Nervous system

Body’s communication systems

• The nervous system and the endocrine system provide means by which organ systems communicate, maintaining homeostasis

• The nervous system controls thoughts, movement, and emotion

• The endocrine system controls growth, development, and digestion

The nervous system

• Works quickly

• Using chemical and electrical signals.

• interconnected network of cells

• signals move through cells

• divide into:

  • Central nervous system (CNS)

  • Peripheral nervous system (PNS)

Divisions of the nervous system

• CNS: interprets information (brain, spinal cord)

• PNS: gathers and transmits information (Cranial nerves, spinal nerves)

• The CNS and PNS pass signals between one another

  • Sensory receptor generates impulse

  • PNS passes impulse to CNS

  • CNS interprets impulse

  • CNS passes impulse to PNS

  • PNS stimulates a response

Divisions of peripheral nervous system

Sensory Division
• Picks up sensory information and delivers it to the CNS

Motor Division
• Carries information to muscles and glands

Divisions of the Motor Division
• Somatic – Carries information to skeletal muscle
• Autonomic – Carries information to smooth muscle, cardiac muscle, and glands

General functions of the nervous system

• 3 general functions:

  • Receiving stimuli = sensory function

  • Deciding about stimuli = integrative function

  • Reacting to stimuli = motor function

Sensory input, Integration and Motor output

• Sensory input:

  • Sensory receptors gather information

  • Information is carried to the central nervous system

• Integration:

  • Process and interpret sensory input and decide if action is needed

  • Sensory information used to create:

    • Sensations

    • Memory

    • Thoughts

    • Decisions

• Motor output:

  • Decisions are acted upon

  • Impulses are carried to effectors

  • The response activates muscles or glands

Functional hierarchy of the vertebrate PNS

Afferent Neurons (Sensory Neurons)

• Carry sensory information from the body to the central nervous system (CNS: brain and spinal cord).

• Detect stimuli such as touch, pain, temperature, sound, and light.

Efferent Neurons (Motor Neurons)

• Carry motor commands from the central nervous system to muscles and glands.

• Control muscle movements and glandular secretions.

Easy Way to Remember:

• Afferent (A) → Arrives at the brain (Sensory input).

• Efferent (E) → Exits the brain (Motor response).

Development aspects of the nervous system

• The nervous system is formed during the first month of embryonic development

• No more neurons are formed after birth, but growth and maturation continues for several years.

• The brain reaches maximum weight as a young adult

• However, we can always grow dendrites.

Cells of the nervous system

• Cell types in neural tissue:

  • Neurons

  • Neuroglial cells

• The signaling activity of the nervous system is made up of electrical activity within neurons and chemical flow between neurons

Neuron

• A cell body: has nucleus and organelles

• An axon: a long membrane-bound projection that transmits information away from the cell body in the form of electrical signals (impulses)

• Dendrites: extend from the cell body and are covered by a membrane; receive impulses

• Neurons have other structures to transmit signals

  • Terminal

  • Synapse

Axons and Synapses

• Axons are covered by a lipid layer called a myelin sheath

  • The myelin sheath insulates the neuron, which speeds up the transmission of action potentials along the axon.

  • The end of an axon is called an axon terminal

• Neurons communicate with each other at special junctions called synapses.

  • These synapses communicate by releasing neurotransmitters into synaptic cleft.

  • The synaptic cleft is a small gap between the axon terminal and the receiving cell.

Myelination

• White matter

  • Contains myelinated axons

  • Considered fiber tracts

• Gray matter

  • Contains unmyelinated structures

  • Cell bodies, dendrites

Classification of Neurons - Functional Differences

• Sensory neurons

  • Carry impulse to CNS

  • Most are unipolar

  • Some are bipolar

• Integrative neurons (Interneurons)

  • Link neurons

  • Multipolar

  • In CNS

• Motor neurons

  • Multipolar

  • Carry impulse away from CNS to effectors

Classification of Neurons: Structural Differences

• Bipolar neurons:

  • 2 extensions (processes)

  • Eyes, ears, nose

• Unipolar neurons

  • One extension

  • Ganglia of PNS

  • Sensory

• Multipolar neurons:

  • 99% of neurons

  • Many extensions

  • Most neurons of CNS

Neuroglial Cells in the PNS

1) Schwann Cells

• Produce myelin found on peripheral myelinated neurons

• Speed up neurotransmission

2) Satellite cells

• Support clusters of neuron cell bodies (ganglia)

How Neurons Function - Nerve impulse

• Neuron has a membrane potential

• A membrane potential is a difference in the electrical charge across a cell membrane.

