Neurons and the Nervous System

Neurons: Functional Units of the Nervous System

• Neurons, specialized cells within the nervous system, conduct electrical and chemical signals and serve as fundamental units of the brain.
• A neuron comprises a nerve cell, responsible for signal conduction.
• Neurons exhibit diverse shapes based on their function and location in the body.
• Neurons contain a nucleated cell body
  • This includes dendrites, the receptive apparatus for nerve cells
  • Axons carry impulses away from the cell body.
• An insulating myelin sheath may cover the axon in vertebrates and some complex invertebrates

Nervous system

• The nervous system has two types
  • The central nervous system (CNS) includes the brain and nerve cord
  • Peripheral nervous system (PNS) consists of afferent (sensory) and efferent (motor) nerves.

Types of Neurons

• Afferent (sensory) neurons conduct sensory information from periphery to the CNS for interpretation.
• Efferent (motor) neurons conduct motor commands from the CNS to peripheral effectors (muscles/glands).
• Interneurons conduct impulses within the CNS and account for 99% of all neurons.

Nerves and Ganglia

• Nerves are collections of axons bound by connective tissue.
• Ganglia are collections of neuronal cell bodies located in the PNS.

Neuroglia

• Neuroglial cells are non-neuronal cells that support neurons.
• Neuroglial cells are numerous in the vertebrate brain, outnumbering neurons by 10 to 1
• Schwann cells form myelin in the PNS.
• Oligodendrocytes form myelin in the CNS.

Astrocytes and Microglial Cells

• Astrocytes provide functional and structural support to neurons.
  • They are star-shaped, serve as nutrient and ion reservoirs, act as scaffolds during brain development and promote regenerative processes after injury.
• Microglial cells are phagocytes in the CNS.

Nerve Action Potential

• All nerve signals share the same type of electrochemical message.
• The action potential is an all-or-none phenomenon.
  • Fibers either conduct an impulse or do not.
• The frequency of the signal is the variation a nerve fiber can accomplish.
• Nerve signals can vary from a low of a few to nearly 1000 per second.

Resting Membrane Potential

• The interstitial fluid outside the neuron contains high concentrations of sodium (Na+) and chloride (Cl-)
  • Low concentrations of potassium (K+).
• Inside the neuron there is low sodium (Na+) and chloride (Cl-) ion concentrations
  • High potassium (K+) levels
• At rest, the neuron's membrane is selectively permeable to K+
  • Minimal permeability to Na+ because Na+ channels are closed.

Potassium Ions

• K+ diffuses outward until the positive charge outside repels further K+ exit.
• Equilibrium is reached when the electrical gradient balances the concentration gradient forcing K+ out.
• Resting membrane potential is typically -70 millivolts with a negative charge inside the membrane.

Sodium Pump

• The sodium pump consists of complex proteins embedded in axonal membranes.
• Sodium pumps transport Na+ out of the axon and K+ back inside, using Na+/K+ ATPase.
• Pumps reestablish resting ionic concentrations and require ATP.

Action Potential

• Action potential involves a rapidly moving change in electrical membrane potential.
• Action potential involves rapid, brief membrane depolarization.
• The neuron's exterior becomes negatively charged relative to the interior.
• Action potentials are self-propagating.

Depolarization and Repolarization

• Depolarization: Change in membrane potential opens voltage-gated Na+ channels causing Na+ to rush into the axon from outside.
  • Channels remain open for less than a millisecond, interior becomes slightly positive, thus depolarizing the membrane
• Repolarization: K+ gates open, K+ ions move out and reestablish the resting membrane potential.
  • Interior becomes negative with respect to the exterior again.
• This begins at the axon hillock and ends at the axon terminals.

Speed of Conduction

• Speed varies from 0.1 meters/second in sea anemones to 120 meters/second in some mammalian motor axons.
• Conduction speed relates to the diameter of the axon.
• Small axons conduct slowly due to high internal resistance to current flow.
• Fast conduction is important for quick responses in most invertebrates, and axon diameters are large.

High-Speed Axons

• Giant axons present in squid are 1 mm in diameter and conduct impulses 10-times faster
• Vertebrates do not possess giant axons but achieve high speed by utilizing myelin sheaths.
• Myelin is formed by wrapping axons with Schwann cells in the PNS or oligodendrocytes in the CNS.
• A single Schwann cell wraps around one portion of an axon
• Oligodendrocytes wrap multiple processes around portions of different axons.

