Neurons and the Nervous System: Structure, Function, and Communication

Neurons

and the Nervous System

2.1 Neuron Structure and Function

Neurons are the fundamental units of the nervous system, specialized for the reception, integration, and transmission of information. Each neuron consists of a cell body and several structures adapted to its function.

• Cell Body (Soma): Contains the nucleus and organelles; responsible for metabolic activities.

• Dendrites: Receive signals from other neurons.

• Axon Hillock: Junction between the cell body and axon; site of action potential initiation.

• Axon: Transmits signals to other cells, often covered by myelin sheath for faster conduction.

• Axon Terminal: Releases neurotransmitters to communicate with other neurons or effectors.

Neuronal functions can be classified as:

• Reception: Dendrites receive signals and send them toward the axon hillock.

• Integration: Signals are summed at the axon hillock; if threshold is reached, an action potential is initiated.

• Conduction: The axon conducts the signal to another neuron or effector.

• Transmission: Signals are sent through axon terminals via neurotransmitters.

• Response: Effectors such as muscles or glands carry out the response.

2.2 Membrane and Action Potentials

Neurons maintain a resting membrane potential due to ion distribution across their membranes. Changes in ion distribution trigger the creation and transmission of action potentials.

• Resting Potential: Typically around -70 mV. Na+ is mostly outside the cell; K+ is mostly inside. The Na+/K+ pump maintains this gradient by pumping Na+ out and K+ in.

• Action Potential: Rapid change in membrane potential that propagates along the axon.

• Impulse Initiation: An impulse is initiated if the membrane potential reaches about -50 or -55 mV (threshold).

• Depolarization: Na+ enters the cell, making the membrane potential more positive.

• Repolarization: K+ exits the cell, returning the membrane potential to a negative value.

• Hyperpolarization: Membrane potential becomes more negative than resting potential before stabilizing.

Key Equation:

$V_m = V_{inside} - V_{outside}$

Where $V_m$ is the membrane potential.

2.3 Synaptic Transmission

Synaptic transmission is the process by which neurons communicate with other neurons or effectors (e.g., muscle fibers, glands).

• Synapse: Junction where a neuron makes a communicating connection with another cell.

• Electrical Synapse: Direct contact between plasma membranes via gap junctions; ions flow directly between cells.

• Chemical Synapse: Neurotransmitters are released from the axon terminal into the synaptic cleft, binding to receptors on the postsynaptic cell.

2.4 Integration of Signals

Neurotransmitters regulate the opening and closing of Na+ and K+ channels in the postsynaptic membrane, influencing the generation of action potentials.

• Excitatory Neurotransmitters: Lower the impulse initiation threshold, making action potentials more likely.

• Inhibitory Neurotransmitters: Raise the impulse initiation threshold, making action potentials less likely.

2.5 Neurotransmitters

Neurotransmitters are chemical messengers used in synaptic transmission. They can be excitatory or inhibitory, depending on their effect on the postsynaptic cell.

Neurotransmitter	Site(s) of Action	Action
Acetylcholine	Between some neurons of CNS and at neuromuscular junctions	Excitatory
Norepinephrine	CNS interneurons; also in diverse brain and body locations	Excitatory or inhibitory
GABA	Many CNS pathways; often acts as the same circuits as glutamate	Inhibitory (main inhibitory neurotransmitter in mammalian CNS)
Endorphins	Most act on CNS and PNS; effects such as muscle, reducing pain, and in some cases, also inducing euphoria	Inhibitory; modulate pain system

2.6 Neuroglia

Neuroglia are supportive cells in the nervous system, each with specialized functions.

• Astrocytes: Maintain ion concentrations in the interstitial fluid surrounding neurons in the CNS; communicate with neurons and influence their activity.

• Oligodendrocytes: Form myelin sheaths in the CNS.

• Schwann Cells: Form myelin sheaths in the PNS.

• Microglia: Act as immune cells in the CNS.

• Ependymal Cells: Line ventricles and produce cerebrospinal fluid.

2.7 Overview of the Nervous System

The nervous system is organized into the central nervous system (CNS) and peripheral nervous system (PNS), each with distinct roles in sensory input, integration, and motor output.

• Central Nervous System (CNS): Consists of the brain and spinal cord; responsible for integrating sensory information and coordinating responses.

• Peripheral Nervous System (PNS): Includes all neural tissue outside the CNS; transmits sensory input to the CNS and motor output to effectors.

• Somatic Nervous System: Controls voluntary movements via skeletal muscles.

• Autonomic Nervous System: Regulates involuntary functions such as heart rate, digestion, and respiratory rate.

Example: Sensory neurons in the PNS detect stimuli and send signals to the CNS, which processes the information and sends motor commands back through the PNS to effectors.

Additional info: Academic context and definitions have been expanded for clarity and completeness. The neurotransmitter table has been recreated and summarized for study purposes.