• A membrane potential can change with an addition or removal of ions within the cell.

• Ions move in and out of the cell by passing through protein act as ion channels.

Resting potential

• A neuron is at rest when it is not sending or receiving a signal

• Polarized membrane:

  • Inside is more negatively charged than the outside

  • More Na+ outside of cell

  • More K+ inside of cell

Action potential

• When a dendrite or cell body is stimulated, the permeability of the neuron’s membrane changes suddenly

• The stimulus depolarizes the neuron’s membrane, allows sodium to flow inside the membrane

• The inside becomes more positively charged than the outside

• The exchange of ions initiates an action potential in the neuron.

Nerve Impulse Propagation

• An action potential is a moving electrical impulse

• After the first segment of the neuron is stimulated, the next segment will become stimulated

  => If the action potential (nerve impulse) starts, it is propagated over the entire axon

• Impulses travel faster when fibers have a myelin sheath

Refractory period

• A neuron cannot generate another action potential until it has returned to its resting potential

• The period in which a neuron cannot send a signal is called the refractory period

• Returning the neuron to its resting potential requires energy

• Potassium ions rush out of the neuron after sodium ions rush in => repolarizes the membrane

• The sodium-potassium pump restores the original configuration => Requiring ATP

Conduction of Action Potentials

• At the site where the action potential is generated, usually the axon hillock, an electrical current depolarizes the neighboring region of the axon membrane

• Action potentials travel in only one direction: toward the synaptic terminals

• Inactivated Na⁺ channels behind the zone of depolarization prevent the action potential from traveling backwards

• Action potentials are formed only at nodes of Ranvier, gaps in the myelin sheath where voltage-gated Na⁺ channels are found

Saltatory conduction

Action potentials in myelinated axons jump between the nodes of Ranvier in a process called saltatory conduction

Continuation of the nerve impulse between neurons

• When an action potential reaches the axon terminal, neurotransmitters are released into the synaptic cleft.

• These neurotransmitters bind to receptors on the next neuron’s dendrites, opening ion channels.

• If enough ion channels open, the action potential continues in the next neuron; otherwise, the signal stops.

• Different neurotransmitters can either open or close ion channels, affecting signal transmission.

• An action potential is started in the dendrite.

• Video:

The synapse

Nerve impulses pass from neuron to neuron at synapses, moving from a pre-synaptic neuron to a post-synaptic neuron.

Central Nervous System

• Develops from the embryonic neutral tube

• The neutral tube becomes the brain and spinal cord

  • The brain is the control center of the nervous system

  • The spinal cord carries nerve signals between the body and the brain

• The opening of the neutral tube becomes the ventricles

  • Four chambers within the brain

  • Filled with cerebrospinal fluid

Regions of the brain

• Cerebral hemispheres controls thought, movement, emotion

• Diencephalon

• Brain stem

• Cerebellum

Cerebral Hemisphere (Cerebrum)

• Paired (left and right) superior parts of the brain

• Include more than half of the brain mass

• The surface is made of ridges (gyri) and grooves (sulci)

• Fissures (deep grooves) divide the cerebrum into lobes

Lobes of the Cerebrum

Surface lobes of the cerebrum:

• Frontal lobe

• Parietal lobe

• Occipital lobe

• Temporal lobe

Cerebral cortex

• The folded outer layer of the cerebrum

• The cerebral cortex is the portion of the cerebrum that controls higher mental functions, general movement, organ function, perception, and behavioral reactions.

• The many folds of the cerebral cortex allow the brain to have a large surface area and still fit into the skull.

Specialized areas of the cerebrum

• Somatic sensory area – receives impulses from the body’s sensory receptors:

  • gustatory area (taste)

  • visual area

  • auditory area

  • olfactory area (smell)

• Primary motor area – sends impulses to skeletal muscles

• Broca’s area – involved in our ability to speak (speech/language region)

• Language comprehension area

• General interpretation area

Layers of the Cerebrum

The cerebral cortex is called gray matter. Beneath the gray matter is white matter.

