The nervous system is the principal regulatory system in animals, which is required to survive and maintain homeostasis. The nervous system along with the endocrine system jointly coordinate and integrate all the activities of the organs and regulate physiological processes so that they function in a synchronised manner. The endocrine system provides relatively slow and long-lasting regulation, whereas the nervous system responds rapidly, but briefly.
Table of Content
- Human Nervous System
- Main Parts Of A Neuron
- Neural Communication
- Conduction Of A Nerve Impulse
- Polarised Membrane And Resting Potential
The neural system provides an organised network of point-to-point connections for quick coordination. The nervous system of all animals is composed of special cells known as neurons, which are the structural and functional unit of the nervous system.
Human Nervous System
The human nervous system has two parts:
- The Central Nervous System: It is a site where the received information is processed and integrated for the action or response by effectors
- Spinal Cord
- The Peripheral Nervous System: All the nerves associated with the CNS. There are two types of nerves present
- Afferent nerve fibres: transmits nerve impulse from organs or tissues to CNS
- Efferent nerve fibres: transmits impulses from the CNS to peripheral organs or tissues
The PNS is divided into two types based on the organs/tissue it transmits the nerve impulse to. These are:
- Somatic nervous system: Impulse is transmitted from CNS to skeletal muscles
- Autonomic nervous system: Impulse is transmitted from CNS to smooth muscles and involuntary organs of the body
Neuron: Structural and Functional Unit of Nervous System
Neurons receive stimuli and transmit neural signals. The neuron is a highly specialized cell that transmits an electrical signal called nerve impulses or action potential.
Main Parts Of A Neuron
- Cell body: cytoplasm with a nucleus, cell organelles, Nissl’s granules. The cell body integrates incoming signals
- Dendrite: short, highly branched fibres that project outwards from the cell body. They are specialised to receive stimuli and signals to the cell body
- Axon: single long fibre, branched at the terminals. Axon conducts nerve impulses away from the cell body to another neuron, muscle or gland.
- Axon terminal ends in a synaptic knob. Synaptic knob contains synaptic vesicles, that release neurotransmitter, chemicals that 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. These are:
- Multipolar: It has one axon and two or more dendrites. It is found in the cerebral cortex
- Bipolar: It has one axon and one dendrite. It is found in the retina of the eye
- Unipolar: It has only one axon. It is found in the embryonic stage
- Myelin sheath: Axon of many neurons are surrounded by a series of cells called Schwann cells. The plasma membrane of these cells contains myelin, a white fatty material. Schwann cells wrap their plasma membrane around the axon, forming an insulated covering called the myelin sheath. Gaps in the myelin sheath are known as nodes of Ranvier.
There are two types of axons:
- Myelinated: It is found in spinal or cranial nerves
- Non-myelinated: It is found in autonomous and somatic nervous systems
An animal receives thousands of stimuli simultaneously. The survival depends on identifying and responding to these stimuli effectively. In most of the animals, neural communication involves four processes. Whether a stimulus originates externally or internally, information must be received, transmitted to the CNS, integrated and transmitted to muscle or glands to carry out some action, the actual response.
- Reception: Reception is the process of detecting a stimulus by neurons or sensory receptors present in sensory organs like skin, eyes, ear, etc.
- Transmission: Transmission is the process of sending signals to and fro from a neuron to another neuron or from neuron to muscles or glands
- Integration: Integration involves sorting and interpreting incoming sensory information and determining the appropriate response
- Action or response: The actual response to the stimulus by muscles or gland
In summary, information flows through the nervous system in the following sequence:
Conduction Of A Nerve Impulse
Polarised Membrane And Resting Potential
- In the resting state, when a neuron is not conducting an impulse, the neuron membrane is in the polarised state. This is due to the following reasons:
- The difference in the concentration of specific ions across the plasma membrane, inside the cell and in extracellular fluid
- Selective permeability of the plasma membrane for different ions
- At the resting state, the membrane has 100 times more permeability to K+ ions as compared to Na+
- The membrane is impermeable to negatively charged proteins present in the axoplasm
As a result, at the resting state, potassium ion (K+) concentration inside the axon in axoplasm is more compared to outside the cell and sodium ion (Na+) concentration is more outside
- The electric charge inside the cell is more negative than the charge of the extracellular fluid and membrane is said to be polarised
- Due to the difference in electric charge across the plasma membrane, there exists a potential difference across the plasma membrane
- The membrane potential at the resting state is called resting potential
- The neuron has a resting potential of -70mV
- The resting membrane potential depends mainly on the diffusion of ions down the concentration gradient
- Neurons have three types of ion channels: passive ion channels, voltage-gated channels and chemically activated ion channels
- Ion channels and pump maintain the resting potential of neurons
- These pumps require ATP to pump Na+ and K+ against their concentration and electrical gradients
- Sodium potassium pump transports 3 Na+ outwards for 2 K+ into the cell
Action Potential, Nerve Impulse and Depolarisation
Neurons are excitable cells. When an electrical, chemical or mechanical stimulus is applied at a site A, voltage-activated Na+ channels open, increasing the permeability of the membrane for Na+ ions. There is a rapid influx of Na+ inside the neuron, which results in the reversal of polarity at the site A, i.e. the outer membrane becomes negatively charged and the inner membrane becomes positively charged. The membrane is said to be depolarised.
When a stimulus is strong enough, a rapid large change in membrane potential occurs, depolarising the membrane to a critical point known as the threshold level.
