Nerve Cells (Neurons): Types, Structure, Action Potential, Functions and Disorders

Introduction to Nerve Cells

Nerve cells, also called neurons, are cells that make up the nervous system.

The main function of nerve cells is to receive, process, and transmit information.

These cells receive signals from different sensory organs or other neurons, process this information, and then transmit signals to target cells, such as other neurons, muscles, or other organs.

This communication is important for different body functions including controlling movement and supporting cognitive processes like memory and learning.

So, nerve cells act as messengers that transmit information from external stimuli to brain which processes information and generates responses to stimuli.

Types of Nerve Cells Based on Function

Nerve cells are classified into three main types based on function.

• 1. Sensory neurons
• 2. Motor neurons
• 3. Interneurons

Sensory Neurons

• Detect and transmit signals from external environment to central nervous system (CNS).
• Gather information from specialized tissues in various parts of body known as sensory receptors.
• When sensory neuron detects signal from receptor, it converts information into electrical impulses, which are then transmitted to CNS.
• Also called afferent neurons.
• Detect wide range of stimuli, including touch, taste, smell, and pain.

Motor Neurons

• Transmit signals from CNS to muscles.
• Initiate and control voluntary and involuntary muscle movements.
• Also called efferent neurons.
• Divided into upper and lower motor neurons:
  • Upper motor neurons transmit signals to interneurons and lower motor neurons.
  • Lower motor neurons receive signals from upper motor neurons or interneurons and convert these signals into actual muscle movements.

Interneurons

• Most abundant type of nerve cells and are only found in central nervous system.
• Act as connectors between sensory and motor nerve cells.
• Receive signals from other interneurons or sensory neurons and transmit them to motor neurons or other interneurons.

Types of Neurons Based on Structure

Nerve cells can also be classified into four types based on structure.

• Unipolar neurons - have single structure extending from cell body which contains one axon with dendrites. Commonly found in invertebrates.
• Bipolar neurons - have two structures – one axon and one dendrite, extending from cell body.
• Multipolar neurons - contain one axon and multiple symmetrical dendrites extending from cell body. This is most common type of neuron.
• Pseudounipolar neurons - have only one process that extends from cell body, which separates into two structures. These nerve cells do not have dendrites.

Anatomy and Structure of Nerve Cells

A nerve cell contains three main parts: cell body, dendrites, and axon.

Cell Body

• Also known as soma, is central part of nerve cell.
• Contains nucleus and various cell organelles like endoplasmic reticulum, Golgi apparatus, mitochondria, and lysosomes.
• These organelles produce proteins and generate energy within cell.
• Gives rise to dendrites and axons.
• Maintains nerve cell and is involved in growth and development of nerve cell.

Dendrites

• Structures like tree branches extending from cell body.
• Receive signals from other neurons acting as receptors and convert them into electrical impulses.
• Branching structure is useful for receiving signals from other neurons.
• Mainly made up of dendritic shafts and tiny extensions known as dendritic spines which contain neurotransmitter receptors on their membranes.

Axon

• Another extension of cell body that carries signals away from cell body.
• Connected to cell body at point called axon hillock.
• Primary function is to transmit electrical signals to other neurons, muscles, or glands.
• Can be covered by protective layer of fatty material called myelin sheath that speeds up transmission of nerve impulses.
• Axon terminal, located at end of axon, is point where information is transmitted to target cells.

Nerve Impulses and Action Potential

Nerve impulses are signals transmitted through nervous system which allows communication between cells.

Nerve impulses from one neuron are passed onto another through sites called synapses.

Generation of Action Potential

Before nerve impulse is transmitted through synapse, action potential must be generated in presynaptic neuron. Action potential refers to rapid change in membrane potential.

• Initially, neurons have resting membrane potential that maintains negative charge inside cell with higher concentration of sodium ions outside and potassium ions inside cells.

• This is maintained by sodium-potassium pump.

• Depolarization:

  • When nerve cell receives stimulus from other neurons, sodium channels on cell membrane open.
  • Allows sodium ions to flow into cell, causing depolarization.
  • Membrane potential changes to positive.

• Repolarization:

  • After depolarization, membrane potential begins process of repolarization.
  • Potassium channels open and allow potassium ions to leave cell, restoring original resting membrane potential.

• Release of neurotransmitters:

  • Action potential reaches end of axon and triggers release of chemical messengers called neurotransmitters into synapse.
  • Neurotransmitters bind to receptors on postsynaptic neuron.
  • In this way, signal is passed on to next neuron.

Electrical Synapses vs Chemical Synapses

• Electrical synapses vs chemical synapses represent two modes of synaptic transmission, with chemical synapses involving neurotransmitter release as described above.

