Nervous System: Structure, Function, and Key Components
The medulla oblongata, adjacent to the spinal cord and the pons, measures approximately 15 to 25 mm in length. Its primary role is regulating heart and respiratory muscles. It is also involved in chewing, vomiting, and coughing.
Pons (Bridge of Variola)
The pons acts as a bridge between the midbrain and medulla oblongata. It facilitates communication through two types of nerve fibers:
- Afferent: Transmit information towards the brain.
- Efferent: Carry signals away from the brain.
The pons plays a crucial role in maintaining posture and the physiological process of respiration (gas exchange).
Spinal Cord
The spinal cord is a part of the nervous system that conducts impulses between the periphery and the central nervous system. Anatomically, it resembles a nerve cord, extending from the first cervical vertebra (atlas and axis) to the sacral region. Its diameter is approximately 1 cm, with two enlargements:
- Cervical enlargement: Supplies nerves to the upper limbs.
- Lumbar enlargement: Provides nerves to the lower limbs.
The spinal cord is protected by three membranes called meninges:
- Dura mater (outermost)
- Arachnoid mater (middle)
- Pia mater (innermost)
Thirty-one pairs of nerves branch out from the spinal cord, encircling the thoracic and abdominal cavities. These nerves serve as:
- Centers for reflex activity (e.g., knee-jerk reflex)
- Conductors of sensory impulses to the brain and motor impulses to effectors.
Autonomic Nervous System (ANS)
The ANS governs involuntary vegetative functions. It comprises two branches:
- Sympathetic nervous system: Consists of 23 interconnected nodes along both sides of the spine. These nodes regulate breathing, circulation, and secretions. They are grouped into cervical, thoracic, lumbar, and sacral regions. Physiologically, the sympathetic system activates during stress, causing responses like inhibiting digestion, dilating pupils, and accelerating heart and respiratory rates.
- Parasympathetic nervous system: Located near the sympathetic system, it consists of fibers and spinal nerves. Its function is antagonistic to the sympathetic system. It has two portions: the craniobulbar portion (housing nerves like the oculomotor, facial, glossopharyngeal, and vagus nerves) and the sacral portion (innervating the rectum and bladder, promoting relaxation and bowel evacuation). This system aids digestion, contracts pupils, and promotes relaxation.
Organs and viscera are innervated by both sympathetic and parasympathetic fibers.
Key Functions of the ANS:
- Regulates vegetative functions.
- Two antagonistic systems.
- Organs receive innervation from both systems.
Functional Systems of the Nervous System
The Neuron
The neuron is the fundamental unit of the nervous system. It consists of:
- Soma (cell body)
- Extensions (axon and dendrites)
Dendrites receive information, which is then transmitted to the soma and subsequently to the axon. The axon relays the signal to another neuron through synapses. Neurons cannot reproduce or regenerate. The myelin sheath acts as an insulator, accelerating nerve impulse transmission. The gaps in the myelin sheath are called Nodes of Ranvier, and the cells forming the myelin sheath are Schwann cells.
Neurons are classified based on their function:
- Sensory neurons: Transmit impulses from sensory receptors to the brain.
- Motor neurons: Carry impulses from the brain to organs, triggering responses.
- Association neurons (interneurons): Connect sensory and motor neurons.
Basic Processes of Neurons (Synapses)
Synapses are the junctions where nerve impulses are transmitted between neurons or between a neuron and an effector cell (e.g., muscle cell). Neurons transmit impulses over long distances without losing power, often at high speeds, following the all-or-none principle.
Synapses can occur between:
- Neurons
- A neuron and a receptor cell
- A neuron and a muscle cell (neuromuscular junction)
- A neuron and an epithelial cell (neuroepithelial junction)
Impulses can be triggered by external stimuli (e.g., touching a hot object) or internal stimuli (e.g., hunger). The typical pathway is: dendrite → soma → axon. The electrical impulse involves the exchange of positive and negative charges. Most synapses (99%) are excitatory, while some have inhibitory functions. Acetylcholine is a neurotransmitter involved in muscle contraction and relaxation.
Peripheral Nervous System
The peripheral nervous system comprises nerve endings, including:
- Afferent fibers: Transmit sensory information to the spinal cord and brain.
- Efferent fibers: Relay information from the brain to muscles.
It consists of:
- Cranial nerves
- Spinal nerves (surrounding the thoracic cavity). The vagus nerve extends into the thoracic and abdominal cavities.
There are 12 pairs of cranial nerves and 31 pairs of spinal nerves. Of the cranial nerves, the vagus nerve plays a significant role in the stomach. Cranial nerves are involved in hearing, vision, balance, head movement, taste, chewing, and swallowing. Spinal nerves form two plexuses:
- Brachial plexus: Supplies the upper extremities.
- Lumbar plexus: Supplies the lower extremities.
Peripheral nerves are categorized as:
- Sensory nerves: Transmit sensory signals.
- Motor nerves: Control muscle movement.
- Mixed nerves: Have both sensory and motor functions.