A neuron is an electrically excitable cell that processes and transmits information through electrical and chemical signaling, forming the fundamental unit of the nervous system. Neurons can be classified based on their processes, axon length, and function, including sensory, motor, parasympathetic, and sympathetic neurons. The neuron consists of three main parts: the cell body, dendrites, and axon, each serving specific roles in communication within the nervous system.

1. NEURON

2. Specific objective:
 Define what is neuron .
 Describe the anatomy of neuron .
 Enumerate the constituents of neuron.
 Enlist the types of neuron .
 Describe the function of neuron .

3. Introduction
Nervous System:
 It is the chief controlling and coordinating system of the body.
 It is the most complex system of the body and contains about 130 billions
neurons in an adult brain( very rich).
 Average weight of adult brain in air is 1500 grams. Since brain floats in
cerebrospinal fluid (CSF) ,it only weighs 50 grams which is comfortable.
 The basic unit of the nervous system is neuron.
Neuron:
A neuron ( also known as a neurone or nerve cell) is an electrically excitable
cell that processes and transmits information by electrical and chemical
signalling

4. a neuron

5. Definition
Neurons (neurones or nerve cells) are the fundamental units of the brain and
nervous system ,the cells are responsible for receiving sensory input from the
external world , for sending motor commands to our muscles , and for
transforming and relaying the electrical signal at every step .

6. Diagram of neuron:

7. PARTS OF NEURON:
Most neuron have 3 parts:
1]Cell body
2]Dentrites
3]Axon
CELL BODY: The cell body is the central region which is the most important
part of the neuron containing the nucleus of the cell. Collectively they form
grey matter and the nuclei in the CNS and ganglia in the peripheral nervous
system.

8. Cell body in CNS:

9. Cell body in PNS:

10. Conti…..
DENTRITES : Are the receiving or input portions of a neuron. They are many
,short, richly branched and often vericose.
AXON : Is a single elongated process. It propagates nerve impulses toward
another neuron , muscle fiber , or a gland cell.
• Collectively the axons form tracts ( white matter) in the CNS & nerves in the
PNS.
• In the CNS ,collection of axons form a fasciculus in the CNS . Number of
fascicle is collectively called as tract and oligodentrocytes , myelinate axons.
• In the PNS , the plasma membrane of schwann cell wraps around a segment
to form myelin sheath .Only a segment of axon is myelinated by one
schwann cell in PNS.

12. The cell body of neuron consists of the following:
 Cytoplasm
 A cell membrane
 Large Central nucleus with a prominent nucleolus -“owl’s eye appearance”.
 Numerous mitochondria.
 Golgi complex
 Nissl substance /Nissl granules –these are intensely staining rough
endoplasmic reticulum
 Neurofibrils- A network of fibrils in the cytoplasm.

13. Constituents of a neuron:

14. Classification of neuron:
The classification is based on the following :
1}According to the no. of their processes.
2}According to the length of axon.
3}Functional classification.

15. Conti….
1}ACCORDING TO THE NO. OF THEIR PROCESSES:
A]Multipolar neurons : Most of the neurons in man are multipolar eg :all
motor and internuncial neurons /interneurons.
B]Bipolar neurons: 1st neuron of retina , ganglia of 8th cranial nerve and the
olfactory mucosa.
C] Pseudounipolar neurons: Are actually unipolar to begin with but become
bipolar functionally .Found in dorsal nerve root ganglia and sensory ganglia of
cranial nerves.
D] Unipolar neurons :Mesencephalic nucleus of trigeminal nerve.

17. Conti……
2}ACCORDING TO THE LENGTH OF AXON:
A]Golgi Type I : These neurons have long axons and neurons short dentrites .
Seen in pyramidal cells , cerebral cortex , purkinjee cells of cerebellum and
anterior horns of spinal cord.
B] Golgi Type II : These are neurons with small axons and establish synapses
with neighbouring neurons .Seen in cerebral cortex and cerebellar cortex.
AMACRINE NEURONS : Without axon , only with dentrite . Seen in retina
of eyeball.

18. Golgi type I neuron:

19. Golgi type II neuron(A) & golgi typeI neuron (B)

20. Conti…
3}FUNCTIONAL CLASSIFICATION :
A]Sensory neurons: It is again divided into following:
Primary or 1st order sensory neurons : Present as spinal or sensory neurons in
the dorsal root ganglion of spinal nerves.
Secondary or 2nd order sensory neurons : Present in the grey matter of spinal
cord and in brain stem .
Tertiary or 3rd order sensory neurons : Seen in the thalamus.

22. Conti…
B] Motor neurons: They carry impulses from CNS to distal part of the
body. These are again divided into following:
1}Upper motor neuron :Present in motor area of brain . Synapse with
cranial nerve nuclei and anterior horn of spinal cord
2} Lower motor neuron : Located in cranial nerve nuclei and anterior horn
of spinal cord .

24. Conti…..
C]Parasympathetic neurons(autonomous):
These are again divided into following:
1}Preganglionic neurons are located in cranial nerves
III , VII , IX , X and also in sacral 2-4 segments of spinal cord.
2} Post ganglionic neurons are located close to the wall or within the wall of
viscera .
3} ‘CRANIOSACRAL OUTFLOW’: The parasympathetic outflow is called
craniosacral outflow.

26. Conti…
D]Sympathetic neurons (autonomic):
1}Preganglionic neuron are located in lateral horn of thoracic , one to
lumbar , two segments of spinal cord .
2} Post ganglionic neuron are located in the ganglia of the sympathetic
trunk away from the viscera .
3}THORACOLUMBAR OUTFLOW : The sympathetic outflow is called
thoracolumbar outflow.

27. Preganglionic &Postganglionic neuron:

29. Functions:
 IRRITABILITY: Ability to initiate nerve impulse.
 CONDUCTIVITY : Ability to transmit an impulse.