Brain Structure and Function: CNS

Central Nervous System (CNS)

• The CNS comprises the brain, spinal cord, and peripheral nervous system.
• The brain continuously communicates with the CNS, which is connected to the peripheral nervous system, to determine responses to stimuli.

Brain Development and Structure

• In early embryological development (gestational age 4), the brain develops three bulges: the hindbrain, midbrain, and forebrain.
• The forebrain divides into the cerebrum (or telencephalon) and the diencephalon.
• The midbrain and hindbrain form the brainstem.
• The hindbrain consists of the cerebellum, pons, and medulla.
• The medulla connects the brain to the spinal cord.

Cerebral Hemispheres

• The brain is divided into two halves along the midline, separated by the longitudinal fissure.
• The two hemispheres communicate via the corpus callosum.
• The outer layer is the cerebral cortex, the site of most neuronal transmissions, and is made up of folds and ridges called sulci.
• Gyri are the parts between the sulci or ridges.
• More ridges and furrows result in a larger surface area and more neurons, enhancing the brain's ability to process information.

Lobes of the Brain

• The cerebral hemisphere divides into 4 lobes.
• Occipital lobe: vision.
• Parietal lobe: goal-oriented behavior.
• Frontal lobe: emotional regulation, personality, executive function, and problem-solving.
• Temporal lobe: walking/balance and helps the frontal lobe with emotional regulation and musical ability.

Cortex Organization and Function

• Cortexes are labeled based on the lobe they reside within.
• The lateral sulcus/Sylvian fissure separates the frontal and temporal lobes.
• The central sulcus separates the frontal and parietal lobes.
• The brain processes external information to decide on behaviors.
• Sensory organs relay information to the brain.
• Stimuli are processed in different brain regions, like the olfactory cortex for smell.
• The cerebral cortex is divisible into regions based on information processing.
• Function is based on external events.

Somatosensory Cortex

• The somatosensory cortex responds to touch and maps anatomical regions of the body.
• The amount of cortex dedicated to a body part is proportional to the amount of sensory information it receives.
• Sensory homunculus represents the body map in the somatosensory cortex.

Primary Motor Cortex

• The primary motor cortex makes decisions on behavior.
• A large portion of the primary motor cortex is dedicated to body parts requiring fine movement control.
• It acts based on events in the external environment and sends information to enact a response.

Ventricles

• Ventricles are fluid-filled sacs that provide nutrients to the brain and aid in navigation.

Neurons: Structure and Function

• The brain responds to stimuli through neuron firing.
• Neurons receive input from the environment via sense organs.
• Neurons interpret signals and respond by firing (or not firing) an action potential and releasing neurotransmitters (NT).

Neuron Structure

• Dendrites: receive input from other neurons, increasing surface area.
• Soma: contains the nucleus (DNA), mitochondria (energy), and endoplasmic reticulum (protein synthesis).
• Axon: covered in myelin sheath, carries the action potential.
  • Afferent neurons carry information into a region.
  • Efferent neurons carry information away from a region.
• Axon terminals: communicate with other neurons through signals and release neurotransmitters.
• Axon terminals consist of vesicles, which are balls of NT.
• When a neuron fires, it releases NT, which signals the next neuron.
• The dendrites of the next neuron pick up this signal.

Glial Cells

• Glial cells make up approximately 85% of the cells in the brain.
• They provide structure to the brain, support nutrients for neurons to function. -Astrocytes: provide physical support, nutrients, and involved in phagocytosis.
  • Microglia: phagocytosis and prevention against infection.
  • Oligodendrocytes: support axons and produce myelin sheath in the CNS. -Schwann cells: support axons and produce myelin sheath in the PNS.

Neuronal Transmissions

• Electrical transmission happens within the neuron.
• Chemical transmission (communication) occurs between neurons.

Cell Membrane

• The neuron's wall comprises a phospholipid bilayer and protein channels.
• The membrane is selectively permeable, allowing only certain substances to pass in and out.
• Sodium, potassium, chloride, and calcium cross through channels that can sometimes be closed.

Neuron at Rest

• Polarized: the resting potential is -70mV.
• Electrical gradient: more negative inside the neuron than outside.

