[U6] Nervous System Physiology, Cranial and Spinal Nerves, Sensory Pathways

Central Nervous System (CNS)

Processing center of the human body, composed of the brain and spinal cord

It integrates received sensory information and coordinates the body's activity

Peripheral Nervous System (PNS)

Communication network of the body, all the nervous tissue outside the CNS

It connects the CNS to the limbs and organs, carrying information to and from the CNS.

Sensory (Afferent) Division

PNS Division

Input pathway; conducts action potentials from sensory receptors to the CNS with sensory neurons

It relays sensory information about both the external (e.g., touch, sight) and internal (e.g., organ condition) environments.

Motor (Efferent) Division

PNS Division

Output pathway; conducts action potentials to effector organs (muscles, glands) via motor neurons

Transmits impulses from the CNS out to peripheral organs (effectors like muscles and glands) to cause an effect or action.

Somatic Nervous System (SNS)

Motor Division of PNS

Voluntary branch, controlling body movement; transmits action potentials from the CNS to skeletal muscles

Autonomic Nervous System (ANS)

Motor Division of PNS

Involuntary branch, controlling automatically occurring processes without conscious effort; transmits action potentials from the CNS to cardiac muscle, smooth muscle, and glands

Divided into the sympathetic and parasympathetic divisions

Enteric Nervous System (ENS)

Motor Division of PNS

Distinct nervous system, only found in the gastrointestinal (GI) tract

Neurons

Nerve cells that are fundamental information-processing units of the nervous system

Receive stimuli, conduct action potentials, and transmit signals to nerve cells or effector organs

Glial Cells (Neuroglia)

Supportive cells of the CNS and PNS that do not conduct action potentials

Carry out different functions that enhance neuron function and maintain normal conditions within the nervous tissue

Cell Body

Part of a Neuron

Contains a single nucleus and the organelles found in other types of cells

Dendrites

Part of a Neuron

Cytoplasmic extensions (branches) from the cell body; input zones that receive information from other neurons, and transmit the information to the cell body

Axon

Part of a Neuron

Single, long cell process extending away from the cell body; output zones that conduct electrical signals (sensory signals to the CNS and motor signals away from the CNS), may branch to form collateral cells

Multipolar Neurons

Structural Type of Neuron

Neurons that have many dendrites and a single axon

Comprises most of the neurons within the CNS and nearly all motor neurons

Bipolar Neurons

Structural Type of Neuron

Neurons that only have two processes: one dendrite and one axon

Specialized sensory neurons located in some sensory organs, such as the retina of the eye and inside the nasal cavity

Psuedo-unipolar Neurons

Structural Type of Neuron

Neurons that have a single process extending from the cell body, which divides into two branches - peripheral process (dendrite) and central process (axon)

Most sensory neurons

Astrocytes

Glial Cells of the CNS

Major supporting cells in the CNS, stimulate or inhibit the signaling activity of neurons and form the blood-brain barrier

Blood-Brain Barrier (BBB)

Formed by astrocytes, controls the passage of substances from the blood into the brain tissue

Ependymal Cells

Glial Cells of the CNS

Line the cavities in the brain that contain cerebrospinal fluid

Microglial Cells

Glial Cells of the CNS

Act in an immune function in the CNS by removing bacteria and cell debris

Oligodendrocytes

Glial Cells of the CNS

Involved in myelination, provide myelin to axons of neurons in the CNS

Schwann Cells

Glial Cells of the PNS

Involved in myelination, provide myelin to axons of neurons in the PNS

Satellite Cells

Glial Cells of the PNS

Found around cell bodies of certain neurons in the PNS

Provide support and nutrition to the neurons and protects the neurons from heavy-metal poisoning

Myelin Sheath

Specialized layers that wrap around the axons of some neurons

Provides protection and insulation, preventing almost all ion movement across the cell membrane

Increases speed and efficiency of action potential transmissions along the axon

Myelinated Neurons

Neurons with myelin sheaths around their axons

It allows electrical impulses to travel very quickly via saltatory (jumping) conduction.

Nodes of Ranvier

Gaps in the myelin sheath that occur about every milimeter (periodic gaps)

Ion movement can occur at these gap

Multiple Sclerosis

Autoimmune disease of the myelin sheaths of the CNS

Causes loss of muscle function

Unmyelinated Neurons

Neurons without myelin sheaths around their axons

It conducts impulses slower via continuous conduction and is common in the gray matter and the autonomic PNS.

Rest in the indentations of the oligodendrocytes (CNS) and the Schwann cells (PNS)

Gray Matter

Organization of Nervous Tissue

Consists of groups of neuron cell bodies their dendrites, and unmyelinated axons

It is the primary site for processing information, synapses, and integration.

