Chapter 12

The nervous system, along with the endocrine system, plays a crucial role in maintaining homeostasis in the body by regulating physiological processes and responding to external and internal stimuli. It operates through electrical impulses, facilitating perceptions, movements, behaviors, and memories, which are essential for survival and interaction with the environment.

Components of the Nervous System

The nervous system is composed of:

1. Brain: The central organ of the nervous system responsible for processing sensory information, controlling motor functions, and managing cognitive functions such as thinking, memory, and emotions.

2. Cranial Nerves: There are 12 pairs of cranial nerves that directly emerge from the brain and serve various sensory and motor functions in the head, neck, and control certain visceral functions.

3. Spinal Cord: Acts as a conduit for signals between the brain and the rest of the body and also plays a role in reflex actions.

4. Spinal Nerves: Composed of 31 pairs of spinal nerves that branch out from the spinal cord, transmitting sensory and motor information to and from the body.

5. Sensory Receptors: Specialized cells that detect internal and external stimuli, including pain, temperature, pressure, and light, enabling organism's awareness of and response to their environment.

Functions of the Nervous System

1. Sensory Function: The nervous system detects stimuli through sensory (afferent) neurons, processing sensory input from the environment to produce a perception of surroundings.

2. Integrative Function: Involves interpreting sensory information, making decisions, and forming perceptual experiences through interneurons that analyze data before a response is initiated.

3. Motor Function: The nervous system responds to stimuli via motor (efferent) neurons, which relay impulses to muscles and glands to elicit actions or physiological responses, such as muscle contraction or secretion of hormones.

Organization of the Nervous System

• Central Nervous System (CNS): Composed of the brain and spinal cord, it serves as the main processing center for the entire nervous system, coordinating messages between sensory input and motor output.

• Peripheral Nervous System (PNS): Encompasses all neurons outside the CNS, which includes:

  • Somatic Nervous System (SNS): Controls voluntary movements by transmitting sensory information and motor commands to skeletal muscles.

  • Autonomic Nervous System (ANS): Controls involuntary actions by regulating bodily functions, including heart rate and digestion, via sensory neurons from visceral organs and motor neurons to smooth muscles and glands. It is further divided into:

    • Sympathetic System: Responsible for the body's ‘fight or flight’ responses, preparing the body for stressful situations.

    • Parasympathetic System: Manages ‘rest and digest’ functions that promote relaxation and conserve energy.

  • Enteric Nervous System (ENS): Operates independently to manage and coordinate functions of the gastrointestinal tract, often referred to as the second brain due to its complexity and autonomy.

Neural and Neuroglial Cells

1. Neurons: Specialized cells responsible for transmitting nerve impulses (action potentials). They consist of:

   • Dendrites: Receive signals from other neurons.

   • Cell Body: Contains the nucleus and organelles.

   • Axon: Transmits impulses away from the cell body to other neurons or muscles. Neurons are electrically excitable, communicating through action potentials to convey information.

2. Neuroglial Cells (Glial Cells): Non-neuronal cells that provide support and protection for neurons. Key types include:

   • Astrocytes: Maintain the blood-brain barrier and provide metabolic support.

   • Oligodendrocytes: Myelinate axons in the CNS, insulating them for faster signal transmission.

   • Microglia: Act as immune cells in the CNS, responding to injury and disease.

   • Ependymal Cells: Line the ventricles of the brain and facilitate cerebrospinal fluid circulation.

   • Schwann Cells (PNS): Myelinate axons in the PNS, also crucial for nerve regeneration.

   • Satellite Cells (PNS): Support neuron bodies in ganglia, providing nutrient exchange.

Myelination Process

• Myelin Sheath: A protective layer that forms around axons, which significantly increases the speed of action potentials. Myelination occurs when Schwann cells wrap around PNS axons, while oligodendrocytes do the same in the CNS.

• Demyelination: The loss or damage of myelin sheaths, which can lead to neurological disorders such as Multiple Sclerosis, characterized by disrupted nerve conduction and physiological symptoms.

Electrical Impulses in Neurons

• Resting Membrane Potential: Typically around -70 mV, it is maintained by ion gradients and sodium-potassium exchange pumps, providing the baseline electrical state of a neuron.

• Graded Potentials: Local changes in membrane potential that vary in amplitude; if they reach a threshold, they trigger action potentials.

• Action Potentials: Rapid changes in membrane potential characterized by three main phases:

  1. Depolarization: Rapid influx of sodium ions, making the inside of the neuron more positive.

  2. Repolarization: Potassium ions exit the neuron, restoring a negative internal charge.

  3. Return to Resting Potential: The membrane potential stabilizes back to resting levels.

• Refractory Periods: Times during which neurons are temporarily less responsive to stimuli after an action potential, preventing the reuse of the same pathway too quickly.

Synaptic Communication

Involves the release of neurotransmitters from presynaptic neurons to postsynaptic neurons through chemical synapses. This communication can create:

• Excitatory Postsynaptic Potentials (EPSPs): Increase the likelihood of action potentials in the receiving neuron.

• Inhibitory Postsynaptic Potentials (IPSPs): Decrease the likelihood of action potentials in the receiving neuron, contributing to neural network dynamics.

Neurotransmitter Types & Functions

Diverse chemicals such as:

• Acetylcholine: Involved in muscle activation and memory.

• Norepinephrine: Plays a role in attention and responding actions, affecting arousal and alertness.

• Dopamine: Associated with pleasure, motivation, and motor control.

• Gamma-Aminobutyric Acid (GABA): Functions primarily as an inhibitory neurotransmitter, regulating neuronal excitability across the nervous system. These neurotransmitters play vital roles in signal transmission, mood regulation, and various bodily functions, influencing both psychological and physiological processes.

Neuronal Circuits & Plasticity

Neuronal circuits can be classified as:

• Diverging Circuits: Spread signals from one neuron to many neurons to amplify a signal.

• Converging Circuits: Receive signals from multiple neurons to a single output.

• Reverberating Circuits: Have feedback loops that maintain a persistent signal.

• Plasticity: Refers to the nervous system's ability to change and adapt based on experience, which includes learning and memory formation. While synaptic plasticity can enhance connections, neuron repair is limited, particularly in the CNS, making recovery from injury challenging.

Disorders of the Nervous System

Several disorders illustrate the complexities and vulnerabilities of the nervous system:

• Multiple Sclerosis: An autoimmune condition impacting myelination, leading to impaired communication between the brain and body.

• Epilepsy: Characterized by abnormal electrical discharges in the brain, resulting in seizures, which can disrupt normal functioning. These disorders highlight the importance of the nervous system's health and the need for ongoing research into effective treatments and interventions.