The nervous system works through an interconnected network of billions of neurons. These neurons transmit information in the form of nerve impulses, across the nervous system and thus, coordinate the various functions of the body.
The human nervous system is a highly specialized network, that contains billions of neurons, and is responsible for controlling and coordinating all the functions of the body. This system enables us to communicate with the outside world and it consists of two components, the central nervous system (CNS) and the peripheral nervous system (PNS).
The central nervous system includes the brain and the spinal cord, while the peripheral nervous system consists of all the neurons of the body, excepting those found in the brain and the spinal cord. The human nervous system is concerned with receiving information from the outside world, processing it, and then generating appropriate responses. It is a network that controls and coordinates all the activities of the body, by transmitting messages or signals from the brain to the different parts of the body and vice versa.
How does the Nervous System Work?
The way the nervous system works is truly unique and complex. It works through a complex network of neurons, which are the basic functioning cells of the nervous system. The neurons conduct the signals or impulses between the two components of the nervous system, i.e., the central and the peripheral nervous system. There are mainly three types of neurons, sensory neurons, motor neurons, and interneurons.
The sensory neurons transmit the stimuli or impulses received from the sensory organs, like the eyes, nose or skin, to the central nervous system, i.e., to the brain and the spinal cord. The brain in turn, processes these stimuli and sends them back to other parts of the body, telling them how to react to a particular type of stimulus. The motor neurons are responsible for receiving signals from the brain and spinal cord, and sending them to other parts of the body.
On the other hand, the interneurons are concerned with reading the impulses, received from the sensory neurons and deciding the responses to be generated. They are mainly found in the brain and the spinal cord. Apart from neurons, the nervous system also contain glial cells, which support and nourish the neurons. The neurons use electrochemical signals, or neurotransmitters for transmitting impulses from one neuron to another. However, the transmission of impulses from one neuron to another is not as simple as it sounds. So, let’s find out how exactly a neuron sends impulses to another neuron.
Transmission of Nerve Impulses
The human nervous system contains billions of nerve cells and about 86 billion of them are found in the brain alone. Each neuron has a cell body, from where numerous branch-like projections emerge, which are known as dendrites. The dendrites usually look like the branches of a tree. At the opposite end of the cell body, a long, slender projection can be found, which is known as axon. The dendrites pick up impulses in the form of electrical signals from other neurons, which are then passed down the axon to another neuron or cell.
The axons of most of the neurons are covered by a myelin sheath, which insulates the nerve cells and speeds up the transmission of nerve impulses. Some axons can travel up to one meter or more in human body, before branching off at the end. The branches arising from an axon are slightly swollen at the tip, and these swollen tips are known as, synaptic knobs or terminal button. For transmitting impulses, neurons form a specialized structure called synapse, with other neurons and cells of the body. A synapse typically serves as the junction, where impulses or information can flow from one neuron to another.
There are basically three elements of a synapse, a presynaptic membrane of the signal-passing neuron (which can be usually found in the synaptic knob of an axon), the postsynaptic membrane located in a dendrite or a target cell, and a synaptic cleft, which is the space between the presynaptic and the post synaptic membranes. There are primarily two types of synapses, chemical synapses and electrical synapses, and both of them vary in the way they send impulses from one neuron to another.
In a chemical synapse, the transmission of nerve impulses takes place with the help of neurotransmitters. In its resting state, a neuron maintains a small voltage difference across its membrane. The inside of the membrane is negatively charged, while the electrical charge outside the membrane is positive, when the neuron is not stimulated by any impulse. This difference in electrical potential or voltage, between the inside and outside of a cell is known as, membrane potential.
