Understanding the Nervous System: Neurons, Nerves, Synapses, and the Central Nervous System
1. Neuron – Electrical Properties
Neurons, the fundamental units of the nervous system, are interconnected nerve cells responsible for transmitting information. They carry signals from sensory systems to nerve centers and elaborate responses sent to organs like muscles and glands.
Structure of a Neuron
Neurons consist of a cell body, extensions called dendrites, and a long axon. Axons are often associated with Schwann cells, which form a myelin sheath around them. The gaps between Schwann cells are known as Nodes of Ranvier.
Electrical Properties
Nerve impulses typically originate in dendrites, travel to the cell body, and then along the axon. Transmission occurs due to changes in the neuron’s membrane potential.
Membrane Potential
The membrane potential is the electrical difference between the inside and outside of the neuron, typically around -70 millivolts. Potassium ions are abundant inside the neuron, while sodium ions are mostly outside. Potassium can passively cross the membrane, but sodium channels are closed at rest. This creates an electrical gradient that attracts potassium ions inward, establishing an equilibrium.
When a stimulus is received, sodium channels open, allowing sodium influx and causing a charge reversal. The inside becomes positive, and the membrane potential rises to +50 mV. This change is brief, lasting only 0.5 milliseconds, after which the resting potential is restored.
2. Nerves and Synapses
Nerves, or fibers, can be categorized into three types based on their function:
Types of Nerves
1. Afferent (Sensory) Fibers
These fibers transmit nerve impulses from sensory organs to the central nervous system.
2. Efferent Fibers
Efferent fibers carry responses from the brain and spinal cord to effector organs. They are further divided into:
- Vegetative: Innervate glands and involuntary muscles (e.g., heart, gut)
- Motor: Innervate skeletal muscles responsible for voluntary movement (e.g., biceps, tongue)
3. Association Neurons
These neurons are responsible for interconnections within the nervous system, enabling functions like memory, thought, and emotion.
The Synapse
Synapses are junctions between neurons or between neurons and other cells (e.g., glands, muscles) that allow for the transmission of nerve impulses. Most synapses are chemical in nature.
Neurotransmitters are chemical substances stored in synaptic vesicles at the ends of axons. They transmit nerve impulses across the synapse.
Stages of Synapse
- Release of Neurotransmitter: When an action potential reaches the end of the presynaptic neuron, calcium channels open, triggering the release of neurotransmitters into the synaptic space.
- Recognition of Neurotransmitters: Neurotransmitters diffuse across the small synaptic space and bind to receptors on the postsynaptic neuron.
- Response of Postsynaptic Neuron: Receptor activation causes ion flux and a change in membrane potential, potentially generating a new action potential that propagates along the neuron.
3. Central Nervous System
The central nervous system (CNS) comprises the brain and spinal cord. It consists of millions of interconnected neurons, glial cells, collagen fibers, and blood vessels.
Cell bodies, which are gray in color, occupy the peripheral part of the brain and the central part of the spinal cord. Dendrites and axons, forming the white matter due to myelin, have the reverse arrangement.
The brain and spinal cord are protected by the skull and spine, respectively, and are further enveloped by three membranes called meninges: the dura mater, arachnoid mater, and pia mater. Cerebrospinal fluid circulates between the arachnoid and pia mater.
The Brain
The brain receives information from the peripheral nervous system, processes it, and generates responses. It connects to the spinal cord through the foramen magnum.
The brain consists of:
- Brain: Cerebral Hemispheres
- Diencephalon (Thalamus, Hypothalamus, Pituitary Gland)
- Cerebellum
- Brainstem