Human circuit – NEURON
Neuron: It’s a nerve cell that transfers information to other cells through synapses.
Nervous System: The networks of nerve cells and fibers which transmit nerve impulses between parts of the body.
⇒ Nervous System Classification
⇒ Neuron Classification
⇒ Structure of neuron
⇒ Action Potential
Nervous system classification:
The nervous system divided mainly into the central & peripheral nervous systems.
⇒ Central Nervous System (Brain & Spinal Cord)
⇒ Peripheral Nervous System (Bundles of nerve fibers)
⇒ Somatic Nervous System
⇒ Automatic Nervous System
⇒ Sympathetic Nervous System
⇒ Para Sympathetic Nervous System
⇒ Enteric Nervous System
Central Nervous System:
⇒ The brain and spinal cord makes up central nervous system. The brain is protected by the skull whereas the spinal cord by vertebrae.
⇒ This nervous system considered central since all the received information is processed here and corresponding activities are coordinated and performed in every part of the body.
⇒ Both the brain and spinal cord are protected by three membrane layers called meninges.
⇒ The entire CNS is present in the dorsal body cavity which means the posterior surface of the human body.
⇒ This dorsal body consists of a cranial cavity (brain resides) and a spinal cavity (spinal cord reside).
Peripheral nervous system:
⇒ PNS consists of only nerves that branch off from the spinal cord and extend to all parts of the body.
⇒ Its function is to connect CNS to each limb and organ of the body. Essentially it serves the information to the entire body from the brain and spinal cord.
⇒ PNS is divided into the somatic and autonomous nervous system.
⇒ The somatic nervous system (Voluntary nervous system) consists of spinal and cranial nerves. It controls all the voluntary functions of muscles and also reflex response.
⇒ The autonomic nervous system is responsible for maintaining and regulating the unconscious function of the body.
⇒ It has two divisions; Sympathetic (fight or flight response) and Parasympathetic (rest and digest)
⇒ Sympathetic consists of preganglionic and postganglionic neurons for transmission of impulse throughout this system.
⇒ Sympathetic plays main role in the control of fight and flight response during threats and maintains homeostatic.
⇒ Parasympathetic deals with the activities which have been stimulated when the body is at rest such as urine, tears, defecation, saliva, sexual arousal.
⇒ Sympathetic prepares the body for intense action whereas parasympathetic for action when the body at rest.
Enteric Nervous System:
ENS also referred to as the second brain. Here neurons control the function of the gastrointestinal tract.
Neuron division based on structure:
The neuron is divided into three types based on the processes count which extending from the cell body.
Unipolar: It has only one process that extends out from the cell body.
Bipolar: It has two processes that extend in the opposite direction from the cell body.
Multipolar: It has three or more process extends out from the cell body. It comprises more than 99% of total neurons in the human body.
Neuron division based on function:
Sensory neuron (Afferent neuron): Information received by sense organs.
Motor neuron (Efferent neuron): Impulse is passed from the brain or spinal cord to muscle or gland.
Interneuron: Neural circuits which connect sensory neuron to motor neuron.
Neural circuit: The interconnections between neurons are called neural circuits.
Structure of Neuron:
The components of a neuron are listed below.
⇒ Cell body
⇒ Myelin sheath
⇒ Nodes of Ranvier
⇒ Glial Cells
Long slender projection of nerve cell which conducts electric signals.
Axon hillock = A part of the cell body connects to axon
Axon initial segment (AIS) = Generates and shapes action potentials
Axon telodendria = End branches of the axon
Axon terminals = Synaptic boutons or terminal boutons. The distal end of telodendria.
Cell body (Soma):
The spherical part of the neuron contains a nucleus. Cell body which connected to dendrite which gathers information from other neurons and to axon which sends information to other neurons.
The components of the cell body are Nucleus, Cell membrane, Endoplasmic reticulum, Golgi apparatus, Mitochondria, and other cellular components.
Dendrites: Tree-like structure receives information from other neurons.
Myelin sheath: The insulating layer covers the nerves. Allows electrical signals to pass through quickly and efficiently through nerve cell.
