Nervous and Endocrine Systems

• All activities of multicellular organisms require synchronisation, be they fast or slow.
• Mammals rely on the nervous and endocrine systems for coordination.

Differences Between Nervous and Endocrine Systems

Similarities Between Nervous and Endocrine Systems

• Both involve cell signalling.
• Both involve a signal molecule binding to a receptor.
• Both involve chemicals.

Structures of Sensory and Motor Neurones

• The mammalian nervous system includes the brain and spinal cord, which form the central nervous system (CNS).
• The cranial and spinal nerves form the peripheral nervous system (PNS).
• Neurones carry information directly to target cells.
• Neurones are sensory (receptor → CNS), intermediate / relay / connector (CNS → CNS), and motor (CNS → effector).
• The nucleus of a neurone resides in its cell body.

Sensory Neurones

• Transmit impulses from receptors to the CNS.
• Has one long axon with a cell body near the source of stimuli or in a spinal nerve swelling known as a ganglion.

Motor Neurones

• Transmit impulses from the CNS to effectors (muscles and glands).
• Has one long axon which conducts impulses over long distances.
• The ends of branches of the axon have many mitochondria and vesicles with neurotransmitters.
• Synaptic knobs are present at the end furthest from the soma.
• The soma lies within the spinal cord or brain.
• The sarcoplasm of the soma contains many mitochondria, rough endoplasmic reticulum, and Nissl's granules.
• Thin, short, and highly branched cytoplasmic processes extend from the soma, called dendrites.
• Numerous highly branched dendrites provide a large surface area for the endings of other neurones.

Relay Neurones

• Transmit impulses from sensory to motor neurones.
• It is also known as intermediate or connector neurones.
• It is entirely within the CNS.

Sensory Receptor Cells

• Respond to a stimulus by initiating an action potential, located mostly in sense organs.
• Transducers convert energy from one form (e.g., light, taste, smell) into energy in an electrical impulse in a neurone.

Role of Chemoreceptor Cells in Human Taste Buds

• Chemicals act as a stimulus (e.g., salty, sweet).
• Chemoreceptors are specific in detecting taste, acting like transducers.
• Na+ diffuses into the cell upon receptor stimulation via microvilli.
• The membrane depolarises.
• Ca2+ channels are stimulated to open.
• Ca2+ enters the cell.
• This causes vesicle movement containing a neurotransmitter.
• The neurotransmitter is released by exocytosis to stimulate an action potential.

Reflex Arc

• The reflex arc is the route along which impulses travel from a receptor to an effector without involving 'conscious' regions of the brain.
• A reflex action is an immediate response to a stimulus without involving 'conscious' regions of the brain.

Transmission of Action Potentials

• In a neurone, an action potential produces a nerve impulse, and in a muscle cell it produces the contraction required for movement.
• Neurones transmit electrical impulses rapidly along the cell surface membrane.
• These signals are brief changes in electrical charge distribution across the cell surface membrane, called action potentials.
• Neurones have a negative resting potential when not firing, with more positively charged ions outside the cell.
• This is typically around -70mV.
• The cell surface membrane is polarised because there is a potential difference across it.

Maintenance of Resting Potential

• There is more Na+ outside than inside the neurone.
• There is more K+ inside than outside the neurone.
• A Na+/K+ pump uses ATP to pump 3Na+ out and 2K+ in, resulting in a more positive charge outside than inside.
• The membrane has more protein channels for K+ than Na+, so K+ leaks out of the cell.
• Due to the higher K+ concentration inside, it diffuses out, lowering the membrane potential more.
• K+ diffuses out of the cell faster than Na+ diffuses in, due to fewer channels for Na+ compared to K+.
• Many negatively charged molecules are present inside the cell (e.g., Cl and organic anions), and the membrane is impermeable to them, making the neurone more negative inside.

Action Potentials

• Action potentials involve a rapid change in potential difference across the membrane due to changes in the permeability of the cell surface membrane to Na+ and K+ ions.
• Action potentials either trigger or do not, and are transmitted only one at a time.
• Response strength depends on the frequency of action potentials transmitted.

Action Potential Initiation

• The five phases - resting potential, threshold potential, depolarisation, hyperpolarisation, the refractory period.

Depolarisation

• Stimulation of the axon causes Na+ voltage-gated channels to open.
• Na+ diffuses into the axon, down its concentration gradient.
• The inside of the membrane gets more positive.
• If the depolarisation reaches the threshold potential (-50 mV), more Na+ voltage gated channels open, generating an action potential.
• The inside reaches a potential of +30mV.
• This is positive feedback.

Repolarisation

• Once +30mV is reached, Na+ voltage gated channels close and K+ ones open.
• K+ diffuses out because more K+ is inside than out.
• Na+/K+ pump restarts, restoring the potential difference back to -70mV.

