Brain Your brain is an essential organ that controls many body functions. Your brain receives and interprets all the sensory information you encounter, like sights, sounds, smells and tastes. Your brain has many complex parts that work together to help you function. What are the main parts of the brain? Your brain’s structure is complex. It has three main sections: Cerebrum: Your cerebrum interprets sights, sounds and touches. It also regulates emotions, reasoning and learning. Your cerebrum makes up about 80% of your brain. Cerebellum: Your cerebellum maintains your balance, posture, coordination and fine motor skills. It's located in the back of your brain. Brainstem: Your brainstem regulates many automatic body functions. You don’t consciously control these functions, like your heart rate, breathing, sleep and wake cycles, and swallowing. Your brainstem is in the lower part of your brain. It connects the rest of your brain to your spinal cord. What are the lobes that make up your brain? Each side of your brain has different lobes (sections). While all the lobes work together to ensure normal functioning, each lobe plays an important role in some specific brain and body functions: Frontal lobes: The frontal lobes are in the front part of your brain, right behind your forehead. This is the largest lobe and it controls voluntary movement, speech and intellect. The parts of your frontal lobes that control movement are called the primary motor cortex or precentral gyrus. The parts of your brain that play an important role in memory, intelligence and personality include your prefrontal cortex as well as many other regions of your brain. Occipital lobes: These lobes in the back of your brain allow you to notice and interpret visual information. Your occipital lobes control how you process shapes, colors and movement. Parietal lobes: The parietal lobes are near the center of your brain. They receive and interpret signals from other parts of your brain. This part of your brain integrates many sensory inputs so that you can understand your environment and the state of your body. This part of your brain helps give meaning to what's going on in your environment. Temporal lobes: These parts of the brain are near your ears on each side of your brain. The temporal lobes are important in being able to recall words or places that you've been. It also helps you recognize people, understand language and interpret other people’s emotions. Limbic lobes: The limbic lobe sits deep in the middle portions of your brain. The limbic lobe is a part of your temporal, parietal and frontal lobes. Important parts of your limbic system include your amygdala (best known for regulating your “fight or flight” response) and your hippocampus (where you store short-term memories). Insular lobes: The insular lobes sit deep in the temporal, parietal and frontal lobes. The insular lobe is involved in the processing of many sensory inputs including sensory and motor inputs, autonomic inputs, pain perception, perceiving what is heard and overall body perception (the perception of your environment). What other parts of the brain send and receive signals? Although most brain cells reside on the surface of your brain (called gray matter) and the cabling (white matter) is deep and connects various parts of your brain, there are some nuclei (collection of brain cells) that reside deep in your brain. They include: Thalamus: Your thalamus is a structure residing deep in your cerebrum and above your brainstem. This structure is sometimes referred to as the switchboard of the central nervous system. It relays various sensory information, like sight, sound or touch, to your cerebral cortex from the rest of your body. Hypothalamus: Your hypothalamus sits below your thalamus. It's important in regulating various hormonal functions, autonomic function, hunger, thirst and sleep. Your hypothalamus and pituitary gland are important structures involved in the control of your hormonal system. Pituitary gland: Your pituitary gland sends out hormones to different organs in your body. Basal ganglia: Your basal ganglia are a group of nuclei deep in your cerebrum that is important in the control of your movement, including motor learning and planning. Brainstem nuclei: There are a number of nuclei situated in your brainstem involved in a variety of different functions including cells that give rise to a number of important cranial nerves, normal sleep function, autonomic functions (breathing and heart rate) and pain. Reticular formation: Your reticular formation is a part of your brainstem and thalamic nuclei. These are a part of your reticular activating system (nuclei plus the white matter connecting these nuclei), which lies in your brainstem, hypothalamus and thalamus. The reticular activating system (RAS) mediates your level of awareness, consciousness and focus. They also help control your sleep-wake transitions and autonomic function. Parts of a Neuron and Their Function Neurons or nerve cells are the basic building blocks or units of the nervous system. Nearly 86 billion neurons work co-ordinately within the nervous system to keep the body organized. They are highly specialized cells that act as information processing and transmitting units of the brain. A group of neurons forms a nerve. Although they have a characteristic elongated shape, they vary widely in size and properties based on their location and type of functions they perform. While they have the