Nervous System: Diagram, Function & Diseases

Have you ever wondered what would it be like if humans had no nervous system? It is quite difficult even to imagine such a situation. The humankind would lose its essence and integrity. We would also fail to choreograph our daily activities and not be able to accomplish our pursuits. It is only because of the nervous system, which acts as an ‘Electrical Wiring,’ that we can walk, talk, eat, read, write, sit, memorise, think, and do our daily work. The human nervous system is an intricate network of nerves. The neurons form the basic unit of the nervous system. The nervous system consists of billions of neurons. They are a special type of cells which help in conveying information.

The nervous system forms the major communication and regulatory centre as well as the control unit. The nervous system, along with the endocrine system, regulates homeostasis. Let us talk more about the brain and the associated organs.

Human Nervous System

The human nervous system is divided into the following:

1. THE CENTRAL NERVOUS SYSTEM (CNS)

• It comprises the brain and spinal cord.
• The CNS is the site for processing the information.

2. THE PERIPHERAL NERVOUS SYSTEM (PNS)

• It comprises all the nerve cells which are associated with the CNS.
• There are two types of nerve fibres associated with the PNS; sensory neurons (afferent fibres) and motor neurons (efferent fibres).
• The PNS is further classified into somatic and autonomic nervous system.
• The autonomic nervous system is, in turn, divided into the sympathetic and parasympathetic nervous system.
• The collection of the peripheral nerve cells along with the afferent and efferent axons and support cells are called ganglia.

Neuron: The Basic Structural And Functional Unit

The neurons form the basic structural and functional unit of the nervous system. These nerve cells are critical for the conduction of the nerve impulse. The neurons are amitotic.

A neuron is composed of three major parts; namely, cell body, dendrites, and axon. 

1. CELL BODY

• The cell body has cytoplasm with a nucleus, cell organelles, and granular bodies.
• These granular bodies are called Nissl’s granules.
• There are no centrioles in the cell body.

2. DENDRITES

• They are the extensions of the cell body.
• They are also known as fibres.
• Dendrites receive nerve impulse and transmit it towards the cell body.

3. AXON

• It is also the cytoplasmic extensions of the cell body.
• It is a long fibre and has a branched distal end.
• A bulb-like structure is present at the end of each branch, which is called synaptic knob.
• The synaptic knob has synaptic vesicles.
• These synaptic vesicles contain neurotransmitters.
• The axon is responsible for transmitting a nerve impulse away from the cell body; to a synapse.

Classification on the basis of structure:

1. UNIPOLAR NEURONS
2. BIPOLAR NEURONS
3. MULTIPOLAR NEURONS

Classification on the basis of the presence or absence of myelin sheath:

1. MYELINATED NEURONS: The neurons are surrounded by Schwann cells. These Schwann cells may develop a myelin sheath around the neurons; such neurons are called myelinated neurons. The gaps which extend between myelin sheath is known as nodes of Ranvier. Such neurons are found in spinal and cranial nerves.
2. UNMYELINATED NEURONS: The Schwann cells which do not develop a myelin sheath around the neuron are called unmyelinated neurons. These neurons are found in the autonomous and somatic neural system.

Classification on the basis of function:

1. SENSORY NEURONS: These transmit the nerve impulses from tissues and organs to the spinal cord and brain.
2. MOTOR NEURONS: These neurons are responsible for transmitting the nerve impulses from the brain and spinal cord to the muscles and glands.
3. INTERNEURONS: The interneurons connect one neuron to another. They transmit signals from sensory neurons and other interneurons to the motor neurons.

Nerve Impulse: Generation & Conduction

The membrane of the neurons is polarised. This can be because of the ion channels which are present on the membrane of the neurons. These ion channels are selectively permeable to different ions. Therefore, the neurons are excitable.

I. When the neuron is in a state of rest (not conducting any impulse):

Because of the varying concentration of the ions inside the cell and in the extracellular fluid, a concentration gradient is formed. Whenever the membrane is at rest, the sodium-potassium pump is responsible for maintaining this concentration gradient. This sodium-potassium pump actively transports the ions.

As a result of the active transport of ions, the membrane of the neuron gets polarised.

RESTING MEMBRANE POTENTIAL: The voltage difference across the membrane of the cell when it is in a state of rest (not conducting an impulse).

II. When the membrane is in a state of electrical activity (conducting an impulse)

The cell membrane becomes freely permeable to Na+ at the site where a stimulus is applied; and

• there is an influx of sodium ions,
• reversal of polarity, &
• the membrane is depolarised (depolarization).

ACTION POTENTIAL & NERVE IMPULSE: The short-term change in the electrical potential difference; at the site where the stimulus is applied is called an action potential. This is what is termed as “NERVE IMPULSE.”

