Neurons: Structure and Function

• A neuron contains the nucleus and controls cell function.
• The cytoplasm around the nucleus contains organelles like mitochondria, endoplasmic reticulum, ribosomes, and the Golgi apparatus.

Dendrites

• Short extensions of the cytoplasm from the neuron cell body, often highly branched.
• Primarily carry messages or nerve impulses into the cell body.

Axon

• Single, long extension of cytoplasm.
• Carries nerve impulses away from the cell body.
• Axon length varies; some in the brain are millimeters long, while others from the spinal cord to the foot can be a meter long.
• Axons branch into smaller parts that conclude at the axon terminal.

Myelin Sheath

• Fatty material layer covering most axons, forming a nerve fiber.
• Myelinated fibers have a myelin sheath, while unmyelinated ones do not.
• Outside the brain and spinal cord, Schwann cells form the myelin sheath by wrapping around the axon, creating gaps called nodes of Ranvier.
• Myelin sheath functions as insulator, protects the axon from damage, and speeds nerve impulse movement along the axon.
• The outermost Schwann cell coil forms the neurilemma, which aids in repairing injured fibers.
• In the brain and spinal cord, oligodendrocytes produce the myelin sheath.
• Myelinated fiber regions appear white and are called white matter, while areas with cell bodies and unmyelinated fibers appear grey due to color.

Synapses

• Nerve impulses pass from neuron to neuron at synapses, where the axon terminal of one neuron connects with a dendrite or cell body of another.
• Neurons do not physically touch at a synapse; neurotransmitters carry across a small gap.
• The neuromuscular junction is a synapse where an axon meets a skeletal muscle cell.

Types of Neurons (Functional)

• Sensory: (afferent/receptor) carry messages from receptors in sense organs or skin to the central nervous system (brain/spinal cord).
• Motor: (efferent/effector) carry messages from the central nervous system to effectors, muscles and glands.
• Interneurons: Located in the central nervous system, linking sensory and motor neurons, also called association, connector, or relay neurons.

Types of Neurons (Structural)

• Multipolar: Have one axon and multiple dendrites extending from the cell body, most common type, including interneurons in the brain and spinal cord, and motor neurons to skeletal muscles.
• Bipolar: Have one axon and one dendrite, branched at their ends, found in the eye, ear, and nose to take impulses from receptor cells.
• Unipolar: Have just one extension (axon), found in insects, not humans or vertebrates.
• Pseudounipolar: Properties of both unipolar and bipolar neurons, a single axon from the cell body that separates into two extensions, mostly sensory neurons carrying messages to the spinal cord.

Nerve Impulses

• Messages that travel along a nerve fibre.
• Transmitted quickly allowing the body to respond fast to changes in surroundings.
• Electrochemical change traveling along a nerve fibre, involving changes in electrical voltage and concentration of ions inside/outside the neuron's cell membrane.

Electrical Charge and Potential Difference

• Two types of electrical charges; positive and negative.
• Like charges repel each other, while opposites attract.
• Separated opposite charges have an electrical force pulling them together; this force increases with closer or larger charges.
• Energy is released when negative and positive charges come together.
• Potential difference between two places is measured as voltage in volts (V) or millivolts (mV), with 1000 mV in 1 V.

The Potential Difference Across a Cell Membrane

• When chemical substances dissolve in water, they break up into electrically charged particles called ions.
• Extracellular fluid= a high concentration of sodium chloride, and so most of its charged particles are positive sodium ions (Na+) and negative chloride ions (CI*).
• Intracellular fluid= a low concentration of sodium ions and chloride ions. Its main positive ions are potassium (K*), and negative ions come from a variety of organic substances made by the cell.
• Differences in ion concentration mean there is a potential difference (membrane potential).
• Resting membrane potential, is measured at -70 mV, this means that the potential of the inside of the membrane is less than that of the outside.
• Instead, they move through protein channels (leakage channels, voltage-gated channels)

