Biological Neural Network

Biological neural network, also called the nervous system, controls our perceptions, feelings, emotions, thoughts, intelligence, decisions and actions. Biological neural network models are extensively used in building artificial neural networks to give artificial intelligence to machines. This paper explains the structure and functions of a biological neural network.

Biological neural network consists of two parts i.e., the central nervous system (CNS) consisting of the brain and the spinal cord and the peripheral nervous system (PNS) consisting of the nerves that goes through the whole body. Fundamental building block of the nervous system is neuro cells or Neurons. 

Nervous system is made of neuro cells also called neurons. A group of neurons form a nerve. Average human has more than 100 billion neurons, Neurons transmit signals from our sensory organs to the brain for understanding and decision making and carry the decision signals back to the muscles for executing the actions. 

Each neuron has mainly three parts i.e., dendrites, cell body and axon. Dendrites are branch like structure, and they receive messages from sensory receptors or from other neurons and allow transmission of those messages to the cell body. Cell body also know as soma has a nucleus and it is the core of the neuron. Cell body carry genetic information, maintains neuron structure and provide energy to drive activities. 

Axon is a long cable that goes away from the main cell and it is where the electrical impulse from the neuron travel away to be received by other neurons. Myelin is an insulating layer that allows electrical impulse to transmit quickly over a long distance. Node of Ranvier are uninsulated and are highly enriched with ion channels allowing them to participate in exchange of ions required to regenerate the action potential. Schwann cells are required for insulating and supplying nutrients to individual axons in the PNS. 

Based on their functions, neurons are classified into three types i.e. sensory neurons, interneurons and motor neurons.

Neurons control all observations, decisions and actions through (a) transmitting information from our sensory organs to the brain (b) processing that information in the brain for arriving at a decision and (c) transmitting those decisions back to the muscles for executing the actions. Based on the above functions, neurons are classified into sensory neurons, motor neurons and Interneurons.

Sensory neurons (or afferent neurons) are the neurons that carry signals from sensory organs like eye, ear, nose, tongue and skin to the brain for interpretation and decision making and motor neurons (or efferent neurons) are the neurons that carry the decisions from the brain to the muscles like hand, leg etc. for executing those actions. Interneurons act as middleman between the sensory neurons and the motor neurons. Sensory neurons allow us to feel sensations, motor neurons control our movement and Interneurons help us process information and make decision. The dendrites of the sensory neurons are generally too long while the axons are short. Motor neuron’ s dendrites on the other hand are small while the axons long.

Interneurons are the neurons that are found exclusively in the central nervous system i.e. in the brain and the spinal cord. In the neocortex (making up about 80% of the human brain), approximately 20 - 30% of neurons are interneurons. On the one hand Interneurons connect to the sensory neurons for receiving signals from the sensory organs and other hand they connect to the motor neurons for sending decision signals to the muscles for executing actions. Interneurons also interact with themselves allowing the brain to perform complex functions like learning, memory, perceptions, emotions, cognition, planning and decision making. More interneurons are activated when the response to a stimulus is required to be complex. Additionally, interneurons also drive our perceptions and emotions.

Interneurons can be broken into local neurons & relay neurons. Local Interneurons have short axons and they create circuits with other neurons to analyse small pieces of information while relay interneurons have long axon, and they connect circuits of neuron in one region of the brain with the other region. Interneurons form key nodes within the neuronal circuitry in the brain and regulate neuronal activity by releasing the neurotransmitter GABA, which inhibits the firing of other neurons. Our brain receives millions of signals from our sensory organs and not all of them require our action. Interneuron play a vital role here by inhibiting other neurons from firing action potentials where no action is required. While most of the time interneurons use GABA to inhibit other neurons, they also release glutamate to excite other neurons in a reflex action.

Sensory neurons begin with the dendrites, as this is where the signal is received. When receptors at the end of the dendrites of the sensory neuron encounter a stimulus, the message is carried to the cell body of the sensory neuron through the dendrites and based on the strength of the message the cell body fires an action potential i.e. it converts the message into an electrical signal for transmitting the message to the Interneurons in the spinal cord of the central nervous system. These inter neuron carry those signals from the spinal cord to the brain and the network of Interneurons in the brain process those signals to reach a decision. Then the Interneurons in the brain carry those decisions from the brain to the spinal cord for transmitting to the motor neurons. Finally motor neurons carry those decisions from the central nervous system to the neuro muscular junctions for transmitting the decision to the muscles for executing the actions.