Now, the site which is immediately ahead is still in a state of rest. This means it has a more positive charge on its outer membrane and more negative charge in the cytosol. Henceforth, the current flow from:

• Inner Surface– from the site of the stimulus to the site immediately ahead.
• Outer Surface– from the site immediately ahead, back to the site of the stimulus.

THE SODIUM-POTASSIUM PUMP: The sodium-potassium pump is a type of active transport (requires energy/ATP) which transports ions/molecules against their concentration gradient. This pump utilises ATP for giving out three sodium ions and taking in two potassium ions.

• A carrier protein is required. Since carrier proteins are employed to transport ions against their concentration gradient, therefore, these carrier proteins are known as pumps.
• Three sodium ions bind to protein pump (present inside the cell). The shape of the protein pump, now, changes after it derives energy from ATP. As soon as it changes its shape, it moves three sodium ions outside the cell.
• The protein pump faces outwards as it releases sodium ions. Now, two potassium ions bind to the pump and are transported inside the cell.

Nerve impulse: Transmission

A synapse is a point of connection and communication between two neurons. It can also be the point of communication between a neuron and a muscle or gland. The word ‘synapse’ is derived from the word ‘syn’; ‘syn’ means together. A synapse has dendrites of one axon and cell body of another axon. The nerve impulse (or information) is carried from the first neuron, called presynaptic neuron, and received by another neuron, called the postsynaptic neuron. The presynaptic and the postsynaptic neuron may or may not be separated by a gap which is known as the synaptic cleft.

There are two types of synapses;

1. ELECTRICAL SYNAPSES

• The electrical synapse is a gap junction.
• These gap junctions have connexon channels which are placed into the plasma membrane.
• The connexon channels have connexin pores which allow ions and signalling molecules to pass directly from one cell to another.
• The membranes of presynaptic and postsynaptic neurons are very close to each other.
• The conduction of impulse is faster in electrical synapses as compared to the chemical synapses.
• The electrical current moves directly through these synapses.
• They are rarely present in humans.

2. CHEMICAL SYNAPSES

• The chemical synapses transmit information through the help of chemical signals (neurotransmitters).
• The membranes of the presynaptic and the postsynaptic neurons are separated by a gap known as synaptic cleft.
• All chemical synapses have membrane-bound organelles called synaptic vesicles; at the presynaptic terminals.
• The synaptic vesicles are filled with neurotransmitters.
• Most of the central nervous system synapses are chemical in nature.

SYNAPTIC NERVE TRANSMISSION

• The arrival of the nerve impulse at the axon terminal triggers the movement of the synaptic vesicles toward the plasma membrane. The vesicles fuse with the plasma membrane.
• After their fusion, the neurotransmitters are released from the presynaptic neurons into the synaptic cleft.
• The neurotransmitters which are released bind to their specific receptors; present on the membrane of the postsynaptic neurons.
• The binding of the neurotransmitters opens ion channels.
• The opening of the ion channels permits the entry of the specific ions. Therefore, a new action potential is generated in the postsynaptic neuron.
• The action potential generated may be excitatory or inhibitory.

The Reflex Arc

The sudden response to a stimulus is known as the reflex action. It occurs involuntarily and requires the involvement of only a part of the central nervous system. There are different types of neurons which work together in a reflex action;

• Receptor- afferent/sensory neuron
• Effector- efferent/motor neuron

Steps involved in the reflex arc:

1. The receptor detects the signal and transmits it to the relay neurons. The relay neurons are located in the spinal cord (part of CNS).
2. The efferent neurons transmit signals to the effector organ. The effector organ produces a response.

The whole process involved in the reflex action is called a reflex arc.

Some of the examples of the reflex action are:

• Withdrawl of hands from a pin.
• Moving your hands away from a hot object or fire.
• Knee-jerks.

Diseases

Some of the diseases which occur because of the anomalies in the nervous system are:

1. Epilepsy: Seizures occur because of the abnormal electrical discharges from neurons.
2. Parkinson’s Disease: A neurodegenerative disorder which affects motor skills.
3. Multiple Sclerosis: The myelin sheath which insulates the neurons is affected.
4. Huntington’s Chorea: An inherited brain disorder which results in the death of the nerve cells.
5. Alzheimer’s disease: A chronic, neurodegenerative and progressive disease which interferes with behaviour, memory, and thinking.

Quick Revision in 5 Points

1. The nervous system is divided into two branches: CNS (central nervous system) & PNS (peripheral nervous system).
2. The neuron is the basic structural and functional unit of the human nervous system.
3. The neurons are classified into different types depending upon their structure, function, and presence of myelin sheath.
4. The neurons are excitable; therefore, the generation and conduction of the nerve impulse are possible.
5. The synapse is the point of communication between two neurons.

Based on the above topic, let’s have a short quiz!