Animals Nervous System: Introduction, Classification, Spinal Cord, Neurons

The main function of the nervous system is to sense changes, integrate information, and provoke suitable responses to maintain homeostasis and coordinate activities.

Neurons transmit through electrical signals called action potentials and through a chemical signal in the form of neurotransmitters at synapses.

The reflex action is a rapid, involuntary response to a stimulus having its pathway as the simple reflex arc.

The common ones include Parkinson's characterised by tremors and rigidity; Alzheimer's by amnesia; and Multiple Sclerosis by muscle weakness.

Neurodegenerative diseases, such as Alzheimer's and Parkinson's, involve the progressive loss of structure or function of neurons. This can lead to problems with movement (ataxia), mental functioning (dementia), or both. These diseases often involve the accumulation of abnormal proteins, disruption of cellular processes, and eventual cell death. The specific symptoms depend on which regions of the nervous system are affected.

Neuroplasticity refers to the brain's ability to change and reorganize itself by forming new neural connections throughout life. It's significant because it allows the nervous system to adapt to new experiences, recover from injuries, and learn new skills. This plasticity is the basis for learning, memory, and the brain's ability to compensate for damage.

The blood-brain barrier is a highly selective semipermeable border of endothelial cells that prevents many substances in the blood from entering the brain. It protects the central nervous system by restricting the passage of potentially harmful substances, pathogens, and even many beneficial drugs. This barrier is crucial for maintaining a stable environment for proper brain function but also presents challenges in treating certain neurological disorders.

Pain is processed through specialized sensory neurons called nociceptors, which detect potentially damaging stimuli. These signals are transmitted through the spinal cord to various brain regions, including the thalamus and cortex, where they're interpreted as pain. The purpose of pain perception is protective – it alerts the body to potential or actual damage, prompting actions to avoid or minimize harm. Chronic pain, however, can persist beyond its protective function and become a disorder in itself.

Neurotransmitter reuptake is the process by which neurotransmitters are removed from the synaptic cleft after signaling. This process is crucial for terminating the signal and preparing the synapse for the next transmission. It helps maintain the proper balance of neurotransmitters, prevents overstimulation of receptors, and allows for the recycling of neurotransmitters, which is energetically efficient for the neuron.

The primary function of the nervous system is to coordinate and control various activities of the body. It receives sensory input from the environment, processes this information, and generates appropriate responses. This system allows animals to interact with their surroundings, maintain homeostasis, and adapt to changes.

The main difference lies in the complexity and organization. Invertebrates generally have a simpler nervous system, often consisting of nerve cords and ganglia. Vertebrates have a more complex system with a centralized brain and spinal cord, known as the central nervous system (CNS), and a peripheral nervous system (PNS) that connects the CNS to the rest of the body.

Glial cells, often called neuroglia, play crucial supporting roles in the nervous system. They provide physical support and insulation for neurons, supply nutrients and oxygen, remove dead neurons and pathogens, and maintain the proper chemical environment for neural signaling. Some types of glial cells, like oligodendrocytes and Schwann cells, produce the myelin sheath that insulates axons.

Astrocytes, a type of glial cell, play multiple crucial roles in supporting neuronal function. They help maintain the blood-brain barrier, regulate the chemical environment around neurons by taking up excess neurotransmitters and ions, provide nutrients to neurons, and help control blood flow in the brain. Recent research has also shown that astrocytes can release signaling molecules that influence neuronal activity and synaptic transmission, suggesting they play a more active role in information processing than previously thought.

The nervous system can adapt to injury or damage through several mechanisms. Neuroplasticity allows for the reorganization of neural pathways, where undamaged areas can sometimes take over functions of damaged regions. Axonal sprouting can occur, where undamaged axons grow new nerve endings to compensate for damaged ones. In some cases, neurogenesis can produce new neurons. The brain can also alter its processing strategies to compensate for lost functions. However, the extent of recovery depends on the severity and location of the damage.

Neurons are specialized cells that transmit electrical and chemical signals in the nervous system. They're considered the basic functional units because they're responsible for receiving, processing, and transmitting information throughout the body. Neurons have unique structures like dendrites, axons, and synapses that allow them to communicate with other cells effectively.

The main types of neurons are sensory neurons, motor neurons, and interneurons. Sensory neurons carry information from sensory receptors to the CNS. Motor neurons transmit signals from the CNS to muscles and glands. Interneurons connect and integrate information between other neurons within the CNS.

Neurotrophic factors are proteins that promote the survival, development, and function of neurons. They play crucial roles in neural development by guiding the growth of axons and dendrites, promoting synapse formation, and regulating neuron survival. In the adult nervous system, they continue to support neuron health, promote plasticity, and can even stimulate the growth of new neurons in certain brain regions.

Neurogenesis is the process of generating new neurons from neural stem cells. While it was once thought to occur only during embryonic development, we now know that neurogenesis continues in certain regions of the adult brain, particularly the hippocampus (important for memory) and the subventricular zone. Adult neurogenesis is believed to play roles in learning, memory, and mood regulation, though its extent and significance in humans are still subjects of ongoing research.

