Source: Nervous System Functions & Structure Guide
Tags: nervous system, CNS, PNS, central nervous system, peripheral nervous system, neurons, neuron structure, afferent, efferent, somatic, autonomic, sympathetic, parasympathetic, nerve structure, epineurium, perineurium, endoneurium, fascicle, axonal transport
Difficulty: Introductory–Intermediate Prerequisites: Basic cell biology (cell membrane, organelles, ions). Familiarity with general tissue types is helpful but not essential.
The nervous system is one of the two major control systems in the body (the other being the endocrine system), and this unit is where most anatomy and physiology courses shift from passive structure into active function. Everything here, from how neurons are classified to how nerves are physically built, underpins the electrical signalling and reflex content that follows. If you are coming in cold, know that the nervous system collects sensory information, decides what to do with it, and sends out commands. The rest is detail on how and where each of those steps happens.
The nervous system splits into a central division (brain and spinal cord) that processes information and a peripheral division (nerves) that carries signals to and from the body. Neurons are the functional cells doing the signalling, classified by shape and by job. Nerves themselves are layered bundles of axons wrapped in protective connective tissue.
Central nervous system (CNS)
The brain and spinal cord considered together. The CNS is the processing and integration centre for all incoming sensory data and outgoing motor commands.
In simple terms, this is the headquarters where decisions get made.
Peripheral nervous system (PNS)
All the nerves and ganglia outside the brain and spinal cord. The PNS connects the CNS to the limbs, organs, and sensory receptors throughout the body.
Think of it as the wiring that runs between headquarters and every outpost in the body.
Afferent (sensory) nerves
Nerve fibres that carry signals from receptors toward the CNS. The word "afferent" comes from Latin for "carrying toward."
In simple terms, these are the inbound lines, reporting what the body senses.
Efferent (motor) nerves
Nerve fibres that carry commands from the CNS out to muscles and glands. "Efferent" means "carrying away."
Think of it as the outbound lines, delivering orders from the brain.
Somatic division
The branch of the PNS that controls voluntary skeletal muscles and processes conscious sensory input (sight, touch, hearing).
In simple terms, this is the part you have conscious control over.
Autonomic division
The branch of the PNS that regulates involuntary functions: heart rate, digestion, gland secretion, smooth muscle activity.
Think of it as the autopilot running in the background.
Sympathetic nervous system
The subdivision of the autonomic division that prepares the body for intense physical activity, commonly called the "fight or flight" response.
In simple terms, this is the system that kicks in when you are stressed or in danger.
Parasympathetic nervous system
The subdivision of the autonomic division that promotes rest, digestion, and energy conservation. Sometimes called "rest and digest."
Think of it as the calm-down system that takes over once the threat has passed.
Neuron
The fundamental signalling cell of the nervous system, specialised for generating and transmitting electrical impulses.
In simple terms, neurons are the cells that carry messages.
Fascicle
A bundle of axons (nerve fibres) grouped together within a nerve, wrapped by a connective tissue sheath called the perineurium.
Think of it as one cable inside a larger cable bundle.
Epineurium
The outermost connective tissue layer that encloses an entire nerve, providing structural support and protection.
Perineurium
The connective tissue sheath that wraps around each individual fascicle within a nerve.
Endoneurium
The delicate connective tissue layer surrounding each individual axon within a fascicle.
In simple terms, epineurium wraps the whole nerve, perineurium wraps each bundle, endoneurium wraps each fibre. Think outside-in: epi → peri → endo.
Multipolar neuron
A neuron with many dendrites and one axon. The most common structural type, found throughout the CNS. Motor neurons and interneurons are typically multipolar.
Bipolar neuron
A neuron with one dendrite and one axon. Found in specialised sensory organs such as the retina and olfactory epithelium.
Unipolar (pseudounipolar) neuron
A neuron with a single process extending from the cell body that splits into a peripheral branch and a central branch. Most sensory neurons are this type.
Anterograde transport
Movement of materials (neurotransmitters, organelles) from the neuron's cell body toward the synaptic terminal.
Think of it as the supply chain sending fresh stock to the front line.
Retrograde transport
Movement of used or damaged materials from the axon terminal back to the cell body for recycling or breakdown.
Think of it as the return line, sending worn-out parts back to the factory.
The nervous system divides into the CNS (brain + spinal cord) and the PNS (everything else).
The CNS is the integrating centre: it receives input, interprets it, and issues commands.
The PNS is the communication network: it relays sensory data inward (afferent) and motor commands outward (efferent).
Somatic division: voluntary control of skeletal muscles, plus conscious sensory input (sight, hearing, touch).
Autonomic division: involuntary regulation of organs, glands, and smooth muscle.
Sympathetic branch: fight or flight. Increases heart rate, dilates pupils, diverts blood to muscles.
Parasympathetic branch: rest and digest. Slows heart rate, stimulates digestion, conserves energy.
Sensory input (collection): receptors detect stimuli (light, sound, pressure, temperature, internal organ signals) and send afferent signals to the CNS.
Integration (processing): the brain and spinal cord interpret incoming signals and decide on a response.
Motor output (response): the CNS sends efferent commands to effectors, which are either muscles (movement) or glands (secretion).
A nerve is a cable-like structure made of bundled axons.
Axons are grouped into fascicles.
Three connective tissue layers protect the nerve, working from outside in:
Epineurium – wraps the entire nerve.
Perineurium – wraps each fascicle.
Endoneurium – wraps each individual axon.
This layered arrangement protects delicate axons from mechanical damage while allowing efficient signal transmission.
Excitability: neurons respond to stimuli by generating electrical signals (nerve impulses).
Conductivity: they propagate those signals along their membranes, sometimes over long distances.
Secretion: at synapses, neurons release neurotransmitters to communicate with other cells.