Gray matter

• Outer layer

• Composed mostly of neuron cell bodies

• Basal nuclei (basal ganglia) – internal islands of gray matter

White matter

• Made up of myelinated axons, which link specific regions of the cortex with each other and with other neural centers

• Example: corpus callosum connects hemispheres

Diencephalon

• Lies on top of the brain stem

• Enclosed by the cerebral hemispheres

• Made of three parts:

  • Thalamus

  • Hypothalamus

  • Epithalamus

Thalamus

• Surrounds the third ventricle

• The relay station for sensory impulses

• Transfers impulses to the correct part of the cortex for localization and interpretation

Hypothalamus

• Under the thalamus

• Important autonomic nervous system center

• Maintains homeostasis and directly controls most of the body’s hormone production:

  • Helps regulate body temperature

  • Controls water balance

  • Regulates metabolism

• An important part of the limbic system (emotions)

• The pituitary gland is attached to the hypothalamus

Epithalamus

• Forms the roof of the third ventricle

• Houses the pineal body (an endocrine gland)

• Includes the choroid plexus – forms cerebrospinal fluid

Brain Stem

• Attaches to the spinal cord

• Controls basic life functions

• Parts of the brain stem:

  • Midbrain

  • Pons

  • Medulla oblongata

• The brain stem also has a network of neurons called the reticular formation. This section helps control respiration and circulation and helps separate signals that are important from those that are not.

Midbrain

• Mostly composed of tracts of nerve fibers

• Reflex centers for vision and hearing

• Cerebral aqueduct – 3rd to 4th ventricles

Pons

• The bulging center part of the brain stem

• Mostly composed of fiber tracts

• Includes nuclei involved in the control of breathing

• Relays (transmits) communications between the cerebral hemispheres and the cerebellum

Medulla Oblongata

• The lowest part of the brain stem

• Merges into the spinal cord

• Includes important fiber tracts

• Serves as both a relay center and a control center for:

  • Heart rate control

  • Blood pressure regulation

  • Breathing

  • Swallowing

  • Vomiting

Cerebellum

• Lies below and behind the cerebral hemispheres

• Two hemispheres with convoluted surfaces

• Provides involuntary coordination of body movements, allowing for balance

• Receives sensory impulses from muscles, tendons, joints, eyes, ears, and other brain centers

Protection of the Central Nervous System

• Scalp and skin

• Skull and vertebral column

• Meninges

• Cerebrospinal fluid

• Blood-brain barrier

Meninges

• Dura mater

  • Double-layered external covering

    • Periosteum – attached to the surface of the skull

    • Meningeal layer – outer covering of the brain

  • Folds inward in several areas

• Arachnoid mater

• Pia mater

Cerebrospinal Fluid

• Similar to blood plasma composition

• Formed by the choroid plexus

• Forms a watery cushion to protect the brain

• Circulated in arachnoid space, ventricles, and central canal of the spinal cord

Ventricles and location of the cerebrospinal fluid

Cerebrovascular accident (CVA)

• Commonly called a stroke

• The result of a ruptured blood vessel supplying a region of the brain

• Brain tissue supplied with oxygen from that blood source dies

• Loss of some functions or death may result

Alzheimer’s Disease

• Progressive degenerative brain disease

• Mostly seen in the elderly, but may begin in middle age

• Structural changes in the brain include abnormal protein deposits and twisted fibers within neurons

• Victims experience memory loss, irritability, confusion, and ultimately, hallucinations and death

Spinal cord

• The spinal cord is a column of nervous tissue that starts at the medulla oblongata and runs throughout the vertebral column to the region of T12 (12th thoracic vertebra)

• Below T12 is the cauda equina (a collection of spinal nerves)

Spinal Cord Anatomy

• Exterior white matter – conduction tracts

• Internal gray matter – mostly cell bodies

  • Posterior (dorsal) horns

  • Anterior (ventral) horns

• Central canal filled with cerebrospinal fluid

• Meninges cover the spinal cord

• Nerves leave at the level of each vertebra

  • Dorsal root: associated with the dorsal root ganglia – collections of cell bodies outside the central nervous system

