Types of neurons

By: Prof. Dr. Fazal Rehman Shamil | Last updated: February 3, 2024

Types of neurons

• Neurons are the fundamental units of the nervous system, responsible for transmitting and processing information through electrical and chemical signals.
• They can be classified into various types based on their structural and functional characteristics.

Sensory neurons

• Sensory neurons, also known as afferent neurons, are specialized cells that play a crucial role in detecting and transmitting sensory information from the body and the environment to the central nervous system (CNS
• Sensory neurons have specialized sensory receptors that detect specific types of sensory information, such as touch, temperature, pain, vision, hearing, taste, and smell.

Structure

• The structure of a sensory neuron typically consists of three main parts: the sensory receptor, the cell body, and the axon.
• The sensory receptor is the specialized structure that detects the sensory stimulus and converts it into an electrical signal.

Cell body:

Cell body receives electrical signals from the sensory receptor and then generates an action potential that travels along the axon.

Axon:

The axon is a long, slender projection that carries the electrical signals from the sensory receptor to the CNS.

Dendrites:

Sensory neurons have dendrites, which are branch-like extensions originating from the cell body. Dendrites serve as the receptive regions of the neuron, receiving sensory stimuli from the environment or the body. They contain specialized sensory receptors that detect specific types of sensory information. The structure and characteristics of the dendrites vary depending on the sensory modality the neuron is specialized for.

Functions

• Sensory neurons detect various types of sensory stimuli from the environment or the body itself.
• Sensory neurons convert sensory stimuli into electrical signals, known as action potentials or nerve impulses through a process known as sensory transduction.
• Sensory neurons transmit the electrical signals generated by sensory transduction to the CNS. The signals travel along the axons of sensory neurons.
• Once the sensory signals reach the CNS, sensory neurons transmit the information to specific regions of the brain and spinal cord for further processing.
• Sensory neurons can also be involved in generating reflex responses. In certain situations, sensory neurons can bypass higher brain processing and directly initiate a rapid and involuntary response. This reflex action is coordinated at the level of the spinal cord, where sensory neurons synapse with motor neurons to elicit a quick motor response, protecting the body from potential harm.

Motor neurons

Motor neurons play a vital role in the control and coordination of voluntary as well as involuntary movements within the body.

Structure:

Motor neurons consist of three main components: the cell body, dendrites and an axon.

Cell Body (Soma):

The cell body of a motor neuron consists of nucleus and other organelles responsible for the cell’s metabolic activities.

Dendrites:

Dendrites are short, branch-like extensions that extend from the cell body. They receive signals from other neurons and transmit those signals to the cell body.

Axon:

The axon is a long, tubular extension of the motor neuron, covered with myelin sheath. It carries electrical impulses away from the cell body. The axon terminates at the neuromuscular junction or it may synapses with other neurons.

Types of Motor Neurons: Motor neurons can be classified into two main types based on their target and function:

Somatic Motor Neurons:

Somatic motor neurons are also known as alpha motor neurons. These neurons innervate skeletal muscles and thus involved in voluntary movements of the body. Each motor neuron can innervate multiple muscle fibers, forming a motor unit.

Autonomic Motor Neurons:

Autonomic motor neurons regulate involuntary functions and control smooth muscles, cardiac muscles, and glands. They are further divided into two subtypes:

Sympathetic Motor Neurons:

These neurons innervate smooth muscles in blood vessels, sweat glands, and other organs. They also control the body’s response to stress or emergencies situations.

Parasympathetic Motor Neurons:

These neurons control smooth muscles in the digestive track, heart, and other organs. They regulate activities during rest and relaxation which are known as the “rest and digest” response.

Function:

• The main function of motor neurons is to transmit signals from the CNS to muscles and glands, allowing for voluntary and involuntary movements and regulating various bodily processes.
• When the motor neuron receives a signal, an action potential is generated and travels down the axon, reaching the neuromuscular junction or synapse, where it releases neurotransmitters that stimulate the target muscle or gland.

Interneurons:

These neurons are also known as association or connector neurons. These are found entirely within the central nervous system (CNS). They play a vital role in aiding communication between sensory neurons and motor neurons.

Structure:

Interneurons share a similar basic structure like other types of neurons and consist of a cell body (soma), dendrites, and an axon.

Cell Body (Soma):

The nucleus and other organelles necessary for the cell’s metabolic processes are found in the cell body. Through its dendrites, it receives input from other neurons, such as sensory neurons and other interneurons.

Dendrites:

Multiple dendrites of interneuron receive signals from neighboring neurons. These dendrites also have dendritic spines which allow them to receive synaptic inputs from other neurons.

Axon:

These neurons have an axon that carries electrical signals away from the cell body. The length of the axon can vary, and it branches extensively to form connections, called synapses, with other neurons in the CNS.

Function:

Interneuron plays a crucial role in the processing and modulation of signals to ensure appropriate responses to various stimuli. Functions of interneurons include:

• Interneurons receive inputs from multiple sources, including sensory neurons and other interneurons. They integrate these inputs and generate outputs based on the pattern and strength of synaptic inputs. This integration allows for the coordination and synchronization of neural activity within specific circuits.
• Interneurons play a vital role in local circuit processing, where they receive signals from sensory neurons and transmit them to motor neurons within the same region of the CNS. They help modulate the strength, timing, and coordination of signals within these circuits, refining the output and ensuring appropriate responses.
• Many interneurons release inhibitory neurotransmitters, such as gamma-aminobutyric acid (GABA), to regulate the excitability of neighboring neurons. These inhibitory inputs can suppress the activity of certain neurons balancing and fine-tuning neural activity.
• Interneurons can provide feedback inhibition by receiving signals from output neurons and feeding inhibitory inputs back into the circuit. This mechanism helps regulate and control the activity of specific neural pathways.
• Pyramidal Neurons:
• Pyramidal neurons are an excitatory neuron subtype that is located in the cerebral cortex which is the brain’s outer layer.
• These neurons have pyramid shaped cell body and also have several dendrites that take in incoming impulses. In addition higher cognitive processes including learning, memory, and information processing also need pyramidal neurons.