Control and coordination 2

Question 1

why are nuerons excitable

Answer 1

Neurons are excitable cells because their membranes are in a polarised
state.Because of the electrical difference across the cell membrane, the cell membrane of the neuron is polarized.

Question 2

is the neuron permeable to all ions

Answer 2

Different
types of ion channels are present on the neural membrane. These ion
channels are selectively permeable to different ions. When a neuron is not
conducting any impulse, i.e., resting, the axonal membrane is
comparatively more permeable to potassium ions (K+
) and nearly
impermeable to sodium ions (Na+
). Similarly, the membrane is
impermeable to negatively charged proteins present in the axoplasm.
Consequently, the axoplasm inside the axon contains high concentration
of K
+
and negatively charged proteins and low concentration of Na+
. In
contrast, the fluid outside the axon (extracellular fluid) contains a low concentration of K
+
, a
high concentration of Na+
and thus form a concentration gradient.

Question 3

how is the k+/na+ gradient maintained

Answer 3

These
ionic gradients across the resting membrane are maintained by the active
transport of ions by the sodium-potassium pump which transports 3
Na+
outwards for 2 K+
into the cell. As a result, the outer surface of the
axonal membrane possesses a positive charge while its inner surface becomes negatively charged and therefore is polarised. The electrical
potential difference across the resting plasma membrane is called as the
resting membrane potential.

Question 4

describe depolarisation and generation of electrical impulse

Answer 4

When a stimulus is
applied at a site (Figure 21.2 e.g., point A) on the polarised membrane,
the membrane at the site A becomes freely permeable to Na+
. This leads
to a rapid influx of Na+
followed by the reversal of the polarity at that site,
i.e., the outer surface of the membrane becomes negatively charged and
the inner side becomes positively charged. The polarity of the membrane
at the site A is thus reversed and hence depolarised. The electrical potential
difference across the plasma membrane at the site A is called the
action potential, which is in fact termed as a nerve impulse.

Question 5

how is the impulse conducted

Answer 5

At sites
immediately ahead, the axon (e.g., site B) membrane has a positive charge
on the outer surface and a negative charge on its inner surface. As a
result, a current flows on the inner surface from site A to site B. On the
outer surface current flows from site B to site A (Figure 21.2) to complete
the circuit of current flow. Hence, the polarity at the site is reversed, and
an action potential is generated at site B. Thus, the impulse (action
potential) generated at site A arrives at site B. The sequence is repeated
along the length of the axon and consequently the impulse is conducted.

Question 6

how does membrane return to resting stage?

Answer 6

The rise in the stimulus-induced permeability to Na+
is extremely shortlived. It is quickly followed by a rise in permeability to K+
. Within a fraction
of a second, K+
diffuses outside the membrane and restores the resting
potential of the membrane at the site of excitation and the fibre becomes
once more responsive to further stimulation.

Question 7

define:
i) synapse
ii) synaptic cleft
iii) post/pre synaptic nuerons
iv) neurotransmitter
v) synaptic vescicles
vi) receptor.

Answer 7

i) the junction between two neurons(axon and dedrite) or a neuron and an effector organ is called a synapse

ii) Synaptic cleft is the gap/space between the axon terminal of one neuron and the dendirte of another neuron, across which neurotransmitters travel

iii) neuron present before the synapse ie releases the neurotransmitter is presynaptic

neuron present after the synapse ie receives the transmitter is post synaptic

iv) neurotransmitter are chemicals that travel across synapse and transmit the electrical impulse

v) these are membrane bound sacs present in the axon terminal of an axon and contain neurotransmitters

vi)receptors are specific proteins detect neurotransmitters.

Question 8

what is a synapse

Answer 8

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 not
be separated by a gap called synaptic cleft.

Question 9

describe electrical synapse

Answer 9

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. Electrical synapses are rare in our
system.

Question 10

what is a synaptic cleft

Answer 10

At a chemical synapse, the membranes of the pre- and post-synaptic
neurons are separated by a fluid-filled space called synaptic cleft

Question 11

explain the transmission of impulse

Answer 11

Chemicals called neurotransmitters are involved in the
transmission of impulses at these synapses. The axon terminals contain
vesicles filled with these neurotransmitters. When an impulse (action
potential) arrives at the axon terminal, it stimulates the movement of the
synaptic vesicles towards the membrane where they fuse with the plasma membrane and release their neurotransmitters in the synaptic cleft. The
released neurotransmitters bind to their specific receptors, present on
the post-synaptic membrane. This binding opens ion channels allowing
the entry of ions which can generate a new potential in the post-synaptic
neuron. The new potential developed may be either excitatory or
inhibitory.