Nuclei

Clusters of neuron cell bodies in the CNS

Ganglia

Clusters of neuron cell bodies in the PNS

White Matter

Organization of Nervous Tissue

Consists of bundles of parallel myelinated axons

It is the tissue responsible for transmitting signals rapidly between different gray matter areas

Nerve Tracts or Conduction Pathways

Formed by white matter of the CNS

Nerves

Formed by white matter of the PNS, consists of bundles of axons and associated connective tissue

Leak Channels

Ion channels that are always open so ions can diffuse across the membrane, down their concentration gradient

Gated Channels

Ion channels that are closed until opened by specific signals

Chemically Gated Channels

Ion channels that are opened by chemicals such as neurotransmitters

Voltage Gated Channels

Ion channels that are opened by a change in the electrical property of the cell membrane

Resting Membrane Potential

Action Potential Phase

Stable, negative voltage across the cell membrane of a neuron when it is not actively signaling

Maintained by Na+/K+ pump and K+ leak channels

Action Potential

Electrical signals that are conducted along the cell membrane similar to electricity traveling along an electrical wire

Threshold

Action Potential Phase

Critical level of depolarization that must be reached for an action potential to be initiated

The voltage at which voltage-gated Na+ channels rapidly open

Depolarization

Action Potential Phase

Phase where the membrane potential becomes less negative (more positive)

Caused by massive, rapid influx of Na+ ions through open voltage-gated Na+ channels, causing the inside of the cell to become temporarily positive

Repolarization

Action Potential Phase

Phase where the membrane potential rapidly returns to a negative value, moving back towards the resting membrane potential

Caused by Na+ channels inactivating and the delayed efflux of K+ ions through open voltage-gated K+ channels

Hyperpolarization

Action Potential Phase

Brief phase where the membrane potential becomes more negative than the resting membrane potential

Occurs because of slow-closing K+ channels that remain open for a short time after RMP has been reached, causing temporary undershoot

Continuous Conduction

Action potentials along unmyelinated axons occur along the entire membrane in this process

Saltatory Conduction

Action potentials along myelinated axons occur in this process, in a jumping pattern at the nodes of Ranvier

Synapse

Junction where the axon of one neuron interacts with another cell - another neuron, muscle cell, or gland cell

Presynaptic Terminal

Specialized ending of the axon of the transmitting neuron

Stores and releases neurotransmitters from synaptic vesicles into the synaptic cleft when an action potential arrives

Neurotransmitters

Chemical substances stored in synaptic vesicles in the presynaptic terminal and released into the synaptic cleft in response to an action potential

Postsynaptic Membrane

Surface of the receiving cell (neuron, muscle, or gland) across the synaptic cleft

Embedded with receptors that bind to released neurotransmitters, initiating a response in the receiving cell

Synaptic Cleft

Gap between presynaptic terminal and postsynaptic membrane

Neurotransmitters diffuse across this space to transmit the signal

Acetylcholine

Primary neurotransmitter for skeletal muscle movement (neuromuscular junction)

Norepinephrine

Key neurotransmitter for the Sympathetic Nervous System

Acetylcholinesterase

Enzyme that breaks down acetylcholine

Converging Neuronal Pathway

Simple neuronal pathway in which two or more neurons synapse with the same postsynaptic neuron

Allows information transmitted in more than one neuronal pathway to converge into a single pathway

Diverging Neuronal Pathway

Simple neuronal pathway in which an axon from one neuron divides and synapses with more than one other postsynaptic neuron

Allows information transmitted in one neuronal pathway to diverge into two or more pathways

Summation

Process by which a neuron adds up all of the electrical signals it receives to determine whether it should fire an action potential

Spatial Summation

Summation that occurs when the local potentials originate from different locations on the postsynaptic neuron

Temporal Summation

Summation that occurs when local potentials overlap in time

This can occur from a single input that fires rapidly, which allows the resulting local potentials to overlap briefly

Receptor

A neuron or the beginning of dendrites that detects a specific stimulus (like heat, light, or pressure) and converts it into an electrical signal (nerve impulse). It is the start of the sensory pathway.

Effector

A muscle, gland, or organ that receives a nerve impulse from a motor neuron and produces a specific response. It is the end of the motor pathway. (e.g., a muscle contracting or a gland secreting a hormone).

