8.9. BIO110 LO8/9: The Nervous System

What is the primary function of the Nervous System?

The general role of the Nervous System in maintaining homeostasis.

What are the four main organs of the Nervous System listed in the sources?

The organs of the Nervous System are:

• the Brain

• Spinal cord

• Nerves (Cranial, Spinal)

• Sensory organs (eyes, ears, specialized sensory organs, free nerve endings (in the skin)).

What are some of the sensory organs associated with the Nervous System?

Sensory organs include Eyes, Ears, Specialized sensory organs, and Free nerve endings (in skin).

What are the two main divisions of the Nervous System?

The two main divisions are the Central NS (CNS) and the Peripheral NS (PNS).

What structures compose the Central Nervous System (CNS)?

The Brain and spinal cord.

What is the functional role of the CNS?

• The CNS is the Integrative & control centre

• it interprets incoming info and dictates responses.

What structures compose the Peripheral Nervous System (PNS)?

Nerves that extend to ‘peripheral’ parts of the body (from brain spinal cord), and All nerves (cranial and spinal).

What is the role of the PNS?

The PNS serves as the Communication network between CNS and the rest of the body.

What are the two subdivisions of the PNS (motor division)?

The two subdivisions of the PNS are the Autonomic Nervous System (ANS) and the Somatic Nervous System (SNS).

What are the characteristics and function of the Autonomic Nervous System (ANS)?

• The ANS is involuntary (Visceral MOTOR nerves)

• Conducts impulses from CNS → cardiac muscle, smooth muscles and glands.

What are the characteristics and function of the Somatic Nervous System (SNS)?

• The SNS is voluntary (Somatic MOTOR nerves)

• Conducts impulses from CNS → skeletal muscles.

What are the two divisions of the Autonomic Nervous System?

The two divisions are the Parasympathetic NS and the Sympathetic NS.

Where do the Parasympathetic NS motor neurons originate, and what is its associated function?

The Parasympathetic NS originates in the Cranial and sacral areas and is associated with ‘rest and digest’

Where do the Sympathetic NS motor neurons originate, and what is its associated function?

The Sympathetic NS originates in the Thoracic lumbar areas and is associated with ‘fight-or-flight’.

What are the two types of cells found in the Nervous System (NS)?

Neurons (nerve cells) and Glia/Neuroglia.

What are Neurons?

Neurons are the structural and functional units of the NS.

What are the three main parts of a Neuron?

Dendrites, Cell body, and Axon.

What are the three ways Neurons are classified according to function (direction they transmit the impulse)?

• Sensory (or afferent)

• Motor (or efferent)

• Interneurons (or central or connecting neurons).

What is the structure and function of Dendrites?

Dendrites are Branching projections that Conduct impulses to the cell body.

What is the Cell body of a neuron?

The Cell body is the Main part of the neuron and Contains the nucleus.

• found in CNS or close by it

What is the structure and function of the Axon?

The Axon is a Long, single projection from the cell body.

• transmits impulses away from cell body

What is the function of Sensory or afferent neurons?

They ‘Sense’ stimuli

• Conduct impulses from receptors all over the body → brain and spinal cord.

What is the function of Motor or efferent neurons?

They conduct impulses away from the brain and spinal cord → muscles and glands (collectively called effectors).

What are the two types of motor neurons based on function and target?

• Somatic motor neurons (→ skeletal muscles)

• Visceral or autonomic motor neurons (→ smooth and cardiac muscle, glands).

What is the function of Interneurons (or central or connecting neurons)?

They Conduct impulses from sensory neurons → motor neurons.

(deeper thought)

What are Glia (neuroglia)?

Glia are cells that provide support and protection for the NS.

What are the functions of Glia/Neuroglia, and how do they differ from Neurons?

-They are the ‘glue’ that binds the cells of nervous tissue together structurally and functionally allowing NS 

• they Do NOT transmit nerve impulses.

How much more abundant are Glia than Neurons?

Glia are 5x more abundant than neurons.

What are the four types of Glia cells and where are they located?

• Astrocytes

• Microglia

• Oligodendrocytes (in the CNS)

• and Schwann cells (in the PNS).

What is the primary function of Astrocytes? “star cell”

• Relatively large compared to other glial cells

• Astrocytes Wrap around and secure small blood vessels to neurons.

• Part of the blood-brain barrier

What is the function of the blood brain barrier?

It Separates blood from brain tissue and Protects the brain from harmful substances that may be in the blood.

What is the function of Microglia?

• Microglia act as the first and main form of active immune defence for CNS tissue (brain and spinal cord)

• Act as ‘macrophages’ (phagocytes) if they sense tissue damage or inflammation to decrease inflammation and/or destroy harmful substances to prevent damage to sensitive CNS tissue

• Usually stationary until trouble is sensed, (ex. CNS tissue becomes inflamed/damaged)

What is the function of Oligodendrocytes/Oligodendroglia? “few branches”

They help hold neurons together by extending branches to the axon of neurons (can wrap branches around up to 50 axons

• Form the myelin sheath (neurilemma) on axons in the CNS.

