Unit 2: Nervous System and Muscle

Neuron A neuron is an electrically excitable cell that receives signals and transfers them in the form of electrical and chemical signals. The neurons are broadly divided into two types: Sensory neuron Motor neuron The sensory neuron is a neuron which responds to stimuli like touch, sound, taste, etc. The neuron which receives signals from the central nervous system (brain + spinal cord) is called a motor neuron. Structure of a Typical Vertebrate's Neuron The structure of a typical vertebrate neuron includes parts like cell body, axon, dendrite. Structure of a Typical Vertebrate Neuron The neurofibrils present in the cell processes are protoplasmic threads, also present in cell processes. The cytoplasm also contains certain prominent granules and a centriole. Cell Processes The neurofilaments/cytoplasm gradually form the cell processes, mostly dendrites and axons. Dendron These are small processes of cyton. These do not contain Nissl's granules and neurofibrils. From each dendron, there arise branching structures called dendrites. These afferent processes conduct the impulse into the neuron. Axon It's a special type of cellular extension arising from the cell body or cyton from a part called axon hillock. It contains neurofibrils, mitochondria, and cytoplasm. It does not contain Nissl's granules. This is an efferent process that receives impulses from cyton. The axon may give certain collateral fibers and may branch. Structure of a Typical Vertebrate Neuron The axon may or may not have the myelin sheath on the myelin sheath; there are two types: myelinated and unmyelinated nerve fibers. If present in brain, spinal cord, cranial nerves, the axon covered with a tubular myelin sheath; it's covered by a sheath called neurilemma. The neurilemma is formed by the cytoplasm of Schwann cells. The flattened nucleated Schwann cells present in the formation of myelin sheath. Types of Nerve Fibers Myelinated/Medullated/White Nerve Fiber: This nerve fiber is smaller and without myelin sheath. The axon is surrounded by a syncytial sheath of Schwann cells with scattered nucleus. The autonomic nerve fibers possess this type of axon. It is present in brain, spinal cord, cranial nerves, and spinal nerves. Non-Myelinated/Gray Nerve Fiber This nerve fiber is smaller and without myelin sheath. The axon is surrounded by a syncytial sheath of Schwann cells with scattered nucleus. The autonomic nerve fibers possess this type of axon. It is present in brain, spinal cord, cranial nerves, and spinal nerves. Neuroglia The neuroglial cells present throughout the brain and spinal cord far outnumber the neuronal cells, which regulate the external chemical environment of the neuron. There are certain macroglial cells: Astrocytes: Stimulates the blood capillaries to develop a tight junction and also provide nutritional support by extracting glucose from the blood vessels. Oligodendrocytes: Help in the synthesis of myelin sheath in the neuron of the central nervous system (CNS). Ependymal cells: These cells are used for production of cerebrospinal fluid. Satellite Cells It provides protection to the cell body and nourishment. Synapse It is the junction between two neurons. The gap present in between the junction is known as the synaptic cleft. Voltage-Gated Ion Channels These channels open and close in response to changes in membrane potential. It involves the generation and conduction of action potential in the excitation of nerve tissue. The neurons are excitable in nature. The conduction of impulses depends upon the distribution of different ions inside and outside of the neural membrane. Resting Membrane Potential The resting membrane potential is the difference in potential between the inside and outside of the neuron membrane. It is maintained by the distribution of ions inside and outside the membrane. The membrane mostly has more K+ leaky channels than Na+ leaky channels. Leakage Channels The leaky channels are randomly open and close. They are always open and constantly alternate. Ligand-Gated Channels These channels open and close in response to certain ligand molecules like neurotransmitters, hormones, and certain ions. For example, the neurotransmitter acetylcholine (Ach) opens the cation channels that allow the Na+ and Ca2+ ions to move inward and the K+ ion to move outward. Mechanically Gated Channels These channels open in response to mechanical stimuli like touch, pressure, tissue stretching, etc. These are found in sensory receptors. Generation of Action Potential in Neurons It is a very rapid change in membrane potential and occurs when there is a stimulus. The potential changes from -70 mV to +30 mV. The resting membrane potential is calculated as -70 mV when there is no impulse. The negative sign represents that there is more K+ ions outside the membrane as compared to inside. At this condition, the membrane is called polarized. This is also due to more Na+ ion concentration outside the membrane at rest. K+ and Na+ ion channels are always open, but some K+ ion channels are open more. Hence, some K+ ions are able to diffuse outside to maintain the resting membrane potential. Threshold Potential The sodium ion channel opens when the membrane potential reaches the threshold potential. Thus, the influx of Na+ ions moves the membrane potential from -70 mV to +30 mV. The potential from -70 mV to +30 mV is called the action potential. Once the neuron reaches the threshold potential, the action potential will follow all or none rule. The neuron follows an all-or-none rule, meaning once the action potential is reached, it will reach its maximum, or it won't occur at all. Depolarization The voltage-gated Na+ ion channels open and allow the Na+ ions to move inside the membrane, and after a certain time, the sodium ion channel becomes inactivated and the influx of Na+ ions stops. Then, the voltage-gated potassium ion channels open, and more K+ ions move outside the membrane, creating a difference between inside and outside, which is called depolarization. Propagation of Action Potential through Myelinated Neuron Once the action potential is generated, it travels from one node of Ranvier to another node of Ranvier through the axon. The mode of conduction is called saltatory conduction and depends upon the positive feedback. The action potential travels along the axon once it is being generated. In myelinated neurons, the action potential propagates more rapidly as compared to non-myelinated neurons. Saltatory conduction is a special type of mode of propagation of impulses along myelinated axons. The region where the myelin sheath covers the exolemma contains few voltage-gated channels, while the region under the myelin sheath contains many voltage-gated channels. Propagation of Action Potential through Myelinated Neuron When an action potential arrives at the first node of Ranvier, the extracellular current depolarizes the membrane to the threshold. This opens the voltage-gated sodium channel at the second node. The flow of the current across the membrane only occurs at the nodes of Ranvier. This has two consequences: The action potential appears to leap from node to node, hence the name saltatory conduction. The impulse travels faster than in unmyelinated neurons of the same length. The opening of sodium gates only at the nodes represents a more energy-efficient mode of conduction because only a small portion of the membrane depolarizes. This requires minimal inflow of sodium and outflow of potassium with each time an action potential travels from one node to another. Propagation of Action Potential through Non-Myelinated Neuron In non-myelinated neurons, the axon does not have the myelin sheath. Hence, there is no node of Ranvier. Saltatory conduction is not possible. The action potential travels continuously throughout the length of the axon. Hence, there are no nodes of Ranvier. Factors Affecting the Propagation of Action Potential The speed of propagation of action potential is affected by a major factor like: Concentration of myelin sheath Diameter of the axon Temperature Concentration of Myelin Sheath The amount of myelination affects the speed of conduction of the nerve fibers. Myelin sheath is present, the conduction is much faster than when myelin sheath is absent. Diameter of Axon The axon having more diameter propagates the action potential faster than the axon of small diameter because of the large surface area. Temperature The propagation of impulse is slower at lower temperatures. Synapse It is the junction between the terminal of one neuron and the dendrite of another neuron. Generally, this junction is called the neuromuscular junction. Synapses are essential for homeostasis because they allow information to be filtered into the neuron. The neuron which gives the signal is called the presynaptic neuron, and the neuron which receives the signal is called the postsynaptic neuron. The gap between the presynaptic and postsynaptic neuron is called the synaptic cleft. Electrical Synapse In an electrical synapse, the action potential can conduct directly between each neuron through gap junctions. Each gap junction contains a large number of tubular connexons. The electrical synapse conducts the impulse through gap junctions and is mostly found in visceral smooth muscle, cardiac muscle, and the central nervous system. In developing embryos, the electrical synapse has two main advantages: Faster communication because the action potential travels directly through the gap junction, which is very less as compared to chemical synapses. Synchronization: The electrical synapses can synchronize the activity of a group of neurons. Types of Synapses Based on the synaptic transmission, there are two types: electrical synapses and chemical synapses. Electrical Synapse Electrical synapses directly transfer action potentials between neurons through gap junctions. A large number of gap junctions can produce action potentials in a group of muscle fibers together.