Fundamentals of Neuroscience: Structure and Function

Posted on Jan 27, 2026 in Biology

Cellular Foundations of the Nervous System

Neurons: Structure and Function

Neurons are the basic units of the nervous system. They have four functional zones:

  • Input: Dendrites receive information from other cells.
  • Integration: The cell body (or soma) integrates the information it receives.
  • Conduction: A single axon conducts output information away from the cell body as an electrical impulse.
  • Output: Axon terminals at the end of the axon communicate activity to other cells.

Neuronal Shapes and Functions

Different shapes of neurons reflect their function:

  • Motor neurons are large, have long axons, and stimulate muscles.
  • Sensory neurons have various shapes that best respond to specific environmental stimuli, such as light, odor, or touch.
  • Interneurons analyze input from one set of neurons and pass the processed information along to other neurons.

Neuronal Classification Types

Neurons are also classified by three general types:

  • Multipolar neurons: one axon, many dendrites; most common.
  • Bipolar neurons: one axon, one dendrite.
  • Unipolar neurons: a single extension branches in two directions, forming an input zone and an output zone.

Synaptic Transmission

The neuronal cell body and dendrites receive information across synapses. Information is transmitted from the presynaptic neuron to the postsynaptic neuron.

Synapse Components

Synapses have three components:

  • Presynaptic membrane: located on the axon terminal of the presynaptic neuron.
  • Postsynaptic membrane: located on the dendrite or cell body of the postsynaptic neuron.
  • Synaptic cleft: the gap that separates the membranes.

Neurotransmitters and Receptors

Synaptic vesicles are small spheres in presynaptic axon terminals that contain a neurotransmitter, a specialized chemical. Neurotransmitters are released in response to electrical activity in the axon.

Neurotransmitter receptors in the postsynaptic membrane are specialized proteins that react to neurotransmitter molecules.

Neural plasticity refers to the continual remodeling of neuronal connections.

Glial Cells and Myelin Sheaths

Glial cells assist neuronal activity by providing raw materials, chemical signals, and structure, and they participate in information processing.

Myelin-Forming Glia

Two types of glial cells wrap around axons to provide a fatty insulation layer called myelin:

  • Oligodendrocytes: form the myelin sheaths in the brain and spinal cord (CNS).
  • Schwann cells: provide myelin to neurons outside the brain and spinal cord (PNS).

Nodes of Ranvier are gaps between sections of myelin where the axon is exposed.

Other Glial Types

  • Astrocytes: star-shaped cells with many processes that stretch around and between neurons and, sometimes, blood vessels; they secrete chemicals and help form the outer membrane around the brain.
  • Microglial cells (or microglia): tiny, mobile cells that remove debris from injured or dead cells.

Organization of the Nervous System

Central and Peripheral Systems (CNS and PNS)

  • The Central Nervous System (CNS) consists of the brain and spinal cord.
  • The Peripheral Nervous System (PNS) includes all other parts of the nervous system outside the brain and spinal cord.

Peripheral Nervous System Components

The PNS consists of nerves, which are bundles of axons.

Motor and Sensory Nerves

  • Motor nerves transmit information from the spinal cord and brain to muscles and glands.
  • Sensory nerves convey information from the body to the CNS.

Somatic Nervous System

The various motor and sensory nerves of the body are divided into two distinct systems:

  • The Somatic Nervous System interconnects the brain and the major muscles and sensory systems of the body.

Autonomic Nervous System (ANS)

  • The Autonomic Nervous System connects to the viscera (internal organs). It is the main system for controlling the body’s organs.

Cranial and Spinal Nerves

The nerves of the somatic nervous system form two anatomical groups:

  • Cranial nerves: innervate the head, neck, and visceral organs directly from the brain.
  • Spinal nerves: connect to the spinal cord.

Cranial Nerves (12 Pairs)

There are 12 pairs of cranial nerves:

Exclusively Sensory:

  • Olfactory (I): smell
  • Optic (II): vision
  • Vestibulocochlear (VIII): hearing and balance

Motor Pathways:

  • Oculomotor (III): eye movement
  • Trochlear (IV): eye movement
  • Abducens (VI): eye movement
  • Spinal Accessory (XI): neck muscles
  • Hypoglossal (XII): tongue

Sensory and Motor Functions:

  • Trigeminal (V): facial sensation, chewing muscles
  • Facial (VII): taste sensation, facial muscles
  • Glossopharyngeal (IX): throat sensation, throat muscles
  • Vagus (X): innervates the heart, liver, and intestines

Spinal Nerves (31 Pairs)

There are 31 pairs of spinal nerves. Each spinal nerve consists of a group of motor fibers that project from the spinal cord and a group of sensory fibers that enter the spinal cord.