At any given point of time our brain simultaneously receives multiple sensory information from multiple sensory neurons from multiple sensory organs like eye, ear nose, mouth and skin. Brain processes this information using multiple Interneurons and transmit the decisions to multiple motor neurons for activating multiple muscles to execute multiple tasks at the same time. So many activities happen so fast with in fraction of a second that we fail to notice them.In case of a reflex action, spinal cord interprets the signal and sends a response without involving the brain. Interneuron plays the role of mediator in this case by receiving the signal from the sensory organ and passing the response to the motor neuron. For example, when accidentally you put your hand on a hot object, you immediately take your hand away. While an inter neuron mediates this action, another inter neuron takes this signal to the brain and you feel the pain.

Motor neurons are of two types i.e. upper motor neurons and the lower motor neurons. The upper motor neurons originate in the cerebral cortex and travel down to the brain stem or spinal cord, while the lower motor neurons begin in the spinal cord and go on to innervate muscles and glands throughout the body. In most of the cases interneurons connect the upper motor neurons with the lower motor neurons for transmission of information. 

Sensory neurons, motor neurons and interneurons  communicate with each other through neuron signals also called action potential. Action potentials are electrical signals which the neurons use to transmit information to the other neurons.

Action potential are neuron signals. Neurons generate and transmit these signals to other neurons. An action potential occurs when a neuron sends information down an axon in the form of an electric signal, to be transmitted to the next neuron. Action potential is caused by a stimulus when the stimulus has a force sufficient to reduce the negativity of the nerve cell to the threshold. 

A resting neuron (non signaling neuron) has a resting membrane potential of -70 mV. This means the inside of the neuron is 70 mV less than the outside. While a neuron is resting, there are relatively more sodium ions (Na+) outside the cell and more potassium (K+) ions inside the cell. 

Action potentials are generated when different ions cross the neuron membrane. When a resting neuron encounters a stimulus the sodium channels open causing more positively charged sodium to rush into the neuron causing the neuron depolarized. When the depolarization reaches the threshold level of around -55 mV, a neuron will fire an action potential. After a little lag potassium channels also open and potassium rushes out of the cell which reverses the depolarization. At this point sodium channels start to close while potassium channels stay open a bit long, this causes the neuron to go past - 70 mV leading to hyperpolarization. 

Gradually the sodium potassium pump makes the ion concentration go back to the resting level and the cell returns to - 70 mV. After firing an action potential, till the time the cell goes back to the resting potential, it cannot fire another action potential. This time is called refractory period.

When ever our sensory organs encounter a stimulus, sensory receptors located in these organs carry the information to the cell body of the sensory neurons and based on the  strength of those signals, sensory neurons fires an action potential. Based on the nature of the stimuli  to which they respond, these sensory receptors are classified into photoreceptors, thermoreceptors, chemoreceptors, mechanoreceptors, proprioceptors and nociceptors.

Photoreceptors - These are special cells in the eye’s retina that is capable of visual phototransduction i.e., they convert light into signals that are sent to the brain for interpretation of the object. The two basic photoceptor cells are rods and cones. Rods contribute to night-time vision while cones contribute to day-time vision.

Chemoreceptors - Chemoreceptors are used by our nose to smell and the mouth to taste. Chemical compounds in the odour and in the food are sensed by the chemoreceptors in our nose and mouth which convert them into nerve signals and send to the brain for interpretation. Arterial chemoreceptors detect the increase in blood level of carbon dioxide or a decrease in blood level of oxygen and send the information to the central nervous system for restoring homeostasis. Chemoreceptors in our stomach transmit information to our brain regarding the nutrient content in the food which is used by the brain to isolate and remove waste from the good nutrients.

Thermoreceptors - Thermoreceptors are the found in the skin to detect temperature in the environment. Thermoreceptors are two types, warmth and cod. Warmth thermoreceptors are excited by rising temperature and inhibited by cold temperature and opposite happens in cold thermoreceptors. Our feeling of warm and cold depends upon the location and number of such receptors in our body. Skin thermoreceptors are concentrated around our moth, lips, tongue, eyes, ears, hands and feet. Thermoreceptors also enable the hypothalamus to maintain internal body temperature through a feed forward mechanism to change internal body temperature in response to environmental changes.

Mechanoreceptors - Mechanoreceptors in the skin are responsible for providing information regarding pressure and texture of the object when our skin encounters an object. Arterial mechanoreceptors (also called baroreceptors) are stimulated by distortion of the arterial wall when blood pressure changes. Baroreceptors continuously transmit information on the average blood pressure and the change in pressure with each arterial pulse, to the central nervous system. This results into reflex responses triggering increase or decrease in heart rate in the autonomic nervous system. Mechanoreceptors are also present throughout our respiratory tract. They transmit information to the brain regarding the status of our chest and lungs, which is used by our brain to control breathing and trigger protective reflex action like cough. Mechanoreceptors in our stomach transmit information to the brain regarding the amount and movement of food as it passes through the gastrointestinal tract and the brain controls our hunger & food intake based this information.

