Unit 5

Communication
What is a Neuron?

 A neuron is a nerve cell, the basic unit of the nervous system.

 It is specialized to carry messages (nerve impulses) from one part of the body to another.
 Neurons help in receiving, processing, and sending information.

 It is responsible for transmitting information throughout the body in the form of

electrical and chemical signals.
 Neurons are highly specialized and do not divide like normal cells.
 Human body contains billions of neurons that work together to control body functions.
Structure of a Neuron

Structure of a Neuron;

The structure of a neuron is specialized to send and receive nerve impulses

(electrical signals). It consists of three main parts:

1. Cell Body (Soma)

1. Contains the nucleus, which controls cell functions.

2. Holds other organelles like mitochondria and ribosomes.
3. Processes incoming signals from the dendrites.
4. Responsible for the general maintenance and health of the neuron.
5. Sends the processed signal to the axon for transmission.

2. Dendrites

1. Branch-like structures extending from the cell body.

2. Receive signals from other neurons or sensory cells.
3. Carry signals toward the cell body.
4. Can have many branches to connect with multiple neurons.
5. Play a key role in how neurons communicate and learn.

3. Axon

1. A long, thin fiber that carries nerve impulses away from the cell body.
2. Can be very short or over a meter long (especially in motor neurons).
3. Only one axon per neuron, but it may branch at the end.
4. Transmits the signal to other neurons, muscles, or glands.
5. Often surrounded by a myelin sheath to speed up signal transmission.
 There are 2 more important parts related to the axon.These are important for the
function of the neuron — they help messages travel faster and get passed to the next cell.

Myelin Sheath (Part of the Axon)

1. A fatty insulating layer that covers the axon.

2. Speeds up the transmission of nerve impulses.
3. Made by Schwann cells (in PNS) or oligodendrocytes (in CNS).
4. Gaps between myelin sections are called Nodes of Ranvier.
5. Helps protect the axon and maintain signal strength.

Axon Terminals (End of the Axon)

1. Located at the end of the axon.

2. Connect to other neurons, muscles, or glands.
3. Release neurotransmitters (chemical messengers).
4. Pass the signal across a synapse (gap between cells).
5. Ensure the message is delivered to the next cell accurately.

Functions of Neurons:

1. Receive Information

 Neurons receive signals from the sense organs (like eyes, ears, skin) or from other
neurons.
 This is done through dendrites, which act like antennae to pick up messages.

2. Process the Information

 The cell body (soma) collects and processes the signals received by the dendrites.
 If the signal is strong enough, the cell body sends it forward to the axon.

3. Carry the Signal

 The axon carries the signal as an electrical impulse (called an action potential) away
from the cell body.
 In many neurons, the axon is covered with a myelin sheath to make the signal travel
faster.

4. Send the Signal to the Next Cell

  At the end of the axon are the axon terminals.
 These release chemicals called neurotransmitters into a small gap (called a synapse) to
pass the message to the next cell.

5. Control Body Activities

 Neurons help the brain and spinal cord control the body:
o Sensory neurons carry information from the body to the brain.
o Motor neurons carry instructions from the brain to the muscles.
o Interneurons connect neurons inside the brain and spinal cord.
 This allows you to think, feel, move, and react.

Neuron Communication :

Definition:

Neuron communication is the process by which neurons send and receive messages to and from
each other, and to other cells in the body. This process uses both electrical and chemical signals.

Two Main Steps in Neuron Communication:

1. Electrical Transmission (Within the Neuron):

 A signal (stimulus) is received by the dendrites.

 The message is passed to the cell body, where it is processed.
 If strong enough, an electrical impulse called an action potential is generated.
 This impulse travels along the axon toward the axon terminals.
 The myelin sheath (if present) helps the signal travel faster.

2. Chemical Transmission (Between Neurons):

 At the axon terminals, the electrical impulse triggers the release of neurotransmitters
(chemical messengers).
 These neurotransmitters cross a tiny gap called the synapse.
 On the next neuron, receptors on the dendrites detect these chemicals.
 The signal is passed on, starting a new electrical impulse in the next neuron.

