Prokaryotes evolved the ability to perceive and react billions of years ago, improving survival and reproductive success in changing circumstances. Modification of simple recognition and response mechanisms later in evolution created a foundation for communication between cells in an animal body. More than 500 million years ago, at the period of the Cambrian explosion, specialized nerve systems had evolved, allowing creatures to detect their environment and respond quickly.

Hydras, jellyfish, and other cnidarians are the most primitive creatures to have nervous systems. Interconnected neurons create a diffuse neural net in most cnidarians, controlling the contraction and expansion of the gastrovascular canal. Multiple neurons' axons are frequently bundled together to produce nerves in more sophisticated organisms.

These fibrous structures direct information flow through the nervous system along specific pathways. Sea stars, for example, contain a network of radial nerves that link to a central nerve ring.

The radial nerve is connected to a neural net within each arm of a sea star, from which it receives input and transmits signals that govern muscle contraction.

Cerebrospinal fluid, which is produced in the brain by filtering arterial blood, fills both the canal and the ventricles. The cerebrospinal fluid delivers wastes and nutrients to the CNS, flowing via the ventricles and central canal before emptying into the veins.

The brain and spinal cord include gray matter and white matter in addition to fluid-filled areas.

The majority of gray matter is made up of neuron cell bodies.

White matter is made up of bundled axons. White matter constitutes the outer layer of the spinal cord, indicating its role in connecting the CNS to sensory and motor neurons in the PNS.

White matter predominates in the inside of the brain, where communication between neurons plays a role in learning, feeling emotions, processing sensory information, and producing orders.

The spinal cord travels longitudinally inside the vertebral column, also known as the spine, invertebrates (as shown in the attached image).

The spinal cord transmits information to and from the brain and creates fundamental locomotor patterns. It also functions independently of the brain as part of the basic nerve circuits that generate reflexes, or the body's instinctive reactions to specific stimuli.

Ventricles, gray matter, and white matter are all components of the brain.

Cerebrospinal fluid is stored in ventricles deep into the brain's core. The majority of gray matter is found on the brain's surface, bordering the white matter.

Sensory signals are carried to the CNS by afferent neurons. Efferent neurons work in either the motor system (which sends signals to skeletal muscles) or the autonomic nervous system (which controls smooth and cardiac muscles). The autonomic nervous system's sympathetic and parasympathetic divisions have antagonistic effects on a wide range of target organs, whereas the enteric nervous system regulates the functioning of numerous digestive organs.

Glia, which includes astrocytes, oligodendrocytes, and Schwann cells, supports vertebrate neurons. Some glia function as stem cells capable of transforming into adult neurons.

The cerebrum is divided into two hemispheres, each with cortical gray matter overlaying white matter and basal nuclei.

The basal nuclei play a crucial role in movement planning and learning. The pons and medulla oblongata serve as relay stations for information flowing from the PNS to the cerebrum. The reticular formation, a network of neurons located in the brainstem, controls sleep and arousal. The cerebellum assists in the coordination of motor, perceptual, and cognitive activities.

The thalamus is the primary center from which sensory information is sent to the cerebrum.

Homeostasis and fundamental survival behaviors are regulated by the hypothalamus. The suprachiasmatic neurons are a group of neurons found in the hypothalamus.

Each side of the cerebral cortex is divided into four lobes: frontal, temporal, occipital, and parietal, which comprise main sensory regions and association areas. Association regions combine information from several sensory areas. Broca's and Wernicke's areas are critical for language generation and comprehension. These functions, as well as math and logic processes, are centered in the left cerebral hemisphere. Pattern identification and nonverbal reasoning appear to be stronger in the right hemisphere.

Neurons in the somatosensory and motor cortex are distributed according to which region of the body provides sensory input or receives motor commands.

Primates and cetaceans, both of which are endangered, Memory and learning are based on changes in synaptic connections.

More neurons and synapses grow throughout development than would exist in adulthood. In embryos, the fundamental structure of the nervous system is established through the controlled death of neurons and the removal of synapses.

Adult nervous system remodeling can entail the loss or addition of synapses, as well as the strengthening or weakening of signals at synapses. This remodeling ability is known as neural plasticity. Temporary connections in the hippocampus are responsible for short-term memory. These transient linkages are replaced in long-term memory by connections inside the cerebral cortex.

• The term Schizophrenia refers to being characterized by hallucinations, delusions, and other symptoms that affect neuronal pathways that use dopamine as a neurotransmitter.

Many neurological diseases may now be described in molecular terms. Bipolar disorder and major depressive disorder can be treated with drugs that enhance the activity of biogenic amines in the brain. Addiction is characterized by compulsive drug use, which indicates abnormal functioning of the brain's reward system, which usually provides an incentive for activities that improve survival or reproduction.

The plaques are made up of -amyloid, an insoluble peptide that is formed when the extracellular part of a membrane protein present in neurons is broken. Secretases, which are membrane enzymes, catalyze the cleavage, causing -amyloid to collect in plaques outside the neurons. These plaques appear to be responsible for the death of neighboring neurons.

The tau protein is the main component of the neurofibrillary tangles seen in Alzheimer's disease. (This protein is unrelated to the tau mutation that impairs hamster circadian rhythm.) The tau protein is typically involved in the formation and maintenance of microtubules, which carry nutrients along axons.

Tau undergoes alterations that cause Alzheimer's disease.

Alzheimer's disease and Parkinson's disease are both neurological diseases that generally develop with age. Alzheimer's disease is a kind of dementia characterized by the formation of neurofibrillary tangles and amyloid plaques in the brain.

• The term Parkinson's disease refers to a movement condition caused by the loss of dopamine-secreting neurons and associative neurons.