The document provides an overview of the nervous system, detailing its components, including the central and peripheral nervous systems, and their roles in maintaining homeostasis and responding to internal and external stimuli. It describes the structure and function of neurons, including the processes of nerve impulse generation and transmission, highlighting the importance of myelin and the roles of axons and dendrites. Additionally, it discusses the physiological mechanisms behind action potentials and the factors affecting the speed of impulse conduction.

1. - BY SHAILESH SANGLE

2.  INTRODUCTION OF THE NERVOUS SYSTEM
 The nervous system detects and responds to changes inside and outside the
body. Together with the endocrine system it controls important aspects of body
function and maintains homeostasis.
 The nervous system consists of the brain, the spinal cord and peripheral nerves.
 Response to changes in the internal environment maintains homeostasis and
regulates involuntary functions, e.g. blood pressure and digestive activity.
 Response to changes in the external environment maintains posture and other
voluntary activities.
 the central nervous system (CNS), consisting of the brain and the spinal cord.
 the peripheral nervous system (PNS) consisting of all the nerves outside the
brain and spinal cord.
 The PNS comprises paired cranial and sacral nerves some of these are sensory
(afferent), some are motor (efferent) and some mixed.

3.  Following two functional parts within the PNS:
• the sensory division
• the motor division
 the motor division is involved in activities that are:
• voluntary — the somatic nervous system (movement of voluntary muscles)
• involuntary — the autonomic nervous system (functioning of smooth and
cardiac muscle and glands).

4.  The nervous system consists of a vast number of cells called neurones
 Supported by a special type of connective tissue, neuroglia.
 Each neuron consists of a cell body and its processes, one axon and many
dendrites.
 Neurons are commonly referred to simply as nerve cells.
 Bundles of axons bound together are called nerves.
 Neurons cannot divide.
 Survival they need a continuous supply of oxygen and glucose. Unlike many
other cells.
 neurons can synthesis chemical energy (ATP) only from glucose.
 The physiological 'units' of the nervous system are nerve impulses, or action
potentials, which are akin to tiny electrical charges.
 neurons are actively involved in conducting nerve impulses.
 Some neurons initiate nerve impulses while others act as 'relay stations' where
impulses are passed on and sometimes redirected.

5.  Neurons have the characteristics of irritability and conductivity.
 Irritability is the ability to initiate nerve impulses in response to stimuli from:
• outside the body, e.g. touch, light waves
• inside the body, e.g. a change in the concentration of carbon dioxide in the
blood
alters respiration; a thought may result in
voluntary movement.
 In the body this stimulation may be described as partly electrical and partly
chemical - electrical.
 Motor neurons and sensory nerve endings initiate nerve impulses.
 And chemical in the transmission of impulses between one neuron and the next
OR between a neuron and an effector organ.
 Conductivity means the ability to transmit an impulse.
Properties of neurons

6.  Cell bodies form the grey matter of the nervous system.
 Are found at the periphery of the brain and in the centre of the spinal
cord.
 Groups of cell bodies are called nuclei in the central nervous system
and ganglia in the peripheral nervous system.

7.  Axons and dendrites are extensions of cell bodies and form the white
matter of the nervous system.
 Axons are found deep in the brain and in groups, called tracts, at the
periphery of the spinal cord. They are referred to as nerves or nerve
fibres, outside the brain and spinal cord.
Each nerve cell has only one axon, carrying nerve impulses away from the cell
body.
They are usually longer than the dendrites, sometimes as long as 100 cm.
 AXON >>>

8.  The membrane of the axon is called axolemma and it encloses the cytoplasmic
extension of the cell body.
 Large axons and those of peripheral nerves are surrounded by a myelin sheath.
 Myelin sheath consists of a series of Schwann cells arranged along the length of
the axon.
 Between the layers of plasma membrane there is a small amount of fatty
substance called myelin.
 The outermost layer of Schwann cell plasma membrane is sometimes called
neurilemma.
 There are tiny areas of exposed axolemma between adjacent Schwann cells,
called nodes of Ranvier, which assist the rapid transmission of nerve impulses.
 Postganglionic fibres and some small fibres in the CNS are non-myelinated.
 The adjacent Schwann cells are in close association and there is no exposed
axolemma. speed of transmission of nerve impulses is significantly
 slower in non-myelinated fibres.

9.  The dendrites are the many short processes that receive and carry incoming
impulses towards cell bodies.
 They have the same structure as axons but they are usually shorter and
branching.
 In motor neurons they form part of synapses and in sensory neurons they form
the sensory receptors that respond to stimuli.
1. Dendrites

10.  An impulse is initiated by stimulation of sensory nerve endings or by the
passage of an impulse from another nerve.
 Transmission of the impulse, or action potential, is due to movement of
ions across the nerve cell membrane.
 In the resting state the nerve cell membrane is polarised due to
differences in the concentrations of ions across the plasma membrane.
 a different electrical charge on each side of the membrane that is called
the resting membrane potential.
 At rest the charge on the outside is positive and inside it is negative.
The principal ions involved are:
• sodium (Na+) the main extracellular cation
• potassium (K+) the main intracellular cation.

11.  In the resting state there is a continual tendency for these ions to
diffuse along their concentration gradients, i.e. K+ outwards and Na+
into cells.
 Initially Na+ floods into the neuron from the ECF causing depolarisation,
creating a nerve impulse or action potential.
 Depolarisation is very rapid, enabling the conduction of a nerve impulse
along the entire length of a neuron in a few milliseconds (ms).
 It passes from the point of stimulation in one direction only, i.e. away
from the point of stimulation towards the area of resting potential.
 The one-way direction of transmission is ensured because following
depolarisation it takes time for repolarisation to occur.
 During this process K+ floods out of the neuron and the movement of
these ions returns the membrane potential to its resting state. This is
called the refractory period.

12.  electrical changes across the membrane can only occur at the gaps
in the myelin sheath, i.e. at the nodes of Ranvier.
 When an impulse occurs at one node, depolarisation passes along
the myelin sheath to the next node so that the flow of current
appears to 'leap' from one node to the next. This is called
saltatory conduction.
 The speed of conduction depends on the diameter of the neuron:
the larger the diameter, the faster the conduction.
 Myelinated fibres conduct impulses faster than un-myelinated
fibres because saltatory conduction is faster than the complete
conduction, or simple propagation.
 The fastest fibres can conduct impulses to, e.g., skeletal muscles
at a rate of 130 metres per second while the slowest impulses
travel at 0.5 metres per second.