35.3 The Central Nervous System

Central Nervous System Structures and Functions

The central nervous system (CNS) consists of two structures: the brain and the spinal cord. Together, they receive sensory input, interpret it, and send out motor commands. Understanding how the CNS is organized helps you connect structure to function, which is a recurring theme in biology.

Structures of the Central Nervous System

The brain contains several distinct regions, each with specialized roles:

• Cerebrum: The largest part of the brain, divided into four lobes (frontal, parietal, temporal, occipital). Handles higher cognitive functions, sensory processing, and motor control.
• Cerebellum: Sits at the back of the brain, below the cerebrum. Coordinates movement, balance, and fine motor skills.
• Brainstem: Connects the brain to the spinal cord. Made up of three parts (midbrain, pons, medulla oblongata) that regulate vital functions like breathing, heart rate, and sleep cycles.
• Thalamus: Acts as a relay station, routing sensory and motor information between the body and the cerebral cortex.
• Hypothalamus: Maintains homeostasis by regulating body temperature, hunger, thirst, and endocrine function. Sits just below the thalamus.
• Pituitary gland: Secretes hormones that control growth, metabolism, and the activity of other endocrine glands. Often called the "master gland" because it directs so many other glands.

The spinal cord extends downward from the brainstem and transmits sensory and motor information between the brain and the rest of the body. It's divided into regions that correspond to different parts of the body:

• Cervical (neck)
• Thoracic (chest)
• Lumbar (lower back)
• Sacral (pelvis)
• Coccygeal (tailbone)

Functions of Brain Regions and Spinal Cord

Cerebrum — Each lobe has distinct responsibilities:

• Frontal lobe: Executive functions (planning, decision-making), voluntary motor control, and language production. Broca's area, located here, is critical for producing speech.
• Parietal lobe: Processes somatosensory information (touch, pressure, temperature) and contributes to spatial awareness.
• Temporal lobe: Processes auditory information and plays a role in memory, language comprehension, and emotion. Wernicke's area, located here, is critical for understanding language.
• Occipital lobe: Primarily responsible for visual processing.

Cerebellum — Think of the cerebellum as a fine-tuning center for movement:

• It takes motor commands from the cerebral cortex and smooths them out so your movements are precise rather than jerky.
• It integrates sensory input from the vestibular system (inner ear) and proprioceptors (sensors in muscles and joints) to maintain balance and posture.
• It supports motor learning, so repeated practice of a skill (like riding a bike) gradually becomes more automatic.

Brainstem — Each of its three parts handles different functions:

• Midbrain: Involved in visual and auditory processing, motor control, and regulating arousal and sleep/wake cycles.
• Pons: Relays information between the cerebral cortex and cerebellum. Also plays a role in sleep, arousal, and respiratory control.
• Medulla oblongata: Regulates vital autonomic functions like heart rate, blood pressure, and breathing. Contains reflex centers for coughing, sneezing, and vomiting.

Thalamus — Nearly all sensory information passes through the thalamus before reaching the cortex (the notable exception is smell, which goes directly to the cortex). It also filters information, helping regulate sleep and wakefulness.

Hypothalamus — A small but powerful region that:

• Maintains homeostasis (body temperature, hunger, thirst, fluid balance)
• Controls the endocrine system by releasing hormones that stimulate or inhibit the pituitary gland
• Influences emotional responses and behaviors

Pituitary gland — Secretes key hormones including growth hormone, thyroid-stimulating hormone (TSH), and adrenocorticotropic hormone (ACTH), which in turn regulate other endocrine glands throughout the body.

Spinal cord — Serves three main functions:

• Transmits sensory information from the body to the brain via ascending pathways
• Transmits motor commands from the brain to muscles via descending pathways
• Acts as a reflex center for simple spinal reflexes (like the knee-jerk reflex) that don't require input from the brain

Sensory Processing and Motor Control in the Central Nervous System

Central Nervous System Processing

The CNS constantly receives information, processes it, and generates responses. This happens through three main pathways: sensory processing, motor control, and reflexes.

Sensory Processing

1. Sensory receptors detect stimuli from the environment or the body. Examples include touch receptors in the skin, photoreceptors in the eye, and chemoreceptors in the nose.
2. Sensory information travels via afferent (sensory) neurons to the spinal cord and brain.
3. The thalamus receives this input and routes it to the appropriate cortical area (somatosensory cortex for touch, visual cortex for sight, auditory cortex for sound).
4. The relevant sensory cortex interprets the information, producing your conscious experience of the stimulus.

Motor Control

1. The motor cortex in the frontal lobe generates commands for voluntary movements.
2. The basal ganglia and cerebellum modulate these commands to ensure smooth, coordinated movement.
3. Motor commands travel via efferent (motor) neurons from the brain down to the spinal cord.
4. Lower motor neurons in the spinal cord directly innervate skeletal muscles, causing contraction.
5. Proprioceptors (muscle spindles, Golgi tendon organs) send feedback to the brain, allowing real-time adjustments to movement.

Reflexes

Reflexes are automatic, rapid responses to specific stimuli that don't require conscious thought. They protect the body by cutting down response time.

• Spinal reflexes (like the knee-jerk reflex) involve a sensory neuron and a motor neuron at the level of the spinal cord, bypassing the brain entirely. That's why you react before you even "feel" the stimulus.
• Cranial reflexes (like the pupillary light reflex or vestibulo-ocular reflex) involve the brainstem and coordinate more complex responses.

Cellular and Molecular Components of the Central Nervous System

Neurons are the primary functional units of the nervous system. Each neuron has three main parts: a cell body (contains the nucleus), dendrites (receive signals), and an axon (transmits signals away from the cell body). Neurons communicate across synapses using chemical messengers called neurotransmitters.

Glial cells support and protect neurons. There are several types, each with a distinct role:

• Astrocytes: Help maintain the blood-brain barrier, regulate neurotransmitter levels, and supply nutrients to neurons.
• Oligodendrocytes: Produce myelin sheaths that insulate axons in the CNS, speeding up signal transmission. (In the peripheral nervous system, Schwann cells perform this role instead.)
• Microglia: Act as the immune cells of the CNS, clearing debris and defending against pathogens.

Three additional concepts tie the CNS together:

• The blood-brain barrier is a selective membrane formed by tightly packed cells lining brain capillaries. It prevents most harmful substances in the bloodstream from reaching brain tissue, while still allowing essential molecules like glucose and oxygen through.
• Cerebrospinal fluid (CSF) surrounds the brain and spinal cord. It cushions these structures against physical impact and helps remove metabolic waste.
• Neuroplasticity is the brain's ability to form new neural connections and reorganize existing ones in response to learning, experience, or injury. This is why recovery after brain damage is sometimes possible and why practice strengthens skills over time.