Hyperpolarisation

• When hyperpolarized, the neurone is in a refractory period.
• This occurs when the membrane potential becomes even more negative than the resting potential.
• It is caused by slow closing K+ channels.

Refractory Period

• The refractory period is the period of inactivity after an action potential, before which the neurone can fire again.
• During this time, the neurone is unresponsive to stimulation and the ion distribution is being restored.
• Na+ and K+ channels are closed.
• The Na+/K+ pump is open.

Purpose of Refractory Period

• It makes action potentials discrete and unidirectional.
• The length of the refractory period determines the maximum frequency at which impulses are transmitted along neurones.
• The purpose is to give the neurone time to replenish neurotransmitter packets at the axon terminal.

Transmission of Action Potential

• Nerve impulses (action potentials) move along the length of the axon as a unidirectional wave of depolarisation.
• Once an area on the axon has been stimulated, a local circuit is set up between the active area and the resting area next to it.
• The flow of some Na+ sideways towards the negative area next to it causes Na+ channels in that area to open and depolarisation.
• The local reversal of the membrane potential is detected by the surrounding voltage-gated ion channels.

Saltatory Conduction/Myelination

• The axons of myelinated neurones are covered in myelin, an insulating layer.
• Myelin is made by Schwann cells, surrounding the axons of a third of motor and sensory neurones.
• The Schwann cell spirals around and encloses the axon in multiple layers of its cell surface membrane.
• This enclosing sheath is called the myelin sheath.
• The uncovered axon areas between Schwann cells are nodes of Ranvier.

Importance of Myelin Sheath

• Myelin increases the speed of impulses via saltatory conduction.
• Action potentials propagate sequentially along the unmyelinated axon in a continuous wave of depolarisation.
• In myelinated neurones, Na+ and K+ cannot flow through the insulating myelin sheath.
• They can only move in and out of the cytoplasm at the nodes of Ranvier.
• Because of this, the action potential will 'jump' from one node to the next, and will travel faster.
• This process is called saltatory conduction.

Summary of Saltatory Conduction

• Action potential jumps from node to node.
• Local circuits are set up between nodes.
• Conduction velocity / speed of impulses becomes faster.

Cholinergic Synapses

• Cholinergic synapses use acetylcholine (ACh) as a transmitter substance.

How a Cholinergic Synapse Functions

• An action potential reaches the pre-synaptic membrane (synaptic knob).
• It stimulates the opening of Ca²+ voltage-gated channels.
• Ca2+ diffuses into the pre-synaptic neuron.
• This causes vesicles containing ACh to move towards the pre-synaptic membrane and fuse with it.
• ACh is released via exocytosis and diffuses across synaptic cleft.
• It binds to receptors on the post-synaptic membrane
• This causes ligand-gated/chemically gated Na+ channels to open and Na⁺ enter post-synaptic
• membrane.
• Na+ depolarizes the membrane, and an action potential is generated.
• ACh is recycled via the steps and transported back to the pre-synaptic vesicles (by Acetylcholinesterase)

Roles of Synapses in the Nervous System

• Ensure one-way transmission: neurotransmitters are released on one side and receptors on the other.
• Allow connections between one neurone and many others (interconnection of nerve pathways): synaptic transmission.
• Allow integration of impulses: ensures the brain is not overloaded with sensory information.

Types of Muscle Tissue

• Striated muscles are one of three types of muscle tissue mammals have, including cardiac and smooth muscle.

Features of Striated Muscles

• Attached to the skeleton via tendons.
• Neurogenic, stimulated by impulses from motor neurones
• Multinucleate 'cells' called syncytium.

Ultrastructure of Striated Muscle

• Muscles like the biceps are made of interconnected muscle fibres.
• Each muscle fibre is a long “cell” with many nuclei - syncytium.
• Parts of the muscle are known by different terms.
• Sarcolemma is a cell surface membrane
• Sarcoplasm is cytoplasm where lots of mitochondria are present for support during ATP requirements for muscle contraction
• Sarcoplasmic reticulum is the endoplasmic reticulum with protein pumps that transport Ca2+ to the cisternae of the SR.
• The function of t-tubules are to allow impulses from the sarcolemma to pass to the SR and to maintains the Ca2+ store in the SR.
• Muscle fibres contain tubular myofibrils that contain rod-like organelles
• Myofibrils responsible for muscle contraction and made of regular arrangement of sarcoplasm

Thick and Thin Filaments

• Each myofibril is made of smaller components of filaments: thick composed of mysosin or thin with actin
• Darker striations correspond to thick myosin filaments, while lighter parts (I-bands) indicate only thin actin filaments.
• A-bands: centre of the sarcomere appear darker of actin and myosin
• H-band: within A-band means the only myosin is present
• M-line: attachment site for thick filaments.
• Z-line/disc: attachment site for thin filaments
• Part of myofibrils between 2-Z lines is sarcomere
• Sarcomeres are repeating contractile unit separated by protein discs known as Z-lines

Muscular Contraction

• The process of muscular contraction can be summarised: a) depolarisation and Ca2+ release, b) sliding filament model, c) sarcomere shortening (muscle contraction).