The electric potential difference at that site A is called the action potential or nerve impulse.
All cells can generate graded potentials, but only neurons, muscle cells and a few cells of the endocrine and immune systems can generate action potentials.
When depolarisation is greater than -55mV, the threshold level is reached and an action potential is generated.
Propagation of Nerve Impulse and Repolarisation
An action potential is self-propagating.
An action potential is an all-or-none response and no variation exists in the strength of a single impulse. The intensity of sensation depends on the number of neurons stimulated and on their frequency of discharge.
Action potential or nerve impulse is an electrical signal that travels rapidly down the axon into the synaptic terminals.
At site B, ahead of where the action potential is generated (site A), the membrane is polarised, i.e. negatively charged inside and positively charged outside so the current flows from A to B at the inner surface and from B to A on the outer surface. This results in the reversal of polarity and the action potential or nerve impulse is generated at site B. The conduction of impulse throughout the length of the axon is the result of a repeated sequence of these steps.
Depolarisation is very rapid so the conduction of nerve impulse along the entire length of axon occurs in a fraction of second.
After a certain period (milliseconds) membrane again becomes impermeable to Na+ as Na+ channels close. Voltage activated K+ channels open resulting in diffusion of K+ outside the membrane and the resting potential is restored. This is called repolarisation.
A wave of depolarisation moves down the membrane of axon and the normal polarised state is quickly re-established behind, i.e. known as repolarisation. The membrane resting potential is restored and the membrane once more becomes responsive to further stimulation. 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 nerve impulse is transmitted from one neuron to another through junction called synapses. The membrane of the presynaptic and postsynaptic neuron make a synapse. Synapse can be between two neurons or between a neuron and effector such as neuron and a muscle cell.
Conduction ends at the axon terminals and neurotransmission begins. At the axon terminal, the neuron sends the signal to other neurons.
Signals across synapses can be electrical or chemical.
At the electrical synapse, an electrical signal is generated and at the chemical synapse, neurotransmitters are secreted.
At electrical synapses, the membrane of pre and postsynaptic neuron are in very close proximity and form gap junctions (<2 nm).
The interiors of the two cells are physically connected by a protein channel.
The transmission of impulse is similar to conduction along a single axon.
Electrical synapses let ion pass from one cell to another resulting in rapidly transmitting an impulse from presynaptic to the postsynaptic neuron.
Electrical synapse transmits signal much faster than chemical synapse, but they are rare in humans.
The escape responses of many animals involve electrical synapses. E.g. the “tail-flick” escape response of the crayfish.
The majority of synapses are chemical synapses.
A fluid-filled space between the pre and postsynaptic neuron is called synaptic cleft (~20 nm).
When an action potential reaches the end of the axon, it cannot jump the gap because depolarisation is the property of the plasma membrane.
The electrical signal has to be converted into a chemical one. Neurotransmitters are involved in the transmission at these synapses.
Axon terminal consists of synaptic vesicles containing neurotransmitters. When an action potential (impulse) reaches axon terminal it stimulates synaptic vesicles to release neurotransmitters in the synaptic cleft.
When an action potential reaches the synaptic terminal, voltage-gated Ca2+ channels open. Ca2+ ion from extracellular fluid enters the synaptic terminal inducing synaptic vesicles to fuse with the presynaptic membrane and release neurotransmitters by exocytosis.
These neurotransmitters bind to specific receptors present on the dendrites or cell body of postsynaptic neurons or on the plasma membrane of the effector cells.
This binding triggers the opening of specific gated ion channels, resulting in changes in the permeability of the postsynaptic membrane.
When 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, it decreases the ability of the neuron to generate nerve impulse and known as inhibitory.
Membrane potential that brings the neuron closer to firing is called an excitatory postsynaptic potential (EPSP).
Unlike action potentials, postsynaptic potentials are graded responses.
Some neurotransmitter-receptor combinations hyperpolarize the postsynaptic membrane. A potential change in this direction is called an inhibitory postsynaptic potential (IPSP).
Excess neurotransmitters from synaptic cleft have to be removed in order to quickly repolarise the postsynaptic membrane. It is either degraded into its component or transported back into synaptic terminals, the process known as reuptake. These are repackaged in the vesicles and recycled.
Many drugs such as antidepressants inhibit the reuptake of neurotransmitters.
In summary, neurotransmission across synapses has the following steps:
Many chemicals are found to act as a neurotransmitter. They can be broadly classified into various chemical groups:
- 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
- Acetylcholine level decreases in the brain during Alzheimer’s disease
- Cells that release acetylcholine are known as cholinergic neurons
- Biogenic amines
- Catecholamines (norepinephrine, epinephrine, dopamine), serotonin and histamine belong to this class
- Neurons that secrete norepinephrine are called adrenergic neurons
- Affect mood, sleep, wakefulness, attention, etc.
- Their imbalance has been linked to various disorders, e.g. 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 in the spinal cord and brain
- Anxiety reducing drugs enhance the action of GABA
- Endorphins and enkephalins act as a neuromodulator
- These bind with opioid receptors and block pain signals
- Gaseous neurotransmitters
- Nitric oxide (NO) acts as a retrograde messenger at some synapses
- It transmits information from postsynaptic to the presynaptic neuron, i.e. the opposite direction
- Carbon monoxide (CO) is shown to function as a neuromodulator