Proprioceptors - Proprioceptors support our body’s ability to perceive its position, movement and acceleration in open space. Proprioceptors enable us for example to touch our right foot while closing our eyes. Proprioceptors enable us to sense the position and movement a body part relative to the other parts in the body. They continuously transmit the position of muscles, tendons and joints to the brain, which help the brain maintain body balance and control body movement.

Nociceptors - Nociceptors are pain receptors located in our skin, muscles, joints and bones which transmit the sense of pain to the brain. They carry pain signals to the brain including information regarding the location and intensity of the pain stimuli. They support the brain in ascertaining the damage caused to the body parts and activating appropriate response mechanism. They keep on sending the pain signals till the damage to the affected area is cured. There are many type of nociceptors like the mechanical nociceptors which respond to extreme stretch or strain, thermal nociceptors which respond to extreme hot and cold temperature and chemical nociceptors which respond to  damages caused from external chemicals. 

Neurons connect to each other through a connection point called synapse. A pre-synaptic neuron transmits the information to the post-synaptic neurons using a synapse.  A single neuron simultaneously sends information to multiple neurons using multiple synapses.

Synapse is a small gap at the end of a neuron that allows passing of signal from one neuron to the next. Each neuron connects to several other neurons for transmission and processing of data and there are more than 100 trillion synapses found in the nervous system. In many synapses, the presynaptic part is located on an axon and the postsynaptic part is located on a dendrite or soma. There are fundamentally two type of synapses i.e., chemical synapse and electrical synapse.

In a chemical synapse, electrical signal from the presynaptic neuron is converted into the release of a chemical called neurotransmitter (also called chemical messengers) that binds to receptors located in the plasma membrane of the post synaptic cell. The neurotransmitter may initiate an electrical response that may either excite or inhibit the postsynaptic neuron. If the postsynaptic neuron is excited, it will send an electrical signal to the next neuron.

Receptors and neurotransmitters use a like a lock-and-key mechanism. A neurotransmitter will bind to a specific receptor only just a key will open a specific lock only. If the neurotransmitter is able to bind the receptor site, it either excite or inhibit an action in the post synaptic neuron.

In an electrical synapse, presynaptic and post synaptic cells are connected by special channels called gap junctions that are capable of passing electrical signals directly from one neuron to the other. The key advantage of an electrical signal is rapid transfer of signals from one cell to next. Reflex actions use electrical synapses where speed in transmission of signal is very essential.

synaptic vesicles (or neurotransmitter vesicles) store various neurotransmitters that are released at the synapse. The release is regulated by a voltage-dependent calcium channel. 

Neurotransmitters are of two types i.e. excitatory and inhibitory. Excitatory neurotransmitters create excitatory effects on the post synaptic neuron. They increase the likelihood that the post synaptic neuron will fire an action potential. On the other hand, inhibitory neurotransmitters create inhibitory effects on the post synaptic neuron. They decrease the likelihood that the neuron will fire an action potential.

Some of the common excitatory neurotransmitters are:

Acetylcholine - Regulates muscle functioning and sleep cycle.

Norepinephrine - Increases alertness and wakefulness. Low level of norepinephrine leads to mood disorder like depression and anxiety whereas high level of it leads to sleep disorder.

Epinephrine - Makes the body ready for fight or flight response with increased heart rate and blood pressure, when encountered with danger.

Dopamine - Improves motivation and enthusiasm to do things. When dopamine is released, it creates feeling of pleasure and reward which motivates to repeat a specific behaviour.

Glutamate - Regulates learning, memory and other cognitive functions. It is used by every major excitatory function in the brain found in more than 90% of the synaptic connections in the brain. 

Following neurotransmitters are Inhibitory neurotransmitters :

Gamma-aminobutyric acid (GABA) - Reduces excitability throughout the nervous system. It reduces excitement by acting as a brake to excitatory neurotransmitters.

Serotonin - Regulates perception of emotion and pain. In sufficient serotonin level leads to depression, anger and even suicidal tendencies.

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Neuroplasticity is the ability of the brain to change the neural network i.e., the ability to create new networks or modify existing networks, because of growth and reorganization.  Or in other words neuroplasticity makes it possible for the brain to be moulded and shaped based on experience. Neural networks in the brain keeps on changing based on new experiences like learning a new skill, learning a new language, meeting new people, travelling to new places and due to influence of the environment and impact of physiological & psychological stress. 

Though changes to the  neural networks continues till our lifetime, this process is high during the growing age of the child and it slows down as we become old. Neuroplasticity has many advantages like it supports learning, memory formation, cognitive improvement, forgetting painful experiences , getting rid of bad habits and recovery from brain injuries. How ever our brain does not have infinite plasticity and certain deficits in our movement, speech and cognition arising from brain damage can not be cured. 

