2: Central and Peripheral Nervous System

Central Nervous System: Brain

The brain is the control center of the body. Its outer surface is the cerebral cortex, a thin, highly folded layer. The cortex is mostly gray matter, made of neuron cell bodies. The folds increase the surface area, allowing more neurons and more complex processing. The cerebral cortex is essential for higher functions such as thinking, planning, language, personality, conscious movement, and conscious sensation.

Different areas, or lobes, of the cortex have different main roles. The frontal lobe is involved in decision‑making, planning, voluntary movement, personality, and social behavior. The parietal lobe processes body sensations such as touch, temperature, and pain, and helps you know where your body is in space. The temporal lobe is involved in hearing, understanding language, and forming memories. The occipital lobe is mainly responsible for vision.

At the very front of the frontal lobe is the prefrontal cortex. This region has many connections to emotion centers in the limbic system and to motor areas. It is important for executive functions such as planning, judgment, impulse control, social behavior, and emotional regulation. Damage to the prefrontal cortex often causes changes in personality and behavior, even if basic movement and sensation remain normal.

Central Nervous System: Spinal Cord

The spinal cord is a long, thin column of nervous tissue inside the vertebral column (spine). In cross‑section, it has gray matter in the center and white matter on the outside. The gray matter contains neuron cell bodies and local circuits that form reflex pathways. The white matter contains myelinated axons that run up and down, forming “highways” for information.

The spinal cord has two major functions. First, it acts as an information highway. Sensory information travels from receptors in the body up the spinal cord to the brain. Motor commands travel from the brain down the spinal cord to muscles and glands. Second, the spinal cord acts as a reflex center. Some fast, automatic responses, such as pulling your hand away from something hot, can be handled at the spinal cord level without waiting for the brain. This allows very rapid, protective reactions.

Peripheral Nervous System: Nerves and Ganglia

The peripheral nervous system includes all nervous tissue outside the brain and spinal cord. Its main parts are nerves and ganglia. A nerve is a bundle of axons wrapped in connective tissue. Nerves may be mostly sensory, mostly motor, or mixed (containing both sensory and motor fibers). Cranial nerves connect directly to the brain. Spinal nerves connect to the spinal cord. The function of nerves is to act as communication lines between the CNS and the rest of the body.

Ganglia are clusters of neuron cell bodies in the PNS. They serve as relay and processing stations, especially in the autonomic nervous system.

Sensory (afferent) fibers in the PNS carry information from receptors in the skin, muscles, joints, and internal organs to the CNS. Motor (efferent) fibers carry commands from the CNS to skeletal muscles, smooth muscle, cardiac muscle, and glands.

Functional Divisions of the PNS: Somatic and Autonomic

The PNS is divided into the somatic nervous system and the autonomic nervous system.

The somatic nervous system is mainly under voluntary control. Its motor neurons go to skeletal muscles and allow conscious movements such as walking, writing, or speaking. Its sensory neurons bring information from the skin, muscles, and joints to the CNS, creating conscious sensations such as touch, pain, temperature, and awareness of limb position.

The autonomic nervous system (ANS) controls internal organs and is mostly involuntary. Its motor neurons go to smooth muscle (for example, in the digestive tract and blood vessels), cardiac muscle (the heart), and glands. The ANS helps regulate heart rate, blood pressure, digestion, pupil size, and many other automatic functions.

The autonomic system has two main divisions: the sympathetic division and the parasympathetic division. The sympathetic division is often called “fight or flight.” It prepares the body for action by increasing heart rate, dilating the airways, and redirecting blood flow to skeletal muscles. The parasympathetic division is often called “rest and digest.” It supports rest, digestion, and energy storage by slowing heart rate, increasing digestive activity, and promoting glandular secretion. These two divisions often have opposite effects on the same organs, which allows fine, balanced control.

Protection and Support of the CNS

The CNS is essential but fragile, so the body provides several layers of protection. The first protection is bone. The skull (cranium) surrounds the brain, and the vertebral column surrounds the spinal cord. These bony structures protect against mechanical injury.

Under the bone are three connective tissue layers called the meninges. The outer layer is the dura mater, a tough, thick membrane attached to the inside of the skull and vertebrae. The dura also forms venous channels called dural sinuses. The middle layer is the arachnoid mater, which has a web‑like appearance. Beneath it is the subarachnoid space, filled with cerebrospinal fluid (CSF). The inner layer is the pia mater, a thin, delicate membrane that closely follows the surface of the brain and spinal cord. Together, the meninges support and stabilize the CNS, divide it into compartments, and create spaces where CSF can circulate.

Blood Supply and the Blood–Brain Barrier

The brain needs a constant blood supply for oxygen and glucose. Arterial blood reaches the brain through the internal carotid arteries and the vertebral arteries. These vessels join to form a circular arterial structure called the circle of Willis. This circle provides alternative pathways for blood flow, helping to maintain perfusion if one part of the system is narrowed or blocked.

The blood–brain barrier (BBB) is a special barrier between the blood and the CNS tissue. It is formed mainly by tight junctions between capillary endothelial cells, supported by glial cells called astrocytes. The BBB limits the exchange of substances between the blood and the interstitial fluid of the brain and spinal cord. It protects the CNS from many toxins and pathogens and from rapid chemical changes in the blood, while still allowing important molecules such as oxygen and glucose to enter. Because of the BBB, the CNS has a “privileged” internal environment that is carefully controlled.

Cerebrospinal Fluid (CSF) and Its Circulation

Cerebrospinal fluid is a clear, watery fluid that surrounds and fills spaces in the brain and spinal cord. It is produced mainly by choroid plexuses, which are networks of capillaries and specialized cells inside the four ventricles of the brain. These structures filter blood to make CSF.

