The nervous system is the primary regulatory system in animals, necessary for survival and maintaining homeostasis. It works in tandem with the endocrine system to coordinate and integrate the activities of the organs and regulate physiological processes, allowing them to function synchronously. The endocrine system provides a slower, more long-lasting regulation, while the nervous system responds quickly, but for a shorter duration.

Table of Contents

Human Nervous System

Main Parts Of A Neuron

Neural Communication

Conduction Of A Nerve Impulse

Polarised Membrane and Resting Potential

The nervous system of all animals is composed of neurons, which are the structural and functional unit of the nervous system. This system provides an organised network of point-to-point connections for quick coordination.

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Human Nervous System

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The human nervous system has two parts:

1. Central Nervous System

2. Peripheral Nervous System

3. The Central Nervous System: It is a site where the received information is processed, integrated, and then used to generate an action or response by effectors.

4. Brain

5. Spinal Cord

6. The Peripheral Nervous System:

   • All the nerves associated with the CNS.
   • There are two types of nerves present.

7. Efferent Nerve Fibres: Transmits nerve impulse from CNS to organs or tissues.

8. Efferent Nerve Fibres: Transmits impulses from the Central Nervous System (CNS) to peripheral organs or tissues.

The PNS can be divided into two types based on the organs/tissues to which it transmits nerve impulses:

1. Somatic Nervous System

2. Autonomic Nervous System

3. Somatic Nervous System: Impulse is transmitted from the Central Nervous System (CNS) to Skeletal Muscles.

4. Autonomic Nervous System: Impulses are transmitted from the Central Nervous System to smooth muscles and involuntary organs of the body.

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Neuron: Structural and Functional Unit of the Nervous System

Neurons are highly specialized cells that receive stimuli and transmit nerve impulses or action potentials, which are electrical signals.

Main Parts of a Neuron

1. Cell Body
2. Dendrites
3. Axon
4. Synaptic Terminals

Cell Body: Contains cytoplasm with a nucleus, cell organelles, and Nissl’s granules. This is where incoming signals are integrated.

Dendrite: Short, highly branched fibres that project outward from the cell body. They are specialized to receive stimuli and signals to the cell body.

Axon: A single, long fibre that is branched at the terminals. It conducts nerve impulses away from the cell body to another neuron, muscle, or gland.

Axon terminal ends in a synaptic knob, which contains synaptic vesicles that release neurotransmitter chemicals. These chemicals transmit signals from one neuron to another neuron, or from neuron to muscle or gland. The junction between the synaptic terminal and another neuron or effector is called a synapse.

Neurons are divided into three types, based on the number of axon and dendrites present in them:

• Unipolar Neurons
• Bipolar Neurons
• Multipolar Neurons

Multipolar: It has one axon and two or more dendrites, and is found in the cerebral cortex.

• Bipolar Cells: They have one axon and one dendrite and are found in the retina of the eye.

• Unipolar: It has only one axon and is found in the embryonic stage.

Myelin Sheath: Axons of many neurons are surrounded by a series of cells called Schwann Cells. These cells have a plasma membrane which is composed of myelin, a white fatty material. The Schwann Cells wrap their plasma membrane around the axon, forming an insulated covering known as the myelin sheath. There are gaps in the myelin sheath which are referred to as Nodes of Ranvier.

There are two types of axons:

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Myelinated: It is found in spinal or cranial nerves.

Non-myelinated: It is found in both the autonomous and somatic nervous systems.

Neural Communication

Animals receive thousands of stimuli at once, and their survival depends on being able to identify and respond to them correctly. The neural communication process in most animals involves four steps: receiving the stimulus (whether internal or external), transmitting it to the central nervous system, integrating it, and then transmitting it to muscles or glands for a response.

1. Detection: Detection is the process of recognizing a stimulus by neurons or sensory receptors located in sensory organs such as skin, eyes, ear, etc.

2. Transmission: The process of sending signals between neurons, muscles, and glands is known as transmission.

3. Integration of Sensory Information: The process of integration involves sorting and interpreting incoming sensory information and determining an appropriate response.

