Central University of South Bihar Panchanpur, Gaya, India

E-Learning Resources

Department of Biotechnology

NB: These materials are taken/borrowed/modified/compiled from various resources like research articles and freely available internet websites, and are meant to be used solely for the teaching purpose in a public university, and for serving the needs of specified educational programmes.

Dr. Jawaid Ahsan

Assistant Professor Department of Biotechnology

Central University of South Bihar (CUSB)

Course Code: MSBTN2003E04 Course Name: Neuroscience

Neurons • Remember: It’s a cell! • Body of neuron

– Cell Body – contains cell organelles – Dendrites- carry messages to cell body – Axons – carry messages away from cell body

• Cell Structures – Large nucleus with easily seen nucleolus – Chromatophilic substance (Nissl bodies) – similar to

rough ER • Scattered throughout cytoplasm, membranous

– Neurofibrils- help support cell shape

Structure of a Neuron

Structure of a Neuron

Myelinated fibers – nerve fibers whose axons are encased in a myelin sheath. Myelin sheath – The insulating envelope of myelin that surrounds the axon and facilitates the transmission of nerve impulses. Myelin – fatty material. Insulates the fibers and greatly increases the speed of neurotransmission by neuron fibers.

Myelin Sheath The neuron is covered with the Myelin Sheath or Schwann Cells. These are white segmented covering around axons and dendrites of many peripheral neurons. The covering is continuous along the axons or dendrites except at the point of termination and at the nodes of Ranvier. The neurilemma is the layer of Schwann cells with a nucleus. Its function is to allow damaged nerves to regenerate.

Axons

• Can be myelinated or unmyelinated

• PNS

– Schwann cells form myelin sheath

– Nodes of Ranvier- small breaks in myelin sheath

• CNS

– Oligodendrocytes form myelin

– Myelinated neurons form white matter

– Unmyelinated neurons form gray matter

4. Synaptic cleft – the

space between two neurons. Neurotransmitters are released from the axon of the presynaptic neuron (sending neuron) into the synaptic cleft, where they float around until they are picked up by receptors on the postsynaptic neuron (receiving neuron), initiating the action potential. Action potential – The change in voltage in a neuron. When the voltage of a cell fluctuates, the cell is said to be "active". The more active a cell, the more often it will "fire".

A Neuron at Rest

Inside the neuron › High in K + › High in negative ions Outside the neuron › High in Na + › High in positive ions Result › K + tends to diffuse out › Na + tends to diffuse in › Negative ions cannot cross

Na/K pump - helps to restore concentration gradient across the cell membrane Resting potential - difference in electrical charge across the membrane › Established by concentration gradients of various ions › Inside of the membrane has a negative charge of 70 mv (-70 mV) › Membrane is said to be polarized

Potential Changes

Stimuli cause changes to the resting potential by making the inside of the membrane less negative

Once a stimulus happens:

› If stimulus is not strong enough to reach threshold potential = cell membrane will return to resting potential

› If stimulus is strong enough to reach threshold potential = start an action potential

Summation - when additive effect of stimuli causes action potential

Action Potential

• Starts at trigger zone of axon

• Threshold stimulus open sodium channels

• Sodium moves into axon

– Because of the concentration gradient

– Because of the negative charge that attracts the positive ions

• Depolarizes the membrane as negative charge diminishes

• Potassium channels open and potassium moves out of the axon, repolarizing the membrane

Action Potential Animation

Nerve Impulse

• Action potential at the trigger zone stimulates the next part of the axon to do a action potential

• Potentials spread along the axon like a wave

• Unmyelinated axons

– Wave continues uninterrupted; relatively slow

• Myelinated axons

– Wave goes through saltatory conduction (jump from one node to the next); very fast

• (Saltatory conduction (from the Latin saltare, to hop or leap))

Neuron Responses

• All-or-nothing effect

– Neuron does not react until a threshold stimulus is applied, but once it is applied it reacts fully

• Refractory period:

– After a action potential

– Brief period of time

– The nerve cannot be stimulated again.