13.2 The Central Nervous System

Anatomy of the Central Nervous System

The central nervous system (CNS) consists of the brain and spinal cord. It receives sensory information from the body, processes it, and sends out motor commands. Understanding the physical structure of the CNS is essential for making sense of how different regions contribute to everything from conscious thought to automatic reflexes like breathing.

Major Regions of the Adult Brain

The adult brain has four major regions: the cerebrum, diencephalon, brainstem, and cerebellum. Each handles different functions, but they all work together through dense networks of nerve fibers.

Cerebrum

The cerebrum is the largest portion of the brain, divided into left and right cerebral hemispheres. Its outer layer, the cerebral cortex, consists of gray matter and is responsible for conscious thought, sensory perception, and voluntary movement.

The cerebral cortex is divided into four lobes, each with distinct functions:

• Frontal lobe — executive functions like decision-making and planning, plus motor control (initiating voluntary movements). The precentral gyrus here is the primary motor cortex.
• Parietal lobe — processes somatosensory information (touch, pressure, temperature) and spatial awareness. The postcentral gyrus is the primary somatosensory cortex.
• Temporal lobe — auditory processing, language comprehension (especially in Wernicke's area on the dominant hemisphere), and memory formation.
• Occipital lobe — primarily handles visual processing, interpreting information sent from the eyes.

Diencephalon

The diencephalon sits between the cerebrum and the brainstem. Its two key structures are:

• Thalamus — acts as a relay station, routing nearly all incoming sensory information (except smell) to the appropriate area of the cerebral cortex. It also relays motor information between the cortex, cerebellum, and brainstem.
• Hypothalamus — regulates homeostasis (body temperature, hunger, thirst), emotional responses (fear, pleasure), and endocrine function by controlling the pituitary gland. Despite its small size, it has an outsized role in keeping internal conditions stable.

Brainstem

The brainstem connects the cerebrum to the spinal cord and contains three regions, listed superior to inferior:

1. Midbrain — involved in visual and auditory reflexes (e.g., pupillary light reflex, startle response) and motor control such as eye movements and body posture.
2. Pons — relays information between the cerebral cortex and cerebellum. Also plays a role in regulating the sleep-wake cycle and level of arousal.
3. Medulla oblongata — controls vital autonomic functions: breathing rhythm, heart rate, and blood pressure. Damage here can be life-threatening because these functions are essential for survival.

Cerebellum

Located at the posterior of the brain, the cerebellum coordinates and fine-tunes motor movements so they're smooth and precise. It also helps maintain balance and posture. The cerebellum doesn't initiate movement; instead, it compares intended movements with actual movements and makes corrections.

Spinal Cord

The spinal cord extends inferiorly from the medulla oblongata through the vertebral canal. It conducts sensory and motor information between the brain and the rest of the body. It also serves as a reflex center, processing spinal reflexes (like the knee-jerk reflex) without requiring input from the brain.

Interconnections of Brain Structures

No brain region works in isolation. Understanding how structures connect helps you see the CNS as an integrated system rather than a collection of separate parts.

• The cerebrum communicates with the diencephalon via nerve fibers. Sensory information heading to the cortex and motor commands leaving it both pass through the diencephalon.
• The thalamus is the central relay point, routing information between the cerebral cortex, brainstem, and cerebellum.
• The hypothalamus connects downward to the brainstem and to the pituitary gland, linking the nervous system to the endocrine system.
• The three brainstem regions (midbrain, pons, medulla) are continuous with each other, forming a pathway that relays information between the cerebrum, cerebellum, and spinal cord.
• The spinal cord is continuous with the medulla oblongata at its superior end. It carries sensory information up to the brain and motor commands down to the muscles.

At the cellular level, neurons are the basic functional units transmitting all of this information. They communicate across synapses, the junctions where signals pass from one neuron to the next.

Role of Basal Nuclei

The basal nuclei (also called basal ganglia) are a group of subcortical nuclei located deep within the cerebrum. They don't initiate movement directly. Instead, they help regulate and refine voluntary movements, contribute to motor learning, and support executive functions like planning.

The major components include:

• Striatum (caudate nucleus + putamen) — receives input from the cerebral cortex and integrates motor and cognitive information. This is the main "input" structure of the basal nuclei.
• Globus pallidus — receives processed information from the striatum and refines motor commands before relaying them to the thalamus. This is the main "output" structure.
• Substantia nigra — provides dopaminergic input to the striatum. Degeneration of dopamine-producing neurons here is the hallmark of Parkinson's disease, which causes tremors and difficulty initiating movement.
• Subthalamic nucleus — modulates the activity of the globus pallidus, particularly through the indirect pathway.

The basal nuclei influence movement through two main pathways:

• Direct pathway — facilitates desired movements by increasing thalamic activity, which in turn excites the motor cortex.
• Indirect pathway — suppresses unwanted movements by decreasing thalamic activity, which reduces cortical excitation.

The balance between these two pathways is what allows you to perform smooth, intentional movements without extraneous motion.

Gray vs. White Matter in the Spinal Cord

In the spinal cord, gray and white matter are arranged differently than in the brain. Gray matter is on the inside, and white matter surrounds it on the outside.

Gray Matter

Gray matter forms a butterfly-shaped (or H-shaped) region in the center of the spinal cord. It contains neuron cell bodies, interneurons, and glial cells. It's organized into functional regions called horns:

• Dorsal (posterior) horn — receives incoming sensory information from the body.
• Ventral (anterior) horn — contains motor neurons whose axons exit the spinal cord to control skeletal muscles.

Think of gray matter as the spinal cord's processing center, where sensory input is integrated and motor output is generated.

White Matter

White matter surrounds the gray matter and consists of myelinated axons bundled into tracts. The myelin gives it its white appearance. These tracts are organized by the direction information flows:

• Ascending tracts (sensory) — carry information from the body up to the brain.
  • Spinothalamic tract: pain and temperature sensation
  • Dorsal column-medial lemniscus pathway: fine touch and proprioception (awareness of body position)
• Descending tracts (motor) — carry commands from the brain down to the spinal cord and muscles.
  • Corticospinal tract: voluntary movements
  • Reticulospinal tract: postural control

White matter functions as the communication highway between the spinal cord and the brain.

Protection and Support of the Central Nervous System

The CNS is delicate tissue that can't regenerate well after injury, so it has multiple layers of protection:

• Meninges — three layers of connective tissue that surround the brain and spinal cord. From outermost to innermost: dura mater, arachnoid mater, and pia mater.
• Cerebrospinal fluid (CSF) — a clear fluid that circulates through the brain's ventricles and the subarachnoid space (between the arachnoid and pia mater). CSF cushions the CNS against mechanical shock and delivers nutrients while removing waste.
• Blood-brain barrier (BBB) — formed by tight junctions between endothelial cells of brain capillaries. It selectively controls which substances can pass from the bloodstream into CNS tissue, blocking many toxins and pathogens while allowing necessary nutrients like glucose and oxygen through.

Neurotransmission

Neurotransmitters are chemical messengers released at synapses to transmit signals between neurons. When an electrical signal (action potential) reaches the end of a neuron, it triggers the release of neurotransmitter molecules into the synaptic cleft. These molecules bind to receptors on the next neuron, either exciting or inhibiting it. This chemical signaling is the basis of all communication within the CNS.