What is a neurilemma?

another word for myelin sheath in CNS

What is Myelin?

Myelin is a fatty substance that electrically insulates axons.

• essential for proper function of nervous system/nerve impulse conduction

What is the function of the oligodendroglia?

forms myelin in CNS

• wrap around neurons, but leave tiny, uncovered ‘gaps’ that are regularly spaced along the neuron (nodes of ranvier)

What is the function of Schwann cells?

Schwann cells Form myelin in the PNS. same function as myelin

(Myelin is a fatty substance that electrically insulates axons.

• essential for proper function of nervous system/nerve impulse conduction)

What are the unmyelinated spaces along the neuron called? what is the purpose?

These spaces are called nodes of Ranvier.

• since nerve is not insulated with myelin in these spaces, it is able to generate electrical activity

What type of nerve impulse transmission occurs due to the nodes of Ranvier?

The rapid and effective transmission of a nerve impulse called saltatory conduction (‘saltatory’ = ‘jumping’).

What is a Nerve?

A Nerve is made up of several bundles of axons (fascicles) in the PNS.

usually covered by a myelin sheath, so it appears as white in colour

What is a fascicle?

A bundle of axons

What is a single axon?

A nerve fibre

What is a Tract?

A Tract is a Bundle of axons in the CNS.

What is White matter composed of, and what color does it appear?

White matter is tissue composed primarily of myelinated axons and appears white in color.

What is Gray matter composed of?

Gray matter is tissue composed primarily of cell bodies and unmyelinated fibers/axons and dendrites.

What are the three layers of fibrous connective tissue that cover a nerve, based on location?

• Endoneurium (surrounds individual axons),

• Perineurium (surrounds individual fascicles), and

• Epineurium (surrounds the entire nerve).

What are constantly being conducted from receptors to effectors over neuron pathways?

Nerve impulses (also called action potentials) are constantly being conducted from receptors to effectors over neuron pathways = neuron→neuron→neuron→neuron→neuron→etc

receptor →neuron pathway→ effector

What is a Reflex arc?

Specialized neuron pathway that results in a rapid involuntary or automatic response to a stimulus (called a reflex).

What are the two types of reflex responses?

A contraction by a muscle or a secretion by a gland.

What are the five components of a reflex arc?

• A receptor (detects change and generates nerve impulse),

• Sensory/afferent neuron (transmits impulse to CNS)

• CNS (integrating centre with one or more synapses - where the interneurons are located)

• Motor/efferent neuron (transmits impulses from CNS to effector)

• Effector (a muscle or gland that performs the “effect”)

What are the two types of reflex arcs based on neuron components?

• Two-neuron arcs (sensory neurons synapsing in spinal cord with motor neurons – no interneuron)

• Three-neuron arcs (sensory neurons synapsing in spinal cord with interneurons that synapse with motor neurons).

Where are Interneurons located?

Interneurons are located entirely within the gray matter of the brain or spinal cord.

In a three-neuron arc, what is the role of the interneurons?

They ‘determine’ what, if any, action needs to happen in response to a stimulus and communicate this information to the motor neuron.

• The more ‘threatening’ the sensory information is, the more likely it will get sent to the brain for more ‘complex’ processing/integration

What are Two-neuron reflex arcs often referred to as, and what are they used for?

They are Simple, unlearned or ‘true reflexes’ (e.g., knee jerk; pupil dilation).

What are Three-neuron arcs associated with?

They are associated with more ‘complex responses such as response to injury/pain (WITHDRAWAL REFLEX).

• provide automatic reactions that occur quickly to ensure “survival”

Reflexes can occur before you are even conscious of the stimulus/pain!

What is a Nerve Impulse (or Action Potential)?

A self-propagating wave of electrical disturbance that travels along the surface of a neuron’s plasma membrane.

What is the charge on the membrane of a “resting” neuron?

The membrane of a ‘resting’ neuron has a slightly positive charge on the outside and a negative charge on the inside (due to excess Na+ on the outside)

What is the initial mechanism that triggers a nerve impulse?

A stimulus triggers the opening of Na+ channels in plasma membrane of the neuron (starting at dendrites) allowing Na+ to rush into neuron

What marks the beginning of a nerve impulse/action potential?

Inward movement of positive sodium ions (Na+) leaves a temporary, slight excess of negative ions on the outside of the neuron’s membrane at the stimulated point

How does the nerve impulse travel along the entire length of the neuron’s membrane?

• This ‘electrical disturbance’ of the neuron’s membrane stimulates Na+ channels in the adjacent section of the membrane to open, and so on, and so on

• - this creates a ‘self-propagating’ wave of electrical disturbance – a nerve impulse – that travels along the entire length of the neuron’s membrane

What is a Synapse?

A microscopic junction between neurons (or a neuron and an effector).

What are the three structures that comprise a Synapse?

Synaptic knob, Synaptic cleft, and Plasma membrane of postsynaptic neuron.

What is the function of a Synapse regarding impulse transmission?

A synapse ensures there is only one-way transmission of a nerve impulse.

What is the Synaptic knob (AKA axon terminal) and what does it contain?