Resting Membrane

• Neurons occur due to differences in potassium (K*) and sodium ions (Na*) distribution on each side of the cell membrane to be more positively charged than the intracellular fluid .
  • Sodium ion concentration is ~10x higher outside the neuron.
    • Membrane is slightly more permeable to sodium ions due to the limited amount of sodium leakage channels.
  • Potassium ion concentration is ~30x greater inside the neuron.
    • Cell membrane is highly permeable to potassium ions, so potassium ions are able to diffuse.
  • Chloride ion concentration is greater outside the neuron.
    • Cell membrane is permeable to allow diffusion of chloride ions.
  • A higher concentration of large, negatively charged organic ions is higher inside the neuron.
    • The cell membrane, is impermeable to these ions; therefore, they remain within the cell.
• Moves two potassium ions into the cell for every three sodium ions that are removed using ATP. This movement goes against the concentration gradient.
• Inside the cell is more negative than the outside.
• The membrane is said to be polarised.

Action Potential

• If a stimulus to a neuron is sufficient, the signal can be passed along the neuron
• Rapid depolarisation and repolarisation of the membrane happens when opening and closing of gates occur for voltage that cause an action potential to occur.

Depolarisation

• Sudden increase of membrane potential when stimulation exceeds ~15 mV (threshold).
• Sodium channels on the nerve open due to it being stimulated by a neurotransmitter or sensory receptor.
• Once open, sodium ions move in the cell to make the intracellular fluid less negative, increasing the potential difference.
• Voltage-gated sodium channels open if stimulus is strong to increase the potential to -55mV, this creates movement that proceeds independently of the stimulus.
• Making sodium ions move inward making the inside of the membrane more positive at +40 mV.
• The membrane becomes depolarised

Repolarisation

• Follows after short period.
• Sodium channels close to stop the influx of sodium ions.
• Voltage-gated potassium channels open increasing the flow of potassium out of the cell.
• The inside of the membrane decreases its potential, making it more negative than the outside.
• Potassium channels remain open longer dropping the membrane to a lower resting point creating hyperpolarisation.

Refractory Period

• Sodium channels quickly become inactive once opened.
• Making them unresponsive to stimulus.
• While unresponsive to stimulus, the membrane will not undergo another action potential.
• This period last until the membrane goes back to the resting point which is after the threshold is met at -55mV.

Transmission of the Nerve Impulse

• Initial action potential occurs in a section of the membrane, and triggers one in the adjacent membrane.
• This continues along the neuron creating the nerve impulse.
• In an unmyelinated nerve fibre, depolarization of a membrane area causes sodium ions to move into adjacent areas.
• Movement stimulates opening of voltage-gated sodium channels and initiates an action potential there.
• This process is repeated along all the membrane for the action potential.
• Body stimulation occurs at the end of a fibre, so the nerve impulse is prevented from going backwards with the refractory period.
• The refractory period won't allow another action potential to be generated.

Transmission Along Myelinated Fibres

• In a myelinated fibre, the myelin sheath stops ions to flow between the inside and outside of the membrane so no action potential can form.
• Instead, the action potential goes from one node of Ranvier to the next knowns as saltatory conduction, and allowing the nerve impulse to speed up.
• Large myelinated fiber can conduct up to 140 m/s unlike unmyelinated fibre can only transmit 2m/s.

Size of Nerve Impulse

• Nerve impulse is always the same size no matter how strong the stimulus may be.
• Either the stimulus is strong to trigger the impulse, or not, resulting in the all-or-none response.
• A strong stimulus causes depolarisation of more nerve fibers than a weak stimulus.
• A strong stimulus produces more nerve impulses unlike a weak one.

Transmission Across a Synapse

• Very small gap between neurons.
• The nerve impulse is transmitted along the neuron membrane with change in ion concentration, but at the synapse something else is used.
• When a nerve impulse reaches axon terminal, calcium ion channels are activated.
• A higher concentration of calcium ions makes ions flow in the cell at the pre-synaptic axon terminal.
• This causes synaptic vesicles and neurotransmitters to fuse by exocytosis.
• Neurotransmitters react with receptors on the other neuron to create new ligand-gated protein channels, and initiates a new action potential.
• Neurotransmitters are disposed of to be reabsorbed by the presynaptic membrane, by being degraded by enzymes, or by moving away through diffusion.
• Including; Acetylcholine, adrenaline, dopamine, histamine

Effect of Chemicals on the Transmission of Nerve Impulses

• Stimulants such as caffeine and benzedrine stimulate transmission.
• Other drugs, such as anaesthetics or hypnotics, depress the transmission.
• Venom from certain species affect the neuromuscular junction.
• Nerve agents (nerve gases) contain organophosphates, causes acetylcholine buildup at the neuromuscular junction.

