Understanding the Human Nervous System: Structure and Function
The Nervous System: Receiving and Responding to Stimuli
The nervous system receives and responds to stimuli, maintaining the body’s homeostasis. It coordinates the activities of tissues, organs, and systems.
Neurons: The Building Blocks
Neurons are the primary cells responsible for transmitting nerve impulses. They consist of:
- Cell Body: Contains the nucleus and cytoplasm.
- Dendrites: Short, numerous extensions that capture nerve impulses.
- Axon: A long extension that conducts impulses to distant areas, forming nerves when bundled together (e.g., the sciatic nerve).
Nerve impulses travel in one direction: dendrites to cell body to axon. The speed of impulse propagation varies based on the presence of Schwann cells, which produce myelin. Myelinated neurons transmit impulses faster due to saltatory conduction (e.g., the optic nerve). Unmyelinated neurons conduct impulses more slowly (e.g., pain receptors).
Synapses: Connecting Neurons
Synapses are junctions where nerve impulses pass between neurons via neurotransmitters, involving a presynaptic neuron, a synaptic space, and a postsynaptic neuron.
Divisions of the Nervous System
Central Nervous System (CNS)
The CNS includes the brain (conscious and voluntary responses) and the spinal cord (unintentional acts). The brain comprises the cerebrum, cerebellum, and medulla oblongata, protected by the meninges and skull. The spinal cord, also protected by meninges and the vertebral column, receives sensory nerves and sends out motor nerves.
Peripheral Nervous System (PNS)
The PNS consists of sensory and motor nerves, categorized as:
- Cranial Nerves: Originating from the brain.
- Spinal Nerves: Originating from the spinal cord (e.g., sciatic nerve).
Functionally, nerves are:
- Somatic: Controls voluntary skeletal muscle movement.
- Autonomic: Controls involuntary internal organ activity.
White matter (myelinated axons) is found inside the brain and outside the spinal cord. Gray matter (unmyelinated axons) is in the brain’s cortex and the spinal cord’s interior.
Spinal Cord and Reflexes
The spinal cord facilitates rapid responses to stimuli through reflex arcs, involving sensory neurons, interneurons, and motor neurons.
Higher Brain Functions
The brain’s cortex regulates higher functions, divided into four lobes controlling muscle movement and senses. The cerebellum controls body movements and posture, ensuring smooth and precise actions. The medulla oblongata manages vital functions like heart rate, respiration, and protective reflexes.
Nervous Tissue
Nervous tissue comprises neurons and glial cells (neuroglia). Glial cells include:
- Astrocytes: Abundant in the CNS, providing nutrients to neurons.
- Microglia: Protective cells defending against harmful substances.
- Schwann Cells: In the PNS, insulating neurons with myelin sheaths.