Spinal nerves are named for the segment of the spinal cord they are connected to:

  • Cervical (neck): 8 segments
  • Thoracic (trunk): 12 segments
  • Lumbar (lower back): 5 segments
  • Sacral (pelvic): 5 segments
  • Coccygeal (bottom): 1 segment

Autonomic Nervous System Divisions

The ANS has two major divisions:

  • The Sympathetic Nervous System has axons that innervate the sympathetic ganglia (small clusters of neurons outside the CNS). Sympathetic innervation often prepares the body for action—the “fight-or-flight” response.
  • The Parasympathetic Nervous System helps the body relax and recuperate. Parasympathetic neurons extend longer distances from the CNS to the parasympathetic ganglia, which are usually close to the organ they innervate.

The sympathetic and parasympathetic nervous systems have different effects on organs due to different neurotransmitters.

Brain Anatomy and Development

Anatomical Terminology and Tissue Types

The anatomy of the human brain is described using specific terminology.

Planes and Directional Terms

Planes: sagittal, coronal, horizontal.

Directional Terms: medial, lateral, ipsilateral, contralateral, superior, inferior, basal, anterior, posterior, proximal, distal, dorsal, ventral.

Afferent and Efferent Pathways

A nerve or pathway is afferent if it carries information into a region of interest, and efferent if it carries information away from a region of interest.

Gray Matter vs. White Matter

Two types of brain tissue:

  • Gray matter: contains more cell bodies and dendrites, which lack myelin.
  • White matter: consists mostly of axons with white myelin sheaths.

Mammalian Brain Structure

The brain has two cerebral hemispheres.

Cerebral Hemispheres and Cortex

The cerebral cortex is the outermost, convoluted layer of the brain (cortex = “bark”). The cortex has gyri (ridged or raised portions) and sulci (furrows).

Four Cerebral Lobes

Each cerebral hemisphere has four lobes:

  1. Frontal: most anterior region.
  2. Parietal: lies between the frontal and occipital lobes.
  3. Occipital: posterior region, responsible for visual processing.
  4. Temporal: lateral region, responsible for auditory processing.

Key Cortical Boundaries and Structures

  • Sylvian fissure: boundary of the temporal lobe.
  • Central sulcus: divides the frontal lobe from the parietal lobe.
  • Corpus callosum: a bundle of axons that connects the two cerebral hemispheres.
  • Postcentral gyrus: a strip of cortex behind the central sulcus, important for touch sensation.
  • Precentral gyrus: located in the frontal lobe, important for motor control.

Embryonic Development

During embryonic development, the neural tube develops three subdivisions: the forebrain, midbrain, and hindbrain.

  • The forebrain develops into the telencephalon and diencephalon.
  • The hindbrain develops into the cerebellum, pons, and medulla.

The Brainstem refers to the midbrain, pons, and medulla combined.

Cortical Organization

Cortical neurons are arranged in six layers, each with a distinct pattern of cell bodies or neuronal processes.

  • Pyramidal cells are the most prominent neurons in the cerebral cortex, typically found in layer III or V.
  • In some regions, cortical columns, perpendicular to the layers, extend the full thickness of the cortex and serve as information-processing units.

Subcortical Structures

Basal Ganglia

The basal ganglia are important in motor control and consist mainly of the caudate nucleus, putamen, and globus pallidus.

The Limbic System

The limbic system includes structures important for emotion and learning:

  • Amygdala: emotional regulation and perception of odor.
  • Hippocampus and fornix: learning and memory.
  • Cingulate gyrus: attention.
  • Olfactory bulb: sense of smell.

Midbrain Systems

The midbrain contains sensory and motor systems:

Tectum Sensory Systems:

  • Superior colliculi: visual processing.
  • Inferior colliculi: auditory processing.

Motor System:

  • Substantia nigra: part of the basal ganglia circuit.

Other Systems:

  • Reticular formation: involved with sleep and arousal.
  • Periaqueductal gray: pain perception.

