For class XI CBSE stuents & competitive examination

2. NEURAL SYSTEM & ENDOCRINE SYSTEM

3.  The neural
system of
all animals
is composed
of highly
specialised
cells called
neurons.

4. Central nervous peripheral
nervous
system (CNS)
system(PNS)


7. The afferent nerve fibres transmit impulses
from tissues/organs to the CNS.
The efferent fibres transmit regulatory
impulses from the CNS to the concerned
peripheral tissues/organs. relays impulses
The somatic neural systemrelays impulses ;-
CNS skeletal muscles.
 The autonomic neural systemtransmits
impulses;- CNS involuntary organs and
smooth muscles of the body.

8. 1) Myelinated Nerve Fibres (with myelin sheth)
2)Nonmyelinated Nerve Fibres(without myelin sheth)

9.  A neuron is a microscopic structure composed of
followings:-
 i) Cell body
 ii) Dendrites
 iii) Axon
 The cell body contains cytoplasm with typical cell
organelles and certain granular bodies called
Nissl’s granules.
 Short fibres which branch repeatedly and project
out of the cell body also contain Nissl’s granules and are
called dendrites.
 These dendrites transmit impulses towards the cell
body.

10.  The axon is a long f iber, t he dist al end of which is
branched. Each branch t erminat es as a bulb-like
st ruct ure called synapt ic knob.
 I t cont ains synapt ic vesicles cont aining chemicals
called neurot ransmit t ers.
 The axons t ransmit nerve impulses away f rom t he cell
body t o a synapse or t o a neuromuscular j unct ion.
 The myelinat ed nerve f ibres are enveloped wit h
Schwann cells, which f orm a myelin sheat h around t he
axon.
 The gaps bet ween t wo adj acent in myeline sheat hs are
called nodes of Ranvier.

14. DIAGRAMMATIC REPRESENTATION OF NERVE
IMPULSE

15.  The elect rical pot ent ial dif f erence across t he plasma
membrane at t he sit e A is called t he action potential,
which is in f act t ermed as a nerve impulse.
 At sit es immediat ely ahead, t he axon (e.g., sit e B)
membrane has a posit ive charge on t he out er surf ace
and a negat ive charge on it s inner surf ace.
 As a result , a current f lows on t he inner surf ace
f rom sit e A t o sit e B. On t he out er surf ace current
f lows f rom sit e B t o sit e A t o complet e t he circuit of
current f low. Hence, t he polarit y at t he sit e is
reversed,
 An act ion pot ent ial is generat ed at sit e B. Thus, t he
impulse (action potential) generat ed at sit e A arrives

16.  The sequence is repeat ed along t he length of the
axon and consequent ly t he impulse is conducted.
 The rise in t he st imulus-induced permeabilit y t o Na+
is ext remely short lived.
 I t is quickly f ollowed by a rise in permeability to
K+. Wit hin a f ract ion of a second, K+ diffuses
out side t he membrane and rest ores t he resting
potential of the membrane at t he sit e of excit at ion
and t he f ibre becomes once more responsive t o
f urt her stimulation.

18.  A nerve impulse is transmitted from one neuron to
another through junctions called synapses.
 A synapse is formed by the membranes of a
 pre-synaptic neuron and a post-synaptic neuron,
which may or may notbe separated by a gap called
synaptic cleft.

19.  At electrical synapses, the membranes of pre- and post-
synaptic neurons are in very close proximity.
 Electrical current can flow directly from one neuron
into the other across these synapses.
 Transmission of an impulse across electrical synapses
is very similar to impulse conduction along a single
axon.
 Impulse transmission across an electrical synapse is
always faster than that across a chemical synapse.