common features of a typical cell, they are structurally and functionally unique from other cells in many ways. All neurons have three main parts: 1) dendrites, 2) cell body or soma, and 3) axons. Besides the three major parts, there is the presence of axon terminal and synapse at the end of the neuron. Dendrites They are specialized extensions that resemble the branch of a tree. Dendrites help to increase the surface area available for connections with the adjacent neurons and thus in receiving incoming signals from them. Some neurons have very small and short dendrites, while in others, they are very long. Most neurons have numerous dendrites, while a few others have only one of them. Functions Acquiring chemical impulse from other cells and neurons Converting the chemical signals into electrical impulses Carrying electrical impulses towards the next part of the neuron, the cell body Cell Body or Soma It is the core of the neuron, similar to a cell that contains the nucleus and all other cellular organelles. The cell body is also the largest part of a neuron enclosed by a cell membrane that protects the cell from its immediate surroundings and allows its interaction with the outside environment. They attach to all the dendrites and thus integrate all the signals. All metabolic activities of the cell take place in the cell body, which also contains DNA, the genetic material of the neuron. Functions Supporting and organizing the functions of the whole neuron Joining the signals received by the dendrites and passing them to the axons, the next part of the neuron Axons They are fine, elongated fiber-like extensions of the nerve cell membrane. Axons run from the cell body of one neuron until the terminal of the next neuron. They are the longest part of the neuron that varies widely in length, with some are as tiny as 0.1 millimeters, while some others can be over 3 feet. The larger the diameter of the axon, the faster is the rate of transmission of nerve signals. Sometimes, a single axon is highly branched to allow better communication with multiple target neurons at the same time. Parts of an Axon a) Axon hillock – The part of the axon which remains attached to the cell body or soma. b) Myelin sheath – The layer of fatty acid produced from specialized cells called Schwann cells that are wrapped around the axon. c) Nodes of Ranvier – The gaps between the discontinuous myelin sheath that is running along the axon. Functions Axons help to receive signals from other neurons and transmit the outflow of the message to the adjacent connected neurons and also to other muscles and glands by changing the electrical potential of the cell membrane called the action potential. Myelin sheath insulates the axon and thus prevents shock similar to an insulated electric wire Myelin sheath also increases the speed of the flow of signals through the axon Nodes of Ranvier allows diffusion of ions in and out of the neuron and thus maintains the electrical potential of the neuron Apart from all the major parts described so far in a neuron, there are other structures (that are not basic parts) found at the junctions between the neurons that help to establish functional links or connections between them. They are described below. Connecting Parts of a Neuron: Axon Terminal and Synapse Axon terminal also called synaptic bouton or terminal button is the terminal branches of the axon located at the very end of the neuron. They are farthest from the soma and contain chemical messengers called neurotransmitters in specialized structures called synaptic vesicles. Synapse or synaptic cleft is the small space or gap between two adjacent neurons. It is formed between the axon terminal of one neuron and the dendrites of the next neurons. Functions Releasing of neurotransmitters through specific transport vesicles, called synaptic vesicles from one neuron to the adjacent connected neurons called exocytosis Synaptic vesicles of one neuron for conducting nerve impulse to the adjacent connected neuron through exocytosis Sending neuronal information from one nerve cell to another and also to other cells of the muscle or gland with the help of neurotransmitters Re-up taking of excessive neurotransmitters from the synaptic cleft References: Overview of neuron structure and function – Khanacademy.org Neurons – Courses.lumenlearning.com/ https://www.khanacademy.org/science/biology/human-biology/neuron-nervoussystem/v/anatomy-of-a-neuron An Overview of the Different Parts of a Neuron – Verywellmind.com What is a neuron? – Qbi.uq.edu.au Spinal Cord The spinal cord is a long, tube-like band of tissue. It connects your brain to your lower back. Your spinal cord carries nerve signals from your brain to your body and vice versa. These nerve signals help you feel sensations and move your body. Any damage to your spinal cord can affect your movement or function. The spinal cord is a continuation of the brainstem. It extends from the foramen magnum at the base of the skull to the L1/L2 vertebra where it terminates as the conus medullaris (medullary cone). A thin thread called filum terminale extends from the tip of the conus medullaris all the way to the 1st coccygeal vertebra (Co1) and anchors the spinal cord in place. You can easy remember the extent of the spinal cord with a mnemonic 'SCULL', which stands for ' Spinal Cord Until L2 (LL)'. Throughout its length, the spinal cord shows two well defined enlargements to accommodate for innervation of the upper and