The nervous system and the neuron · Biology Class 11–12 | StemPy Lab
Course 17.1·Lesson 1 of 3
The nervous system and the neuron
The nervous system carries messages as electrical signals. In humans it has a central part, the brain and the spinal cord, and a peripheral part, the nerves. The nerve cell, or neuron, is its working unit. It has a cell body, short dendrites and a long axon, and some axons are wrapped in a sheath of myelin.
about 35 min1 simulation1 practice question
Lesson 1 of 3Show all lessonsHide
Try the code and the simulations beside the lesson.
A message with no delay
Touch a hot cup. Your hand moves away before you feel the heat. A message ran from your skin to your spinal cord, and a reply ran back to your muscles. This took a few hundredths of a second. The system that does this work is the nervous system. It finds a change, carries the news and starts a response.
Simple and complex nervous systems
The plan of the nervous system is different in different animals.
Hydra has a network of neurons spread through the body. There is no brain.
Insects have a brain, a ventral nerve cord and ganglia along the cord. A ganglion is a group of nerve cell bodies.
Vertebrates have a more developed system, with a large brain and a spinal cord.
The human nervous system
The human nervous system has two parts.
The central nervous system (CNS) is the brain and the spinal cord. It is the place where information is processed, and it controls the body.
The peripheral nervous system (PNS) is all the nerves that join the CNS to the rest of the body. A nerve is a bundle of nerve fibres wrapped together in connective tissue. NCERT uses the word without a definition, so this is the usual meaning.
The nerves of the PNS have two kinds of fibres.
Afferent fibres bring impulses from tissues and organs to the CNS.
Efferent fibres carry impulses from the CNS to tissues and organs. The word effector names the organ that responds.
A way to remember them: afferent arrive at the CNS, and efferent exit from it.
The PNS has two divisions.
The somatic neural system relays impulses from the CNS to the skeletal muscles.
The autonomic neural system sends impulses from the CNS to involuntary organs and smooth muscles. It has two parts, the sympathetic and the parasympathetic neural system.
The visceral nervous system is also a part of the PNS. It is the whole complex of nerves, fibres, ganglia and plexuses by which impulses travel from the CNS to the internal organs (the viscera) and from the viscera to the CNS.
A tree diagram of the human nervous system. At the top, the nervous system divides into the central nervous system, which is the brain and the spinal cord, and the peripheral nervous system, which is all the nerves joined to the central nervous system. The peripheral system has afferent fibres, which carry impulses from tissues and organs to the central nervous system, and efferent fibres, which carry impulses from the central nervous system to tissues and organs. The efferent fibres go to two divisions: the somatic neural system, which serves the skeletal muscles, and the autonomic neural system, which serves involuntary organs and smooth muscles. The autonomic system has a sympathetic and a parasympathetic division. A note says that the visceral nervous system is the part of the peripheral system made of the nerves, fibres, ganglia and plexuses of the internal organs.
You should find that a signal is called afferent when it runs towards the CNS and efferent when it runs away from it. Commands to the heart are efferent and autonomic.
What the two autonomic parts do
NCERT names the sympathetic and the parasympathetic parts but does not list what they do. This part is not in the NEET syllabus, but A-Level and IB ask it. The two parts usually work in opposite ways.
Heart rate: the sympathetic part raises it, and the parasympathetic part lowers it.
Pupil of the eye: the sympathetic part widens it, and the parasympathetic part narrows it.
Airways: the sympathetic part widens them, and the parasympathetic part narrows them.
Digestion: the sympathetic part slows it, and the parasympathetic part speeds it up.
The sympathetic part prepares the body for action, as in fear or in a race. The parasympathetic part is the part of rest and digestion.
The neuron
The neuron is the structural and functional unit of the nervous system. A neuron has three parts.
The cell body, also called the cyton, holds the nucleus. Its cytoplasm contains Nissl granules, which are small dark granules.
The dendrites are short fibres that branch from the cell body. They bring impulses towards the cell body.
The axon is a long fibre. It carries the impulse away from the cell body. It ends in fine branches, and each branch ends in a swollen synaptic knob. The knob holds synaptic vesicles, small sacs filled with chemicals called neurotransmitters.
