Neurophysiology

Nervous system

From a structural point of view, the nervous system is divided into two major components: the central nervous system (CNS) and the peripheral nervous system (PNS).

• The CNS includes the brain and spinal cord, serving as the main control center. The brain, housed within the skull, is the most complex organ, containing billions of neurons communicating through trillions of synapses. It is responsible for processing sensory information, generating thoughts, and issuing commands. The spinal cord, protected by the vertebral column, is the primary pathway for transmitting signals between the brain and the rest of the body.
• The PNS consists of nerves extending from the brain and spinal cord to other body parts, including the limbs and organs. It is divided into the somatic nervous system, which controls voluntary movements and relays sensory information, and the autonomic nervous system (ANS), which regulates involuntary functions like heart rate and digestion. The ANS is further subdivided into the sympathetic and parasympathetic systems, which typically work in opposition. The sympathetic nervous system is primarily responsible for the body's "fight-or-flight" response. Activation of the sympathetic nervous system leads to physiological changes such as an increase in heart rate, dilation of the pupils, and the release of adrenalin/epinephrine. These changes optimize the body for rapid, intense physical activity, enhancing alertness and responsiveness to external stimuli. In contrast, the parasympathetic nervous system governs the "rest-and-digest" functions. It slows the heart rate, reduces blood pressure, and stimulates digestive processes.

Homeostasis

One of the most crucial functions of the nervous system is homeostasis. Homeostasis is the body's ability to maintain a stable internal environment despite changes in external conditions. This balance is crucial for survival and is tightly regulated by the nervous system. The system monitors physiological variables like temperature, pH, and blood pressure and responds appropriately when these variables deviate from normal ranges.

The nervous system uses feedback mechanisms to maintain homeostasis. For instance, in temperature regulation, receptors detect changes in body temperature and send signals to the hypothalamus, the brain's thermoregulatory center. If the body is too hot, the hypothalamus triggers cooling mechanisms like sweating and vasodilation (widening of blood vessels) to release heat. If the body is too cold, it triggers shivering and vasoconstriction (narrowing of blood vessels) to conserve heat. These negative feedback loops are essential for keeping the body functioning within optimal parameters.

Motor control

Motor control refers to the nervous system's ability to regulate and guide muscles and limbs to perform desired movements. It involves the brain, spinal cord, and peripheral nerves.

The spinal cord is a key player in motor control, acting as a major conduit for motor information. It transmits signals between the brain and muscles and contains reflex arcs, which are simple neural circuits that control reflex actions. For instance, the knee-jerk reflex is mediated by a reflex arc in the spinal cord, allowing the leg to extend rapidly in response to a tap on the knee without involving the brain.

Voluntary movements are primarily controlled by the motor cortex in the frontal lobe. When a person decides to move, the motor cortex sends signals through the corticospinal tract to the spinal cord, which then directs the appropriate muscles.

The basal ganglia and cerebellum are crucial for fine-tuning movements. The basal ganglia help initiate movements and regulate their intensity, while the cerebellum monitors ongoing movements, comparing intended actions with actual ones and making adjustments to ensure smooth and accurate motion.
Motor learning, the process of acquiring new motor skills, involves the cerebellum and changes in synaptic strength within neural circuits.

Autonomic responses are involuntary actions regulated by the autonomic nervous system (ANS), which governs the activity of smooth muscles, cardiac muscles, and glands. Unlike voluntary motor control, autonomic responses manage essential bodily functions such as heart rate, digestion, and respiratory rate. The ANS is divided into two components:

• The sympathetic nervous system initiates the "fight-or-flight" response, preparing the body for intense physical activity by increasing heart rate, dilating pupils, and redirecting blood to muscles.
• The parasympathetic nervous system supports the "rest-and-digest" response, slowing heart rate, facilitating digestion, and conserving energy during relaxed states.

While the somatic nervous system controls voluntary movements, the autonomic nervous system regulates these involuntary responses, ensuring the body maintains homeostasis during both stressful and restful periods.

Senses

The sensory systems allow the body to perceive and interpret the environment, enabling appropriate reactions and adaptations. Sensory information is collected by specialized receptors and transmitted to the CNS, where it is processed and integrated to form a coherent picture of the world.

Neuronal signaling

At the core of neurophysiology, lie all the mechanisms that control how neurons communicate with each other and with other cells in the body. This communication which is also known as neuronal signaling occurs through the transmission of electrical impulses, known as action potentials, along the different parts of a neuron. When an action potential reaches the end of an axon, it triggers the release of neurotransmitters, chemical messengers that cross the synapse, the gap between neurons. These neurotransmitters bind to receptors on the adjacent neuron, leading to the generation of a new electrical signal in that neuron. This intricate process allows the nervous system to rapidly process and respond to information, enabling everything from reflex actions to complex thought processes.

Neurotransmitters and their roles

Neurotransmitters are the chemical messengers that transmit signals across synapses from one neuron to another. They play a crucial role in regulating a wide range of bodily functions and behaviors, from mood and sleep to heart rate and digestion.

Neurotransmitters can be broadly classified into two categories based on their effects on the postsynaptic neuron:

• Excitatory neurotransmitters, such as glutamate, increase the likelihood that the neuron will fire an action potential. Glutamate is the most abundant excitatory neurotransmitter in the brain and is involved in cognitive functions like learning and memory.
• Inhibitory neurotransmitters, such as gamma-aminobutyric acid (GABA), decrease the likelihood of an action potential. GABA plays a key role in reducing neuronal excitability and preventing overstimulation, which is crucial for maintaining balance in brain activity.