The dorsal horn of the spinal cord processes sensory information, including pain, and is a key target for analgesic development. Neurons in the dorsal horn receive input from primary afferents, which respond to noxious and non-noxious stimuli. These afferents terminate in specific regions of the dorsal horn, and their input is processed by complex circuits involving excitatory and inhibitory interneurons, which then relay information to projection neurons. Nociceptive information also contributes to spinal reflexes. The balance between excitation and inhibition is crucial for normal sensory function, and disruptions in this balance can lead to pain conditions such as allodynia and hyperalgesia. The dorsal horn contains several neuronal populations, including interneurons and projection neurons. Interneurons are mainly inhibitory and use GABA or glycine as neurotransmitters, while projection neurons are excitatory and use glutamate. These neurons are organized into different laminae, with laminae I-III being the most studied. Lamina I contains the major termination zone for nociceptive primary afferents, while lamina III contains interneurons that modulate pain processing. Primary afferents can be classified based on their sensory modality, conduction velocity, and neurochemical properties. They include myelinated (Aβ and Aδ) and unmyelinated (C) fibers, which respond to different types of stimuli. The distribution of these afferents in the dorsal horn is determined by their functional class. For example, Aδ and C fibers innervate lamina I and much of lamina II, while myelinated low-threshold mechanoreceptive afferents arborise in lamina II-VI. Descending inputs from the brainstem, such as serotonergic and noradrenergic pathways, modulate pain processing in the dorsal horn. These inputs can act through volume transmission, affecting the function of interneurons and projection neurons. The dorsal horn also receives inputs from other regions of the brain, which contribute to the processing of pain and temperature. Neuronal circuits in the dorsal horn are complex, with interneurons and projection neurons interacting to process sensory information. The function of these circuits is influenced by the balance between excitation and inhibition, as well as the activity of specific neurotransmitters and ion channels. Disruptions in these circuits can lead to chronic pain conditions, such as neuropathic and inflammatory pain. Recent studies have begun to identify the specific roles of different neuronal populations in the dorsal horn, including the effects of neurotransmitters, ion channels, and receptor expression. These studies have also revealed the importance of intrinsic plasticity in the dorsal horn, which can contribute to the development of chronic pain. Understanding the organization and function of these circuits is essential for developing new treatments for pain.The dorsal horn of the spinal cord processes sensory information, including pain, and is a key target for analgesic development. Neurons in the dorsal horn receive input from primary afferents, which respond to noxious and non-noxious stimuli. These afferents terminate in specific regions of the dorsal horn, and their input is processed by complex circuits involving excitatory and inhibitory interneurons, which then relay information to projection neurons. Nociceptive information also contributes to spinal reflexes. The balance between excitation and inhibition is crucial for normal sensory function, and disruptions in this balance can lead to pain conditions such as allodynia and hyperalgesia. The dorsal horn contains several neuronal populations, including interneurons and projection neurons. Interneurons are mainly inhibitory and use GABA or glycine as neurotransmitters, while projection neurons are excitatory and use glutamate. These neurons are organized into different laminae, with laminae I-III being the most studied. Lamina I contains the major termination zone for nociceptive primary afferents, while lamina III contains interneurons that modulate pain processing. Primary afferents can be classified based on their sensory modality, conduction velocity, and neurochemical properties. They include myelinated (Aβ and Aδ) and unmyelinated (C) fibers, which respond to different types of stimuli. The distribution of these afferents in the dorsal horn is determined by their functional class. For example, Aδ and C fibers innervate lamina I and much of lamina II, while myelinated low-threshold mechanoreceptive afferents arborise in lamina II-VI. Descending inputs from the brainstem, such as serotonergic and noradrenergic pathways, modulate pain processing in the dorsal horn. These inputs can act through volume transmission, affecting the function of interneurons and projection neurons. The dorsal horn also receives inputs from other regions of the brain, which contribute to the processing of pain and temperature. Neuronal circuits in the dorsal horn are complex, with interneurons and projection neurons interacting to process sensory information. The function of these circuits is influenced by the balance between excitation and inhibition, as well as the activity of specific neurotransmitters and ion channels. Disruptions in these circuits can lead to chronic pain conditions, such as neuropathic and inflammatory pain. Recent studies have begun to identify the specific roles of different neuronal populations in the dorsal horn, including the effects of neurotransmitters, ion channels, and receptor expression. These studies have also revealed the importance of intrinsic plasticity in the dorsal horn, which can contribute to the development of chronic pain. Understanding the organization and function of these circuits is essential for developing new treatments for pain.