CSF flows from the ventricles through connecting openings into the subarachnoid space between the arachnoid and pia mater. It then circulates around the brain and spinal cord. CSF acts as a shock absorber, cushioning delicate nervous tissue. It also helps maintain a stable chemical environment and carries away metabolic wastes from CNS cells.

Waste removal occurs as CSF is reabsorbed into the venous blood through structures called arachnoid granulations, which project into the dural sinuses. The dural sinuses are venous channels located within the dura mater. From the dural sinuses, blood (now mixed with reabsorbed CSF and dissolved wastes) drains out of the skull through the jugular veins and returns to the heart.

Phineas Gage: Linking Brain Structure and Behavior

The famous case of Phineas Gage shows clearly how specific brain structures are linked to behavior and personality. In 1848, Gage, a 25‑year‑old railroad construction foreman, suffered a severe accident. An explosion drove a heavy iron rod up through his face and skull, entering below his left cheekbone and exiting through the top of his head. He survived and remained conscious, but the damage to his brain was major.

Modern imaging and modeling suggest that only about 4 percent of his cerebral cortex was directly destroyed, but about 11 percent of his white matter, the brain’s “wiring” that connects different regions, was damaged. Important connections between the left frontal lobe, temporal lobe, both frontal lobes, and limbic (emotional) structures were cut. This widespread disconnection of neural pathways was as important as the direct injury to the cortex.

Before the accident, Gage was described as responsible, balanced, and a natural leader. After the accident, his personality changed dramatically. He became impulsive, rude, unreliable, and socially inappropriate. Friends said he was “no longer Gage.” These changes match the functions of the prefrontal cortex and its connections with emotion and decision‑making centers. Damage to these areas and pathways impaired his judgment, impulse control, and emotional regulation.

Before Gage’s case, the frontal lobes were often thought to be “silent” areas with no major function. His case was one of the first strong pieces of evidence that particular brain regions and their connections are necessary for normal personality and higher mental functions. It helped establish the idea that brain structure determines behavioral function and inspired later research into localization of function and the role of the prefrontal cortex.

Today, similar behavioral changes are seen in patients with traumatic brain injuries and in neurodegenerative diseases such as Alzheimer’s disease and frontotemporal dementia. These conditions also damage frontal brain areas and their white matter connections. This confirms that both specific regions and the networks connecting them are essential for normal behavior, supporting what was first learned from Phineas Gage’s injury.

Summary: How CNS and PNS Work Together

The CNS, with its folded cortex and organized spinal tracts, is specialized for integration, decision‑making, and reflex control. It is well protected by bone, meninges, cerebrospinal fluid, and the blood–brain barrier. The PNS, with its long, branching nerves and specialized sensory endings, is specialized for sensing the external and internal environment and carrying out the body’s actions.

Sensory information from the PNS enters the CNS, where it is processed and integrated. The CNS then sends motor commands back through the PNS to skeletal muscles and internal organs. The somatic division of the PNS allows voluntary movement and conscious sensation. The autonomic division regulates involuntary functions of the heart, smooth muscles, and glands through its sympathetic and parasympathetic branches.

Together, the structure of the CNS and PNS allows the nervous system to sense, think, decide, and act, and to maintain the body’s internal balance (homeostasis) while supporting complex behaviors and mental functions.

• 2.1: Overview( Delete)

  Different structures of the nervous system perform different functions. The structures of the nervous system were discovered through dissection, while functions of those regions were discovered through lesion case studies. In these studies, injuries or illnesses of the nervous system are studied to understand the relationship between the injured area and the function of it. One famous lesion case study was Phineas Gage, an American railroad worker.

• 2.2: Support and Protection of the Brain

  The CNS is crucial to the operation of the body and any compromise of function in the brain and spinal cord can lead to severe difficulties. The CNS is protected by the skeletal system (skull and vertebral column), and underneath by membranes of connective tissue, called meninges. In addition, the CNS has a privileged blood supply, as suggested by the blood-brain barrier. Because of this privilege, the CNS needs specialized structures for the maintenance of circulation.

• 2.3: Brain- Cerebrum

  The brain is divided into four major regions: cerebrum, diencephalon, brainstem, and cerebellum. The cerebrum is divided into different regions called lobes: frontal, parietal, occipital, temporal and insula. Each lobe performs a specialized function through their cerebral cortex. Overall, the functions of the cerebrum are motor initiation and coordination, processing of general and special senses, and high level functions such as judgment, reasoning, problem solving, and learning.

• 2.4: Brain- Diencephalon, Brainstem, Cerebellum and Limbic System

  Deep and inferior to the cerebrum, the diencephalon, brainstem and cerebellum compose the rest of the brain. These regions are responsible for various functions including maintaining homeostasis, relaying sensory information, carrying vital functions through the autonomic system. Most of the cranial nerves originate from these regions. The limbic system comprehends structures from the cerebrum and diencephalon that are associated with emotions.

• 2.5: Cranial Nerves

  The Peripheral Nervous System include nerves and ganglia. Nerves are organized into structures by layers of connective tissue that cover them. The epineurium covers the nerve, the perineurium covers the fascicles and the endoneurium covers the individual axon. Cranial nerves originate from the brain and carry sensory, motor or mixed information. There are twelve pairs of cranial nerves. Cranial nerve ganglia can be part of the somatic sensory NS or autonomic NS.

• 2.6: Spinal Cord and Spinal Nerves

  The spinal cord transmits sensory information from the periphery to the brain and motor information from the brain to the periphery. The spinal cord is divided into grey horns that house interneurons, autonomic neurons and somatic motor neurons as well as glial cells, and white columns that house ascending and descending tracts of axons. Spinal nerves originate from the spinal cord, carry both sensory and motor information and connect to the skin to form a map of dermatomes.

• 2.7: Key Terms

   

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