Resting Potential Importance

• Maintaining resting potential requires energy for the sodium-potassium pump.
• It prepares the neuron to respond quickly to a stimulus.
• It maintains the concentration gradient for sodium.

Maintaining Resting Potential

• The sodium-potassium pump removes 3 sodium ions from the cell and brings in 2 potassium ions.
• Results in more positive ions outside and fewer positive ions inside, making the inside more negative.
• This process requires energy and happens continuously.
• Sodium wants to re-enter because of the electrical and concentration gradients.
• Ion channels prevent sodium from going back in.
• A small amount of potassium leaves.
• Sodium ions are larger than potassium ions, resulting in some potassium leakage.

Action Potential

• Sodium and potassium channels are voltage-gated. -Sodium channels: closed at rest -Potassium channels: almost closed at rest

• Sodium and potassium start to enter, making the neuron more positive (from -70 to -55 mV).

• The neuron becomes depolarized: sodium and potassium channels start to open further.

• Sodium enters the cell but has a minimal effect on potassium.

• The inside becomes positive from -70 to -55 mV.

• If the threshold of excitation is reached, sodium channels fully open.

• Sodium rushes into the cell.

• Reaching the -55 mV threshold causes sodium channels to fully open and allow sodium to enter.

Repolarization and Hyperpolarization

• Now that the membrane is now more positive, potassium channels open.
• Potassium leaves the cell.
• Sodium channels close.
• This results in +30 mV = depolarization.
• Resting potential = -70 mV.
• Threshold of excitation = -55 mV.
• Endpoint of depolarization = +30 mV.
• Hyperpolarization prevents part of the cell from depolarizing, ensuring the action potential travels down the axon.

Ion Balance Restoration

• Potassium ions re-enter = repolarization.
• The potential goes below -70mV, hyperpolarization, to ensure the following neurons fire only when required.
• Too much sodium in the top half causes it to travel down the neuron because it is more negative.
• As sodium moves down the neuron and the -55 threshold of excitation is hit, more sodium rushes in, moving down the neuron.

Action Potential Rules

• Myelin sheath is impermeable to sodium.
• Little gaps in the myelin sheaths speed up action potential propagation jumping from node to node (saltatory conduction).
• Action potentials travel in one direction (refractory period).
• All or nothing law: neurons fire only if the excitation threshold is reached, releasing neurotransmitters (NT), or nothing happens.
• The way intensity is perceived translates into how we feel; brain processes stimuli by simultaneously firing more neurons (Rate Law).

Chemical Communication Between Neurons: The Synapse

Process:

1. The action potential reaches the terminal button in the presynaptic neuron. -The terminal button contains vesicles with different kinds of NT depending on what triggered it.
2. Depolarisation of the terminal button
3. Synaptic vesicles migrate, fuse with the presynaptic membrane
4. Vesicles release chemical neurotransmitters into the synaptic cleft between the two neurons.
5. NTs attach to the postsynaptic membrane and fuse with the dendrites of the next neuron.
6. Enzymes break down the NTs. -If there are too many leftover NTs, they go back into the previous neuron.
7. NTs are taken back up by the presynaptic terminal.
8. Auto-receptors on the presynaptic membrane decrease neurotransmitter synthesis or release.

Post-Synaptic Potentials

• The neuron emits either an excitatory (depolarizing) or an inhibitory AP (hyperpolarizing) signal.
• The postsynaptic potential is determined by postsynaptic receptors. -Sodium channels open: this causes an excitatory post-synaptic potential. -Partial depolarization (decrease in negative state i.e. more positive). -Increases the likelihood that the postsynaptic neuron will fire. -Potassium channels open: this causes an inhibitory post-synaptic potential. -Hyperpolarization (increase in negative state i.e. becomes more negative). -Decreases the likelihood that the postsynaptic neuron will fire.

Integration of Post-Synaptic Potentials

• Each postsynaptic neuron has synapses with many presynaptic neurons.
• Neural integration: interaction of excitatory and inhibitory synapses on a particular neuron.
• The neuron firing rate depends on the relative activity of excitatory and inhibitory synapses on postsynaptic dendrites.