Nodes

• Nodes of Ranvier are exposed areas of the axon found between adjacent regions of myelin.
• Myelin insulation prevents depolarization, thus depolarization only occurs at the nodes, increasing conduction speed.
• Saltatory conduction describes the action potential leaping from node to node.
• A frog myelinated axon with a 12µm diameter conducts nerve impulses as fast as a squid axon (unmyelinated) of 350µm.

Temperature and Conduction

• Temperature also regulates the conduction velocity
• Endotherms have high conduction velocity
• Ectotherm conduction velocity fluctuates with environmental temperatures.

Synapses: Junctions Between Nerves

• During An action potential, it must cross a small gap known as a synapse.
• This junction can be between two neurons or a neuron and an effector (muscle/gland).
• Neurons carrying action potentials to the synapse are presynaptic neurons.
• Postsynaptic neurons carry action potentials away from the synapse.
• A synaptic cleft is about 20 nanometers wide.
• Electrical and chemical synapses are the two synapse Types.

Electrical Synapses

• Electrical synapses occur in both invertebrates and vertebrates.
• Ionic currents flow directly across a narrow gap junction from one neuron to another.
• There is no time lag and they are important in escape reactions.
• Signals are often bidirectional.
• Unidirectional synapses are found in crustaceans.
• They are important for communication between cardiac muscle cells.

Chemical Synapses

• Communication depends on the use of chemicals in chemical synapses.
• Presynaptic knobs of axons contain neurotransmitters in packets.
• Axon terminals may input on multiple dendrites of a single postsynaptic neuron.
• The fluid-filled gap situated between presynaptic and postsynaptic membranes prevents the action potential from proceeding.

Synaptic Transmission

• Neurotransmitters include Acetylcholine, GABA, dopamine, serotonin, and norepinephrine.

Changes After Transmission

• The magnitude of voltage change depends on the quantity of neurotransmitter released and number of ion channels open.
• Acetylcholinesterase rapidly converts acetylcholine into acetate and choline.
• Acetylcholinesterase prevents acetylcholine from continuing to stimulate the postsynaptic membrane.
• Organophosphate insecticides block acetylcholinesterase.
• Choline is reabsorbed and reused to resynthesize acetylcholine.

Evolution of the Nervous System

• Protozoa are unicellular and lack nerves.
• Nerve nets are the simplest nervous system pattern and are found in jellyfish, hydra, sea anemones and comb jellies.
  • An extensive network is found in the epidermis.
  • Signals travel in all directions
  • one-way synapses do not occur here.
• This system survives in advanced animals as a nerve plexus for intestinal movements.

Bilateral Nervous Systems

• Flatworms contain the simplest system with bilateral nervous systems.
  • Two anterior ganglia lead into two main nerve trunks that run down the body
  • Lateral branches give the systems a ladder-like appearance
  • Simplest system that has a PNS and a CNS.
• Annelids possess segmented ganglia where relays for regional activity coordination occur
  • Advanced afferent and efferent neurons are found here

Nervous Systems of Molluscs and Arthropods

• Molluscan plan centers are situated on three pairs of well-defined ganglia
  • In cephalopods,the ganglia have burgeoned into nervous centers of over 160 million cells
• Arthropods have larger ganglia than annelids.
  • They have well developed sense organs
  • The social behavior is elaborate and includes some examples of learning.

Vertebrate Nervous System

• Vertebrates display encephalization.
  • An increase and greater complexity of the brain.
• The CNS consists of the brain and the spinal cord.
  • Begins as an ectodermal neural groove and folds into a long neural tube.
  • Unlike invertebrates, segmental nerves here are separated into dorsal and ventral roots and meet to form a mixed spinal nerve

Spinal Cord

• The spinal cord resides in the protected space of the vertebrae in the column with protective membranes called meninges.
• In cross-section there is an inner zone or gray matter, and an outerzone of white matter.
  • Gray matter contains the cell bodies of the motor neurons,
  • White matter contains many axons and dendrites

Reflex Arc

• In a spinal chord, neurons work most often as reflex arcs of at least two neurons.
• This is the fundamental unit of neural operation.

Parts of a Reflex Arc

• Receptor: Sense organs in the skin, muscles, etc.
• Afferent (sensory) neuron: Transports impulses to the CNS.
• Central Nervous System: Consisting of synaptic connections between sensory neurons and interneurons.
• Efferent (motor) neuron: Carries the message from the CNS to the effectors.
• Effector: Allow changes for a muscle, gland, or other organ.