Neurogenesis:

New neurons are formed in the brain through a process called neurogenesis. Neurogenesis in a large scale happens when an embryo is developing, and this process continues throughout our lifetime. However, this process generally slows down as the person ages. Neurogenesis in the adult human brain plays a critical role in memory, learning and cognition. Various studies now have proved that exercise has a positive impact on neurogenesis. Activities such as learning new skill, learning new language, making deeper social connections increase neurogenesis. Diet also plays a big role in neurogenesis, consuming foods rich in omega 3 fatty acid also increase generation of new neurons in the brain. On the contrary, acute and chronic stress are known to decrease neurogenesis. 

Synaptogenesis:

Synaptogenesis is the process of formation of new synapses (i.e., connections) between neurons. Though synaptogenesis is extremely high during the early brain development, synapse development continues throughout the lifetime of a person. Formation of functional synapses is the basis for forming neural circuits which are critical for executing complex tasks. Learning is the key to synaptogenesis. When we learn something new like when we meet a new person or visit a new place or learn a new skill, new synapses are created in our brain which are stored as memory and the same synapses are activated when we try to remember them in future.
Besides synapse development, synapse maintenance (i.e., synaptic pruning) also takes place in our brain throughout our lifetime whereby frequently used synapses are strengthened and synapses which are not used for long time are removed. Synaptic pruning plays a big role in our memory and those memories which we use frequently gets strengthened whereas memories which are not used for a long time are lost. Synaptic pruning is extremely essential as with out them our brain would have been overloaded with information & analysis resulting into lot of confusion and inefficiency.

Our brain is structured in such a way that there are different regions in the brain and each region perform a set of specific tasks. Each region executes its tasks using multiple neural circuits and the neural circuit of each region is connected with the other regions through interneurons, thus creating a large and complex neural network. Neural circuits in the brain use multiple types of specialised neurons which are capable of executing highly specialized tasks. Based on the expertise and functions, our brain is broadly classified into three regions i.e., Forebrain, Midbrain and Hindbrain.

Forebrain - Forebrain includes the cerebral cortex, cerebrum, thalamus, hypothalamus, pituitary gland and the limbic system (covering our emotion, behaviour, long term memory and olfaction). The forebrain drives our sensory and associative activities, complex cognitive activities and the voluntary motor activities. Sensory inputs from all over the body reach to the forebrain where these information are interpreted; decisions are taken and motor commands for the voluntary movement of the body is sent.

Midbrain - Midbrain serves as the vital connection point between the forebrain and the hindbrain. Midbrain is the shortest part of the brainstem. It contains the relay nuclei involved in the processing of the auditory and visual information. Nuclei of three cranial nerves (oculomotor nerve, trochlear nerve and trigeminal nerve) resides the midbrain which controls the sensation in the face and movement in the eye. Midbrain provides the passageways for the information that travels between the cerebral cortex and the spinal cord.

Hindbrain - Hindbrain  is also referred to as the brainstem which connects the brain with the spinal cord. It is located at the back of the head and it looks like an extension of the spinal cord. It consists of the pons, medulla and the cerebellum. Hindbrain primarily coordinates the autonomic nervous system and plays an important role in breathing, heart rate, blood pressure, swallowing, digestion and sleep cycle. Hindbrain continuously receives messages from the muscles, joints, tendons and the internal parts of the ear to control our posture, balance and movement during walking, running, cycling and swimming. Hindbrain also facilitates our motor learning & sequence learning and plays an important role in processing our procedural memory and reflex memory. The cerebellum in the hindbrain fine tunes the motor commands and adjusts voluntary movements in coordination with the motor cortex in the cerebral cortex.

Cerebral Cortex & Cerebrum - Cerebral cortex is the outer layer of the cerebrum. Cerebral cortex comprises of the dendrites and cell bodies of the neurons whereas cerebrum consists of the cell bodies and nerve fibres. The cerebral cortex is composed of the four lobes (frontal lobe, parietal lobe, occipital lobe & temporal lobe) and the inner cerebrum is divided into two hemispheres (left hemisphere & right hemisphere). In this way each hemisphere contains the four lobes. 

· Frontal Lobe - Responsible for personality, judgement, reasoning, planning, analysis, cognition, expressive language, body movement and emotions. 

· Parietal Lobe - Responsible for processing of sensory information like touch, taste, pressure, temperature and pain. Somatosensory cortex is located in this lobe which interprets these sensations.

· Occipital Lobe - Responsible for processing visual information. Primary visual cortex located in in this lobe receives and interprets signals from the retinas of the eyes.

