Summary
Highlights
The nervous system controls all organs, physiological, and psychological reactions. It is essential for our existence and processes all sensory input, integrates information, and executes motor output. All actions, from dreaming to making toast, are examples of the nervous system at work through electrical and chemical signals.
The nervous system performs three main functions: sensory input (detecting stimuli), integration (processing the input and deciding on a response), and motor output (executing the response). An example used is a spider on the knee: sensory receptors detect the spider (sensory input), the nervous system decides how to react (integration), and the hand lashes out (motor output).
The nervous system is divided into two main parts: the Central Nervous System (CNS), consisting of the brain and spinal cord, which is the control center; and the Peripheral Nervous System (PNS), which includes all nerves branching off the brain and spine, enabling communication with the rest of the body. The PNS has sensory (afferent) and motor (efferent) divisions. The motor division further subdivides into somatic (voluntary) and autonomic (involuntary) systems, with the autonomic system having sympathetic and parasympathetic divisions.
Nervous tissue is densely packed with cells, primarily neurons and glial cells. Neurons (nerve cells) respond to stimuli and transmit signals. Glial cells, such as astrocytes, microglial cells, ependymal cells, oligodendrocytes (in the CNS), satellite cells, and Schwann cells (in the PNS), support and protect neurons, outnumbering them significantly and performing crucial functions like anchoring neurons, immune defense, circulating cerebrospinal fluid, and forming myelin sheaths.
Neurons are long-lived, irreplaceable (mostly amitotic), and have a high metabolic rate, requiring constant glucose and oxygen. Their basic structure includes the soma (cell body) for life support, dendrites for receiving messages, and an axon for transmitting electrical impulses away from the cell body to other cells.
Neurons are classified by the number of processes extending from the cell body: multipolar (most common, one axon, many dendrites), bipolar (one axon, one dendrite, rare), and unipolar (one process, mostly in sensory receptors). Functionally, neurons are classified by the direction of impulse travel: sensory (afferent) transmit to the CNS, motor (efferent) transmit away from the CNS, and interneurons transmit between sensory and motor neurons within the CNS.
The video summarizes the learning by revisiting the spider-on-the-knee example. Unipolar sensory neurons in the skin detect the spider, sending a signal to the spinal cord. Interneurons in the spinal cord might directly trigger a motor response (kicking leg) or send the signal to the brain for recognition and a more complex reaction (screaming or remaining calm). The connections between neurons dictate the final output.