Organ Systems That Coordinate

Overview of Coordination Systems

The human body relies on two major organ systems to coordinate activities and maintain homeostasis: the nervous system and the endocrine system. Each system has distinct mechanisms and response characteristics.

• Nervous system: Fast and brief responses; utilizes electrical and chemical signals.

• Endocrine system: Slower but longer-lasting responses; uses hormones.

• The nervous system is the most complex organ system in the body.

Nervous System Functions

Primary Roles

The nervous system is responsible for monitoring, integrating, and coordinating responses to internal and external environments.

• Monitoring: Detects changes in the body and environment.

• Integration: Processes and interprets sensory information.

• Coordination: Directs voluntary and involuntary responses.

Central Nervous System (CNS)

Structure and Function

The CNS consists of the brain and spinal cord, serving as the main processing center for sensory input and motor output.

• Integrates and coordinates sensory processing and motor transmission.

• Location of higher functions: intelligence, memory, emotion.

Peripheral Nervous System (PNS)

Components and Communication

The PNS includes all nervous tissue outside the CNS and acts as a communication link between the CNS and the rest of the body.

• Connects CNS to limbs and organs.

• Facilitates sensory and motor information transfer.

PNS Functional Divisions

Information Flow and Effectors

The PNS is divided functionally into afferent and efferent divisions, each with specific roles in information transmission.

• Afferent division: Brings sensory information to the CNS from receptors in body tissues and organs.

• Receptors: Sensory structures that detect changes and respond to stimuli.

• Efferent division: Carries motor commands away from the CNS to effectors (muscles and glands).

Efferent Division Subdivisions

• Somatic nervous system (SNS): Controls voluntary skeletal muscle contractions.

• Autonomic nervous system (ANS): Regulates involuntary functions of smooth muscle, cardiac muscle, and glands.

• ANS includes three parts:

  1. Sympathetic division

  2. Parasympathetic division

  3. Enteric nervous system (ENS)

Nervous Tissue

Neurons and Neuroglia

Nervous tissue is composed of neurons and neuroglia, each with specialized functions.

• Neurons: Communicate with each other and other cells; transmit electrical impulses.

• Neuroglia: Support and regulate the environment around neurons; more numerous than neurons and capable of division.

Neuron Cell Body

• Contains a large, round nucleus with a nucleolus.

• Cytoplasm includes mitochondria, ribosomes, and rough ER (Nissl bodies).

• Nissl bodies give gray color to areas with many cell bodies (gray matter).

Axon Hillock

• Thickened region marking the beginning of an axon.

• Originating point for electrical impulses (action potentials).

Neuron Regeneration

• Most neurons lack centrioles and cannot divide.

• Typical CNS neurons cannot be replaced if destroyed by injury or disease.

Structural Classification of Neurons

Types Based on Structure

Neurons are classified by the relationship of dendrites to the cell body and axon.

• Multipolar neurons: Two or more dendrites and one axon; most common in CNS; include all motor neurons controlling skeletal muscles.

• Unipolar neurons: Dendrites and axons are continuous; cell body off to one side; most sensory neurons of the PNS.

• Bipolar neurons: One dendrite and one axon; cell body between dendrite and axon; rare, found in special sense organs.

Functional Classifications of Neurons

Types Based on Function

• Sensory neurons (afferent): Receive information from sensory receptors; about 10 million in number.

• Motor neurons (efferent): Carry instructions away from the CNS to effectors.

• Interneurons (association): Located entirely within the CNS; interconnect with other neurons; about 20 billion, play a role in higher functions.

Somatic Sensory Receptors

• External receptors: Monitor external environment; sensations of touch, pressure, taste, smell, sight, hearing.

• Proprioceptors: Monitor position and movement of skeletal muscles and joints.

• Visceral receptors: Monitor activities of internal organs; sensations of distention, deep pressure, pain.

Neuroglia

Types and Functions

Neuroglia make up about half of all neural tissue and are essential for neuron support and maintenance.

• Four types in CNS: Astrocytes, oligodendrocytes, microglia, ependymal cells.

• Two types in PNS: Satellite cells, Schwann cells.

Astrocytes

• Star-shaped, largest and most numerous in CNS.

• Maintain the blood-brain barrier; isolate CNS from general circulation.

• Provide structural support and regulate ion/nutrient concentrations.

Oligodendrocytes

• Form myelin sheaths around axons in CNS.

• Myelin is lipid-rich and appears white.

• Areas covered in myelin are called internodes; gaps are nodes of Ranvier.

White Matter and Gray Matter

• White matter: Dominated by myelinated axons.

• Gray matter: Contains neuron cell bodies, dendrites, and unmyelinated axons.

Microglia

• Smallest and least numerous in CNS.

• Phagocytic cells derived from white blood cells.

• Eliminate pathogens and consume cellular waste.

Ependymal Cells

• Line cavities in CNS with cerebrospinal fluid (CSF).

• Form the ependyma lining.

• Involved in circulating CSF around the CNS.

Satellite Cells

• Surround and support neuron cell bodies in PNS ganglia.

• Similar in function to astrocytes in CNS.

Schwann Cells

• Cover every axon in the PNS.

• Outer surface is the neurolemma.

• One Schwann cell per myelinated axon segment; can encircle several unmyelinated axons.

Neuron Organization in the PNS

Ganglia and Nerves

• Collections of neuron cell bodies (gray matter) are located in ganglia, surrounded by satellite cells.

• Bundles of axons are called nerves, containing both sensory and motor components.

