Neuroscience Chapter: Nerve Cell Functions

Homeostasis

• Homeostasis is a self-regulating process in biological systems to maintain balance for survival. This regulation occurs in a defined internal environment.
• The body constantly tries to maintain a stable internal environment, including a temperature of 37°C, 100 mg/mL of glucose, and pH of 7.4.
• A variety of organs and systems work together to achieve homeostasis.

Homeostasis Feedback Mechanisms

• Negative Feedback: A mechanism in homeostasis where the output reverses the variable, bringing it back to a normal state of balance. An example is body temperature regulation.
• Positive Feedback: A mechanism in homeostasis where the output strengthens and increases the change in the variable. This is often a rapid process used in specific circumstances, such as childbirth. Examples include childbirth, breastfeeding, and blood clotting.

Feedback Systems

• When the internal environment changes, sensors (like nerve cells in skin and brain) detect the change.
• Sensory information is relayed to the control center in the brain.
• The control center sends signals to effectors (like sweat glands or blood vessels).
• Effectors make necessary adjustments to maintain homeostasis.

Negative Feedback Example: Internal Temperature

• Stimulus: Body temperature exceeds 37°C
• Sensor: Nerve cells in the skin and brain
• Control center: Temperature regulatory center in the brain
• Effector: Sweat glands throughout the body
• Response: Sweating cools the body, thus decreasing the body temperature

Negative Feedback Example: Blood Glucose Levels

• Stimulus: Blood glucose level decreases.
• Sensor: Beta cells in the pancreas
• Effector: Insulin release into the blood
• Response: Liver breaks down glycogen, releasing glucose into the bloodstream, increasing blood glucose levels

Positive Feedback Example: Childbirth

• Stimulus: Head of the fetus pushes against the cervix.
• Sensor: Nerve impulses from the cervix transmitted to the brain.
• Control center: Brain stimulates the pituitary gland to secrete oxytocin.
• Effector: Oxytocin stimulates uterine contractions.
• Response: Uterine contractions increase, pushing the fetus toward the cervix, leading to childbirth.

Positive Feedback Example: Breastfeeding

• Stimulus: Baby suckles at the nipple.
• Sensor: Suckling impulses to the hypothalamus.
• Control center: Hypothalamus signals posterior pituitary to release oxytocin.
• Effector: Oxytocin released into the bloodstream stimulates the mammary gland.
• Response: Milk is released and the baby continues to feed.

Positive Feedback Example: Blood Clotting

• Stimulus: Damaged blood vessel
• Sensor: Platelets
• Effector: Formation of platelet plug; development of a clot
• Response: Stops bleeding

The Nervous System

• The nervous system includes the central nervous system (CNS) and the peripheral nervous system (PNS).
• The CNS is comprised of the brain and spinal cord.
• The PNS connects the CNS to the rest of the body.
• Different types of neurons perform specialized functions.

Neuron Structure

• Dendrite: Receives signals.
• Soma: Cell body.
• Axon: Carries signals away from the soma.
• Node of Ranvier: Gaps in the myelin sheath.
• Myelin Sheath: Insulation around the axon.
• Axon Terminal: Releases neurotransmitters.
• Schwann Cell: Produce myelin.

Types of Neurons

• Glial Cells: Support and protect neurons.
• Sensory/Afferent Neurons: Carry signals from sensory receptors to the CNS.
• Motor/Efferent Neurons: Carry signals from the CNS to effectors (muscles or glands).
• Interneurons: Relay signals between sensory and motor neurons within the CNS.

Neural Signalling

• Afferent neurons carry signals from sensory receptors to the CNS.
• Interneurons integrate and process information within the CNS.
• Efferent neurons carry information from the CNS to effector organs.

Brain Hemispheres

• Left Hemisphere: Analytical, detail-oriented, ordered, rational thought, verbal, cautious planning, math, science, right field vision.
• Right Hemisphere: Intuitive, holistic, random, sequences, emotional thought, non-verbal, adventurous, impulsive, creative writing/art, left field vision.

Corpus Callosum

• A structure that connects the left and right hemispheres of the brain.

The Brain (Central Nervous System)

• The brain is the control center of the body, responsible for maintaining homeostasis, controlling emotions, thought, and relaying information from sensory neurons to effectors that respond effectively.
• Main sections: Forebrain, Midbrain and Hindbrain

Brain Structures (detailed)

• Medulla Oblongata: Connects spinal cord to cerebellum. Important in autonomic nerve control.
• Cerebellum: Important for muscle movement and balance.
• Pons: Transfers nerve signals between cerebellum and medulla.
• Pituitary Gland: Regulates growth, metabolism, and reproduction through hormones produced.
• Cerebrum: Involved in motor activities and sensory information.
• Thalamus: Interprets sensory input and signals the cerebrum.
• Hypothalamus: Produces hormones that regulate heart rate, body temperature, hunger, and sleep-wake cycle.
• Blood-Brain Barrier: Protects brain from harmful substances by forming a barrier of endothelial cells in brain capillaries.

Brain - Lobes

• Frontal Lobe: Motor functions, higher level reasoning, planning, personality
• Parietal Lobe: General sensory functions
• Temporal Lobe: Auditory, language processing, memory, facial recognition
• Occipital Lobe: Visual processing

Neural Circuits

• Neural circuit: Coordination of receptors, afferent neurons, interneurons, efferent neurons, and effectors in response to a stimulus.
• Reflex arc: A neural circuit that travels through the spinal cord, not the brain, allowing for coordinated quick responses, such as withdrawing a hand from a hot stove.

Synaptic Transmission

• Nerve cells don't transmit signals directly.
• The signal is passed through a gap called the synapse via neurotransmitters from the presynaptic terminal to the postsynaptic receptors.

Chemical vs. Electrical Synapse

• Chemical synapse: Neurotransmitters diffuse across the gap.
• Electrical synapse: Signals transmit directly across gap junctions.

Transmitting Signals in the Nervous System (Details)

• Resting potential: The cell is negatively charged (-70 mV) inside compared to the outside.

• Action potential: The potential changes from negative to positive.

• Depolarization: The inside of the nerve cell becomes positive, caused by the influx of Na+ions.

• Repolarization: The inside of the nerve cell returns to being negative, caused by the effluxes of K+ ions.

• Hyperpolarization: The potential dips briefly below resting potential.

• Refractory period: A period after an action potential where it is difficult to initiate another action potential.

• Ion channels and the Sodium-Potassium pump are important in maintaining resting potential.

Myelinated Nerve Impulses

• Myelinated nerves impulses travel faster.
• Saltatory conduction is the process by which action potentials "jump" between the nodes of Ranvier.