Nervous system:
- Mammalian nervous system includes: Central nervous system (CNS) and peripheral nervous system (PNS).
  - CNS comprises the brain and the spinal cord.
  - PNS comprises the nerves that connect to CNS: Cranial nerves and spinal nerves.
    - Cranial nerves transmit information to the brain.
    - Spinal nerves transmit information to the spinal cord.

Neurones and their structures:
- Neurones are a part of the nervous system.
- They are individual nerve cells that transmit electrical impulses throughout the body as a nerve fibre.
- Neurones carry information directly to their target cells.
- They coordinate the activities of sensory receptors (in the sense organs), decision-making centres (in the CNS), and effectors (in muscles and glands).
- There are three different kinds of neurones: Sensory neurones, intermediate/relay neurones and motor neurones.
Motor neurones:
- They carry impulses from the CNS (brain or spinal cord) to the effector organs (muscles or glands).
- The cell body lies within the brain or spinal cord and consists of a nucleus and a rough endoplasmic reticulum (that synthesise proteins).
- Dendrons are the thin cytoplasmic processes that extend from the cell body.
- Dendrons are branched into dendrites, which are very short and often have many branches.
- Highly branched dendrites provide a large surface area for the attachment of other neurones.
- Axon is much longer and conducts impulses in long distances (some motor neurones with a cell body in the spinal cord have the axon running all the way up to the toes).
- Axons consist of mitochondria in their cytoplasm and some are covered by dark rings called myelin.
  - Myelin is made up of specialised cells known as Schwann cells.
  - Schwann cells spiral around and enclose the axon in many layers which is called myelin sheath.
  - The myelin sheath is largely made up of lipids and proteins, and it helps increase the speed of conduction of nerve impulses.
  - Small uncovered areas of axon between Schwann cells are called nodes of Ranvier (occurs about every 1 to 3 mm in human neurones).
- End branches of the axon have a large number of mitochondria and vesicles containing neurotransmitters.
- These vesicles are involved in passing impulses to an effector cell (muscles or glands).
Sensory neurones:
- Sensory neurones carry impulses from sensory organs (eye, ear, skin, nose and tongue) to the CNS.
- They have the same basic structures as a motor neurone.
- The only difference is, it has one long dendron and an axon. Axon is often shorter than the dendron.
- Dendrites of a sensory neurone are found outside the spinal cord in the skin, muscle or gland of their particular sensory receptor.
- The axon lies in the spinal cord where they connect with the dendrites of other neurones.
Intermediate/relay neurones:
- Relay neurones are mainly present in the CNS and allow the communication of the sensory and the motor neurones.
- They have very short and unmyelinated axons.
- About two-thirds of the motor and sensory neurones are unmyelinated due to the nature of adaptation of the neurones.
- Damaged myelin sheath causes problems in the nerve impulse transmission.
Summary table for the features of sensory and the motor neurones:
Reflex action:
- It is a pathway along which impulses are transmitted from a receptor to an effector without the involvement of the conscious regions of the brain (involuntary control).
- An example of a reflex arc when a finger is pricked by a needle:
  - First, sensory receptor cells on the skin pick up a change (stimulus) in the skin such as pricking of the finger.
  - The stimulus is then converted into electrical impulses and is passed through a sensory neurone to the spinal cord.
  - In the spinal cord, signal is passed through an intermediate neurone to a motor neurone as well as the brain.
  - Signal transferred from the motor neurone then reaches the muscle cells causing it to contract, as a result the finger is withdrawn.

Electrical impulses:
- Electrical impulses do not involve the movement of electrons.
- They are very small changes in the distribution of electrical charge across the cell surface membrane called action potentials.
- Action potential is caused by the very rapid movement of sodium ions and potassium ions into and out of the axon.
- It is also caused due to channels that are open all the time.
- Movement of the ions occurs due to potential difference across the membranes.
- Potential difference in the axon at rest is called resting potential.

Measurement of potential difference:
- Potential differences could be measured in some organisms such as squids and earthworms, whose axons are very wide.
- Tiny electrodes are inserted into their cytoplasm to measure the changes in electrical charge.
- When the test is done in an unmyelinated neurone and the charge is measured through voltmeter:
  - Before the electrode touches the axon, voltmeter reads 0 mV.
  - After the electrode touches the axon, voltmeter reads -70 mV.
  - Inside of the axon has slightly negative charge than the outside which cause potential difference
Maintenance of resting potential:
- Resting potential is produced and maintained by sodium-potassium pumps present in the cell membrane of the axon.
- Three sodium ions are pumped out of the cell while two potassium ions enter the cell using one ATP.
- In addition, there are many large, negatively charged molecules inside the cells that attract the potassium ions which reduces their outward diffusion.
- This way more positive ions are removed compared to the positive ions entering the cell.
- This maintains the negative charge on the inside of the axon compared to the outside.

Action potential:
- The rapid fleeting change in potential difference across the membrane is the action potential.
- It can be measured using an oscilloscope.
  - Oscilloscope measures the changing voltage of an electrical signal over a time period.
- Voltage-gated channels also play a role in action potential by letting sodium and potassium ions pass through.
  - They open and close depending on the electrical potential across the membrane.

Generation of action potential:
- In the axon, more sodium ions are present outside whereas more potassium ions are present inside.
- First, either a natural or artificial stimulus, such as electric current is used to stimulate the axon.
- The threshold value to generate the axon potential is between -60mV and -50mV.
- Stimulation of the axon causes the cell membrane to reach a certain value of charge, which opens the voltage-gated sodium channels to pass sodium ions inside from the outside.
- In the beginning, only a few channels are open to let the sodium ions in.
- Entry of sodium ions reduces the negative charge inside the axon and changes the potential difference across the membrane.
- This change in potential difference causes depolarisation of the membrane and triggers to open up more voltage-gated sodium channels.
- Entry of more sodium ions causes more depolarisation and increases the inside potential from -70 mV to +30 mV.
- After the inside potential becomes positive all the voltage-gated sodium ion channels get deactivated and voltage-gated potassium ion channels open.
- Therefore, potassium ions diffuse out of the axon, down their concentration gradient.
- This decreases the positive charge inside the axon and brings the potential difference back down to -70 mV. This process is called repolarisation.
- Infact, the potential difference becomes even more negative than the normal resting potential. This process is called hyperpolarisation.
- More negative charge inside the axon causes voltage-gated potassium ion channels to close.
- Finally, the sodium-potassium pump works to maintain the normal resting potential.

Refractory period:
- The period of recovery after the action potential is generated in the axon making it unresponsive is called refractory period.
- Importance of refractory period:
  - gives time to prepare the axon for another action potential
  - limits the frequency of impulses that occur at the same period of time
  - allows impulses to travel in only one direction

15.01 - Nervous System

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