· Temporal Lobe - Responsible for processing auditory information. Auditory cortex located in the lobe receives and interprets signals received from our ear. 

Hippocampus is also located in this lobe which is responsible for formation of memories through integrating the sensations of taste, smell, touch, sight and sound.

Cerebral cortex has three regions i.e., sensory regions, associative regions and motor regions. Sensory nerves bring the sensory information to the sensory region where these information are received and then sent to the associative regions. Associative regions interpret and process the sensory information and this region is also responsible for learning, memory, planning, analysis and overall decision making. The decisions are then conveyed to the motor region which is responsible for planning, coordination and execution of the muscle movement. Cerebrum on the other hand coordinates and controls the fine muscle movement of the body. Cerebral cortex with its sensory and motor functions is responsible for our consciousness.

Corpus Callosum - Corpus Callosum is the bridge between the left hemisphere and the right hemisphere of the brain and shares the information from one hemisphere with the other hemisphere. Left hemisphere controls the sensation and movement of the right side of the body and the right hemisphere controls the left side. Corpus callosum plays an important role here in smooth functioning of the entire body by connecting both side of the brain together and efficient sharing of information between them. Corpus callosum is a thick bundle of nerve fibres which carries sensory, motor and cognitive information from one side of the brain to the other.

Thalamus -  Thalamus translate neuron signals from the sensory receptors to the cerebral cortex. It relays the visual, auditory, gustatory ad somatosensory information to the cerebral cortex. Sensory impulses travel from the sensory organs towards the thalamus which receives them as sensation. These sensations are then transmitted to the cerebral cortex for interpretation as touch, temperature or pain. Thalamus acts as a traffic controller and decides which sensory signals to be sent to the cerebral cortex for further processing. Our body is transmitting millions of sensory and motor signals to the thalamus and all of them do not require our attention and thalamus plays an important role here by segregating the information and transmitting only that information to the cerebral cortex that require our attention. As such thalamus controls our level of consciousness and alertness. The thalamus consists of different types of nuclei which are responsible for the relay of different signals. Thalamus acts like a relay station that filters information between the brain and the body. Except for olfaction (sense of smell), each sensory system has a thalamic nucleus that receives, filters and relays information to the corresponding cortical area.

Hypothalamus - Hypothalamus controls hormones in the body. Hypothalamus works with the pituitary gland which creates and sends many important hormones to the body. Together with the pituitary gland, hypothalamus controls many of the glands that produce hormones in the body, called the endocrine system. Hypothalamus acts like a connector between the nervous system and the endocrine system. The major function of the hypothalamus is to maintain homeostasis i.e., to keep the body in stable condition.  To do this, hypothalamus stimulate and inhibit many important body functions like temperature, blood pressure, hunger, thirst, fluid & electrolyte balance, sleep cycle and sexual behaviour. Hypothalamus produces many stimulating and inhibiting hormones to stimulate and inhibit production of other hormones in the body. Hypothalamus also controls the growth and stress hormones in the body.

Hippocampus - Hippocampus plays an important role in memory formation. Hippocampus is the place where impressions such as visual inputs, auditory inputs, touch and smell are interlinked and stored on a short-term basis as working memory. Since we come across thousands of impressions each day and the capacity of the hippocampus is limited, these impressions are transferred from the working memory of the hippocampus to the long-term memory storage area in the neocortex in the cerebral cortex. For this purpose, hippocampus synchronises with the cerebral cortex during our sleep to transfer the long-term memory. This is the reason why good sleep improves our memory. Hippocampus also encodes emotional context from the amygdala. This is the reason why a particular emotion is triggered when we remember a particular event. 

Basal Ganglia - Basal ganglia facilitates desired movement and inhibit unwanted or competing movements. They receive the impulses of the desired movement from the motor cortex in the cerebral cortex and after adjusting the movements they send the information to the thalamus which in return relays the information back to the cortex. Finally, the adjusted motor commands are sent to the skeletal muscles for execution. Basal ganglia refine the motor commands received from the cerebral cortex to ensure highly effective body movements to achieve the desired outcome. As a part of the motor system, basal ganglia play an important role in executing habitual actions, learning new actions and forming new habits. 

Amygdala - Amygdala is responsible for processing our emotions. It is where our emotions are generated and remembered. It controls our emotional response (like fear, anger, anxiety, disgust, sorrow and happiness) to situations. Amygdala also plays an important role is associating our emotions to memories and triggering those emotions when a memory is recalled. Amygdala drives our emotional memory formation, storage and recall. This is the reason why we remember those events very well which are attached with strong emotions. While our frontal lobe which is associated with reasoning, planning and decision making, provides a logical response to stimuli, amygdala provides an emotional response to stimuli. 