  • Ventral root

Peripheral nervous system

• Sensory Pathways

  • Picks up sensory information and delivers it to the CNS

• Motor Pathways

  • Carries information to muscles and glands

  • Somatic nervous system – carries information to skeletal muscle

  • Autonomic nervous system – carries information to smooth muscles, cardiac muscles, and glands

    • Sympathetic divisions

    • Parasympathetic divisions

• Nerves and ganglia outside the central nervous system

• Nerve = bundled axon and dendrites of many neurons

• Neuron fibers are bundled by connective tissue

Structure of a Nerve

• Endoneurium surrounds each fiber

• Groups of fibers are bound into fascicles by perineurium

• Fascicles are bound together by epineurium

Classification of Nerves

• Mixed nerves – both sensory and motor fibers

• Afferent (sensory) nerves – carry impulses toward the CNS

• Efferent (motor) nerves – carry impulses away from the CNS

Spinal nerves

• There is a pair of spinal nerves at the level of each vertebra

• Spinal nerves are made up of both a dorsal and ventral root

• The dorsal roots carry signals from sensory receptors

• Ventral roots contain the axons of motor neurons

• Interneurons can be found in both sections of the nervous system

• Interneurons can relay information between other neurons

Sensory Division

• Contains sensory receptors and the interneurons that connect them to the central nervous system

• Sensory receptors receive information from the body’s external and internal environments

Motor Division

• Allows the body to react to sensory information

• The motor division is composed of the somatic nervous system and the autonomic nervous system

Comparison of Somatic and Autonomic Nervous Systems

• Somatic Nervous System (SNS)

  • Generally voluntary

  • Controls skeletal muscles

• Autonomic Nervous System (ANS)

  • Generally involuntary

  • Controls smooth muscle, cardiac muscle, and glands

  • Subdivided into sympathetic and parasympathetic divisions

Somatic Nervous System

• Contains motor neurons that control the movement of skeletal muscles

• Considered voluntary, but can operate without conscious control (e.g., reflexes)

• Also relays signals in reflex pathways

The Reflex Arc

• Reflex = rapid, predictable, and involuntary response to a stimulus

• Reflexes are often self-protective

• The patellar reflex is an example of a spinal reflex (the impulse bypasses the brain)

• A reflex arc is the direct route from a sensory neuron to an interneuron to a motor neuron and then to the effector

Types of Reflexes and Regulation

• Autonomic reflexes

  • Smooth muscle regulation

  • Heart and blood pressure regulation

  • Regulation of glands

  • Digestive system regulation

• Somatic reflexes

  • Activation of skeletal muscles

Autonomic Nervous System

• The involuntary branch of the nervous system

• Controls internal body conditions by regulating smooth muscles in blood vessels and organs

• Consists of only motor nerves

• Divided into two divisions:

  • Sympathetic division

  • Parasympathetic division

Anatomy of the Autonomic Nervous system

Autonomic Functioning

Sympathetic – "fight-or-flight," prepares the body when activated by physical or emotional stress (unusual stimulus). For example, pupils dilate and heart rate increases.

• Takes over to increase activities.

• Remember as the "E" division = exercise, excitement, emergency, and embarrassment.

Parasympathetic – housekeeping activities, controls the internal environment during routine conditions.

• For example, pupils constrict and heart rate decreases.

• Conserves energy.

• Maintains daily necessary body functions.

• Remember as the "D" division = digestion, defecation, and diuresis.

Physiological Effects of the Autonomic Nervous System

Sense Organs

• In order to detect environmental changes

• Sense organs receive stimuli and give rise to the senses such as sight, smell, taste, hearing, and pain.

• Sense organs are part of the sensory division of the peripheral nervous system.

Receptors and Sense Organs

• Sensory receptors are found in higher concentrations in the sense organs than in other parts of the body.

• When stimulated, these sensory receptors convert the stimulus into electrical signals and send those signals to the brain.

• Each signal that is sent to the brain is similar but may be sent to different parts of the brain to be interpreted.

Types of Sensory Receptors