Difficulty: Introductory | Prerequisites: Basic cell biology (cell membrane, organelles).
Tags: nervous system, CNS, PNS, central nervous system, peripheral nervous system, somatic nervous system, autonomic nervous system, neuroglia, glial cells, astrocytes, oligodendrocytes, microglia, ependymal cells, Schwann cells, satellite cells, afferent, efferent, sensory pathways, motor pathways
The nervous system is the body's coordination network. It detects changes inside and outside the body, processes that information, and tells muscles and glands what to do about it. Everything from pulling your hand off a hot stove to forming a memory depends on this system working properly.
Before diving in, you should be comfortable with basic cell structure (membrane, nucleus, organelles) and the idea that cells can be specialised for different jobs. If those feel shaky, review your cell biology notes first.
This topic sits right at the start of the neuroscience block. Every later topic (action potentials, synapses, brain anatomy, spinal cord reflexes) builds on the divisions and cell types covered here.
The nervous system splits into two main parts: the CNS (brain and spinal cord) for processing, and the PNS (everything else) for communication with the body. The PNS further divides into voluntary (somatic) and involuntary (autonomic) branches. Supporting the neurons throughout both divisions are neuroglia, six main types of non-neuronal cells that insulate, protect, feed, and clean up after neurons.
Central nervous system (CNS)
The brain and spinal cord. The main processing centre where sensory information is integrated and motor commands originate. Think of it as the headquarters that receives reports and issues orders.
Peripheral nervous system (PNS)
All the nerves and ganglia outside the brain and spinal cord. In simple terms, the PNS is the wiring that connects headquarters to every other part of the body.
Afferent (sensory) pathways
Nerve fibres that carry information from sensory receptors toward the CNS. "Afferent" and "arrival" both start with "a," which is a useful memory hook.
Efferent (motor) pathways
Nerve fibres that carry commands from the CNS outward to muscles and glands. "Efferent" and "exit" both start with "e."
Somatic nervous system
The voluntary motor division of the PNS. It controls skeletal muscles and mediates conscious sensation. In simple terms, if you decided to move it, the somatic system carried the signal.
Autonomic nervous system (ANS)
The involuntary division of the PNS. It regulates smooth muscle, cardiac muscle, and glands without conscious effort. Think of it as the autopilot that handles digestion, heart rate, and similar background tasks.
Sympathetic nervous system
The "fight or flight" branch of the ANS. Prepares the body for stress or danger by increasing heart rate, dilating airways, and redirecting blood flow to muscles.
Parasympathetic nervous system
The "rest and digest" branch of the ANS. Conserves energy and maintains routine body functions such as digestion and steady heart rate.
Neuroglia (glial cells)
Non-neuronal support cells found in both the CNS and PNS. They outnumber neurons and handle insulation, nutrition, immune defence, and structural support. Think of them as the maintenance crew that keeps the neural network running.
Astrocytes
Star-shaped CNS glia that maintain the blood-brain barrier, supply nutrients to neurons, and regulate the extracellular environment. In simple terms, they are the housekeepers and gatekeepers of the brain's internal environment.
Oligodendrocytes
CNS glia that wrap myelin sheaths around axons. Each oligodendrocyte can myelinate segments of several axons at once, speeding up signal conduction.
Microglia
The immune cells of the CNS. They patrol for pathogens, clear debris, and respond to injury or infection. Think of them as the brain's resident immune patrol.
Ependymal cells
Ciliated cells lining the ventricles of the brain. They help produce and circulate cerebrospinal fluid (CSF).
Schwann cells
PNS glia that form the myelin sheath around peripheral nerve fibres. Unlike oligodendrocytes, each Schwann cell wraps a single axon segment. They also assist in nerve regeneration after injury.
Satellite cells (nervous system)
PNS glia that surround neuronal cell bodies in ganglia. They regulate the chemical environment around those cell bodies, similar to what astrocytes do in the CNS.
The top-level split is structural:
CNS – brain and spinal cord. Integrates sensory input, coordinates motor output, and handles higher functions (thought, memory, emotion).
PNS – cranial nerves, spinal nerves, ganglia, and sensory receptors. Carries signals to and from the CNS.
Within the PNS, the functional split matters more for exams:
Sensory (afferent) division – detects stimuli (touch, temperature, pain, stretch) and relays signals inward to the CNS.
Motor (efferent) division – carries commands outward. This divides again:
Somatic – voluntary control of skeletal muscle, plus conscious sensation.
Autonomic – involuntary control of smooth muscle, cardiac muscle, and glands.
Sympathetic – mobilises energy (fight or flight).
Parasympathetic – conserves energy (rest and digest).
