The Nervous System

• The nervous system regulates and coordinates the body's functions through communication.
• It transmits signals between different areas of the body.
• The system is composed of the brain and spinal cord.
• Both nervous and endocrine systems coordinate the body's activities.

Overlapping Functions

• Sensory input involves gathering information through receptors.
• Integration processes and interprets sensory information in the brain.
• Motor output involves sending signals to appropriate effectors.

Neuron Types

• There are many types of neurons but their structural and functional characteristics are similar
• Neurons are electrically excitable cells that transmit information through electrical and chemical signals.
• The nervous system encompasses the brain, spinal cord, and peripheral ganglia.
• The human brain has billions of neurons connected by synapses.

Neuron Structure

• Neurons have a cell body (soma), dendrites, and an axon.
• The cell body is the neuron’s central and most vital region.
• The nucleus of the neuron also resides inside the cell body.
• Metabolic activity of the neuron happens in the soma.

Nissl's Granules

• The protoplasm of the cell body contains distinctive Nissl bodies.
• These granules temporarily disappear during fatigue or nerve stimulation.
• These granules store nervous energy and are deficient/absent in mental diseases.
• They are involved in neurotransmitter synthesis, like acetylcholine.

Dendrites

• Dendrites extend from the cell body toward impulses.
• They are also called afferent fibers.
• Dendrites facilitate communication with other neurons as they enhance the neuronal surface area .

Axons

• Axons protrude from the cell body, in contrast to dendrites.
• Axons carry electrical impulses away from the soma.
• Axons are distinguishable dendrites by their shape, length, and function .
• Axons originate at the cell body at the axon hillock.
• Axoplasm represents the cytoplasm inside the axon.

Axolemma

• The axolemma serves as the cell membrane around the axon.
• The axolemma maintains neuron's membrane potential and contains ion channels.
• Vertebrate axons undergo myelination .
• Schwann cells myelinate peripheral neurons.
• Oligodendrocytes insulate the central nervous system's neurons.

Myelin Sheaths

• Myelin sheaths protect and electrically insulate the axon.
• Myelin sheaths increase the rate of action potential transmission via saltation.
• Nodes of Ranvier are the regularly spaced gaps of myelination along nerve fibers.

Axon Terminals

• Axons branch into collaterals, forming telodendria.
• Synaptic knobs are located at the ends of telodendria.
• Synaptic vesicles containing neurotransmitters (NTs) are inside Synaptic knobs.
• Synapses form junctions between neurons and other cells.
• Synaptic clefts form the region between connecting neurons.

Classification of Neurons Based on Polarity

• Unipolar neurons have one protoplasmic process from the cell body and are found in invertebrates and dorsal root ganglia.
• Pseudo-unipolar neurons' axon splits into two branches, in which one runs to the periphery and the other to the spinal cord.
• Bipolar neurons contain an axon and a dendrite on opposite ends of the soma and are specialized sensory neurons, abundant in sensory pathways.
• Multipolar neurons have one axon and multiple dendrites, and make up the majority of brain neurons in the brain, including motor neurons and interneurons.

Classification Based on Function

• Afferent neurons, also called sensory neurons, carry information from tissues and organs to the central nervous system.
• Efferent neurons, also called motor neurons, carry nerve impulses from the central nervous system to muscles or glands.
• Motor neurons are classified into somatic motor neurons, special visceral motor neurons, and general visceral motor neurons.
• Interneurons connect afferent and efferent neurons in neural pathways.

Classification Based On Neurotransmitters

• Cholinergic neurons secrete acetylcholine.
• GABAergic neurons secrete gamma aminobutyric acid.
• Glutamatergic neurons secrete glutamate.
• Dopaminergic neurons secrete dopamine; loss of dopamine neurons are linked to Parkinson's disease.
• Serotonergic neurons secrete serotonin; lack of serotonin at postsynaptic neurons has been linked to depression.

Nerve Fiber vs. Neuron

• A single emanating is the nerve fiber, it emenates from its cell body and transmits electrical impulses to other neurons, muscles, or glands and is also known as an axon.
• A neuron is the functional unit of the nervous system, it has a cell body, dendrites and an axon, and can be motor, sensory, or interneuron.

