12.3 The Function of Nervous Tissue

Nervous Tissue Communication and Function

Nervous tissue has two main jobs: transmit signals quickly across long distances and pass those signals between cells with precision. It accomplishes this through a combination of electrical signals (within neurons) and chemical signals (between neurons). Understanding how these two signal types work together is the foundation for everything else in this unit.

Communication in nervous tissue

Neurons use electrical signals to carry information along their length, then switch to chemical signals to pass that information to the next cell. This electrical-to-chemical handoff is central to how the nervous system works.

Electrical signals

When a neuron is at rest (not firing), it maintains a stable charge difference across its membrane called the resting membrane potential (around $-70 \text{ mV}$). The inside of the cell is negative relative to the outside. This baseline charge is what makes signaling possible.

When a stimulus is strong enough to push the membrane potential to a critical level called threshold (around $-55 \text{ mV}$), the neuron fires an action potential. An action potential is a brief, rapid reversal of the membrane's charge that travels down the axon. Here's how it propagates:

1. A stimulus brings the membrane to threshold.
2. Voltage-gated sodium ($Na^+$) channels open, and $Na^+$ rushes into the cell, making the inside positive (depolarization).
3. Sodium channels inactivate, and voltage-gated potassium ($K^+$) channels open. $K^+$ flows out, restoring the negative charge (repolarization).
4. This cycle repeats at the next segment of the axon, so the action potential moves from the cell body toward the axon terminal.

Action potentials are all-or-nothing: once threshold is reached, the signal fires at full strength every time.

Chemical signals

Once the action potential reaches the axon terminal (synaptic terminal), the neuron switches to chemical signaling. The neuron releases molecules called neurotransmitters into the gap between cells. These neurotransmitters bind to specific receptors on the next cell (the postsynaptic cell) and produce one of two effects:

• Excitatory neurotransmitters make the postsynaptic cell more likely to fire an action potential. Examples: glutamate, acetylcholine.
• Inhibitory neurotransmitters make the postsynaptic cell less likely to fire. Examples: GABA, glycine.

Whether the postsynaptic neuron actually fires depends on the balance of excitatory and inhibitory signals it receives at any given moment.

Neuromodulators are a related category of chemicals that don't directly trigger or block action potentials but instead modify how strongly a neuron responds to other neurotransmitters. Think of them as adjusting the volume rather than flipping the switch.

Sensory input to motor output sequence

The nervous system follows a three-step pathway: detect a stimulus, process it, and respond. This sequence applies whether you're pulling your hand off a hot stove or deciding to raise your hand in class.

1. Sensory input

   • Sensory receptors detect stimuli from the internal or external environment. These are specialized structures tuned to specific stimulus types (light, sound, touch, temperature, etc.).
   • The receptor converts (transduces) stimulus energy into an electrical signal called a receptor potential.
   • Sensory neurons then carry these signals to the CNS (brain and spinal cord).

2. Processing in the CNS

   • The CNS receives sensory information and processes it through neural circuits that analyze, compare, and interpret the signals.
   • Much of this higher-order processing occurs in the cerebral cortex, the brain's outermost layer, which handles decision-making and complex interpretation.
   • Based on this processing, the CNS determines an appropriate response.

3. Motor output

   • The CNS sends commands through motor neurons to effector organs (muscles and glands).
   • Motor neurons transmit action potentials to these effectors, triggering a response such as muscle contraction or gland secretion.
   • The neuromuscular junction is the specialized synapse where a motor neuron communicates with a skeletal muscle fiber to initiate contraction.

Key structures of the nervous system

Sensory receptors are the entry point for all nervous system activity. They detect environmental changes and convert stimulus energy into receptor potentials, starting the chain of events that sends information to the CNS.

Neurons are the basic functional units of the nervous system. They generate and transmit action potentials and communicate via neurotransmitters. There are three functional types:

1. Sensory (afferent) neurons carry information from receptors toward the CNS.
2. Interneurons exist entirely within the CNS and are responsible for processing and integrating information. They connect sensory and motor pathways.
3. Motor (efferent) neurons carry commands from the CNS out to effector organs.

Synapses are the junctions where information passes from one neuron to the next (or from a neuron to a target cell). Each synapse has three components:

• The presynaptic neuron (sends the signal)
• The synaptic cleft (the tiny gap between cells)
• The postsynaptic cell (receives the signal)

Neurotransmitters are released from the presynaptic terminal, diffuse across the synaptic cleft, and bind to receptors on the postsynaptic cell.

Cerebral cortex is the outermost layer of the brain and is divided into four lobes, each associated with different functions:

The cortex plays a central role in integrating sensory information, generating responses, and coordinating complex behaviors.

Supporting cells and plasticity

Neuroglia (glial cells) are the support staff of the nervous system. They don't generate action potentials themselves, but they protect neurons, provide structural support, insulate axons, and help maintain the chemical environment neurons need to function.

Graded potentials are small, localized changes in membrane potential that vary in size depending on stimulus strength. Unlike action potentials, they are not all-or-nothing. Multiple graded potentials can add together (summation), and if their combined effect reaches threshold, they trigger an action potential. This is how the neuron "decides" whether a signal is strong enough to pass along.

Neuroplasticity refers to the nervous system's ability to reorganize and form new neural connections in response to learning, experience, or injury. This is why practice strengthens skills and why the brain can sometimes recover function after damage.