Dr. Meenakshi Assistant Professor,

Department of Psychology Magadh University, Bodh Gaya

The nervous system can be divided into two main parts:

Central Nervous System (CNS)

Peripheral Nervous System (PNS)

There are three main types of neurons in the nervous system:

Sensory Neurons

Relay Neurons

Motor Neurons

Transmission of a nerve impulse within a neuron occurs via the movement of an electrical potential along the length of the neuron

When a neuron is not firing, the charge difference across the membrane is negative (-70 mV) - this is known as the resting potential

When a neuron is firing, the charge difference changes to become slightly positive (~ 30 mV) - this is known as the action potential

The change in membrane polarity when the neuron is firing (from resting potential to action potential) is called depolarisation

Restoration of the resting potential is known as repolarisation

The sodium-potassium pump (Na+/K+ pump) maintains the electrochemical gradient of the resting potential (-70 mV)

It is a transmembrane protein that uses active transport to exchange Na+ and K+ ions across the membrane (antiport mechanism)

It expels 3 Na+ ions for every 2 K+ ions admitted (in addition, some of the K+ ions will leak back out of the cell)

This makes the inside of the membrane relatively negative when compared to the outside (-70 mV = resting potential

Sodium and potassium channels in nerve cells are voltage-gated,

meaning they can open and close depending on the voltage across the membrane

In response to a signal at a sensory receptor or dendrite, sodium channels open and sodium enters the neuron passively

The influx of sodium (Na+ in) causes the membrane potential to become positive (depolarisation)

If a sufficient change in membrane potential is achieved (threshold potential), adjacent voltage-gated sodium channels open, generating a wave of depolarisation (action potential) that spreads down the axon

The change in membrane potential also activates voltage-gated potassium channels, causing potassium to exit the neuron passively

The efflux of potassium (K+ out) causes the membrane potential to become negative again (repolarisation)

Before the neuron can fire again, the original distribution of ions (Na+ out, K+ in) must be re-established by the Na+/K+ pump

The inability to propagate another action potential during this time (refractory period) ensures nerve impulses only travel in one direction

The junction between two neurons is called a synapse, it forms a physical gap between the pre-synaptic and post-synaptic neurons

An action potential (electrical signal) cannot cross the synaptic gap, so it triggers the release of chemicals (neurotransmitters) to continue the signal

When an action potential reaches the axon terminal, it triggers the opening of voltage-gated calcium channels

Calcium ions (Ca2+) diffuse into the cell and promote the fusion of vesicles (containing neurotransmitters) with the plasma membrane

The neurotransmitters are released from the axon terminal by exocytosis and cross the synaptic cleft

Neurotransmitters bind to appropriate neuroreceptors on the post-synaptic membrane, opening ligand-gated channels

Excitatory neurotransmitters (e.g. noradrenaline) open ligand-gated sodium channels (depolarisation)

Inhibitory neurotransmitters (e.g. GABA) open ligand-gated potassium or chlorine channels (hyperpolarisation)

The combination of chemical messengers received by dendrites determines whether the threshold is reached for an action potential in the post-synaptic neuron

Neurotransmitter molecules released into the synapse are either recycled (by reuptake pumps) or degraded (by enzymatic activity)

1. Receptors-

a. thermal receptors

b. mechanical receptors

c. chemical receptors

d. photic receptors.

2.Effectors-:- There are 2 main classes of effectors –

a.Muscles:- 1. Smooth muscle cell 2. Straiated muscles 3. Cardiac muscle.

b.Glands:- The 2 major classes of glands are – the duct glands and the ductless. Adjustors

3.Adjustors

http://psychologicalresources.blogspot.com/2010/02/peripheral-response-mechanism.html

http://www.vce.bioninja.com.au/aos-2-detecting-and-respond/coordination--regulation/nervous-system.html

https://toxtutor.nlm.nih.gov/14-004.html