Nervous System

Table of contents

Introduction

The nervous system is a highly specialized and complex network of cells and organs that coordinates and integrates the body’s response to internal and external stimuli.
It manages everything from sensory input, motor output, and cognitive functions to regulating vital processes like heart rate, blood pressure, and respiration.
It comprises two major divisions: the central nervous system (CNS), the brain and spinal cord, and the peripheral nervous system (PNS), which includes all the other neural structures throughout the body.

Anatomy of the Nervous System

Central Nervous System (CNS)

The CNS is the control center of the body, integrating information from the entire body and coordinating the body’s responses. It consists of two main components: the brain and the spinal cord.

Brain: The brain is the largest and most complex organ of the CNS and is divided into several regions, each responsible for different functions:
- Cerebrum:
  - The cerebrum is the largest part of the brain and is responsible for most higher functions, such as reasoning, problem-solving, speech, emotions, and sensory processing. It is divided into two hemispheres (right and left), and each hemisphere is further divided into lobes:
    - Frontal Lobe: Responsible for voluntary movement, reasoning, planning, and higher cognitive functions such as decision-making, problem-solving, and speech production (via Broca’s area).
    - Parietal Lobe: Involved in processing sensory information, such as touch, temperature, pain, and pressure. It also contributes to spatial awareness and coordination.
    - Temporal Lobe: Responsible for processing auditory information, memory, and language comprehension (via Wernicke’s area).
    - Occipital Lobe: Primarily responsible for processing visual information.
- Cerebellum:
  Located at the back of the brain, the cerebellum coordinates voluntary movements, balance, posture, and motor learning. It ensures smooth, controlled movements and helps maintain equilibrium during physical activity.
- Brainstem:
  The brainstem connects the brain to the spinal cord and controls many basic life-sustaining functions. It is divided into:
  - Midbrain: Involved in vision, hearing, eye, and body movement.
  - Pons: Relays signals between the forebrain and cerebellum and regulates breathing and sleep cycles.
  - Medulla Oblongata: Controls vital autonomic functions such as heart rate, blood pressure, respiration, and digestion.
- Diencephalon:
  Located deep within the brain, the diencephalon includes the thalamus, which processes sensory information before it reaches the cerebral cortex, and the hypothalamus, which plays a crucial role in maintaining homeostasis by regulating body temperature, hunger, thirst, and circadian rhythms.
Spinal Cord:
- The spinal cord is a long, cylindrical structure extending from the brainstem to the vertebral column.
- It is the primary pathway for information travelling between the brain and the body.
- It is also the site of reflex arcs, allowing for rapid, involuntary responses to stimuli.
- The spinal cord is protected by vertebrae, cerebrospinal fluid, and meninges (protective membranes).
- It is segmented into regions that correspond to the regions of the body it serves: cervical, thoracic, lumbar, sacral, and coccygeal.

Peripheral Nervous System (PNS)

The PNS consists of all the nerves outside the brain and spinal cord and connects the CNS to the rest of the body. The PNS is divided into the somatic and autonomic nervous systems (ANS).

Somatic Nervous System (SNS): The SNS controls voluntary movements and relays sensory information from the skin, muscles, and joints to the CNS. It enables conscious control over skeletal muscles and the sensation of touch, temperature, pain, and proprioception (awareness of body position).
Autonomic Nervous System (ANS): The ANS regulates involuntary functions and maintains homeostasis. It controls the activity of smooth muscles, cardiac muscles, and glands. The ANS is further divided into:
- Sympathetic Nervous System:
  Often described as the “fight or flight” system, it prepares the body for stressful situations. It increases heart rate, dilates pupils, redirects blood flow to muscles, and stimulates the release of glucose for energy. It is activated during physical or emotional stress.
- Parasympathetic Nervous System:
  Often called the “rest and digest” system, the parasympathetic nervous system promotes relaxation, energy conservation, and digestion. It slows heart rate, stimulates digestive processes, and helps the body return to a state of rest after a stress response.
- Enteric Nervous System:
  Sometimes considered a third division of the ANS, the enteric nervous system controls the gastrointestinal system. It contains a vast network of neurons that regulate digestion, bowel movements, and nutrient absorption.

Physiology of the Nervous System

The nervous system transmits signals across the body using electrical impulses (action potentials) and chemical messengers (neurotransmitters). The key functions of the nervous system rely on the interaction between neurons and the brain’s control over various body systems.