Many times, amygdala takes control over us, by passing the frontal lobe and respond to situations without any logical reasoning, which may not be the appropriate response. For example, when we suddenly react to situations out of our fear, anger or sorrow without thinking about their consequences. However, role of amygdala cannot be undermined when we face with extreme danger. In situations like this when there is no time for any logical thinking, amygdala takes control and creates a sense of intense fear and makes the body ready for a fight or flight response. Our heartbeat and respiration rate increases to provide the body with the additional energy and oxygen required to fight the situation, our muscles tensed and become ready for action, our blood flow to the surface area of the body is reduced while the blood flow to the muscles, legs and arms is increased to fight or easily escape the situation. 

Pons - Pons includes neural pathways that conduct signals from the cerebrum down to the cerebellum and medulla. Pons plays an important role in the functioning of the autonomous nervous system that controls our breathing, heartbeat, digestion and sleep cycle. Several cranial nerves originate in the pons. Trigeminal curve which is the largest cranial nerve assist in facial sensation and chewing. Eye movement is assisted by the abducens nerve. Facial movement and expressions are assisted by the facial nerve. Vestibulocochlear nerve assists in hearing and maintenance of equilibrium. Pons also contributes to our sense of taste and supports swallowing.

Cerebellum - Cerebellum is responsible for motor skills such as coordination, balance and posture and plays an important role is coordinating voluntary movements. Cerebellum also plays a critical role in motor learning, for example in learning how to drive a car or how to play basketball through trial-and-error process. Cerebellum does not initiate the motor commands, but the higher-level commands are fine-tuned by the cerebellum to make the motor commands more accurate and effective. Cerebellum continuously receives inputs from the vestibular system and the proprioceptors and modulate motor commands to adjust to change in muscle position and the load on the muscles. In addition to contributing to motor control, cerebellum also play an important role in cognitive functions like language.

Medulla - Medulla is a conduit of nerve fibres that carry the information between the brain and the spinal cord. Our brain and the spinal cord communicate through columns of nerve fibres that run through the medulla called spinal tracts. These tracts are two types i.e., ascending tracts sending information towards the brain and descending tracts sending information towards the spinal cord. Medulla is continuous with spinal cord in the sense that spinal cord gradually transitions into medulla. Medulla also regulates many vital functions of the body like the breathing, heart rate and blood pressure. When the baroreceptors detect changes in the blood pressure through monitoring the expansion and contraction in the walls of the blood vessels, they send the information to the specialized neurons in the medulla which immediately trigger reflex actions to restore the blood pressure to normal level. Chemo receptors in the blood vessels also send the information to the medulla regarding the level of oxygen and carbon dioxide, and the neurons in the medulla respond to the oxygen need of the body by increasing the respiration.

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How do we recognise objects? How do we recognise a person or understand something as a dog, or a table or a building or a tree when we see them? How to we understand a sound as dog barking, or a child crying or a bell ringing or specific music playing when we hear them? How do we understand some thing as salty, sweet, sour or bitter when we taste them, how do we understand some thing as cold, hot, smooth or rough when we touch them? 

Before we answer these questions let us understand why we need to understand and recognise these things when we encounter them. This is because with out our ability to recognise them, we will cease to exist. They are essential for our survival. For example, if we fail to recognise the sound of a wild animal or the hot sensation when we accidentally touch fire or we do not recognise some fruit as poisonous, our life will be in danger.

Now coming back to the question how we recognise an object, the answer lies with our nervous system i.e., the neural network with in us. When we encounter with an object, a network of neurons carries the information to the brain and the brain recognise the object after a set of specialised neurons process this information. Based on a specific neural network pattern, a mental representation of the object takes place in the brain and if that mental representation matches with the stored object in our memory, we recognise the object. Specific neural network pattern in this case is a pattern of connection between neurons in the network, pattern of processing of information at each neuron in the network and the pattern of the force, frequency and the speed of sending the output information by one neuron to the other neurons in the network.

In the example of recognising a face, as soon as we see someone, photoreceptors in our eye which are the end points of the dendrites of the sensory neurons carry the image of the face to the interneurons in the brain. Feature extraction takes place at this stage and the face is broken down to various broad features like the hair, forehead, temple, eyebrow, eye, ear, nose and mouth. Each of this feature is further broken down into sub features like the eye is broken down into eyelid, pupil. Iris and sclera. Each of this sub feature is further broken down into minute sub-sub feature like the pupil is broken down into edges, corners, colours, textures etc. 

Specialised neurons in the first layer with already having the memory of the sub-sub feature from the earlier encounter of the face will capture the corresponding sub-sub feature from the incoming image. These first layer of neurons will transmit the sub-sub features like the edges, colour, texture etc. of the pupil in the eye to the 2nd layer of neurons and the neuron holding the complete pupil in the 2nd layer will respond and receive all the different parts of the pupil from the first layer. Then the 2nd layer of neurons will transmit the various sub features like pupil, eyelid, iris etc to the 3rd layer of neurons. 