4. Response: The actual reaction of muscles or glands to the stimulus.

In summary, the sequence of information flow through the nervous system is as follows:

Conduction of a Nerve Impulse

Polarized Membrane and Resting Potential

The Polarised State of a Neuron Membrane in the Resting State is due to the following Reasons:

The difference in the concentration of specific ions across the plasma membrane, within the cell and in the extracellular fluid

Selective Permeability of the Plasma Membrane for Different Ions

At the resting state, the membrane has a permeability to K+ ions that is 100 times greater than its permeability to Na+ ions.

The membrane is impermeable to negatively charged proteins present in the axoplasm.

At the resting state, the concentration of potassium ion (K+) inside the axon in axoplasm is greater than the concentration of sodium ion (Na+) outside the cell.

The cell is said to be polarised due to the electric charge inside the cell being more negative than the charge of the extracellular fluid and membrane.

There is a potential difference across the plasma membrane due to the difference in electric charge across it.

The resting state of the membrane potential is referred to as the “resting potential”

The neuron has a resting potential of -70mV

The diffusion of ions down the concentration gradient is mainly responsible for the resting membrane potential.

Neurons have three types of ion channels:

• Passive ion channels
• Voltage-gated channels
• Chemically activated ion channels

Ion channels and pumps help to sustain the resting potential of neurons.

These pumps require ATP to pump Na+ and K+ against their respective concentration and electrical gradients.

The Sodium Potassium Pump transports 3 Na+ outwards and 2 K+ into the cell.

Check out the EPSP full form here!

Action Potential, Nerve Impulse, and Depolarization

The membrane at site A is depolarised due to the influx of Na+ ions when an electrical, chemical or mechanical stimulus is applied. This results in a reversal of polarity, with the outer membrane becoming negatively charged and the inner membrane becoming positively charged. This response is characteristic of excitable cells, such as neurons.

When a stimulus is strong enough, a rapid large change in membrane potential occurs, resulting in the depolarisation of the membrane to a critical point, known as the threshold level.

The electric potential difference at that site A is referred to as the action potential or nerve impulse.

Only neurons, muscle cells and a few cells of the endocrine and immune systems can generate action potentials, whereas all cells can generate graded potentials.

When depolarisation is greater than -55mV, the threshold level is reached and an action potential is generated.

Propagation of Nerve Impulse and Repolarization

An action potential is self-propagating.

An action potential is an all-or-none response, meaning that no variation exists in the strength of a single impulse. The intensity of sensation is determined by the number of neurons stimulated and their frequency of discharge.

An electrical signal known as an action potential or nerve impulse travels rapidly down the axon into the synaptic terminals.

The membrane at site B, ahead of where the action potential is generated (site A), is polarised, with a negative charge inside and a positive charge outside. This causes the current to flow from A to B on the inner surface and from B to A on the outer surface, reversing the polarity and generating an action potential or nerve impulse at site B. The conduction of this impulse along the length of the axon is the result of a repeated sequence of these steps.

Depolarization is very rapid, so the conduction of nerve impulse along the entire length of an axon occurs in a fraction of a second.

This process of restoring the resting potential is known as Repolarisation. It occurs when voltage activated K+ channels open, resulting in the diffusion of K+ outside the membrane. After a certain period (milliseconds), Na+ channels close, causing the membrane to become impermeable to Na+ again.

The repolarisation of the axon membrane occurs when a wave of depolarisation moves down the membrane, quickly restoring the normal polarised state. This restores the membrane resting potential and renders the membrane once more responsive to further stimulation. In fact, most neurons can transmit several hundred impulses per second.

In summary, conduction of impulse along the axon proceeds as follows:

Neurotransmission Across the Synapse

A synapse is a junction through which a nerve impulse is transmitted from one neuron to another. The membrane of the presynaptic and postsynaptic neuron form the synapse, which can be between two neurons or between a neuron and an effector, such as a neuron and a muscle cell.

At the axon terminal, conduction ends and neurotransmission begins. The neuron sends the signal to other neurons from the axon terminal.

Signals across synapses can be either electrical or chemical.

At the electrical synapse, an electrical signal is generated and at the chemical synapse, neurotransmitters are released.

Electrical Synapse

At electrical synapses, the membrane of pre- and postsynaptic neurons are in very close proximity and form gap junctions (separation of less than 2 nm).

The interiors of the two cells are physically connected by a protein channel.

The transmission of impulse is akin to conduction along a single axon.