Flow of Neuron Communication

The message (nerve impulse) flows in the following order:

1. Stimulus

 A signal is received from the environment (e.g., touch, sound, pain) or from another
neuron.
2. Dendrites

 The dendrites of the neuron receive the signal.

3. Cell Body (Soma)

 The cell body processes the signal and checks if it is strong enough to send.

4. Axon

 If the signal is strong, an electrical impulse (action potential) is generated and travels
down the axon.

5. Axon Terminals

 The impulse reaches the axon terminals (end of the neuron).

6. Synapse

 The neuron releases neurotransmitters into the synapse (tiny gap between neurons).

7. Next Neuron

 The next neuron’s dendrites receive the neurotransmitters, and the signal continues.

Functions of Neuron Communication:

 Allows the brain to send and receive signals

 Controls body movements and sensations
 Helps us react to stimuli
 Supports thinking, memory, and learning

Resting Membrane Potential (RMP)

Definition

The Resting Membrane Potential (RMP) is the electrical potential difference across the
plasma membrane of a neuron or excitable cell when it is not transmitting an impulse.

 It is typically around –70 millivolts (mV) in neurons, indicating the inside of the cell is
negatively charged relative to the outside.

Ionic Basis of RMP

RMP arises due to the unequal distribution of ions and the selective permeability of the
neuronal membrane.

Main contributing factors:

1. Differential Ion Concentrations:

o K⁺ (Potassium): High inside the cell, low outside.
o Na⁺ (Sodium): High outside the cell, low inside.
o Cl⁻ (Chloride) and large negatively charged proteins: Higher inside (proteins)
and outside (Cl⁻), respectively.
2. Selective Membrane Permeability:
o The membrane is more permeable to K⁺ than Na⁺ at rest.
o K⁺ tends to diffuse out, leaving behind negative charges (proteins), contributing
to negativity inside.
3. Sodium-Potassium ATPase Pump (Na⁺/K⁺ Pump):
o Actively transports 3 Na⁺ ions out and 2 K⁺ ions in, against their concentration
gradients.
o Maintains the electrochemical gradient.
o Requires ATP — hence, an energy-dependent process.
4. Anionic Proteins:
o Large, negatively charged intracellular proteins cannot cross the membrane.
o They add to the net negative charge inside the cell.

Why is RMP Important?

 RMP keeps the neuron ready to fire an impulse when stimulated.

 It sets up the condition for action potentials (nerve impulses).
 Without RMP, the neuron wouldn't be able to communicate effectively.

Significance of RMP

 Prepares neurons for rapid response: The RMP sets up the neuron to generate action
potentials.
 Maintains cellular homeostasis and electrochemical gradients.
 Necessary for functions like muscle contraction, nerve impulse transmission, and
hormone release.

AMP – Action Membrane Potential (Action Potential)

What is an Action Potential (AMP/AP)?

An Action Potential (AP) is a rapid, temporary reversal of the resting membrane potential
(RMP) in a neuron or excitable cell.
  It is an electrochemical signal that allows neurons to transmit information over long
distances.
 During an AP, the inside of the neuron becomes briefly positive compared to the
outside.

Phases of Action Potential

1. Resting Phase

o Membrane at –70 mV (RMP).

o Na⁺ and K⁺ channels are closed.
o Maintained by Na⁺/K⁺ pump.

2. Depolarization

o Stimulus opens voltage-gated Na⁺ channels.

o Na⁺ rushes in due to electrochemical gradient.
o Membrane potential becomes less negative, and eventually positive (~+30 mV).

3. Repolarization

o Na⁺ channels close, and K⁺ channels open.

o K⁺ ions exit the cell, restoring the negative charge inside.
o Membrane potential drops back toward –70 mV.

4. Hyperpolarization (Afterpotential)

o K⁺ channels remain open longer, causing excessive K⁺ outflow.

o Membrane becomes more negative than RMP (~–80 mV).
o Returns to RMP as K⁺ channels close and Na⁺/K⁺ pump restores balance.

