4.5 Nervous Tissue Mediates Perception and Response

Neurons are the stars of the nervous system, transmitting signals throughout your body. These specialized cells, with their unique structures like dendrites and axons, work together to process information and control bodily functions.

Action potentials are the electrical signals that neurons use to communicate. These all-or-nothing events involve rapid changes in ion concentrations across the cell membrane, allowing messages to zip along nerve fibers at lightning speed.

Nervous Tissue Structure and Function

Structure and function of neurons

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• Neurons are the functional units of the nervous system that transmit signals throughout the body
  • Consist of a cell body (soma) which contains the nucleus and organelles necessary for cellular function
  • Dendrites are branched extensions that receive signals from other neurons (sensory input)
  • Axon is a long, thin projection that transmits signals to other neurons or effector cells (motor output)
• Neurons are classified by structure and function to reflect their diverse roles
  • Unipolar neurons have a single process extending from the cell body (sensory neurons in the PNS)
  • Bipolar neurons have two processes extending from the cell body (retinal cells and olfactory neurons)
  • Multipolar neurons have multiple dendrites and a single axon (motor neurons and interneurons)
  • Sensory neurons transmit signals from sensory receptors to the central nervous system (CNS) (touch, vision, hearing)
  • Motor neurons transmit signals from the CNS to effector cells (muscles or glands) to initiate movement or secretion
  • Interneurons transmit signals between neurons within the CNS to integrate and process information (brain and spinal cord)
• Neurons communicate through synapses to transmit signals throughout the nervous system
  • Synapses are the junctions between neurons or between neurons and effector cells (neuromuscular junction)
  • Presynaptic neuron releases neurotransmitters into the synaptic cleft (acetylcholine, dopamine, serotonin)
  • Postsynaptic neuron or effector cell receives the neurotransmitters via neurotransmitter receptors, triggering a response (excitation or inhibition)

Generation of action potentials

• Neurons maintain a resting membrane potential of around -70 mV due to unequal distribution of ions
  • The inside of the neuron is negatively charged relative to the outside due to high $K^+$ and low $Na^+$ inside the cell
  • Resting membrane potential is maintained by the sodium-potassium pump ($Na^+/K^+$ ATPase) and selective permeability of the cell membrane
• Action potentials are generated when the neuron is sufficiently stimulated, causing a rapid change in membrane potential
  • Stimulation causes the cell membrane to depolarize (become less negative) due to opening of ligand-gated or voltage-gated ion channels
  • If the depolarization reaches the threshold potential (around -55 mV), an action potential is triggered
• Action potential propagation involves changes in membrane permeability to $Na^+$ and $K^+$ ions

1. Voltage-gated sodium channels open, allowing $Na^+$ to rush into the cell, further depolarizing the membrane (rising phase)
2. Voltage-gated potassium channels open, allowing $K^+$ to rush out of the cell, repolarizing the membrane (falling phase)
3. The action potential propagates along the axon in a self-regenerating manner due to local current flow

• Action potentials are "all-or-none" events, meaning they occur at full strength or not at all
  • Once the threshold potential is reached, the action potential will propagate to completion regardless of stimulus strength
  • The strength of the stimulus does not affect the amplitude or speed of the action potential
• Myelination increases the speed of action potential propagation by insulating the axon
  • Myelin is an insulating layer formed by glial cells (oligodendrocytes in CNS, Schwann cells in PNS) around the axon
  • Myelin sheaths are interrupted by nodes of Ranvier, where voltage-gated ion channels are concentrated
  • Action potentials "jump" from one node to the next (saltatory conduction), increasing conduction velocity up to 120 m/s
• After an action potential, there is a brief refractory period during which the neuron cannot generate another action potential, ensuring unidirectional signal propagation

Types and roles of glial cells

• Astrocytes provide structural support and maintain homeostasis in the CNS
  • Form the blood-brain barrier, regulating the exchange of substances between blood and brain tissue
  • Regulate neurotransmitter levels in the synaptic cleft by uptake and recycling (glutamate, GABA)
  • Provide nutrients to neurons (glucose, lactate) and help maintain the extracellular environment ($K^+$, $pH$)
• Oligodendrocytes form myelin sheaths around axons in the CNS to increase conduction velocity
  • Myelin insulates axons and increases the speed of action potential propagation via saltatory conduction
  • Each oligodendrocyte can myelinate multiple axon segments, up to 50 at a time
• Schwann cells form myelin sheaths around axons in the peripheral nervous system (PNS)
  • Each Schwann cell myelinates a single axon segment between nodes of Ranvier
  • Schwann cells also aid in axon regeneration after injury by forming bands of Büngner to guide axon growth
• Microglia are the immune cells of the CNS that protect against infection and damage
  • Constantly monitor the brain for damage or infection using ramified processes
  • Phagocytose debris and pathogens to clear the CNS of potentially harmful substances
  • Release cytokines (IL-1, TNF-α) to promote inflammation and repair in response to injury or disease
• Ependymal cells line the ventricles of the brain and central canal of the spinal cord
  • Produce cerebrospinal fluid (CSF) which cushions and nourishes the brain and spinal cord
  • Facilitate the circulation of CSF throughout the CNS via beating of cilia on their apical surface

Neuronal Plasticity and Signal Integration

• Synaptic plasticity refers to the ability of synapses to strengthen or weaken over time in response to increased or decreased activity
• Graded potentials are small changes in membrane potential that can summate to trigger an action potential
• Neuroplasticity is the brain's ability to reorganize itself by forming new neural connections throughout life, which is crucial for learning and memory