Classification of nerve endings
For the nervous system to function as a single entity, specialized structures at the ends of neuronal processes are required. Depending on their role and exact localization, histology generally distinguishes four functional types of nerve endings:
- Receptor (sensory or afferent).
- Interneuronal synapses (contacts between the nerve cells themselves).
- Effector (motor or secretory).
- Neurosecretory (releasing active substances into the blood).
Each of these types possesses a unique morphology and strict attachment to specific parts of the neuron (axons or dendrites).
Type I: Receptor (afferent) endings
Receptor endings represent the initial link in the reflex arc.
- Localization: These structures are always located at the ends of dendrites of sensory (afferent) neurons.
- Function: The main task of receptors is the adequate perception of information. They record stimuli coming both from the external environment (via sense organs) and from the body's internal receptors.
- Purpose: After perceiving a stimulus, the signal is transformed and transmitted for further complex analysis to the structures of the central nervous system (CNS).
Type II: Interneuronal synapses
This type of ending is essential for the formation of neural networks.
- Function: The direct transmission of a nerve signal from one neuron to another. Interneuronal synapses ensure a reliable connection between all existing types of nerve cells (afferent, associative, and efferent neurons).
- Structural components: A synapse consists of two key parts. The transmitting part is typically the axon. The receiving part can be the dendrite or the body (soma) of the next neuron.
Depending on which specific parts of the cells come into contact, the following morphological types of synapses are distinguished:
- Axodendritic — contact between the axon of the first neuron and the dendrite of the second.
- Axosomatic — signal transmission from the axon directly to the cell body (soma).
- Axoaxonal — a specific contact between two axons.
- Somatodendritic — a rare type of connection in which the neuron body contacts a dendrite.
Type III: Effector endings
Effector endings serve as the final link, transmitting a command from the nervous system to the effector organ.
- Localization: Located exclusively at the axon terminals of efferent neurons. They logically complete the efferent (descending) nerve pathways.
- Structure: Together with the plasma membrane of the executive cell (muscle fiber or glandulocyte), they form specialized neuroeffector synapses.
- Function: The transmission of the nerve impulse to effector organs. Such target organs are usually muscles (skeletal or smooth) and various glands.
Type IV: Neurosecretory endings
This type of ending connects the nervous system with the endocrine system.
- Localization: Found at the axon terminals of specialized neurosecretory neurons.
- Structure: A distinctive feature of these endings is that they directly contact blood vessels (capillaries). As a result of this contact, so-called axovasal synapses are formed.
- Function: Targeted release of synthesized hormones (neurosecretion) directly into the bloodstream to regulate the functions of the entire organism.
Briefly: answers to topical questions
How are receptor nerve endings classified according to their morphological structure?
According to their morphological structure, receptor nerve endings can be:
- Free nerve endings — terminal branches of axis cylinders lacking glial sheaths.
- Unencapsulated (non-free) endings — branches of dendrites of sensory neurons accompanied by glial cells (lemmocytes); in the papillary layer, they perceive temperature and pain.
- Encapsulated endings — receptors possessing a connective tissue capsule; these include Meissner's corpuscles, as well as Vater-Pacini corpuscles and Ruffini corpuscles. Meissner's corpuscles comprise three components: terminal branches of the dendrite, modified lemmocytes, and a thin connective tissue capsule.
Which neurotransmitters ensure signal transmission in interneuronal synapses?
Signal transmission in chemical interneuronal synapses is carried out using mediators (neurotransmitters) that diffuse across the synaptic cleft from the presynaptic terminal to the postsynaptic membrane.
By type of mediator, cholinergic, adrenergic, GABAergic, serotonergic, dopaminergic, and glutamatergic synapses are distinguished.
The sources explicitly indicate the following mediators:
- Acetylcholine — the mediator of cholinergic synapses.
- GABA — the mediator of GABAergic synapses, of an inhibitory type.
- Glycine — the mediator of glycinergic synapses, also of an inhibitory type.
In many interneuronal synapses of the brain, there may be multiple co-mediators: usually one of them is non-peptidergic, while the others belong to neuropeptides.
Where are the bodies of neurosecretory neurons located whose axons form axovasal synapses?
The bodies of neurosecretory neurons whose axons form axovasal synapses are localized in the magnocellular nuclei of the hypothalamus. These include:
- Supraoptic nuclei — located above the optic chiasm (chiasma opticum).
- Paraventricular nuclei — located in the lateral walls of the third ventricle of the brain.
Axons extend from the bodies of these cells and descend into the posterior pituitary gland (neurohypophysis), where they form synapses on blood capillaries to release hormones.
To memorize the four types, use the acronym RMEN: Receptor (perceive), Interneuronal (transmit within the network), Effector (make organs work), Neurosecretory (release into the blood).
Class and exam questions
On which processes are receptor nerve endings located?
Receptor endings are localized on the dendrites of sensory (afferent) neurons.
What is an axovasal synapse?
This is a contact between the axon terminal of a neurosecretory neuron and a blood capillary, serving to release hormones into the blood.
Which morphological types of interneuronal synapses are the rarest?
According to morphological classification, somatodendritic synapses (contact between a cell body and a dendrite) are the rarest.
What else to study on this topic
- Mechanisms of synaptic delay in interneuronal synapses
- The process of action potential generation
- Structural features of neuroeffector synapses in muscle tissue
- Specifics of axonal transport in neurosecretory cells
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