Depolarisation and Ca2+ Release

• Impulses cause acetylcholine released and diffuses receptors.
• • Post-synaptic receptors lead to depolarisation from ion channels in sarcolemma of t-tubules towards the centre.
• Arrival of impulses causes Ca2+ ion channels in SR membrane to open: Ca2+ diffuses and this causes a pivotal role in contracting muscles.

Sliding Filament Model

1. Ca2+ are released from stores in SR and bind to troponin, changing its shape
2. troponin and tropomyosin move to different positions on thin filament, exposing myosin binding sites on the actin chain
3. myosin heads then bind to exposed binding sites, forming cross-bridges between thick and thin filaments
4. ATP then binds to the myosin head, breaking the cross-bridge between actin and myosin
5. each myosin head is an ATPase – when ATP is hydrolysed to ADP and Pi, the energy released is used to carry out the power stroke
6. the myosin heads bind to the new actin site and return to their original conformation
7. This reorientation drags the actin along the myosin in sliding mechanism

Sarcomere Shortening

1. in repeated reorientation, myosin heads drag actin filaments along the length of myosin causing actin.
2. Filaments are anchored to the Z-lines which are dragged closer to another when another filament pulls the length of sarcomere.
3. As muscle fibers continue to contract, the individual sarcomeres become shorter.

Energy for Muscular Contraction

• 1. requires aerobic respiration in mitochondria, 2) lactic acid fermentation in sarcoplasm. Creatine Phosphate is an immediate source of energy stored that results in a reversible reaction once ATP runs out. creatine phosphate + ADP = creatine + ATP
• At less demands ATP recharges creatine with creatine + ATP → creatine phosphate + ATP where as when demand is high but no ATP creatinine = creatinine excreted through urination

Hormones and Menstrual Cycle

• Hormones are signaling molecules in endocrine or ductless glands and are secreted in blood streams depending on solubility level. Soluble in water :bind receptors on target cells that activates second messengers such as adrenaline or glucagon Lipid Soluble: able to cross membrane and activate transcription nucleus

The Menstrual Cycle: A Hormone Orchestration

• The hormones in the menstrual cycle, such as steroids, synchronize uterine function with hormonal rhythms.
• The menstrual cycle coordinates glycoprotein hormones secreted from the anterior pituitary gland (FSH, LH) and the ovaries (Estrogen and Progesterone)

Contraceptive Pills

• Act by artificially manipulating progesterone levels/blood concentration which suppresses FSH in the anterior pituitary gland when in negative feedback.
• This prevents the ovulation and the development of Graafian follicles ,LH not secreted
• And forms a thick mucus cervical which inhibits ability of sperm to penetrate
• Prevents implanting if the endometirum is not implanted or developed sufficiently

Plant Coordination: Venus Fly Trap

• Carnivorous plant obtains nutrition by trapping small animals such as insects

Anatomy/Mechanism of Venus Fly Trap

• Trigger on outer edges.
• Nectar for luring prey
• Stiff hairlike triggers
• Digestion cells midrib/hinge and digestive glands

Rapid Response and role of leaf

• Sensory hairs once deflected open Ca that generates the receptor and Action Post
• A higher stimulation causes higher voltage and the deflections stimulated with calcium entering glands.
• Calcium then stimulates with exo of digestive enzymes.
• With digestion there is an increase growth

Role of cell wall acid

• Binds to receptors, then membranes that causes cells to enter wall for water with pH
• Expansions are a lower amount of pH

Regulation of Plant Growth by Auxins

• They are synthesized in meristem and transport with hormones from sap and in phloem with auxins. Auxin binds with the receptors to water and membrane of turgor and this makes cells cause wall stretch

Gibberellin

• A plant growth regulator in plant stems, leaves,seeds
• In the state of dormancy during adverse conditions and metabolically inactive that allows survival

Gibberalin to activate germination in barley

• Absorption and germination occurs as embryo and water enters
• Gibbehellin then stimulates aleurone to make amylase that break down starch to glucose to use for germination

Genes control plant height

• Dominant gene that has the functioning to stimulate to make an enzyme and turn gibberellin on.
• Where is recessive codes for what is non functional which can cause slow mutations making always short from absent gibberellin

Auxin and protein growth in Gene

• Protein is a division from the growth of plants that causes the destruction of gene and results in cell function which then causes the international link length in the stem