Specific neuron in the 3rd layer holding one of the complete features of the face like the eye will receive all the different parts of the eye from the 2nd layer. Now the 3rd layer of neurons will transmit the different features of the face like the eye, nose, ear etc. to the 4th layer. A specific neuron in the 4th layer having the complete image of the face in its memory from earlier encounter will respond and receive all the features of the face from the 3rd layer. That specific neuron in the 4th layer will combine all the incoming feature like the eye, ear, nose etc. and if the combined features match to the stored image of the face in the neuron, we will recognize the face. 

All these steps happen in such high speed (like in milliseconds) that we do not notice them. In just a fraction of a second, visual information from the retina flows through the ventral visual stream to the brain’s inferior temporal cortex where neurons contain the necessary information required to classify the objects. Neurons at each stage in the ventral visual stream process different type of information. Ventral visual stream is built in a hierarchical way where early-stage neurons respond to simple features and the later stage neurons responds to complex objects. V1 is the first stage in the pathway that contains neurons that respond to simple features like lines and edges and as we move along the pathway, more complex neurons respond to complex features and objects.

Neurons in our auditory cortex, olfactory cortex, gustatory cortex use the same principle of feature extraction and feature consolidation for recognising sound, smell and taste in the same way neurons in the visual cortex use this principle to recognise visual objects as explained in this example.

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Now let us take one example of how our biological neural network works when we are taking a basic voluntary decision. Suppose you along with your friends have gone to a restaurant and you are deciding what to order i.e., sandwich or pizza. You think for a moment and order pizza. From the outset it looks like a simple process. But this decision and subsequent action is the output of an overly complex neural network (i.e., our nervous system) where thousands of neurons interacted with each other in an extraordinarily complex manner to deliver this decision. Various components of the neural network and how they have worked in the above decision is given below:

Overall structure-

There are three layers in this neural network, Input layer which receives inputs (i.e., stimuli) from the internal body parts and from the external world through our sense organs, processing layer consisting of the neurons in the peripheral nervous system and the central nervous system which process those inputs to arrive at a decision and the output layer consisting of our muscles who execute the decision. 

Sensory receptors-

In this example, our neural network received the inputs from the following stimuli using separate sensory receptors specifically designed for each stimulus.

· Menu card - Photoreceptors in your eyes captured the image of the sandwich and the pizza and send the signals to the brain.

· Weather - Thermoreceptors in your skin captured the cold sensation from the cold weather in the winter and send the signal to the brain.

· Ambiance - Photoreceptors in your eyes captured the party ambiance and decor in the restaurant and send those signals to the brain.

· Music - Mechanoreceptors in your ear captured the disco music playing in the restaurant and send the signal to the brain.

· Smell - Chemoreceptors in your ear captured the junk food smell in the restaurant and send the signal to the brain.

· Hunger - Mechanoreceptors in your stomach captured the low level of food in your stomach and send the feeling of hunger to the brain.

Sensory Neurons-

Sensory receptors (i.e., the end points of the dendrites of the sensory neurons) converted the above stimuli into nerve signals and carried the message to the concerned cell body of the sensory neurons located in the spinal cord. Assuming the signals are strong, these neurons have fired in this example and the messages are carried through the axons to the interneurons in the spinal cord.

Synapse-

At the synapse (i.e., gap junction), electrical signals from the sensory neurons has been converted into chemical signals through release of neurotransmitters to the interneurons located in the spinal cord. Since you are in the process of choosing an option, these neurotransmitters are excitatory in nature which has excited the interneurons to transmit the signals to the next neuron.

Interneurons-

Interneurons in the spinal cord has carried the signals to the brain for connecting with other neurons in the brain to execute the “intelligence process” for arriving at a decision (i.e., to choose between the sandwich and the pizza). This intelligence process is executed by the brain through connecting neurons with each other forming multiple neural circuits and connecting the circuits with each other forming a large scale neural network. 

Neurons in this network received the information from the presynaptic neurons, processed them and passed the output to the post synaptic neurons and these steps continued till the decision is arrived. Human intelligence system has two parts i.e., “intelligence drivers” which influence our intelligence and the “intelligence process” which builds, activates and applies these drivers to arrive at a decision.

Our intelligence drivers are four types i.e., our memory, beliefs, motives and emotions. As soon as we come across any stimulus, different connections between neurons and the distinct communication between them activates our different memory, beliefs, motives & emotions and all of these drive our decision making through a network of neurons communicating with each other. 