Electrical synapses allow ions to pass from one cell to another, resulting in a rapid transmission of an impulse from the presynaptic to the postsynaptic neuron.

Electrical synapse transmits signal much faster than chemical synapse, however they are rare in humans.

Many animals exhibit “tail-flick” escape responses involving electrical synapses, such as the crayfish.

Chemical Synapse

The majority of synapses are chemical synapses.

The synaptic cleft (~20 nm) is a fluid-filled space located between the pre and postsynaptic neuron.

When an action potential reaches the end of the axon, it cannot jump the gap because depolarization is a property of the plasma membrane.

Neurotransmitters are required to convert the electrical signal into a chemical one, which is then used for transmission at the synapses.

When an action potential (impulse) reaches the axon terminal, it triggers the synaptic vesicles containing neurotransmitters to release them into the synaptic cleft.

When an action potential reaches the synaptic terminal, voltage-gated Ca²⁺ channels open, allowing Ca²⁺ ions from extracellular fluid to enter the synaptic terminal. This induces synaptic vesicles to fuse with the presynaptic membrane and release neurotransmitters by exocytosis.

These neurotransmitters bind to specific receptors located on the dendrites, cell body of postsynaptic neurons, or on the plasma membrane of effector cells.

The opening of certain gated ion channels is triggered by this binding, leading to alterations in the permeability of the postsynaptic membrane.

When the postsynaptic neuron reaches its threshold level of depolarisation, it transmits an action potential.

The new potential developed may be either excitatory or inhibitory.

When the depolarisation initiates transmitting a neural impulse it is known as excitatory, whereas when the membrane potential becomes more negative than the resting potential, it is said to be hyperpolarised, which decreases the ability of the neuron to generate nerve impulses and is known as inhibitory.

An excitatory postsynaptic potential (EPSP) is a membrane potential that brings the neuron closer to firing.

Unlike action potentials, postsynaptic potentials are graded responses that vary in magnitude depending on the strength of the stimulus.

Check out this link for the RMP of Skeletal Muscles and Cardiac Muscles.

An inhibitory postsynaptic potential (IPSP) is a potential change in the direction of hyperpolarization of the postsynaptic membrane caused by some neurotransmitter-receptor combinations.

The reuptake process is necessary to quickly repolarise the postsynaptic membrane by removing excess neurotransmitters from the synaptic cleft. This is accomplished by either degrading them into their component parts or transporting them back into the synaptic terminals. The neurotransmitters are then repackaged in vesicles and recycled.

Many drugs, such as antidepressants, inhibit the reuptake of neurotransmitters.

In summary, neurotransmission across synapses involves the following steps:

Neurotransmitters

Many chemicals have been found to act as neurotransmitters, which can be broadly classified into various chemical groups:

Acetylcholine

Released from motor neurons and by some neurons in the brain and autonomic nervous system

Triggers muscle contraction

Excitatory effect on skeletal muscles

Inhibitory Effect on Cardiac Muscles

• The level of Acetylcholine in the brain decreases during Alzheimer’s Disease.

Cholinergic neurons are cells that release acetylcholine.

Biogenic Amines

Catecholamines (norepinephrine, epinephrine, dopamine), serotonin and histamine all belong to the same class.

Adrenergic neurons are neurons that secrete norepinephrine.

Affecting mood, sleep, wakefulness, attention, etc.

Their imbalance has been linked to various disorders, such as anxiety, depression, ADHD and schizophrenia.

Amino Acids

Glutamate is an excitatory neurotransmitter in the brain

Glutamate receptor is the target of several drugs, such as Angel Dust.

Glycine and GABA (gamma-aminobutyric acid) have an inhibitory effect on the spinal cord and brain.

Anxiety-reducing drugs enhance the action of GABA

Neuropeptides

Endorphins and enkephalins act as a neuromodulator.

These bind with opioid receptors and block pain signals

Gaseous Neurotransmitters

Nitric oxide (NO) functions as a retrograde messenger at certain synapses.

It transmits information from presynaptic to the postsynaptic neuron, i.e. the opposite direction.

Carbon monoxide (CO) has been demonstrated to act as a neuromodulator

Also check:

Neuromuscular Transmission Steps

Difference between Synapse and Synapsis

Neural Control and Coordination