The nervous system is a complex system of cells and nerves that sends and receives messages to and from the brain, spinal cord, and other body components. It coordinates and controls body functions and responses to internal and external stimuli. The peripheral nervous system (PNS) and the central nervous system (CNS) are the two primary parts of the nervous system.
Central Nervous System (CNS):
The central nervous system (CNS) is a primary component of the nervous system that consists of the brain and spinal cord. It serves as the control center for the body, processing sensory information, coordinating body functions, and facilitating cognition and behaviour. Here’s a detailed look at the key components and functions of the CNS:
Components of the CNS:
1. Brain:
The brain is the most complex organ in the body, responsible for various high-level functions. It has been divided up into various main sections:
Cerebrum: The largest part of the brain, the cerebrum is divided into two hemispheres (left and right) and four lobes (frontal, parietal, temporal, and occipital). It is responsible for voluntary movements, speech, sensory perception, thought, memory, and emotion.
Frontal Lobe: Involved in reasoning, planning, movement, emotions, and problem-solving.
Parietal Lobe: Processes touch, temperature, and pain-related sensory data in the parietal lobe.
Temporal Lobe: Responsible for processing auditory information and is also important for memory and emotion.
Occipital Lobe: Primary area for processing visual information.
Cerebellum: Located under the cerebrum, the cerebellum coordinates voluntary movements, posture, balance, and motor learning.
Brainstem: The brainstem connects the brain to the spinal cord and controls many automatic functions necessary for survival, such as heart rate, breathing, and digestion. It is composed of up of the medulla oblongata, pons, and midbrain.
Midbrain: A region that controls movement of the body, eyes, hearing, and vision.
Pons: Relays signals between the cerebrum and the cerebellum, and plays a role in sleep and respiration.
Medulla Oblongata: Heart rate, blood pressure, and breathing are all regulated by the medulla oblongata.
Diencephalon: Located above the brainstem, it includes the thalamus and hypothalamus.
Thalamus: Serves as a hub for the transmission of motor and sensory impulses to the cerebral cortex.
Hypothalamus: Regulates homeostasis by controlling the autonomic nervous system, endocrine system, and functions such as hunger, thirst, sleep, and temperature regulation.
2. Spinal Cord:
The spinal cord is a cylindrical structure that extends from the brainstem down through the vertebral column. It is engaged in reflex actions as well as acting as a signal channel between the brain and the rest of the body.
Gray Matter: Contains neuronal cell bodies and is involved in muscle control and sensory perception.
White Matter: Composed of myelinated nerve fibers (axons) that transmit signals between different parts of the CNS.
Functions of the CNS:
Sensory Processing: The CNS receives and interprets sensory information from the peripheral nervous system (PNS), allowing the body to respond appropriately to stimuli.
Motor Control: The CNS sends signals to muscles to initiate and control movement.
Cognition: Includes functions such as thinking, reasoning, problem-solving, and memory.
Emotion Regulation: The brain processes and regulates emotions and emotional responses.
Homeostasis: The CNS helps maintain internal stability by regulating body functions such as temperature, hunger, and thirst.
Protection of the CNS:
The CNS is well-protected by several structures:
Skull and Vertebral Column: The brain is encased in the skull, and the spinal cord is protected by the vertebrae.
Meninges: Three protective membranes (dura mater, arachnoid mater, and pia mater) cover the brain and spinal cord.
Cerebrospinal Fluid (CSF): This fluid surrounds and cushions the CNS, providing mechanical protection and maintaining a stable chemical environment.
CNS Disorders:
Various disorders can affect the CNS, including:
Neurodegenerative Diseases: These include multiple sclerosis, Parkinson’s disease, and Alzheimer’s disease.
Traumatic Injuries: Like concussions, spinal cord injuries, and traumatic brain injuries.
Infections: Including meningitis and encephalitis.
Stroke: Disruption of blood flow to the brain leading to tissue damage.
Tumors: Abnormal growths within the CNS, either benign or malignant.
Peripheral Nervous System (PNS):
An essential component of the nervous system that connects the central nervous system (CNS) to the rest of the body is the peripheral nervous system, or PNS. It is located outside the CNS. It is responsible for transmitting information between the CNS and various parts of the body, thereby enabling the body to respond to environmental changes and maintain homeostasis. The somatic nervous system and the autonomic nervous system are the two primary parts of the PNS.
Components of the PNS:
1. Somatic Nervous System:
The somatic nervous system sends sensory data to the central nervous system (CNS) and regulates voluntary movements. It consists of:
Sensory (Afferent) Neurons: These neurons carry sensory information from sensory receptors in the skin, muscles, and joints to the CNS. They detect stimuli such as touch, pain, temperature, and proprioception (sense of body position).
Motor (Efferent) Neurons: These neurons transmit signals from the CNS to skeletal muscles, causing them to contract and produce movement.
2. Autonomic Nervous System (ANS):
The autonomic nerve system controls involuntary body processes like breathing, digestion, and heart rate. It operates subconsciously and is divided into three parts:
Sympathetic Nervous System: Prepares the body for stressful or emergency situations, often referred to as the “fight or flight” response. It promotes heart rate, dilates pupils, and inhibits digestion.
Parasympathetic Nervous System: Promotes relaxation and recovery, often called the “rest and digest” response. It decreases heart rate, constricts pupils, and stimulates digestion.
Enteric Nervous System: Sometimes considered a part of the ANS, it is a complex network of neurons that governs the function of the gastrointestinal tract. It can operate independently of the brain and spinal cord but communicates with them.
Functions of the PNS:
Sensory Input: The PNS collects sensory information from the environment and the body’s internal state and sends it to the CNS for processing.
Motor Output: The PNS conveys motor commands from the CNS to muscles and glands, controlling voluntary and involuntary actions.
Regulation of Homeostasis: Through the ANS, the PNS helps regulate vital body functions such as blood pressure, heart rate, respiratory rate, digestion, and body temperature.
Nerves in the PNS:
There are twelve pairs of cranial nerves and thirty-one pairs of spinal nerves in the PNS:
Cranial Nerves: These nerves emerge directly from the brain (including the brainstem) and are involved in sensory and motor functions of the head and neck. Examples include the optic nerve (vision), olfactory nerve (smell), and vagus nerve (regulation of internal organ functions).
Spinal Nerves: These nerves emerge from the spinal cord and innervate various parts of the body, including the trunk, arms, and legs. They are responsible for transmitting sensory information to the CNS and carrying motor commands to the muscles.
Neurotransmitters in the PNS:
Neurotransmitters are chemical messengers that transmit signals across synapses. Key neurotransmitters in the PNS include:
Acetylcholine: Used by motor neurons in the somatic nervous system to stimulate muscle contraction. It is also used in the ANS, where it acts on parasympathetic neurons.
Norepinephrine: Primarily used by the sympathetic nervous system to prepare the body for action.
Disorders of the PNS:
Disorders of the PNS can affect sensory and motor functions and include:
Peripheral Neuropathy: Damage to peripheral nerves causing weakness, numbness, and pain, often in the hands and feet. Causes include diabetes, infections, and injuries.
Guillain-Barré Syndrome: An autoimmune disorder where the body’s immune system attacks peripheral nerves, leading to muscle weakness and paralysis.
Carpal Tunnel Syndrome: Hand and finger discomfort and numbness due to compression of the median nerve in the wrist.