NEURONS

various sizes and shapes
the basic functions of all neurons are more or less similar

1) receive and integrate inputs
2) relay their output to some other target cell

cell that has an excitable cell membrane
capable of producing an electrical impulse
communication between neurons

passing of a chemical message from one nerve cell to another
across space between them called the synapse

NEURON ANATOMY

1) Soma

Nissl bodies
Nucleus

2) Dendrites

3) Axon

neurolemma
Schwann cell
Node of Ranvier

4) Synapse

synaptic knob
synaptic cleft
synaptic vesicles

1) SOMA (Cell body)

lack spindle fibers necessary for cell division
acts as a bridge between the dendrite and the axon
site of an extremely high rate of metabolism
numerous mitochondria, ribosomes and endoplasmic reticulum

NISSL BODIES

clusters of ER and ribosomes

GANGLIA AND NUCLEI

grouped soma having similar functions

ganglia (along spinal cord)
nuclei (within CNS)

2) DENDRITES

short, branched arms that stick out from the soma
receive incoming signals and carry them to the soma
cell membrane of soma and dendrites sensitive to chemical, mechanical or electrical stimulation
stimulation leads to generation of action potential (nerve impulse) conducted along the axon

3) AXON

conduct impulse away from the soma

neurolemma
Schwann cell
Node of Ranvier

NEUROLEMMA

axonal membrane found only in myelinated neurons
primarily in peripheral nervous system

SCHWANN CELL

cells that wrap around the axons of nerves
form myelin

a fatty protein sheath
whitish in color
acts as an electrical insulator
increase the speed and efficiency at which nerve impulses may be transmitted

NODE OF RANVIER

gaps located between neighboring Schwann cells on myelinated neurons
nerve transmission occurs at the node (gap) only
skips over the insulated portion of the axon
the current jumps from node to node

4) SYNAPSE

gap that acts as a junction between axon of presynaptic neuron and dendrite of postsynaptic neuron
presynaptic axon ends in synaptic knob

SYNAPTIC KNOB

contains numerous mitochondria and synaptic vesicles full of neurotransmitters
neurotransmitters act to excite or inhibit neighboring neurons

SYNAPTIC CLEFT

short distance between synaptic knob and postsynaptic dendrite is very small

place for regulation of transmission

If a signal is too weak, it will not traverse the synaptic gap
If the signal is strong enough, it will go on to excite the postsynaptic membrane, and thereby continue the transmission
many drugs that act on the nervous system interfere with activity in the synaptic cleft

EVENTS OF SYNAPSE

STEP 1

STEP 2

STEP 3

STEP 4

SYNAPSE

ACTION POTENTIAL

Java applet

FUNCTIONAL CLASSIFICATION OF NEURONS

1) Sensory neurons

receptors

2) Motor neurons

effectors

3) Interneurons

SENSORY NEURONS

app. 10 million sensory neurons in body
also known as afferent fibers
carry info from receptors to central nervous system

RECEPTORS

may be a process of a sensory neuron
may be a specialized cell which communicates with a sensory neuron

Extroreceptors

touch, temperature, pressure, sight, smell, touch, hearing

Proprioreceptors

monitor position of skeletal muscles and joints

Interoreceptors

monitor the activities of the viscera, taste, pain

MOTOR NEURONS

app. 1/2 million motor neurons in body
also known as efferent fibers
carry signals from the CNS to the effector organs (muscles and glands)

EFFECTORS

peripheral targets of motor neurons
change their activities in response to motor neuron impulse

skeletal muscle, cardiac muscle, smooth muscle, glands

INTERNEURONS

app. 20 billion interneurons in body
located entirely within the CNS
interconnect other neurons
analysis of sensory input
coordination of motor output