Receptors

• Structure that detects changes in the body's external or internal environment.
• Sometimes receptors are grouped in a sense organ like light receptors in the eye, or sound vibrations in the ear.
• Other receptors are basic nerve endings like pain receptors in the skin, that spread through most of the body.
• After being stimulated the body reacts.

Types of Receptors

• Changes in the environment can come from so many places.

Thermoreceptors

• Respond to heat and cold.
• Skin receptors inform the brain (hypothalamus/ cerebrum) of changes in the temperature.
• Peripheral thermoreceptors in the skin are nerve endings and respond to either hot or cold.
• Monitor the core temperature by thermoreceptors in the hypothalamus by detecting the temperature of the blood flowing to the brain.

Osmoreceptors

• Osmotic pressure depends on the fluids concentration in the blood plasma.
• Located in hypothalamus and respond to changes in osmotic pressure.
• They stimulate of the hypothalamus to stay within certain limits.

Chemoreceptors

• Stimulated by certain chemicals, like odours or sensitivity to tastes.
• Internal are sensitive to the pH of the blood or the concentrations of oxygen and carbon dioxide.
• Help regulate the heartbeat and breathing.

Touch Receptors (Mechanoreceptors/Pressure Receptors)

• Found in the skin.
• A number of receptors sensitive to light touch are in areas such as the lips, fingertips, eyelids and external genetalia and are associated to hair follicles.
• Short time wearing clothing and then being unaware of it after a while.
• Other touch receptors are in the skin and are sensitive to pressure and vibrations.

Pain Receptors (Nociceptors)

• Triggered by damage to the tissues, poor blood flow in tissue to a poor blood flow to a tissue, or by excessive stimulation from stimuli like heat or chemicals.
• Concentrated in the skin and mucous membranes.
• Present in most organs, but not in the brain.
• Important for well being warning about possible tissue loss.
• Pain receptors adapt unlike other ones, so long as stimulus is present, resulting in making it worse.
• Not adapting let's the person know a possible tissue losing area.

Reflexes

• Quick, automatic response to a change in the external or internal environment.
  • Stimulus is to trigger the response
  • Involuntary, without any conscious thought
  • Rapid- involves few neurons. - Stereotyped- occurs in the same way each time it happens.
• Spinal cord mostly coordinates the action.
• In the case you step on something sharp, the messages will be sent to the brain to be withdrawn immediately.
• The receptor reacts to a change in the internal or external environment by creating nerve impulse in the sensory neuron.
• A sensory neuron carries impulses from the receptor to the spinal cord or brain
• There can be possible one or more interneurons that direct the impulse to the right motor neuron.
• A motor neuron carries the nerve impulse to an effector.
• An effector gets the nerve and goes through with the response.

Comparison of Hormonal and Nervous system Coordination

• Both help communication around the body but do not act the same rather complement and support each other. -Nervous responses are faster than the hormones, traveling quickly and the other traveling through blood. the nervous system responds in milliseconds, but the hormones take few seconds to days. -When stimulus is gone the, hormones can last years and, in the nervous system response is instant which only lasts for short amount of time.
  • Endocrine messages are hormones which are transferred by blood and a electrochemical change in the neutron creates the change in the nervous system. -Nerve impulses travel along a specific area but hormones travel everywhere throughout the body.
• These are basic ideas but do not account for generalisation
• Differences have important overlaps
  • Hormones/ substances are both and are classified as neurotransmitters
  • Same hormones like oxytocin and dopamine can be secreted into extracellular fluids
  • Both hormones can effect the same target cells.