Brainstem Components

Certain vital body functions are controlled by the brainstem:

  • Cerebellum: attached to the brainstem; crucial for motor coordination and control; participates in some types of learning.
  • Pons: contains sensory and motor nuclei; origin of some cranial nerves.
  • Medulla: marks the transition from brain to spinal cord; drives essential processes such as respiration and heart rate; origin of some cranial nerves.

Neural Networks and the Connectome

Different brain regions that have distinct functions collaborate in larger networks. Tracts of axons that connect regions can be local or long.

The Connectome is the network map describing all functional connections among brain regions.

Protection and Support Systems

Meninges: Protective Membranes

The brain and spinal cord are surrounded by three protective membranes, the meninges:

  • Dura mater: tough outermost layer.
  • Pia mater: delicate innermost layer.
  • Arachnoid membrane: lies between the other two; filled with cerebrospinal fluid (CSF).

Meningitis is an acute infection of the meninges. Meningiomas are tumors formed in the meninges.

The Ventricular System and CSF

The ventricular system is a series of chambers filled with cerebrospinal fluid (CSF).

  • The lateral ventricle in each hemisphere extends into all four lobes and is lined with the choroid plexus, a membrane that produces CSF.
  • CSF flows into the third ventricle at the midline, then into the fourth ventricle where it exits to circulate over the brain and spinal cord.

The brain has a glymphatic system that drains waste in CSF-derived fluids during sleep.

Blood Supply and Stroke

Besides CSF, the brain depends on an ample supply of oxygenated blood from the cerebral arteries.

A Stroke is caused by the rupture or blockage of blood vessels, leading to insufficient oxygen supply.

Methods for Studying the Brain

Histological Techniques

Histology is the study of tissues:

  • Cell bodies can be visualized using Nissl stains.
  • Golgi stains completely label only a few cells.
  • Autoradiography can reveal cells that exhibit a specific property.
  • Tract tracers help visualize neural pathways.

Structural Imaging (CT and MRI)

  • Computerized Axial Tomography (CAT or CT): measures X-ray absorption at several positions around the head. CT scans generate an anatomical map of the brain based on tissue density.
  • Magnetic Resonance Imaging (MRI): provides higher resolution images and fewer damaging effects than CT. When protons relax, they emit radio waves that differ by tissue density.

Functional Imaging (fMRI, fNIRS, PET)

  • Functional MRI (fMRI): detects small changes in brain metabolism, such as oxygen use, in active brain areas. fMRI can show how networks of brain structures collaborate.
  • Functional Near-Infrared Spectroscopy (fNIRS): sends light through superficial brain tissue and detects changes in neural activity and blood flow.
  • Positron Emission Tomography (PET): gives images of brain activity. Emissions of radioactive chemicals in the bloodstream indicate their destination, identifying which brain regions contribute to specific functions.

Magnetic Stimulation and Measurement

  • Transcranial Magnetic Stimulation (TMS): briefly stimulates discrete cortical regions. Scientists can then record observed changes in behavior.
  • Magnetoencephalography (MEG): measures the tiny magnetic fields given off by active neurons during cognitive processing.

Experimental Approaches in Neuroscience

Three types of studies are used to research the biological bases of behavior:

Somatic Intervention

  • Involves altering a structure or function to see how behavior is altered.
  • Independent variable: the structure or function that is being altered.
  • Dependent variable: the response that is measured, such as a behavior.

Behavioral Intervention

  • Involves intervening on a behavior to see how structure or function is altered.
  • Behavior is the independent variable.
  • Body changes are dependent variables.

Correlation Studies

  • Compares how much a body measure varies with a behavioral measure.
  • Negative correlation: one measure goes up while the other goes down.
  • Positive correlation: both measures increase or decrease together.

Note: A correlation experiment may show an association between variables, but cannot prove causality.

Research Principles

Control Groups and Experimental Design

A control group is identical to the experimental group except that they do not experience any alteration or treatment.

  • In a within-subjects experiment, the control group is the same set of subjects tested before alteration or treatment.
  • In a between-subjects experiment, the experimental group is compared to a separate control group that has been treated identically except for the manipulation.

Reductionism and Levels of Analysis

Reductionism is a method that breaks a system down into increasingly smaller parts in order to understand it.

Levels of analysis refers to the scope of an experimental approach, which ranges from social interaction down to the molecular level (this differs from the use of the term in evolutionary biology).