When an impulse reaches the synapse of a presynaptic neuron, it changes the membrane potential of the neuron, which causes calcium channels in the presynaptic membrane to open. The opening of these channels allows calcium ions to enter through the presynaptic membrane and thus, increase the concentration of these ions in the cytoplasm. This induces the synaptic vesicles to fuse with the plasma membrane of the presynaptic neuron, and release neurotransmitter molecules into the synaptic cleft. The neurotransmitter molecules then diffuse and float across the synaptic cleft, to bind to the receptors embedded in the plasma membrane of the postsynaptic cell or neuron.
The process of binding neurotransmitters to the receptors, triggers an electrical response in the postsynaptic neuron and changes its membrane potential, which carry the impulse forward until it reaches its destination. Some neurotransmitters can excite the postsynaptic neuron, while others can inhibit its activity. The neurotransmitter molecules get removed from the receptors due to thermal shaking. They are either broken down, or reabsorbed by the presynaptic cell.
In electrical synapses, the impulses are transferred from one neuron to another neuron or cell, through a specialized intercellular connection, known as gap junction. Here, a direct connection is established between the cytoplasm of the two cells, so that various ions and molecules can travel easily from one cell to another. Nerve impulses can also flow from one neuron to another through the gap junction. Basically, the process begins when an impulse travels along the presynaptic cell and causes voltage changes in that cell. This allows the charged ions to travel through the gap junction and induce voltage changes in the postsynaptic cell, and thus, carry the nerve impulses.
The Central Nervous System
The human brain is the site, where the nerve impulses coming from different parts of the body are processed, in order to generate appropriate responses. The human brain can be divided into three parts, forebrain, midbrain, and hindbrain. The forebrain consists of the cerebrum, which is the largest part of the brain and is the center of memory, thought, emotion, reasoning, problem solving, planning, intelligence, movement and orientation, perception of stimuli, speech, and the ability to feel.
The outer layer of cerebrum is cerebral cortex, which is concerned with thinking, producing and understanding languages, and receiving information collected by the sensory nerves and sending them to other regions of the brain, for further processing. The inner part consists of the thalamus, hypothalamus, and the pituitary gland. The thalamus transmits sensory and motor signals to the cerebral cortex, while the hypothalamus is responsible for regulating pulse, body temperature, appetite, secretion of pituitary hormones, and autonomic processes. Pituitary glands secrete hormones that regulate growth, metabolism, sexual maturity, and response to stress.
The midbrain is located underneath the middle of the forebrain, and is the coordinator of the messages or impulses coming in and out of the brain. On the other hand, the hindbrain, which is located below the back end of the cerebrum, consists of the cerebellum, the pons, and the medulla oblongata. The cerebellum processes signals coming from other parts of the brain and controls movement, posture, and balance, while the pons and the medulla coordinate messages and autonomic functions, like breathing, heart rate, digestion, and blood pressure.
The other component of the central nervous system is the spinal cord, which is a long and cylindrical bundle of nerve tissues, that extends from the medulla oblongata of the hindbrain to the lower back. Spinal cord is the main pathway, along which information or nerve impulses travel back and forth, between the brain and the peripheral nervous system.
The Peripheral Nervous System
The peripheral nervous system can be divided into somatic nervous system and autonomic nervous system, depending on their specific functions. The somatic or voluntary nervous system receives external stimuli and coordinates body movements, while the autonomic or involuntary nervous system is responsible for those functions, that are not under conscious control, such as heartbeat and metabolic processes.
The autonomic nervous system can again be classified into sympathetic, parasympathetic, and enteric division. The sympathetic nervous system responds to stress or anxiety, while the parasympathetic nervous system works when a person is sleeping, or when the body is at rest. The enteric nervous system, on the other hand, manages every aspect of digestion.
The nervous system can transmit impulses at the speed of 100 meters per second, and the transmission of impulses across electrical synapses is much faster and energy efficient than chemical synapses. But, the impulses get weaker as they travel from one neuron to another through electrical synapses. On the other hand, chemical signaling is characterized by ‘gain’, which can be described as the ability to transmit an electrical impulse with constant or greater strength. Chemical synapses are more common than electrical synapses.