Nerve Impluse/Action Potential

Electrochemical charge that travels along a nerve fiber, travels about 200 m/s

Synapse

Gap between an axon of one neuron and the dendrite of another neuron

Gliomas

One of the most common types of brain tumors

Reflex Arcs

Specialized type of neuron pathway that allows a nerve impulse to travel in only one direction

Endoneurium

Connective tissue that surrounds and protects each individual axon within the fascicle

Perineurium

Connective tissue that tightly wraps around a bundle of axons, forming a fascicle

Creates a protective barrier for the axons within the bundle

Epineurium

Tough, outermost sheath of connective tissue that surrounds the entire nerve

Provides structural support and contains blood vessels that nourish the nerve

Fast Axonal Transport

Bidirectional (axonal terminal to cell body; cell body to axonal terminal) transport in axons

Move various materials that form membranes of the axolemma (cell membrane of axon)

Moves materials at a distance of 200-400 mm/day

Slow Axonal Transport

Unidirectional (cell body to axonal terminal only) transport in axons

Supplies and replenishes axoplasm (cytoplasm of axon)

Moves at 1-5 mm/day

Tetanus (Disease)

Disease characterized by painful muscle spasms and stiffness (lockjaw)

Caused by neurotoxin released by Clostridium tetani

Clostridium tetani

Anaerobic bacteria that produce the neurotoxin that affects the nerves via blockage of inhibitory neurotransmitters, causing spastic paralysis (muscle spasms)

Clostridium botulinum

Anaerobic bacteria that forms a neurotoxin that affects the nervous system via flaccid paralysis, clinically used as Botox

Sensory Neuron

Neurons that relay messages from receptors to the brain or spinal cord

Receptor to CNS

Afferent Neuron

Neurons that move signals away from a central organ or point towards the CNS

Signal to CNS

Motor Neurons

Neurons that relay messages from the brain or spinal cord to the muscles and organs

CNS to Muscles/Organs

Efferent Neurons

Neurons that move signals toward a central organ or point from the CNS

Signals away from CNS

Interneuron (Relay Neuron)

CNS Neurons that connects sensory and motor neurons; responsible for integrating processing, and relaying information between both neurons

Acetylcholine

Neurotransmitter

Associated with nerve impulse conduction at synapse, belongs to a chemical group called cholinergics

Norepinepherine

Neurotransmitter

Involved in the Sympathetic Nervous System

Belongs to a chemical group called adrenergics and to the chemical family known as catecholamines

Associated with sleep, mood, motor function, and pleasure recognition

Dopamine

Neurotransmitter

Involved in the brain’s reward system

A catecholamine that is associated with sleep, mood, motor function, and pleasure recognition

Serotonin

Neurotransmitter

A catecholamine that is primarily associated with sleep, mood, motor function, and pleasure recognition

Endorphins

Neurotransmitter

Body’s natural pain relievers 

Inhibits pain-type nerve impulses

Enkephalins

Neurotransmitter

Body’s response to pain and stress

Inhibits pain-type nerve impulses

Somatic Reflex

Reflex mediated by the somatic nervous system

Autonomic Reflex

Reflex mediated by the autonomic nervous system

Reflex

Rapid, involuntary, predictable, programmed motor response to a stimulus

Delta Waves

1-3 Hz, Slowest brain wave frequency

Associated w/ deep, dreamless sleep

Theta Waves

4-8 Hz, Second slowest brain wave frequency

Associated w/ deep relaxation and meditation, mental imagery

Alpha Waves

9-13 Hz, moderate frequency brain waves

Associated w/ relaxed, calm, lucid wakefulness, not thinking

Beta Waves

14-30 Hz, highest frequency brain waves

Associated w/ awake, normal alert consciousness, active thinking

Concussion

Mild traumatic brain injury resulting from a blow to the head or violent shaking

Causes temporary alteration of brain function e.g. confusion, headache, dizziness, and sometimes brief loss of consciousness

Contussion

Serious traumatic brain injury where there is actual bruising of the brain tissue

Results in permanent tissue damage and can lead to immediate and prolonged loss of consciousness

Cerebral Edema

Swelling of the brain caused by excessive accumulation of fluid, dangerous as the skull cannot expand, therefore causing increased intracranial pressure that can compress and damage brain tissue

Stroke

Condition where blood circulation to a brain area is blocked or interrupted by a rupture, leading to brain tissue death due to lack of oxygen and nutrients

Paralysis

General term for the loss of motor function in a body part, resulting from damage to the motor cortex, spinal cord, or peripheral nerves

Paraplegia

Loss of motor funnction of the lower body

Quadriplegia

Loss of motor function of all four limbs

Transient Ischemic Attack (TIA)

Often called a “mini-stroke”, a temporary episode of cerebral ischemia (reduced blood flow) that causes short-lived symptoms e.g. dizziness, temporary blindness, numbness, but no permanent damage

Alzheimer’s Disease

Progressive neurodegenerative disease that leads to severe memory loss, cognitive decline, and behavioral changes

Huntington’s Disease

Progressive, inherited neurodegenerative disorder that affects muscle coordination (hyperkinetic) and leads to cognitive decline and psychiatric problems