A Tiny bulge at the end of a terminal branch of a presynaptic neuron’s axon that Contains vesicles full of neurotransmitters.

What is the Synaptic cleft?

The Microscopic space between a synaptic knob and the plasma membrane of a postsynaptic neuron.

How does a nerve impulse get across the synapse?

With the help of chemical compounds called neurotransmitters.

What are Neurotransmitters?

Chemicals by which neurons communicate.

How is neurotransmitter action ended in the synaptic cleft?

By Reabsorption back into the synaptic knob.

There are at least 30 different types of neurotransmitters, which are specifically localized in discrete groups of neurons and released in specific pathways.

Which 3 are the major ones?

▪ Acetylcholine (ACh)

• Major neurotransmitter of autonomic nervous system responses

▪ Catecholamines (noradrenaline/norepinephrine, adrenaline/epinephrine, dopamine, serotonin)

• Sleep, mood, pleasure recognition, motor function

▪ Endorphins and enkephalins

• Morphine-like

• Natural pain killers (inhibit conduction of pain impulses)

What is the major neurotransmitter of autonomic nervous system responses?

Acetylcholine (ACh).

What are the functions associated with Catecholamines (noradrenaline/norepinephrine, adrenaline/epinephrine, dopamine, serotonin)?

Sleep, mood, pleasure recognition, motor function.

What is the function of Endorphins and enkephalins?

They are Morphine-like/ natural pain killers (inhibits conduction of pain impulses)

What two major structures compose the Central Nervous System (CNS)? What are they protected by?

The Brain and Spinal cord.

• protected by meninges

Which structures are the nervous system protected by?

Nervous system protected by structures of the skeletal system & integumentary system!

What structures provide protection for the brain and spinal cord?

The cranial bones/skull, vertebral column/spine, and meninges.

What are the four main divisions of the Brain?

Brainstem, Cerebellum, Diencephalon, and Cerebrum.

What are the three parts of the Brainstem?

Medulla oblongata

• Lowest part (enlarged upward extension of spinal cord)

Pons “bridge"

• Connects medulla oblongata with surface of cerebrum (cerebral cortex)

Midbrain

• near the centre of the brain

What is the midbrain also known as?

mesencephalon

What substance is the reticular formation in the medulla oblongata composed of?

Bits of scattered white gray matter that are closely mixed together.

What is the function of the gray matter in the Brain Stem?

It functions as autonomic reflex centres (e.g., heartbeat, respiration, blood vessel diameter), which are called ‘vital centres’ (located in the medulla oblongata).

where do the sensory tracts in the brain stem conduct impulses to?

To higher parts of the brain

Which part of the brain stem conduct impulses from higher parts of the brain to the spinal cord

Motor tracts in the brain stem conduct impulses from higher parts of the brain to the spinal cord

What is the Cerebellum?

The 2nd largest part of the brain.

What is the cerebellum connected to?

to the brainstem

In the cerebellum, what is the outer and inner layer composed of?

Outer layer - gray matter

Inner layer - white matter

What is the primary function of the Cerebellum?

It helps control muscle contractions to produce coordinated movements so that we can maintain balance, move smoothly, and sustain normal posture.

What are the two major structures that compose the Diencephalon (“High traffic area”)?

The Hypothalamus and Thalamus.

Describe the hypothalamus in terms of its structure and location

• Posterior pituitary gland

• Pituitary stalk

• Areas of gray matter

• One of the smallest parts

Describe the thalamus in terms of its structure and location

• Dumbbell shaped

• Mass of grey matter in each cerebral hemisphere

• Mostly dendrites and cell bodies of neurons that have axons extending upward into sensory areas of cerebrum

What is the Hypothalamus often referred to as functionally?

‘The Heart of Subconsciousness’.

What are the two major functional roles of the Hypothalamus?

• It is the Major centre for controlling the ANS (involuntary actions)

• Connects nervous and endocrine system.

What specific hormones are affected by the Hypothalamus?

It Controls hormone secretions via anterior and posterior pituitary glands (making hormones like ADH secreted by the posterior pituitary gland).

What other functions does the Hypothalamus control?

Appetite control, body temp, water balance, sleep & wakefulness, pleasure, anger, sexual arousal, and pain.

What is the structure of the Thalamus?

A Dumbbell shaped mass of grey matter in each cerebral hemisphere.

What is the main function of the Thalamus regarding sensory information?

It Receives all sensory information (except for smell), then directs impulses from sensory organs to sensory areas in the brain for conscious interpretation.

What emotional and survival functions is the Thalamus associated with?

Association of sensations with emotions (pleasant or unpleasant) and playing a role in the body’s arousal/alertness mechanism (alerting us to danger).

What is the Cerebrum?

The Largest and uppermost part of the human brain.

What is the structure of the Cerebrum's outer surface?

It has many ridges (gyri) and grooves (sulci).

What connects the two halves (cerebral hemispheres) of the Cerebrum?

The corpus callosum.

What is the cerebral cortex, and what is it composed of?

The Outer layer of gray matter of the cerebrum, composed mainly of dendrites and cell bodies of neurons.