There is a still a higher-level driver i.e., our “ego” which drives the above four drivers. Our ego which is the sense of self or identifying self as different from others, classifies all our memory into good or bad based on how it has affected our self. With respect to beliefs, our ego makes us belief something as true or false based on what our self has understood as true or false irrespective of the reality. Regarding motives, all our desires or motives i.e., what we want to achieve are all driven by our ego i.e., they are based on all those things which we want to satisfy our self. Lastly our emotions are also driven by our ego. All our feelings like happiness, sadness, disgust, anger and fear are based on how something has affected our self. 

Finally, our ego is driven by our “consciousness” which is at the highest level. When we are unconscious, there is no ego i.e., no sense of self and hence there is no memory, no beliefs, no motives and no emotions. When we are in the sub conscious stage, we have sense of self or ego in the background and all our memory, beliefs, motives and emotions are driven subconsciously by our sense of self. When we are conscious our sense of self comes to the forefront and we consciously intervene in our memory, beliefs, motives and emotions to decide what is best for our self. At the super conscious stage, our ego disappears, and our sense of self also disappears. We do not identify ourselves with the body rather we identify ourselves with our soul and we identify ourselves with the soul inside all other living beings. At this moment all our memories, beliefs, motives and emotions do not exist.

Coming back to our example, assuming you are in sub the conscious stage where your ego is working in the background, let us see how our memory, beliefs, motives and emotions have driven our intelligence process to reach to a decision. Our intelligence process has four sub processes i.e.,  observation, diagnosis, prediction and decision making. While observation makes us understand what has happened, diagnosis makes us understand why it has happened, prediction makes us understand what is going happen and decision making makes us understand what needs to be done. In this example, there is no requirement of diagnosis and prediction and hence only observation and decision making takes place.

Observation - During this process, interneurons in your brain interpreted the signals received from the sensory neurons and understood the objects in the menu as sandwich and pizza, the weather as cold, the ambiance as energetic, the music as disco music, the smell as junk food smell and the level of hunger as high. 

Network of neurons in the brain, through receiving, processing and communicating messages to other neurons extracted the detailed features and consolidated them using multiple layers to finally make a mental representation of the objects. Specialised neurons then compared these mental representations with the objects stored in other neurons as memory and when there was an exact match or near match we remembered and understood the objects.

Decision Making - In this process, neurons in your brain formed multiple circuits connected with each other in a larger network to activate and apply the associated memories, beliefs, motives and the emotions to arrive at a decision. 

· Memory - You have remembered the last time you had sandwich here and they were fresh & tasty. Using the calculation process stored in your memory, you have calculated the total cost of sandwich and the total cost of pizza for all your friends, and you understood sandwich will be much cheaper than pizza. Based on your memory, you preferred sandwich.

· Beliefs - Based on your experience you believed that pizza is unhealthy. However, you also  believed that your friends would enjoy pizza more than sandwich. In these contradictory beliefs, the neurons in the brain who are in the process of choosing between the two would access the force of the signals received from the other neurons who are holding those beliefs and the signals with the highest force would be considered. In this case assuming the 2nd belief is stronger than the Ist one, based on your beliefs you preferred pizza.

· Motives - Your motive is to stay healthy and look good, hence sandwich is a better option. On the other hand, your motive is to impress upon your friends and pizza is a better choice. Your motive now is also to eat something heavy since you are feeling  hungry, and pizza again is a better choice.  As the force of your motives associated with pizza is higher, based on this dimension, you preferred pizza.

· Emotions - Party ambiance and disco music in the restaurant has created a feeling of excitement and happiness in you. In this mood when pizza came to your mind, it has increased your excitement. Also in this cold weather, thinking about hot pizza, has increased your excitement further. Based on your emotions, you preferred pizza over sandwich.

Finally with a complex network of neurons in your brain, where thousands of neurons processed and communicated information to each other, executed the above steps in milli seconds and decided on ordering pizza since pizza was the most preferred choice. This decision and the subsequent effects also went on to build our memory, beliefs, motives and emotions further which will influence the next time we are trying to take a similar decision.

Motor neurons-

Once your brain decided on ordering pizza, the message is communicated by the upper motor neurons (located in the cerebral cortex in the brain) to the lower motor neurons located in the spinal cord, through the inter neurons. Lower motor neurons whose axons extend to the neuromuscular junctions carried the message to the muscles for execution of the actions.

Neuromuscular Junctions & Muscles -

Once the lower motor neurons carried the message to the neuromuscular junctions (i.e., a chemical synapse between a motor neuron and a muscle fibre), the signals are transmitted to the muscles in your tongue, lips and jaw to move in a particular way and make the vocal cord vibrate so that spoken words are produced and you ordered the pizza.