• Spinal nerves connect to the spinal cord; cranial nerves connect to the brain.

Neuron Organization in the CNS

Tracts, Columns, and Cortex

• Bundles of axons are called tracts in the spinal cord, forming large groups called columns.

• Neural cortex: Thick layer of gray matter covering the brain surface.

• Pathways: Link centers of brain with rest of body.

• Sensory (ascending) pathways: Carry information to CNS.

• Motor (descending) pathways: Carry information away from CNS.

Membrane Potential

Definition and Resting State

Membrane potential is the potential difference between the inside and outside of a cell, resulting from unequal distribution of ions.

• Living cells have polarized plasma membranes.

• Resting membrane potential: Potential of undisturbed cell, typically -70mV.

Factors Influencing Membrane Potential

• Imbalance in electrical charges.

• Different composition of fluids: ECF high in Na+ and Cl-; ICF high in K+ and proteins (pr-).

• Proteins are non-permeating, staying in ICF.

• Ions enter/leave via channels or carrier proteins.

• Leak channels: Always open.

• Gated channels: Open/close under specific circumstances.

Passive Ion Movement Across Membrane

• Chemical and electrical gradients influence movement.

• Sodium moves into the cell: Higher concentration outside, attracted to negative charge inside.

• Potassium moves out of the cell: Higher concentration inside, diffuses out faster than sodium enters.

Maintaining Resting Membrane Potential

• Active process required: Sodium-potassium pump exchanges 3 Na+ out for every 2 K+ in.

• Results in net loss of positive ions; resting potential of -70mV.

Disturbances to Resting Membrane Potential

• Stimuli alter permeability to Na+ or K+ or pump activity.

• Opening gated Na+ channels: Depolarization (membrane potential becomes more positive).

• Opening gated K+ channels: Hyperpolarization (membrane potential becomes more negative).

Graded Potential

Local Changes in Membrane Potential

• Also called local potentials.

• Changes cannot spread far from site of stimulation.

• Degree of depolarization decreases with distance from stimulus.

• Occur in plasma membrane of all cells; can trigger activation in large neurons and skeletal muscle.

Action Potential

Propagation and Characteristics

• Propagated change in membrane potential of excitable cells.

• Requires voltage-gated channels.

• Travels the entire length of the cell; in neurons, called a nerve pulse.

• Threshold: Level of depolarization required to initiate action potential.

• All-or-none principle: Any stimulus that reaches threshold produces identical action potential.

• Refractory period: Time from sodium channel opening until repolarization is complete; membrane cannot respond to further stimulation.

Continuous Propagation of Action Potential

• Occurs along unmyelinated axons.

• Slower conduction speed compared to myelinated axons.

Saltatory Propagation

• Occurs along myelinated axons.

• Action potential jumps from node to node (nodes of Ranvier).

• Much faster conduction (up to 140 m/s).

Synapse and Neurotransmitters

Synaptic Transmission

• Information transmitted via release of chemicals called neurotransmitters.

• Communication occurs only in one direction: presynaptic to postsynaptic neuron.

• Types of synapses: neuromuscular junction (neuron to muscle), neuroglandular junction (neuron to gland).

Key Neurotransmitters

• Acetylcholine (ACh): Binds to receptors, causes depolarization; removed by acetylcholinesterase (AChE).

• Norepinephrine (NE): Important in brain and ANS.

• Dopamine, GABA, Serotonin: Modulate neural activity; can be excitatory or inhibitory.

• Excitatory neurotransmitters: (ACh, NE) increase likelihood of action potential.

• Inhibitory neurotransmitters: (Dopamine, GABA, Serotonin) decrease likelihood of action potential.

Postsynaptic Neuron Response

• Multiple presynaptic neurons can synapse with one postsynaptic neuron.

• Excitatory and inhibitory effects can summate or cancel each other.

Neuronal Pools

Functional Groups

• Groups of interconnected neurons with specific functions.

Meninges

Protective Membranes

• Three layers: Dura mater (outer), Arachnoid (middle), Pia mater (inner).

• Surround brain and spinal cord; provide stability and shock absorption.

• Dura mater: Tough, outermost layer; contains dural folds and venous sinuses.

• Arachnoid: Squamous cells; subarachnoid space filled with CSF.

• Pia mater: Thin, vascular layer; closely adheres to CNS surface.

Spinal Cord

Structure and Segmentation

• Major neural pathway between brain and PNS.

• Consists of segments, each giving rise to a pair of spinal nerves.

• Gray matter: cell bodies, organized into horns.

• White matter: myelinated and unmyelinated axons, organized into columns.

Brain

Major Regions and Functions

• Contains 97% of neural tissue; average weight ~1.4 kg.

• Six major regions: cerebrum, diencephalon, midbrain, pons, medulla oblongata, cerebellum.

• Cerebrum: Largest region; responsible for thought, sensation, memory, voluntary movement.

• Cerebral cortex: Superficial layer of gray matter; forms gyri and sulci.

• Cerebrospinal fluid (CSF): Cushions and supports CNS; produced by choroid plexus.

Cerebral Hemispheres and Lobes

• Separated by longitudinal fissure.

• Lobes named for overlying bones: frontal, parietal, occipital, temporal.

• Motor and sensory areas divided by central sulcus.

• Primary motor cortex: controls voluntary movements.

• Primary sensory cortex: receives somatic sensory information.

Association and Processing Centers

• Association areas interpret sensory and motor information.

• Processing centers integrate information from multiple association areas.

• Examples: General interpretive area (Wernicke's), speech center (Broca's), prefrontal cortex.