressure, and coughing. It is also a home of the respiratory center and vomiting center.
- Area postrema is known as the vomiting center. It is triggered in response to toxins
Nervous System – CNS and PNS
The nervous system has a presence all over our body. It is divided into two primary divisions: Central nervous system and peripheral nervous system
Central Nervous System
The central nervous system (CNS) includes the brain and spinal cord.
Peripheral Nervous System
The peripheral nervous system (PNS) is separated further into the somatic system and autonomic nervous system.
- Somatic system works to control voluntary muscle action. It consists of different types of nerves whose duties are to either provide inputs to the CNS or output from the CNS. Nerves that provide inputs to the brain are sensory afferent. Nerves that provide outputs to the target cells in the PNS are motor efferent.
- Spinal nerves are known as “mixed” nerves because they have both input and output functions.
- Cranial nerves are very diverse. There are a total of 12 cranial nerves in our body. Some have mixed functions, and some only have input or output functions. Some of these cranial nerves also work on glands and organs and are considered part of the autonomic system.
- Autonomic nervous system focuses on controlling smooth muscle activities through the actions of sympathetic and parasympathetic division.
- The sympathetic division triggers the secretion of epinephrine, resulting in an increase in heart rate and blood pressure.
- The parasympathetic division triggers actions that would conserve the body’s energy, such as slowing the heart rate and respiration.