5. Refractory Period

o Time when the neuron cannot fire another AP:

 Absolute Refractory Period: No AP possible (Na⁺ channels inactivated).
 Relative Refractory Period: Very strong stimulus needed (during
hyperpolarization).

Importance of Action Potential

 Allows for long-distance neural communication (brain to body).

 Triggers muscle contraction, reflexes, and hormonal responses.
 Essential for processes like thinking, sensation, learning, and movement.

Action Potential as a Graph:

  A typical action potential has a sharp rise and fall (like a spike) when graphed.
 Key phases: Resting → Rising (Depolarization) → Falling (Repolarization) →
Recovery (Resting again)

Hormones in Vertebrates and Invertebrates

Vertebrates (animals with backbone)

Examples: Fish, Amphibians, Reptiles, Birds, Mammals

 Have well-developed endocrine glands that secrete hormones directly into the
bloodstream.
 Hormones regulate growth, metabolism, reproduction, and stress response.

🔹 Major Hormones in Vertebrates:

1. Pituitary Hormones (Master gland)

 Growth Hormone (GH): Stimulates growth and cell division.

 Thyroid-Stimulating Hormone (TSH): Stimulates thyroid gland to release thyroxine.

2. Thyroid Hormones

 Thyroxine (T₄) and Triiodothyronine (T₃): Regulate metabolism, growth, and

development.

3. Adrenal Hormones

 Adrenaline (Epinephrine): Triggers “fight or flight” response — increases heart rate

and energy availability.
 Cortisol: Manages long-term stress, increases blood sugar, affects metabolism.

4. Pancreatic Hormones

 Insulin: Lowers blood glucose by promoting uptake into cells.

 Glucagon: Raises blood glucose by stimulating glycogen breakdown in the liver.

5. Sex Hormones

 Testosterone (males): Controls male reproductive functions and secondary sexual traits.
 Estrogen & Progesterone (females): Regulate menstrual cycle, pregnancy, and female
traits.
Invertebrates (animals without backbone)

Examples: Insects, Mollusks, Annelids (worms), Crustaceans

 Endocrine systems are less centralized, often involve neurosecretory cells.

 Hormones control molting, metamorphosis, reproduction, and other vital functions.

🔹 Major Hormones in Invertebrates:

1. Ecdysone (Molting hormone – insects & crustaceans)

 Initiates molting (ecdysis) and metamorphosis.

2. Juvenile Hormone (JH) (Insects)

 Maintains the larval stage.

 When JH levels fall, the insect transitions into the adult form.

3. Neurohormones (From brain-like structures)

 Example: Prothoracicotropic Hormone (PTTH) in insects

→ Stimulates the release of ecdysone.

4. Crustacean Hormones

 Crustacean Hyperglycemic Hormone (CHH): Increases blood glucose.

 Gonad-Inhibiting Hormone (GIH): Suppresses reproductive activity.

5. Mollusk Hormones

 Gonadotropin-like Hormones: Control gamete development and reproduction.

 Growth Factors: Aid in shell formation and tissue repair.

Feedback Mechanism – just take Overview

What is it?

A feedback mechanism is a biological control system that helps maintain homeostasis by

regulating hormone levels or physiological processes based on internal conditions.

Types of Feedback Mechanisms:

1. Negative Feedback (Most common)

 Function: Reverses a change to bring the system back to normal.

  Purpose: Maintains stability and balance.
 Example:
o Blood glucose regulation:
 High glucose → Insulin release → Lowers glucose → Stops insulin.
o Thyroid hormone regulation:
 Low T3/T4 → More TSH → Raises T3/T4 → Inhibits TSH.

2. Positive Feedback ( Less common)

 Function: Amplifies the original change.

 Purpose: Drives processes to completion.
 Example:
o Childbirth (labor):
 Uterine contractions → Oxytocin release → Stronger contractions →
More oxytocin.
o Blood clotting:
 Platelets release chemicals to attract more platelets.

Completed by:
Aqsa Naseer