Functions of Nerve Cells

• Receive and transmit information in form of electrical signals throughout body.

• Allows communication within nervous system which is vital for different body functions.

• Sensory neurons receive stimuli from external environment.

  • Detect changes in surroundings and transmit signals to central nervous system.
  • Allows us to respond to our surroundings.

• Motor neurons receive signals from brain and transmit these signals to muscles.

  • Control muscle movements and allow voluntary movements like walking and running.

• Interneurons act as mediators and transmit signals between other neurons.

• Nerve cells are also involved in maintaining concentrations of ions inside and outside their membranes which is important for generating and transmitting nerve impulses.

Diseases and Disorders of Nerve Cells

Neurodegenerative Disorders

Include several diseases that lead to progressive damage of nerve cells. These diseases are closely linked to aging. Neurodegenerative diseases are currently incurable. However, treatments focus on managing symptoms and improving quality of life.

Alzheimer’s Disease

• Neurodegenerative disease that leads to gradual decline in memory and other cognitive abilities.
• As disease progresses, individuals may experience problems with communication, decision-making, and completing everyday tasks.

Parkinson’s Disease

• Neurological disorder characterized by difficulties in movement. Over time, it also causes decline in cognitive function.
• Symptoms include tremors, muscle stiffness, and impaired balance.
• Both genetic and non-genetic factors lead to development, with age being main risk factor.

Huntington’s Disease

• Genetic disorder that involves gradual loss of nerve cells.
• Affects parts of brain that control voluntary movement.
• Common symptoms include involuntary movements (chorea), unusual body postures, and problems with cognitive functions.

Motor Neuron Diseases

Cause degeneration of motor neurons that control movement. This results in muscle weakness and eventually paralysis.

• Includes several diseases, including:
  • Amyotrophic lateral sclerosis (ALS)
  • Primary lateral sclerosis
  • Progressive muscular atrophy
  • Spinal muscular atrophy
  • Post-polio syndrome
• Symptoms may also include breathlessness, chest infections, and disturbed sleep.

Peripheral Neuropathy

Nerve cell disease that affects peripheral nerves.

• Various factors can lead to this condition, such as:
  • Diabetes
  • Alcohol abuse
  • Infections
  • Autoimmune diseases
  • Genetic disorders
  • Vascular disorders
• Symptoms can include:
  • Muscle weakness
  • Uncontrolled muscle movements
  • Numbness
  • Imbalance
  • Changes in blood pressure
  • Bowel problems
• Treatment options include:
  • Medications such as antidepressants, anticonvulsants, and pain relievers
  • Surgery in case of severe nerve injuries
  • Physical therapy and devices like braces and walkers can help with mobility

Demyelinating Diseases

Conditions that damage myelin sheath surrounding nerve cells.

• Common demyelinating diseases include:
  • Multiple sclerosis (MS)
  • Transverse myelitis (TM)
  • Guillain-Barré syndrome (GBS)
  • Charcot-Marie-Tooth disease (CMT)
  • Chronic inflammatory demyelinating polyneuropathy (CIDP)
• Symptoms include vision changes, tingling, muscle weakness, and difficulty walking.
• Caused by:
  • Immune system attacks on healthy myelin
  • Viral or bacterial infections
  • Genetic factors
  • Other medical conditions
• Treatment options include medications and physical therapy.

Summary

Nerve cells or neurons receive, process and transmit information as messengers from external stimuli to brain. Functionally classified into sensory or afferent neurons detecting touch, taste, smell and pain converting signals to electrical impulses to CNS, motor or efferent neurons divided into upper and lower transmitting CNS signals to muscles for voluntary and involuntary movement, and interneurons most abundant in CNS connecting sensory and motor neurons. Structurally classified into unipolar with single axon-dendrite process common in invertebrates, bipolar with one axon one dendrite, multipolar with one axon multiple dendrites most common, and pseudounipolar without dendrites. Structure includes soma containing nucleus, ER, Golgi, mitochondria, dendrites with dendritic shafts and spines bearing neurotransmitter receptors, and axon with axon hillock, myelin sheath and axon terminal. Nerve impulse transmission requires action potential: resting negative potential maintained by sodium-potassium pump, depolarization via sodium influx, repolarization via potassium efflux, and neurotransmitter release at synapses. Functions include sensory detection, motor control and ion concentration maintenance. Disorders include Alzheimer’s with memory decline, Parkinson’s with tremors and stiffness, Huntington’s genetic chorea, motor neuron diseases like ALS causing paralysis, peripheral neuropathy from diabetes and alcohol, and demyelinating diseases like MS, GBS, CMT and CIDP causing vision changes and weakness.

References

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