Nervous system | 3A: Structure and functions of nervous and endocrine systems | Bio/biochem

Major functions

Major functions of the nervous system involve controlling and integrating body processes, responding to external stimuli, and enabling both sensory (afferent) input and motor (efferent) output. This framework supports higher-level integrative and cognitive activities essential for complex behaviors.

Nervous system organization

The nervous system in vertebrates has 2 divisions:

CNS- the brain and spinal cord PNS- all other neural elements

Within the PNS, the somatic nervous system governs voluntary control over skeletal muscles, while the autonomic nervous system regulates involuntary functions of visceral organs.

Autonomic nervous system

The autonomic system splits into sympathetic (fight or flight- increasing heart rate and blood pressure, redirecting blood to muscles, dilating pupils, and breaking down glycogen for glucose release) and parasympathetic (rest- reducing heart rate and blood pressure, channeling blood toward digestion, constricting pupils, and converting glucose to glycogen for storage) divisions. These systems are often referred to as antagonistic, as they are opposite reactions.

Sympathetic nervous system effects on target organs

Within the ANS:

Sensor neurons detect changes and relay information to the CNS
Effector neurons convey commands from the CNS to target tissues. :::

Reflex arcs and feedback loops

Positive feedback loops amplify an initial event. Examples are when uterine contractions trigger oxytocin release, further intensifying contractions, or when activated platelets attract more platelets for clot formation.

Negative feedback loops counteract an event Negative feedback is illustrated by the regulation of blood pressure: a drop in blood pressure triggers ADH release to raise it, while an increase reduces ADH secretion.

A reflex arc is typically an example of negative and rapid feedback. A typical reflex arc includes:

A receptor detecting a stimulus
A sensor neuron transmitting signals to an integration center (often in the spinal cord)
A motor neuron directing the response
An effector (such as a muscle)

Common examples are the knee-jerk and withdrawal reflexes, both negative feedback mechanisms designed to safeguard the body.

The Golgi tendon reflex prevents excessive muscle tension by easing contraction when forces rise too high.

Although most spinal reflexes occur independently of direct brain input, efferent control means the brain can override them when a conscious decision is made (e.g., holding still or not yelling when getting your ears pierced).

Nerve Cell

Cell body: site of nucleus, organelles
A nerve cell includes a cell body, which houses the nucleus and other organelles. This region synthesizes numerous proteins, supported by extensive rough endoplasmic reticulum and Golgi complexes. Attached to the cell body are dendrites, branching structures that serve as the neuron’s receptive region, boosting surface area for incoming signals.

Extending away from the cell body is a single axon, which conducts electrical impulses toward the axon terminals—sometimes called synaptic knobs or boutons—where neurotransmitters are released.

The axon may be wrapped in a myelin sheath, produced by Schwann cells in the peripheral nervous system or by oligodendrocytes in the central nervous system. This myelin sheath, composed of fatty layers, acts as insulation at intervals along the axon, leaving exposed gaps known as nodes of Ranvier. Because these nodes lack myelin, the action potential jumps from one node to the next, greatly accelerating nerve impulse conduction.

Parts of a neuron including dendrites, axon, and myelin sheath

A synapse is a specialized junction enabling impulses to travel from one neuron to another (propagation).

Signals may pass from a presynaptic axon terminal to:

A postsynaptic dendrite (axodendritic)
Cell body (axosomatic)
In rare cases, another axon (axoaxonic)

When an action potential arrives at the presynaptic terminal, it triggers neurotransmitter release into the synaptic cleft by exocytosis. This exocytosis occurs as synaptic knob vesicles fuse with the presynaptic membrane after calcium influx.

The neurotransmitters diffuse across the cleft, binding to receptors on the postsynaptic membrane and opening ligand-gated ion channels to generate a localized change in potential, a graded potential. If strong enough, this potential reaches threshold, producing a new action potential in the postsynaptic neuron.

Neurotransmitters, such as acetylcholine, norepinephrine, dopamine, and serotonin, are then removed or broken down to prevent continual stimulation.

Over time, continuous synaptic activity may deplete neurotransmitter stores, causing temporary “fatigue.” Despite these complexities, the resulting post-synaptic action potential is all-or-nothing, matching the strength of the original impulse and preserving signal fidelity as it propagates through the nervous system.

Stages of an action potential in neurons unfold as follows:

Resting: The sodium-potassium pump maintains a resting membrane potential of about –70 $mV$ , with sodium concentrated outside and potassium concentrated inside the cell. In this state, ion channels remain closed to prevent ions from leaking across the membrane.
Depolarization: Stimulus-driven ion channels for sodium open, allowing positively charged sodium ions to rush inward, causing the membrane potential to rise to about +30 $mV$ . At this point, sodium is abundant inside the cell, whereas potassium is still mostly inside.
Repolarization: Sodium channels close, while potassium channels open. Positively charged potassium flows outward, bringing the membrane potential back downward. Now, sodium ends up inside, and potassium ends up outside—opposite of the initial resting distribution.
Hyperpolarization: Potassium channels do not close immediately, so the potential temporarily dips below the resting level, creating a slight overshoot where the inside is more negative than usual.
Refractory period: The sodium-potassium pump works to restore the original ion balance, moving three sodium ions out for every two potassium ions pumped in, until normal resting conditions are reestablished. During the absolute refractory period, no new action potential can fire, while in the relative refractory period, a sufficiently strong stimulus can generate another impulse.

Threshold and all-or-none behavior: If a stimulus pushes the membrane potential past a critical threshold, an action potential is triggered. Because this response is all-or-nothing, once the threshold is crossed, the resulting spike in membrane potential has a uniform magnitude regardless of whether the stimulus just barely crosses the threshold or surpasses it substantially.