Definition and Classification of Nervous System
Introduction to Nervous System
The nervous system detects and responds to changes inside and outside the body. Together with the endocrine system, it coordinates and controls vital body functions and maintains homeostasis. While the nervous system provides immediate responses, endocrine activity is usually slower and more prolonged.
The nervous system consists of the brain, spinal cord, and peripheral nerves. Its structure allows rapid communication between all body parts. For descriptive purposes, it is grouped as:
- Central Nervous System (CNS): Brain and spinal cord.
- Peripheral Nervous System (PNS): All nerves outside the brain and spinal cord.
Peripheral Nervous System (PNS)
The PNS includes paired cranial and spinal nerves. Some nerves are sensory (afferent) transmitting impulses to the CNS, some are motor (efferent) transmitting impulses from the CNS, and others are mixed.
Functionally, the PNS is divided into:
- Sensory division: Transmits sensory information to the CNS.
- Motor division: Controls effector organs and is subdivided into:
- Somatic Nervous System: Voluntary control of skeletal muscles.
- Autonomic Nervous System: Involuntary control of heartbeat, peristalsis, and glands; further divided into sympathetic and parasympathetic systems.
The CNS integrates sensory input and sends responses via motor nerves to muscles and glands, maintaining both voluntary and involuntary body functions.
Neuroglia and Neurons:
Neuroglia
Neuroglia cells support neurons and make up about half the CNS. There are six types:
- Osteocytes: Metabolize neurotransmitters, maintain potassium balance, assist brain development, and help form the blood-brain barrier.
- Oligodendrocytes: Form myelin sheaths around CNS neurons.
- Microglia: Phagocytic cells that protect CNS from infection and remove debris.
- Ependymal cells: Line brain ventricles, produce and circulate cerebrospinal fluid.
- Neurolemmocytes (Schwann cells): Produce myelin sheaths around PNS neurons.
- Satellite cells: Support neurons in PNS ganglia.
Neurons
Neurons consist of three main parts: cell body, dendrites, and axon.
- Cell body: Contains the nucleus, cytoplasm, mitochondria, Golgi complex, chromatophilic substance (Nissl bodies), and neurofibrils (cytoskeleton for support).
- Dendrites: Short, branched, non-myelinated processes that receive signals.
- Axon: Long cylindrical process that transmits impulses to neurons, muscles, or glands. Axons have axolemma (plasma membrane), axoplasm, axon collaterals, and axon terminals containing synaptic vesicles with neurotransmitters.
Classification of Neurons:
Structural Classification
- Unipolar neurons: Single process from cell body, sensory in nature, branches into dendritic (sensory) and axonal (motor) pathways.
- Bipolar neurons: One dendrite and one axon; found in retina, inner ear, and olfactory regions.
- Multipolar neurons: Several dendrites and one axon; most common in brain and spinal cord.
Functional Classification
- Sensory (afferent) neurons: Transmit impulses from sensory receptors to CNS.
- Motor (efferent) neurons: Transmit impulses from CNS to muscles and glands.
- Association (interneurons): Connect sensory and motor neurons; form 90% of neurons in the body.
Functional Subtypes of Neurons
- General somatic sensory: pain, temperature, touch, vibration, pressure.
- Special somatic sensory: vision, hearing, balance.
- General visceral sensory: internal organ status and chemical changes.
- Special visceral sensory: taste and smell.
- General somatic motor: skeletal muscles.
- General visceral motor: smooth muscles, cardiac muscles, glands.
- Special visceral motor: skeletal muscles for facial expression, jaw, neck, larynx, pharynx.
Grey and White Matter
In the brain and spinal cord, white matter consists of myelinated axons, while grey matter consists of neuron cell bodies, dendrites, unmyelinated axons, and neuroglia. Grey matter is found in the cerebral cortex, basal ganglia, and spinal cord horns, whereas white matter forms tracts connecting different CNS regions.
Detailed Notes
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