4.5 Nervous Tissue Mediates Perception and Response

Nervous Tissue Structure and Function

Nervous tissue is one of the four primary tissue types, and it's responsible for detecting stimuli, processing information, and coordinating the body's responses. Understanding how neurons and their supporting cells work at the tissue level sets the foundation for everything you'll study later about the brain, spinal cord, and peripheral nerves.

Structure and Function of Neurons

Neurons are the functional units of the nervous system. Each neuron has three main structural components:

• Cell body (soma) contains the nucleus and organelles needed for cellular function. This is the metabolic center of the neuron.
• Dendrites are branched extensions that receive incoming signals from other neurons or sensory receptors.
• Axon is a single long projection that carries signals away from the cell body toward other neurons, muscles, or glands.

Structural classification groups neurons by how many processes extend from the cell body:

• Multipolar neurons have multiple dendrites and one axon. Most neurons in the brain and spinal cord are multipolar, including motor neurons and interneurons.
• Bipolar neurons have one dendrite and one axon extending from opposite sides of the soma. You'll find these in specialized sensory organs like the retina and olfactory epithelium.
• Unipolar (pseudounipolar) neurons have a single process that splits into two branches. Most sensory neurons in the PNS are this type.

Functional classification groups neurons by the direction they carry signals:

• Sensory (afferent) neurons carry signals from receptors toward the CNS (touch, vision, hearing).
• Motor (efferent) neurons carry signals from the CNS to effector cells like muscles or glands.
• Interneurons connect neurons within the CNS, integrating and processing information. They're found in the brain and spinal cord.

Synaptic communication is how neurons pass signals to one another or to effector cells. A synapse is the junction where this transfer happens. The process works like this:

1. A signal arrives at the end of the presynaptic neuron.
2. The presynaptic neuron releases chemical messengers called neurotransmitters (such as acetylcholine, dopamine, or serotonin) into the synaptic cleft, the tiny gap between the two cells.
3. The postsynaptic cell has receptors that bind those neurotransmitters, triggering either an excitatory or inhibitory response.

A familiar example is the neuromuscular junction, where a motor neuron synapses with a skeletal muscle fiber to trigger contraction.

Generation of Action Potentials

At rest, a neuron's membrane sits at about $-70 \text{ mV}$, called the resting membrane potential. The inside of the cell is negative relative to the outside because of an unequal distribution of ions: high $K^+$ and low $Na^+$ inside, with the reverse outside. The sodium-potassium pump ($Na^+/K^+ \text{ ATPase}$) actively maintains this gradient by pumping 3 $Na^+$ out and 2 $K^+$ in per cycle, and the membrane is selectively permeable, mostly allowing $K^+$ to leak out at rest.

An action potential is the rapid, temporary reversal of that membrane potential. It's how neurons send signals over distance. Here's the sequence:

1. Stimulus and depolarization to threshold: A stimulus (chemical, mechanical, or electrical) opens ion channels, allowing $Na^+$ to enter. The membrane becomes less negative (depolarizes). If it reaches the threshold potential (around $-55 \text{ mV}$), an action potential fires.
2. Rising phase (depolarization): Voltage-gated $Na^+$ channels open rapidly. $Na^+$ rushes into the cell, driving the membrane potential toward about $+30 \text{ mV}$.
3. Falling phase (repolarization): Voltage-gated $Na^+$ channels inactivate, and voltage-gated $K^+$ channels open. $K^+$ flows out of the cell, bringing the membrane potential back toward resting levels.
4. Undershoot (hyperpolarization): $K^+$ channels are slow to close, so the membrane briefly dips more negative than $-70 \text{ mV}$ before returning to rest.
5. Propagation: The local depolarization from step 2 triggers voltage-gated $Na^+$ channels in the adjacent section of the axon, so the action potential regenerates itself down the length of the axon.

Two key principles to remember:

• All-or-none: Once threshold is reached, the action potential fires at full strength every time. A stronger stimulus doesn't produce a bigger action potential. Instead, stronger stimuli increase the frequency of action potentials.
• Refractory period: After firing, there's a brief window during which the neuron cannot fire again (absolute refractory period) or requires a stronger-than-normal stimulus (relative refractory period). This ensures the signal travels in one direction along the axon.

Myelination and saltatory conduction dramatically increase signal speed. Myelin is a fatty insulating layer wrapped around the axon by glial cells: oligodendrocytes in the CNS and Schwann cells in the PNS. The myelin sheath is interrupted at regular intervals by gaps called nodes of Ranvier, where voltage-gated ion channels are concentrated. Because ions can only flow at the nodes, the action potential effectively "jumps" from node to node. This is called saltatory conduction, and it can increase conduction velocity up to about 120 m/s compared to roughly 1–2 m/s in unmyelinated fibers of similar diameter.

Types and Roles of Glial Cells

Glial cells (neuroglia) outnumber neurons and provide essential support. They don't transmit action potentials themselves, but neurons can't function without them.

Astrocytes (CNS) are the most abundant glial cells and serve several roles:

• Form the blood-brain barrier by wrapping their foot processes around capillaries, tightly regulating what substances pass from blood into brain tissue.
• Regulate neurotransmitter levels in the synaptic cleft by taking up and recycling neurotransmitters like glutamate and GABA.
• Supply neurons with nutrients (glucose, lactate) and help maintain the extracellular environment, including $K^+$ concentration and pH.

Oligodendrocytes (CNS) form myelin sheaths around axons in the brain and spinal cord. A single oligodendrocyte can myelinate segments of up to 50 different axons.

Schwann cells (PNS) form myelin in the peripheral nervous system. Unlike oligodendrocytes, each Schwann cell wraps around just one segment of one axon. Schwann cells also play a critical role in nerve repair after injury: they form structures called bands of Büngner that guide a regenerating axon back toward its target.

Microglia (CNS) are the resident immune cells of the brain and spinal cord. They constantly survey their surroundings with branching processes, and when they detect damage or pathogens, they activate: phagocytosing debris, releasing cytokines (such as IL-1 and TNF-α) to promote inflammation, and recruiting repair processes.

Ependymal cells line the ventricles of the brain and the central canal of the spinal cord. They produce cerebrospinal fluid (CSF), which cushions and nourishes the CNS. Cilia on their surface help circulate CSF through the ventricular system.

Neuronal Plasticity and Signal Integration

Not every signal a neuron receives triggers an action potential. Graded potentials are small, localized changes in membrane potential that vary in size depending on stimulus strength. They can be either excitatory (depolarizing) or inhibitory (hyperpolarizing). When multiple graded potentials arrive at the axon hillock close together in time or space, they can add together, a process called summation. If the combined depolarization reaches threshold, an action potential fires.

Synaptic plasticity is the ability of synapses to become stronger or weaker based on how much they're used. Synapses that are repeatedly activated tend to strengthen (a process related to long-term potentiation), while underused synapses may weaken. This is a cellular basis for learning and memory.

Neuroplasticity refers more broadly to the nervous system's ability to reorganize by forming new synaptic connections throughout life. This capacity is what allows the brain to adapt after injury or to change in response to new experiences.