Central Nervous System Overview

The Nervous System

• The nervous system allows animals to adjust to internal and external environmental changes. It acts as a control system.
• The two main cell types in the nervous system are neurons and glial cells.
• Neurons are the functional units of the nervous system
• Glial cells support and protect neurons in the central and peripheral nervous systems.
• Human brains contain approximately 100 million neurons and ten times more glial cells.

Neuron Structure

• A neuron has a cell body, dendrites, and an axon.
• Dendrites receive impulses toward the cell body.
• Axons transmit impulses away from the cell body.
• A nerve cell process is a dendrite if it conducts impulses toward the cell body, and an axon if it conducts impulses away from the cell body.

Nerve Structure

• A nerve is a bundle of axons surrounded by connective tissue and blood vessels.
• Axons, connective tissue, and blood vessels are contained within a nerve.

Neuron Types

• Sensory neurons transmit impulses from sensory receptors to the central nervous system.
• Interneurons connect sensory and motor neurons in the central nervous system.
• Motor neurons transmit impulses from the central nervous system to effector organs (e.g., muscles).

Neurons and Synapses

• A neuron has only one axon but typically many dendrites.
• Dendrites receive information from other neurons to provide a large surface area for communication.

Neuronal Polarity

• The polarity of a neuron refers to the number of poles or processes that extend from the cell body.
• Bipolar neurons have one axon and one dendrite extending from the cell body (found in the retina of the eyes and olfactory region of the nose).
• Pseudounipolar neurons have a single process near the cell body that branches into an axon and dendrite (commonly primary afferent neurons).
• Multipolar neurons have many dendrites and a single axon extending from the cell body.

The Axon and Myelin Sheaths

• The axon, often with a myelin sheath, is sometimes called a nerve fiber.
• The axolemma covers the axon.
• Myelin, formed by oligodendrocytes in the central nervous system (CNS) and Schwann cells in the peripheral nervous system (PNS), surrounds the axon and acts as insulation.
• Interruptions in the myelin sheath are called nodes of Ranvier.
• A group of nerve cell bodies in the CNS is called a nucleus, while outside of the CNS it's called a ganglion.
• A bundle of parallel neuron fibers within the brain or spinal cord is a tract.

The Synapse

• The synapse provides continuity from one neuron to the next.
• There is no physical contact; a synaptic gap separates neurons.
• Impulses are transmitted chemically across the synaptic gap via neurotransmitters.
• Three characteristics of the synapse are one-way conduction, facilitation (with repeated impulses), and greater fatigability than a neuron.

Neurotransmitters

• Neurons do not touch each other; instead chemicals called neurotransmitters are released.
• Neurotransmitters are released from vesicles in the presynaptic neuron.
• Neurotransmitters bind to receptors on the postsynaptic neuron, altering its membrane permeability.

Glial Cells

• Glial cells are non-neuronal cells in the central nervous system.
• The vast number of glial cells makes up about half the volume of nervous tissue.
• Glial cells are metabolically active.
• Macroglia, microglia, and ependymal cells are the classification for glial cells.
• Types of macroglial cells are: Oligodendrocytes-myelin sheath forming in the CNS. Astrocytes-most prominent glial cells; their processes surround blood vessels, synaptic structures, and nerve cell bodies and processes; also have phagocytic function. Other functions include: maintaining homeostasis, forming myelin, and providing support and protection for neurons.
• Microglia-phagocytic function.

Central Nervous System (CNS)

• Composed of the brain and spinal cord.
• Structures of the brain: Cerebrum (paired cerebral hemispheres) Cerebellum (unpaired) Brain stem (midbrain, pons, medulla oblongata)

Peripheral Nervous System (PNS)

• Consist of spinal nerves and cranial nerves.
• Spinal nerves arise from spinal cord (7cervical, 13thoracic, 7lumbar, 3 sacral and 20 caudal) and carry afferent/efferent fibers (mixed nerves).
• Cranial nerves (12 pairs) innervate structures in the head and neck. The vagus nerve is an exception, as it innervates thoracic and abdominal viscera in addition to the head and neck.

Autonomic Nervous System (ANS)

• The involuntary nervous system; associated with homeostatic functions.
• Has three parts: Sympathetic (SNS) - associated with body response to stress; Parasympathetic (PSNS) – associated with homeostatic functions in the absence of stress; Enteric (ENS) – associated with regulation of digestive system;
• ANS innervates smooth muscle, cardiac muscle, and glands.