Nerve Impulse

• The nervous system's communication and signal relay consists of neurons.
• Stimulated neurons transmit information in two ways.
• Travel through their own body, or pass across a space to transmit electric and chemical signals.
• Travel from one end to the other through action potential generation.
• Transmission occurs chemically through via neurotransmitters across a synaptic cleft.

Defining Nerve Impulses

• A nerve impulse is a wave of electrical and chemical changes across a neuron that generates an action potential.
• Transmission of a nerve impulse across the neuron membrane due to membrane potential change is the nerve impulse conduction.
• Nerve impules initiate a change in the resting state of the neuron.
• The resting potential changes to an action potential to conduct signals in response to a stimulus.

Action Potentials

• Movement of ions in and out of the cell results in the action potential.
• Sodium and potassium ions get propagated using specific sodium and potassium pumps in the neuron membrane.
• Synaptic connections achieve transmission of nerve impulses from one neuron to another .
• Nerve impulses transmit as a mode of cellular communication.

Nerve Impulse Conduction

• Nerve impulse conduction is critical and organized within the body's functions.
• Nerve impulses get conducted by two mechanisms: continuous and saltatory.

Continuous Conduction

• Continuous nerve impulse conduction occurs in non-myelinated axons.
• Action potentials travel the axon's full length.
• The transmissions take time for generating and conveying impulses during an action potential.
• The continuous conduction requires energy and is a slow process at approximately 0.1 m/s.

Wire Configuration

• Axons can act as electrical wires that transmit nerve impulse, carrying charged particles like sodium and potassium ions.
• Nerve impulse conduction happens when there is a stimulus of about -55 millivolts.

Saltatory Conduction

• It is faster than continuous conduction and happens in myelinated neurons
• Myelinated neurons possess myelinated sheaths separated by unmyelinated gaps known as Nodes of Ranvier.
• Nerve impulses propagate by jumping from one node of Ranvier to the next.
• This process is faster, as the nerve impulse does not travel the full length of the axon occurs.
• Nerve impulses travel at 100 m/s in saltatory conduction.

Nerve Impulse

• The steps involved in in this process are polarization, depolarization, repolarization, the refractory period, and synapse.

Resting Membrane Potential

• The resting membrane potential refers to the nerve cell's non-excited state at rest when no nerve impulse gets being conducted.
• The resting membrane potential of a nerve cell rests -70 mV.
• Sodium and potassium gate keep the state statuc through closed channels.

Action Potential state

• Neurons channels get open. This leads to a huge influx (+40mV) of sodium ions.
• Neuron’s action relies as +40 mV which occurs during an excited state while conducting nerve impulses.

Polarized State

• During polarization, the membrane is electronically charged but non-conductive.
• Neuron doesn't conduct nerve impulses in polarized state.
• Sodium is 16x more outside than inside
• Potassium is 25x more inside than outside

Depolarization State

• About -55mV causes a change in the membrane potential, referring to the cell's grade.
• Changes must be strong enough to alter it into an action membrane potential.
• The stimulus here will prompt a 10x sodium influx through its voltage grated paths.

Repolarization

• Axons have an electrical balance, where sodium is inside and outside the membrane.
• Potassium will begin opening and an efflux occurs through that path leading to voltage changes.
• No more sodium moves as those channels will close.

Saltatory Conduction

• Following success of a nerve impusle, the original resting state now persists.
• ATPase pump allows the re-establishment of the distribution of sodium and potassium ions.
• Driven by ATP, it hekps restore a second charge on the neuron.

Synapses

• After a reaching the synaptic terminal of a neuron, it can travel.
• Synapses are responsible for the end interaction between presynaptic and postsynaptic neurons.
• Neurons are not connected by space, but instead clefts.
• Response gets prompted near the target site, and must pass this final connection.

Types of Synapses

• Electrical Synapses
• Chemical Synapses

Electrical Synapses

• Electrical Synapses pass across protein channels with their membranes.
• This impulses travel in form of an electrical signal
• Faster chemical signaling due to the gaps resting 0.2 nm apart.

Chemical Synapses

• This transmission occurs from chemical form .
• This process occurs between 10 and 20 nm which slows down this process