lower limbs: one at the cervical level (upper limbs), and one at the lumbosacral level (lower limbs). Like the vertebral column, the spinal cord is divided into segments: cervical, thoracic, lumbar, sacral, and coccygeal. Each segment of the spinal cord provides several pairs of spinal nerves, which exit from vertebral canal through the intervertebral foramina. There are 8 pairs of cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal pair of spinal nerves (a total of 31 pairs). Cross section The spinal cord is made of gray and white matter just like other parts of the CNS. It shows four surfaces: anterior, posterior, and two lateral. They feature fissures (anterior) and sulci (anterolateral, posterolateral, and posterior). Spinal meninges The spinal cord and spinal nerve roots are wrapped within three layers called meninges. The outermost is the dura mater, underneath it is the arachnoid mater, and the deepest is the pia mater. Dura mater has two layers (periosteal and meningeal), between which is the epidural space. Between the arachnoid and pia mater is the subarachnoid space, it is filled with cerebrospinal fluid. Blood supply The spinal cord is supplied by branches of the vertebral and segmental arteries. The vertebral artery gives rise to anterior and posterior spinal arteries. Segmental arteries, such as the deep cervical, ascending cervical, and posterior intercostal give rise to 31 pairs of radicular arterial branches which supply the roots of spinal nerves. Similar named veins accompany the arteries. Anterior and posterior spinal veins drain into radicular veins, which then empty into the (internal and external) vertebral venous plexus. This network eventually empties into the vertebral (neck) and segmental (trunk) veins. Blood supply is always an inevitable part of any anatomy study unit. Spinal cord tracts Spinal cord neural pathways are found within the spinal cord white matter. On each side, the white matter is divided into three funiculi: anterior, lateral, and posterior. Ascending tracts convey information from the periphery to the brain. On the other hand, the descending tracts carry information from the brain to the periphery. The spinal cord is more than just a conduit, as it also modifies and integrates the information that pass through it. Spinal nerves Spinal nerves are grouped as cervical (C1-C8), thoracic (T1-T12), lumbar (L1-L5), sacral (S1-S5), and coccygeal (Co1), depending from which segment of the spinal cord they extend. Segmentation of the spinal cord corresponds to the intrauterine period in which the spinal cord occupies the entire vertebral canal. For this reason in adulthood, where the vertebral column is longer than the cord, each spinal cord segment is located higher than its corresponding vertebra. These differences become more obvious distally towards the lumbar and sacral segments of the spinal cord–for example spinal cord segment L5 is at the level of the L1 vertebra. Spinal nerves, however, exit the vertebral column at their correspondly numbered vertebrae. Cervical spinal nerves exit through the intervertebral foramina directly above their corresponding vertebrae, whilst thoracic, lumbar and sacral spinal nerves exit directly below. In order for the more distal spinal nerves to exit they must first descend through the vertebral canal. Since the lumbar and sacral spinal nerves are the farthest from their intervertebral foramina, they are the longest. While descending towards their corresponding intervertebral foramina, lumbosacral spinal nerves form a bundle called the cauda equina (meaning horse’s tail). Each spinal nerve has an anterior and posterior root. Anterior roots transmit motor information, and they originate from the anterior horns of the gray matter and exit the spinal cord through the anterolateral sulcus. The posterior roots transmit sensory information and have sensory ganglion attached to them. They originate from the posterior horns of gray matter and exit through the posterolateral sulcus of the spinal cord. The anterior and posterior roots merge just before the intervertebral foramen, and form the trunk of the spinal nerve. The trunk is very short, and soon after exiting the vertebral column, it divides into four branches: anterior ramus, posterior ramus, communicating ramus, and meningeal ramus. Reflex arc A huge part of spinal cord function is under the influence of the brain, as it functions to relay information to and from the periphery. But there are many reflexes that are generated in the spinal cord independently from the brain. Spinal reflexes are either monosynaptic or polysynaptic. Monosynaptic reflexes play out with only two neurons participating in the reflex arc, one sensory and one motor. The first-order neuron (sensory) is in the spinal ganglion, while the second-order neuron (motor) is in the anterior horn of the spinal cord). The sensory neuron gathers impulses from the muscle and sends this information to the motor neuron which innervates the same muscle. The motor neuron then causes contraction of the innervated muscle. An example of a monosynaptic reflex is the stretch reflex. Polysynaptic reflexes on the other hand have multiple neurons participating. Besides one sensory and one motor neuron, there are also one or more interneurons between them making this communication indirect. They are more complex than monosynaptic reflexes as they involve muscle groups instead of a single muscle. An example is the withdrawal reflex.
