Anatomy and Organization of the Nervous System

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Introduction to the Nervous System

Overview and Divisions

The nervous system is the master controlling and communicating system of the body. It is divided into two main parts: the Central Nervous System (CNS) and the Peripheral Nervous System (PNS). The CNS consists of the brain and spinal cord, serving as the integration and command center. The PNS includes all neural tissue outside the CNS, connecting the CNS to limbs and organs.

CNS: Brain and spinal cord; responsible for processing and integrating information.
PNS: Cranial nerves, spinal nerves, and sensory organs; relays information to and from the CNS.

Diagram of CNS and PNS in the human body Flowchart of nervous system divisions and functions Anatomical diagram of CNS and PNS structures

Functional Organization

The PNS is further divided into sensory (afferent) and motor (efferent) divisions. The motor division is subdivided into the somatic nervous system (voluntary control of skeletal muscles) and the autonomic nervous system (involuntary control of smooth muscle, cardiac muscle, and glands). The autonomic system is further divided into sympathetic and parasympathetic divisions.

Sensory (Afferent) Division: Transmits impulses from sensory receptors to the CNS.
Motor (Efferent) Division: Transmits impulses from the CNS to effector organs.
Somatic Nervous System: Voluntary control of skeletal muscles.
Autonomic Nervous System (ANS): Involuntary control; includes sympathetic (activates body systems) and parasympathetic (conserves energy) divisions.

Detailed flowchart of CNS and PNS with somatic and autonomic divisions

Histology of Nervous Tissue

Cell Types in Nervous Tissue

Nervous tissue is highly cellular, with little extracellular space. There are two principal cell types: neuroglia (glial cells) and neurons.

Neuroglia (Glial Cells): Support, protect, and insulate neurons. Types include:
- CNS: Astrocytes, microglial cells, ependymal cells, oligodendrocytes
- PNS: Satellite cells, Schwann cells
Neurons: Excitable cells that transmit electrical signals. They have extreme longevity, are mostly amitotic, and have a high metabolic rate.

Structural and Functional Classification of Neurons

Neurons are classified structurally by the number of processes extending from the cell body (multipolar, bipolar, unipolar) and functionally by the direction of impulse transmission (sensory, motor, interneurons).

Table comparing structural classes of neurons Diagram of neurons classified by function

Gray Matter vs. White Matter; Nucleus vs. Ganglion

Gray matter consists of neuron cell bodies, dendrites, and unmyelinated axons, while white matter is composed mainly of myelinated axons. In the CNS, a nucleus is a collection of neuron cell bodies, and a tract is a bundle of axons. In the PNS, a ganglion is a collection of neuron cell bodies, and a nerve is a bundle of axons.

Term	Definition
Nucleus	A collection of neuron cell bodies in the CNS
Ganglion	A collection of neuron cell bodies in the PNS
Tract	A bundle of axons in the CNS
Nerve	A bundle of axons in the PNS

Table of CNS vs PNS terminology

Central Nervous System: Brain Regions and Organization

Major Brain Regions

The adult brain is divided into four main regions: cerebral hemispheres, diencephalon, brain stem, and cerebellum. The complexity of neural wiring, rather than size, determines functional capacity.

Cerebral Hemispheres
Diencephalon: Thalamus, hypothalamus, epithalamus
Brain Stem: Midbrain, pons, medulla oblongata
Cerebellum

Labeled diagram of brain regions

Gray and White Matter Distribution

The basic pattern in the CNS is a central cavity surrounded by gray matter, with white matter external to gray matter. The brain stem contains additional gray matter nuclei within white matter, and the cerebrum and cerebellum have an outer cortex of gray matter.

Pattern of gray and white matter in CNS Gray and white matter in brain and spinal cord

Ventricles of the Brain

The brain contains four ventricles filled with cerebrospinal fluid (CSF) and lined by ependymal cells. The lateral ventricles communicate with the third ventricle, which connects to the fourth ventricle. The fourth ventricle is continuous with the central canal of the spinal cord and connects to the subarachnoid space via apertures.

Diagram of brain ventricles

Surface Features of the Cerebral Hemispheres

The surface of the cerebral hemispheres is marked by gyri (ridges), sulci (shallow grooves), and fissures (deep grooves). Sulci divide each hemisphere into five lobes: frontal, parietal, occipital, temporal, and insular.