Motor Neurons of Autonomic and Somatic Nervous Systems

• Autonomic: preganglionic and postganglionic neurons Neurotransmitter: acetylcholine (ACh) or norepinephrine
• Somatic: have one neuron from neuron to effector

Sympathetic and Parasympathetic Nervous Systems (ANS)

• The sympathetic system prepares the body for "fight or flight" responses.
• The parasympathetic system is active when the animal is at rest or not under stress (e.g., "rest and digest").
• The ganglia of the sympathetic system are located in the thoracic and lumbar regions of the spinal cord.
• The ganglia of the parasympathetic system are located in or near the effector organs.

Neurotransmitters in the Nervous System

• Somatic: Acetylcholine (ACh)
• Sympathetic: Acetylcholine (ACh) as a preganglionic transmitter, and norepinephrine (NE) as a postganglionic transmitter.
• Parasympathetic: Acetylcholine (ACh) as both preganglionic and postganglionic transmitters.
• Central Nervous System: Acetylcholine (ACh), norepinephrine (NE), dopamine, gamma-aminobutyric acid (GABA), and glycine

Reflexes

• Unconditional reflexes are inherent and invariable.
• Conditional reflexes are learned and can be modified by experience.

Spinal Reflexes

• Types of spinal reflexes: flexor, extensor pushing, crossed extensor, itching, stepping, and withers in horses, plantar.

Neural Conduction

• Irritability: Ability to respond to stimuli.
• Conductivity: Ability to transmit impulses (nerve impulses from receptors to effectors).
• Correlation: Integration of impulses (decision-making process by synapses.
• Reaction: A response to the integrated impulses.

Cerebrospinal Fluid

• Thin, watery fluid derived from blood plasma.
• Functions: acts as a shock absorbing fluid for the brain and spinal cord

Central Nervous System (CNS) Metabolism

• The CNS's primary energy source is glucose.
• Glucose is transported into the CNS by simple diffusion.
• CNS has relatively high metabolism
• Blood delivers the nutrients it needs for high rate of metabolism.
• The tolerance of an adult brain to hypoxia is lower than the tolerance of a newborn brain.

Blood-Brain Barrier

• The blood-brain barrier protects the brain from many substances in the blood.
• Tight junctions between the endothelial cells of blood vessels in the CNS limit diffusion of many substances from blood capillaries into the brain.
• Astrocytes play a role in the movement of substances across this barrier.

The Nerve Impulse and its Transmission

• Stimulation causes electrical signals in nerve cells.
• These signals are based around changes in the permeability of the cell membrane to certain ions.
• The nerve signal flows along the axon away from the cell body.

Action potential

• Changes in the membrane potential along the axon are called action potentials.
• Changes in membrane potential are based on changes in the permeability of the neuron's cell membrane to ions (i.e., sodium and potassium).
• Depolarization of the membrane occurs when the membrane becomes more permeable to Na+ which flows into the neuron; this event results in a positive charge inside the neuron and a negative charge outside of the neuron.
• Repolarization is the process in which the membrane returns to its original charge across the neuron's membrane by allowing K+ to flow out of the neuron.

Excitatory and Inhibitory Postsynaptic Potentials (EPSPs and IPSPs)

• Graded potentials that cause depolarization (EPSPs) or hyperpolarization (IPSPs) cause changes in the neuron's membrane potential. This process is what makes the neuron's membrane more or less excitable.
• Neurotransmitters like acetylcholine (ACh) and glutamate cause an EPSP by opening ligand-gated Na+ channels and thus bringing about an influx of Na+ into the cell thus creating a positive charge inside the cell.
• Neurotransmitters like glycine and GABA cause an IPSP by increasing the permeability of the membrane to Cl- which thus brings about an influx of Cl- into the cell thus creating a negative charge inside the cell.

Saltatory Conduction

• Saltatory conduction is the mode of action potential transmission that involves leaping from node of Ranvier to the next node on the unmyelinated axon. The myelination of the axon prevents the flow of current to the areas of the axon membrane between the nodes.
• The myelination of the axon permits the leaping from one node to the next.
• This method is faster than continuous conduction.

Postsynaptic Potentials

• Generated at the cell body or dendrites.
• Two main types: Excitatory postsynaptic potential (EPSP)--depolarizing (more positive) Inhibitory postsynaptic potential (IPSP)--hyperpolarizing (more negative)
• The sum of EPSPs and IPSPs in the neuron's cell body determine if the action potential has been reached. If the potential is sufficiently depolarized, then the neuron's threshold is reached triggering the generation of an action potential.

Test your knowledge on the components and functions of the Central Nervous System (CNS). This quiz covers various aspects, including brain regions responsible for memory, problem-solving, and involuntary functions. Explore how the nervous system is structured and its critical roles in bodily functions.