Four main types:
Astrocytes – most abundant. Maintain the blood-brain barrier by wrapping end-feet around capillaries, shuttle nutrients to neurons, buffer extracellular potassium, and recycle neurotransmitters.
Oligodendrocytes – produce myelin in the CNS. A single oligodendrocyte extends processes to myelinate portions of multiple axons. Myelin speeds conduction via saltatory conduction (signals jump between gaps).
Microglia – derived from immune cell lineages. They phagocytose pathogens and dead cells. In disease states they can become overactive, contributing to neuroinflammation.
Ependymal cells – line the ventricles and central canal of the spinal cord. Their cilia help circulate CSF, which cushions the brain and removes metabolic waste.
Two main types:
Schwann cells – each wraps one segment of one axon. Crucial for peripheral nerve repair: after injury they form a regeneration tube that guides regrowth. This is why PNS nerves can sometimes recover while CNS damage is often permanent.
Satellite cells – surround neuron cell bodies in dorsal root ganglia and autonomic ganglia. They regulate the local chemical environment, controlling what reaches the neuron.
Structural support (physical scaffolding for neurons)
Electrical insulation (myelin)
Maintenance of the extracellular environment (ion and neurotransmitter regulation)
Immune defence (microglia in CNS, plus general inflammatory responses)
Repair and regeneration (Schwann cells in PNS especially)
Damage to Schwann cells is what underlies conditions like Guillain-Barré syndrome, where the immune system attacks peripheral myelin and disrupts motor and sensory signals. Understanding which glial cell does what helps clinicians localise the problem.
The blood-brain barrier maintained by astrocytes is also the reason many drugs cannot reach the brain easily, which is a central challenge in treating brain tumours and neurological infections.
Students often confuse oligodendrocytes and Schwann cells. Oligodendrocytes are CNS only and myelinate multiple axon segments. Schwann cells are PNS only and each one wraps a single segment.
"Neuroglia are just packing material." They are not passive. Astrocytes actively regulate neurotransmission, and microglia mount genuine immune responses. Exam questions frequently test whether you know glia have active roles.
"Afferent" and "efferent" get mixed up constantly. Lock in the mnemonic: Afferent = Arriving at the CNS, Efferent = Exiting the CNS.
⚠️ Be able to name all six neuroglia types, state their location (CNS vs PNS), and give at least one function each. This is a standard table question.
⚠️ Know the full hierarchy: Nervous System → CNS / PNS → Sensory / Motor → Somatic / Autonomic → Sympathetic / Parasympathetic. Drawing this tree from memory is a common short-answer task.
⚠️ Understand why PNS nerves can regenerate (Schwann cells) while CNS nerves generally cannot (oligodendrocytes do not support regrowth in the same way).
True or false: Microglia are found in the PNS.
Fill in the blank: The _______ division of the PNS controls voluntary skeletal muscle.
True or false: Each oligodendrocyte myelinates only one axon segment.
Fill in the blank: Ependymal cells line the brain's _______ and help circulate CSF.
True or false: Afferent pathways carry signals away from the CNS.
Answers: 1. False (CNS only). 2. Somatic. 3. False (multiple segments on multiple axons). 4. Ventricles. 5. False (toward the CNS).
Q: List the four types of neuroglia found in the CNS and state one function of each.
A: Astrocytes (maintain the blood-brain barrier and supply nutrients), oligodendrocytes (form myelin sheaths around CNS axons), microglia (immune defence, phagocytosis of pathogens and debris), ependymal cells (line ventricles, produce and circulate CSF).
Q: Explain the difference between the somatic and autonomic divisions of the PNS.
A: The somatic division controls voluntary skeletal muscle movement and mediates conscious sensation. The autonomic division controls involuntary functions (smooth muscle, cardiac muscle, glands) and is further divided into sympathetic (fight or flight) and parasympathetic (rest and digest) branches.
Q: A patient has damage to Schwann cells in a peripheral nerve. Why might they still have hope for recovery, compared with a similar injury in the CNS?
A: Schwann cells support peripheral nerve regeneration by forming a tube that guides axon regrowth. In the CNS, oligodendrocytes do not provide this regenerative scaffolding, so recovery is far less likely.
Q: What is the role of the sodium-potassium pump in maintaining the resting membrane potential, and which glial cell helps buffer extracellular potassium in the CNS?
A: The Na⁺/K⁺-ATPase pump moves 3 Na⁺ out and 2 K⁺ in, contributing to the negative resting potential. Astrocytes help buffer extracellular K⁺ levels in the CNS.
This material connects directly to the next topic on resting membrane potential and action potentials, because myelination by oligodendrocytes and Schwann cells is what enables saltatory conduction and fast signal transmission. The autonomic subdivisions (sympathetic and parasympathetic) reappear in detail when studying organ-level physiology, particularly the cardiovascular and digestive systems.
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