Lobes and surface features of the cerebral hemispheres

Functional Areas of the Cerebral Cortex

The cerebral cortex is the site of conscious mind activities, including awareness, sensory perception, voluntary motor initiation, communication, memory storage, and understanding. It contains three types of functional areas: motor, sensory, and association areas. Each hemisphere controls the contralateral side of the body, and functions are lateralized but integrated.

Brodmann areas of the cerebral cortex Motor and sensory regions of the cerebral cortex

Motor Areas of the Cortex

Primary Motor Cortex: Located in the precentral gyrus; controls voluntary movements via pyramidal cells. The motor homunculus represents the body regions controlled by this area.
Premotor Cortex: Plans and coordinates complex movements.
Broca’s Area: Motor speech area, usually in the left hemisphere.
Frontal Eye Field: Controls voluntary eye movements.

Motor and sensory regions of the cerebral cortex Motor homunculus in precentral gyrus

Sensory Areas of the Cortex

Primary Somatosensory Cortex: Receives sensory input from skin, muscles, and joints; enables spatial discrimination.
Somatosensory Association Cortex: Integrates sensory input for interpretation.
Visual Areas: Primary visual cortex and visual association area for processing visual information.
Auditory Areas: Primary auditory cortex and association area for processing sound.
Vestibular Cortex: Awareness of balance.
Olfactory Cortex: Perception of odors.
Gustatory Cortex: Perception of taste.
Visceral Sensory Area: Perception of visceral sensations.

Sensory regions of the cerebral cortex

Multimodal Association Areas

These areas integrate input from multiple sensory modalities and are involved in higher-order functions such as intellect, cognition, memory, and emotion. Major association areas include the anterior (prefrontal cortex), posterior, and limbic association areas.

Lateralization of Cortical Function

Lateralization refers to the specialization of functions in each hemisphere. The left hemisphere is typically dominant for language, math, and logic, while the right hemisphere is dominant for visual-spatial skills, intuition, and creativity.

Cerebral White Matter

White matter consists of myelinated fibers and tracts responsible for communication within the brain and between the brain and spinal cord. Types of fibers include commissural (between hemispheres), association (within a hemisphere), and projection fibers (to/from lower CNS centers).

Basal Nuclei

Basal nuclei are deep gray matter structures involved in the regulation of movement, cognition, and emotion. They help initiate and modulate motor activity and filter out inappropriate responses.

Diencephalon

Thalamus

The thalamus acts as a relay station for sensory and motor signals to the cerebral cortex. It sorts, edits, and relays information and is involved in sensation, motor activities, learning, and memory.

Hypothalamus

The hypothalamus is the main visceral control center, regulating homeostasis, the autonomic nervous system, emotional responses, body temperature, hunger, thirst, sleep-wake cycles, and the endocrine system.

Epithalamus

The epithalamus contains the pineal gland, which secretes melatonin to regulate sleep-wake cycles, and the choroid plexus, which produces cerebrospinal fluid.

Brain Stem

Structure and Functions

The brain stem consists of the midbrain, pons, and medulla oblongata. It controls automatic behaviors necessary for survival and serves as a pathway between higher and lower brain centers. It is also the origin of most cranial nerves.

Midbrain

The midbrain contains important motor tracts, visual and auditory reflex centers, and nuclei involved in pain suppression and motor control (e.g., substantia nigra, red nucleus).

Pons

The pons contains conduction tracts and nuclei involved in respiration and communication between the cerebrum and cerebellum.

Medulla Oblongata

The medulla oblongata is the autonomic reflex center for homeostasis, controlling cardiovascular, respiratory, and other vital functions. It is the site of decussation for motor tracts, allowing contralateral control.

Cerebellum

Structure and Function

The cerebellum coordinates voluntary movements, balance, and posture. It processes input from the cerebral cortex, brain stem, and sensory receptors to ensure smooth, coordinated muscle activity. It is divided into anterior, posterior, and flocculonodular lobes, and connected to the brain stem by three pairs of cerebellar peduncles.

Functional Brain Systems

Limbic System

The limbic system is involved in emotion, motivation, and memory. It includes structures such as the amygdala, hippocampus, and cingulate gyrus, and interacts closely with the hypothalamus.

Reticular Formation

The reticular formation is a network of neurons that maintains brain arousal and consciousness (reticular activating system, RAS). It filters sensory input and is essential for maintaining alertness.