Nodes of Ranvier: Gap in the myelin sheath of a nerve, between adjacent Schwan cells(one type of glial cells)
They provide insulation support by surrounding neurons. Types of glial cells are ependymal cells, Schwann cells, microglia, astrocytes, oligodendrocytes, and satellite cells.
Action potential (called excitable cell) occurs in different cell types, such as neurons, muscle cells, glomus cells, and endocrine cells.
Before getting to know Action Potential. Let us know what is MEMBRANE POTENTIAL.
The membrane potential is the difference in electrical charge between in and out of a cell. Also called membrane voltage. Voltage-gated ions are present in the cell membrane.
POTENTIAL OR CHARGE OR VOLTAGE ARE SAME.
RESTING MEMBRANE POTENTIAL:
⇒ Generally, cells maintain an internally negative charge depending on their outer voltage or environment.
⇒ It’s about -70 mV (mV=millivolt) – this means that the inside of the neuron is 70 mV less than the outside.
ACTION MEMBRANE POTENTIAL:
⇒ The action potential is generated by voltage-gated ion channels in the cell by depolarization.
⇒ Depolarization: it is a shift in the electric charge of a cell from being negative to more positive.
⇒ Also called Hypopolarization.
⇒ It is more essential to make cells perform the various function and transmit information between cells.
Terms Related to Membrane Potential:
Depolarization/ Hypopolarization: When an electric charge of the cell becomes more positive and become an action potential.
Repolarization: This occurs after depolarization. Here electric charge moves negative and gets into resting potential state.
Hyperpolarization: In this, membrane potential becomes more negative.
Action potential in neuron:
When an action potential occurs in a neuron, it transmits it from the axon of nerve and reaches till synapses. Here action potential movement means the transmission of data across other neurons to perform a function.
Nerve Impulse or Spike: Action potential which occurs in neurons.
Spike train: Its a sequence of action potential generated by a neuron.
Fire: When neuron emitting an action potential /Nerve Impulse/ spike
An action potential cannot cross synaptic cleft and here neurotransmitter helps to transfer to other neurons.
When an impulse is generated in neuron
Impulse move from the axon and reaches its end synapses
Neurotransmitter helps to transfer the impulse from one neuron to target neuron.
The synapse is the junction at which a chemical or electrical exchange occurs between the presynaptic and postsynaptic cells. Neurotransmitters are chemicals that transmit signals from one neuron to the target neuron across a synapse cleft.
One to one communication between neurons occurs in synapses.
SIGNAL (CHemical Or ELectrical) = NEUROTRANSMITTER
Neurotransmitters: It’s an impulse that is passed in the form of chemical or electrical messages across the synaptic cleft.
Presynaptic membrane: Its the neuron in which action potential is triggered and ready to transfer impulse across synapses. The impulse moves from the cell body through the axon and finally, it reaches synapses in the axon terminal.
Postsynaptic membrane: Its the receiving neuron which is ready with its neurotransmitter receptors to absorb the impulse from the presynaptic membrane.
Synaptic cleft: Space between two structures. between the presynaptic and postsynaptic endings. It is about 20nm wide.
Synaptic terminal: A synaptic terminal is the ending of the presynaptic neuron where the electrical signal is converted to a chemical signal.
Synaptic vesicles: Known as neurotransmitter vesicles which store various neurotransmitters at the axon terminal that are released into the synaptic cleft. Once these vesicles are open, the transmission starts. Also, these are recreated constantly by neurons for each impulse.
How Impulse gets transmitted??
⇒ Impulse reaches the end of a pre-synaptic neuron (or axon terminal), which consists of voltage-gated channels or neurotransmitters in synaptic vesicles.
⇒ Next, synapses vesicles open and release the neurotransmitter and it is scattered across the synaptic cleft.
⇒ Now, voltage ions are absorbed by the neuroreceptor of the postsynaptic membrane.
⇒ And thus action potential is triggered in other neuron and this transmission continues between neurons till it reaches the target function to perform.