Long-term potentiation is a persistent strengthening of synapses based on recent patterns of activity. It's believed to be one of the main cellular mechanisms underlying learning and memory. When neurons repeatedly fire together, the connections between them are strengthened, making future communication more efficient. This process involves changes in receptor density, neurotransmitter release, and even structural modifications of synapses.

The spinal cord serves as a conduit for nerve signals between the brain and the rest of the body. It also contains neural circuits that can independently control certain reflexes and simple movements without direct input from the brain. Additionally, the spinal cord plays a crucial role in processing and modulating sensory information before it reaches the brain.

A reflex arc is a neural pathway that produces a rapid, automatic response to a stimulus without involving the brain. It typically involves a sensory neuron, an interneuron in the spinal cord, and a motor neuron. Reflex arcs are important because they allow for quick responses to potentially harmful stimuli, enhancing survival and preventing injury.

The nervous system maintains homeostasis by constantly monitoring the body's internal environment through various sensors and receptors. When deviations from the normal state are detected, the nervous system initiates appropriate responses. For example, it can adjust heart rate, breathing rate, or hormone release to maintain stable conditions. This involves complex feedback loops and integration of information from multiple body systems.

The nervous and endocrine systems work together in a complex interplay called the neuroendocrine system. The hypothalamus, a part of the brain, acts as a major link between these systems. It can produce hormones and also regulate the pituitary gland, often called the "master gland" of the endocrine system. This interaction allows for both rapid (nervous) and sustained (endocrine) responses to environmental changes and internal needs.

The nervous system processes and integrates information from multiple sensory modalities through a process called multisensory integration. This occurs at various levels, from early sensory processing areas to higher-order association cortices. Neurons in these areas can respond to inputs from multiple senses, combining this information to create a coherent perception of the environment. This integration allows for more accurate and robust representations of the world, enhancing our ability to interact with our surroundings effectively.

The CNS consists of the brain and spinal cord, which are the main processing and control centers. The PNS includes all the nerves that connect the CNS to the rest of the body. The PNS is further divided into the somatic nervous system (voluntary control) and the autonomic nervous system (involuntary control).

The somatic nervous system controls voluntary movements and carries sensory information from the body to the CNS. It includes motor neurons that innervate skeletal muscles. The autonomic nervous system, on the other hand, regulates involuntary functions like heart rate, digestion, and respiration. It's further divided into the sympathetic ("fight or flight") and parasympathetic ("rest and digest") systems.

Different brain regions specialize in various functions through a combination of evolutionary development and experience-dependent plasticity. For example, the occipital lobe primarily processes visual information, while the frontal lobe is involved in executive functions like planning and decision-making. This specialization allows for efficient processing of complex information and behaviors. However, it's important to note that most complex functions involve networks spanning multiple brain regions.

The cerebellum plays a crucial role in motor control, balance, and motor learning. It receives input from sensory systems and other parts of the brain and integrates these inputs to fine-tune motor activity. The cerebellum is involved in the timing and precision of movements, allowing for smooth, coordinated actions. It's also important for motor learning, helping to adjust movements based on past experiences and errors.

The sympathetic and parasympathetic systems are two branches of the autonomic nervous system. The sympathetic system prepares the body for "fight or flight" responses, increasing heart rate, dilating pupils, and diverting blood flow to muscles. The parasympathetic system promotes "rest and digest" functions, slowing heart rate, stimulating digestion, and promoting relaxation. These systems often have opposing effects on the same organs, allowing for fine-tuned control of bodily functions.

Neurons communicate through a process called synaptic transmission. When a neuron is stimulated, it generates an electrical impulse (action potential) that travels along its axon. At the synapse (the junction between two neurons), this electrical signal triggers the release of chemical messengers called neurotransmitters. These neurotransmitters cross the synaptic cleft and bind to receptors on the receiving neuron, potentially triggering a new electrical signal.

Neurotoxins are substances that can damage or destroy nerve tissue. They can work in various ways, such as blocking neurotransmitter receptors, interfering with ion channels, or disrupting the synthesis or breakdown of neurotransmitters. Some neurotoxins, like tetrodotoxin, block sodium channels, preventing the generation of action potentials. Others, like botulinum toxin, prevent the release of neurotransmitters. The effects of neurotoxins can range from mild symptoms to paralysis and death, depending on the toxin and exposure.

Inhibitory interneurons play crucial roles in shaping and regulating neural activity. They release inhibitory neurotransmitters like GABA, which decrease the likelihood of the postsynaptic neuron firing. This inhibition is important for creating contrast in sensory processing, sharpening the timing and specificity of neural responses, and preventing over-excitation in the nervous system. Inhibitory interneurons are also key components in generating rhythmic activity in neural circuits, which is important for various brain functions.