A labelled drawing of a multipolar neuron. On the left is the cell body, also called the cyton, with a nucleus and small dark Nissl granules. Several short branched dendrites leave the cell body. One long axon leaves it on the right. The axon is wrapped in a myelin sheath made by Schwann cells, with gaps called nodes of Ranvier between neighbouring parts of the sheath. The axon ends in fine branches, and each tip is a swollen synaptic knob that holds synaptic vesicles. An arrow shows that the impulse travels from the dendrites and the cell body along the axon to the synaptic knobs.
Three kinds of neuron
NCERT groups neurons by the number of processes that leave the cell body.
Three neurons side by side. The multipolar neuron has one axon and two or more dendrites, as in the cerebral cortex. The bipolar neuron has one axon and one dendrite, as in the retina of the eye. The unipolar neuron is a cell body with one axon only, as in the embryonic stage.
A multipolar neuron has one axon and two or more dendrites. It is found in the cerebral cortex.
A bipolar neuron has one axon and one dendrite. It is found in the retina of the eye.
A unipolar neuron has a cell body with only one axon. NCERT says it is found usually in the embryonic stage. Some textbooks use the words pseudo-unipolar for the sensory neurons of the adult, which have a single process that divides into two.
Myelinated and non-myelinated fibres
An axon may be wrapped in a myelin sheath. The sheath is made by Schwann cells, which wind round the axon. The narrow gaps between neighbouring parts of the sheath are the nodes of Ranvier.
Two nerve fibres drawn from the side. The upper one is a myelinated fibre: an axon wrapped by several Schwann cells, each making a segment of myelin sheath, with the narrow gaps between neighbouring segments marked as nodes of Ranvier. The lower one is a non-myelinated fibre: the axon is enclosed in a Schwann cell that does not form a myelin sheath around it, and there are no nodes.
Myelinated fibres have the sheath. They are found in the spinal and cranial nerves.
Non-myelinated fibres are enclosed by a Schwann cell that does not form a myelin sheath around the axon. They are common in the autonomic and somatic neural systems.
In the brain and the spinal cord, other cells called oligodendrocytes make the myelin. NCERT does not name them, so this is for A-Level and IB.
Try it in Python
The program sorts neurons into the three kinds by counting their axons and dendrites.
Practise with new numbers
Common mistakes
Mixing up afferent and efferent. Afferent fibres bring impulses to the CNS. Efferent fibres carry them away.
Saying that dendrites carry the impulse away. Dendrites bring it to the cell body. The axon carries it away.
Calling the brain part of the PNS. The brain and the spinal cord are the CNS. The nerves form the PNS.
Counting the nodes of Ranvier as the segments. A row of segments has one node less than the number of segments.
Saying that myelinated fibres belong to the autonomic system. NCERT places the myelinated fibres in the spinal and cranial nerves.
Remember these
CNS: brain and spinal cord. PNS: the nerves. Afferent: to the CNS. Efferent: from the CNS.
Somatic: to skeletal muscles. Autonomic: to involuntary organs and smooth muscles, in a sympathetic and a parasympathetic part.
Neuron: cell body (cyton, Nissl granules), dendrites, axon, synaptic knobs with vesicles.
Multipolar: one axon and two or more dendrites. Bipolar: one axon and one dendrite. Unipolar: one axon only.
Myelin is made by Schwann cells. Nodes of Ranvier lie between the parts of the sheath.
Check yourself
Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.
Name the two parts of the human nervous system, and say what each contains.
A nerve fibre carries an impulse from a sense organ in the skin to the spinal cord. Is it afferent or efferent? Which division of the PNS is it in?
Which part of a neuron holds the Nissl granules, and which part holds the synaptic vesicles?
A neuron has one axon and one dendrite. Name its kind and one place where it is found.
What is the difference between a myelinated and a non-myelinated fibre?
References
Clark M. A., Douglas M. and Choi J. (2018). Biology 2e, OpenStax, chapter 35, The Nervous System, sections 35.1 and 35.4.
NCERT (2023). Biology, Textbook for Class XI, chapter 18.
Enroll free in The Neuron and the Nerve Impulse
Every lesson is open to read. Enrolling also lets you:
Mark lessons done and carry on where you stopped.
Take the tests of this lesson and its course.
Count the course towards the certificate for its chapter.