Extreme longevity: neurons can survive for the entire lifespan of the individual.
Amitotic nature: most neurons lose the ability to divide after foetal development. Damaged neurons generally cannot be replaced, which is why spinal cord and brain injuries can be permanent.
Neurons typically produce two or more neurotransmitters, allowing nuanced communication.
By structure (number of processes):
Multipolar: many dendrites, one axon. Most common. Motor neurons and interneurons.
Bipolar: one dendrite, one axon. Found in the retina and olfactory epithelium.
Unipolar (pseudounipolar): single process that branches. Most sensory neurons.
By function:
Sensory neurons (afferent): carry signals from receptors to the CNS. Usually unipolar.
Motor neurons (efferent): carry commands from the CNS to effectors. Usually multipolar.
Interneurons (association neurons): sit entirely within the CNS, integrating and relaying information between sensory and motor pathways. Usually multipolar.
Because neurons can be extremely long (some motor neurons stretch over a metre), they need an active internal transport system.
Anterograde transport moves freshly made materials (neurotransmitter vesicles, mitochondria, enzymes) from the cell body toward the axon terminal. Driven by the motor protein kinesin along microtubules.
Retrograde transport moves worn-out organelles and signalling molecules back to the cell body for recycling. Driven by the motor protein dynein.
Both directions depend on the cytoskeleton, specifically microtubules running the length of the axon.
The layered connective tissue structure of nerves is why peripheral nerve injuries often heal better than CNS injuries: the endoneurium and perineurium can guide regrowing axons back to their targets. This is the basis for microsurgical nerve repair.
The amitotic nature of neurons is also why neurodegenerative diseases (Parkinson's, Alzheimer's) are so difficult to treat: lost neurons are, for the most part, gone for good.
Students often think "afferent" and "efferent" refer to specific nerves. They refer to the direction of signal travel, not a physical structure. A single nerve can contain both afferent and efferent fibres (a mixed nerve).
Students frequently confuse the somatic and autonomic divisions by assuming "somatic" means the whole body. Somatic specifically means voluntary skeletal muscle control and conscious sensation. Autonomic handles the involuntary side.
The sympathetic system is often reduced to "stress only." It is active at a baseline level all the time, maintaining blood pressure and vascular tone even when you are relaxed.
"Unipolar" and "pseudounipolar" are used interchangeably in many textbooks. Strictly, human sensory neurons are pseudounipolar (they start bipolar in development and fuse), but most exams accept either term.
⚠️ Know the three functions of the nervous system (sensory input, integration, motor output) cold. This is a classic short-answer or matching question.
⚠️ Be able to trace a signal path: receptor → afferent nerve → CNS → efferent nerve → effector. Many exam questions are variations on this sequence.
⚠️ The three connective tissue layers (epineurium, perineurium, endoneurium) are a favourite labelling or ordering question. Remember outside-in: epi → peri → endo.
⚠️ Structural vs. functional classification of neurons is commonly tested as a table or matching exercise. Know which structural type maps to which functional type (e.g., sensory = usually unipolar, motor = multipolar, interneuron = multipolar).
⚠️ Anterograde vs. retrograde transport: know the direction and what each carries. A common trick question asks which direction neurotransmitters travel (anterograde, toward the terminal).
True or false: The CNS consists of the brain, spinal cord, and peripheral nerves. False. The CNS is only the brain and spinal cord. Peripheral nerves belong to the PNS.
Fill in the blank: Nerves that carry signals toward the CNS are called _______ (or sensory) nerves. Afferent.
True or false: The parasympathetic nervous system is responsible for the fight-or-flight response. False. That is the sympathetic nervous system. The parasympathetic system handles rest and digest.
Fill in the blank: The connective tissue layer that wraps each individual fascicle is the _______. Perineurium.
True or false: Most neurons can divide to replace themselves after injury. False. Most neurons are amitotic after foetal development and cannot divide.
Q: Name the two major divisions of the nervous system and state the main components of each.
A: The central nervous system (CNS), comprising the brain and spinal cord, and the peripheral nervous system (PNS), comprising the cranial and spinal nerves that extend throughout the body.
Q: Distinguish between the somatic and autonomic divisions of the PNS.
A: The somatic division controls voluntary skeletal muscles and processes conscious sensory input. The autonomic division regulates involuntary functions of organs, glands, and smooth muscle, and is further divided into sympathetic (fight or flight) and parasympathetic (rest and digest) branches.
Q: List the three connective tissue layers of a nerve from outermost to innermost, and state what each layer surrounds.
A: Epineurium (surrounds the entire nerve), perineurium (surrounds each fascicle), endoneurium (surrounds each individual axon).
Q: A neuron has many dendrites and a single axon. What is its structural classification, and where is this type most commonly found?
A: Multipolar. It is the most common neuron type, found throughout the CNS, including motor neurons and interneurons.
Q: Explain the difference between anterograde and retrograde axonal transport.
A: Anterograde transport moves materials from the cell body toward the synaptic terminal (e.g., neurotransmitter vesicles). Retrograde transport moves used or damaged materials from the axon terminal back to the cell body for recycling.
Q: Why is the amitotic nature of neurons clinically significant?
A: Because most neurons cannot divide after foetal development, damage to neurons (as in spinal cord injuries or neurodegenerative diseases) tends to result in permanent loss of function, since the lost cells are generally not replaced.
This material connects directly to the next unit on neural signalling (action potentials, graded potentials, synaptic transmission), which explains how the structures described here generate and propagate electrical signals. It also ties into the muscular system, since the somatic motor pathway is the route by which voluntary movement is initiated. The autonomic division resurfaces in cardiovascular and digestive physiology, where sympathetic and parasympathetic regulation of heart rate, blood pressure, and gut motility are tested in detail.
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