Neurons and Neural Transmission:

Structure of Neurons: Neurons are the basic units of the nervous system. They are specialized cells designed to transmit electrical impulses and communicate with other cells. Each neuron has three main parts:
- Cell Body (Soma): The cell body contains the nucleus and is responsible for the metabolic functions of the neuron.
- Dendrites: Short, branched extensions that receive signals from other neurons and transmit them to the cell body.
- Axon: A long, slender projection that transmits electrical impulses away from the cell body to other neurons, muscles, or glands.
- Axon Terminals: These are the endings of the axon, where electrical impulses are converted into chemical signals (neurotransmitters) that cross synapses to reach the next cell.
Action Potential:
Action potentials are electrical impulses that travel along the axon. They occur when a neuron’s membrane potential rapidly changes, allowing ions like sodium (Na+) and potassium (K+) to move in and out of the neuron, generating an electrical signal.
Synaptic Transmission:
Electrical signals are converted into chemical signals at the synapse (the gap between two neurons). Neurotransmitters (chemical messengers) are released from the axon terminals and bind to receptors on the next neuron, either exciting or inhibiting the transmission of the signal. Some common neurotransmitters include:
- Acetylcholine: Involved in muscle activation and memory.
- Dopamine: Important for mood regulation, pleasure, and motor control.
- Serotonin: Affects mood, sleep, and appetite.
- Gamma-Aminobutyric Acid (GABA): An inhibitory neurotransmitter that calms neural activity.
- Glutamate: An excitatory neurotransmitter involved in memory and learning.

Reflex Arc:

A reflex arc is a simple, involuntary neural pathway controlling reflex action.
It allows for rapid responses to stimuli without the need for conscious thought.
For example, when you touch something hot, sensory neurons detect the pain and transmit it to the spinal cord, where it is processed.
A motor signal is returned to the muscles to withdraw the hand before the brain registers the pain.

Components of a Reflex Arc:
- Receptor: Detects the stimulus (e.g., pain).
- Sensory Neuron: Transmits the sensory information to the CNS.
- Interneuron: (In the spinal cord) processes the information and forms a response.
- Motor Neuron: Carries the motor command from the CNS to the muscles or glands.
- Effector: The muscle or gland that produces the reflex action.

Functions

The nervous system performs several critical functions, ensuring the proper coordination of bodily activities.

Sensory Input:

The nervous system receives sensory input from various receptors located in the skin, muscles, joints, and organs.
These sensory signals are sent to the brain for processing, allowing the body to respond appropriately to external and internal stimuli.

Integration and Processing:

Once sensory information is received, the CNS processes and integrates this data to determine an appropriate response.
This can involve complex cognitive functions like thinking, reasoning, and emotional regulation.
The brain also interprets sensory information, allowing for perception of the environment.

Motor Output:

The nervous system produces motor output by sending commands from the brain or spinal cord to the muscles and glands, allowing for voluntary and involuntary movements.
This includes everything from walking to involuntary processes like digestion and heart contraction.

Homeostasis:

The nervous system plays a critical role in maintaining homeostasis.
It regulates key physiological processes such as temperature, heart rate, respiration, blood pressure, and hormone secretion.
The autonomic nervous system adjusts these parameters to ensure a stable internal environment.

Cognitive Functions:

The brain is responsible for higher cognitive functions, such as memory, decision-making, reasoning, language, and learning.
These functions are enabled by the complex interactions between different brain regions, including the prefrontal cortex (for decision-making) and the hippocampus (for memory formation).

Diseases and Disorders

The nervous system is susceptible to various diseases and disorders that can affect its ability to function properly. Some of these include:

Neurodegenerative Diseases: Conditions like Alzheimer’s disease, Parkinson’s disease, and Huntington’s disease involve the progressive degeneration of neurons, leading to cognitive, motor, and behavioral impairments.
Stroke: A stroke occurs when blood flow to the brain is interrupted, leading to brain cell damage and loss of function in the affected area.
Multiple Sclerosis (MS): MS is an autoimmune disease where the immune system attacks the myelin sheath, leading to communication problems between the brain and body.
Epilepsy: Epilepsy is characterized by abnormal electrical activity in the brain, leading to recurrent seizures.
Mental Health Disorders: Imbalances in neurotransmitters or brain structure/function contribute to conditions like depression, anxiety, schizophrenia, and bipolar disorder.