Simplest Reflex Arc

• Simplest Reflex Arcs have one sensory and one motor neuron
  • For example, is the knee-jerk response.
• Input or output are multiplied along the arc with the help of interneurons being interposed between sensory and motor neurons.
• Reflex acts consist of:
  • Involuntary responses to a stimuli
  • A vitial process, which is innate

Brain

• Spinal cord evolution has minimal change.
• There are dramatic changes during the evolution of the brain
• Early vertebrate brains were similar to the fish brains of today.
• The average human brain has 35 billion nerve cells
  • Each one can receive tens of thousands of synapses
• The ratio of brain-weight to spinal cord-weight can indicate approximate species intelligence
  • Amphibians are 1:1, and humans are 55:1 approximately.

Brains of Early Vertebrate Fishes

• There were three principal areas of the early vertebrate brain
  • The hindbrain (Rhombencephalon) supported hearing and balance
  • The midbrain (Mesencephalon) supported vision
  • The forebrain (Prosencephalon) supported smell

Divisions of the Vertebrate Brain

• The telencephalon becomes the cerebrum.
• The diencephalon becomes the thalamus and hypothalamus.
• The mesencephalon becomes the optic lobes and midbrain nuclei.
• The metencephalon becomes the cerebellum and pons.
• The myelencephalon becomes the medulla oblongata.

Hindbrain

• Medulla oblongata:
  • Most posterior division and has many functions including continuation of the spinal cord
• Medulla and anterior midbrain:
  • The “brain stem” supports:
    • Heartbeat control control, respiration, vascular tone, gastric secretions, and swallowing.
• Pons, which
  • Connects both halves of the cerebellum together
  • Connects the medulla and cerebellum with other regional nerves

Cerebellum

• Dorsal to the medulla.
• Controls equilibrium, posture, and movement.
• More agile bony fish with developed Cerebellums.
• Non avian animals and reptiles possess less developed.
• Birds and mammals exhibit highly developed Cerebellums.
• It does not initiate movement, but can fine tune that which already exists.

Midbrain

• It contains nuclei of the tectum
• Nuclei and centers support:
  • Visual and auditory reflexes.
  • Fishes and amphibians use vision, sense or touch, and hearing to do complex actions.
• These functions have been assumed by the forebrain in amniotes

Forebrain

• Posterior with connection to the forebrain is called:
  • Thalamus and hypothalamus
• Egg comes after the posterior zone.
  • Analyzes, and forwards sensory information
• Hypothalamus
  • Controls various body characteristics
  • Produces hormones that are neurosecretory
  • Emotion control

Cerebrum

• Divided into:
  • Paleocortex
    • Where the limbic system resides
  • Neocortex
    • Involved in behaviors, feeding, mating etc
• The limbic includes;
  • The hippocampus
  • Spatial awareness, learning etc.

Neocortex

• The cerebral cortex consist of neocortex
  • Envelops the brain like a large cape
  • Includes discrete areas for controlling movement or reacting to senses.
• Motor area for body control
• Sensory perception for reactions with outside world.

The Mammalian Brain

• The two types:
  • Separate areas that influence; conscious thought and the unconscious
• The Cortex is linked to thinking
• The corpus, a bridge on the left and right, enables thoughts and function.
• Humans, with varying functions for the left from and right:
• Left:
  • Used in math, reading etc, used in capabilities, math, language -right:
  • Intuitive side, perceptual behavior, spatial etc

Peripheral Nervous System

• Peripheral Nervous System (PNS):
  • includes nervous tissue located outside of the CNS
• Two types:
  • The sensory division - collects info
  • Motor division - conveys the major commands for many glands

Divisions of the PNS

• Sensory (Afferent) Division
  • Conducts sensory information from peripheral body parts to the CNS.
• Motor (Efferent) Division
  • Somatic Nervous System
    • Conducts motor messages to skeletal muscle.
  • Autonomic Nervous System
    • Conducts motor messages to smooth muscle, cardiac muscle, and glands.
    • Controls involuntary, internal functions that do not affect consciousness.
• Two Divisions
  • Sympathetic Nervous System
  • Parasympathetic Nervous System

Sense Organs

• Specialized receptors designed to detect environmental status and change.
• Stimulus can be:
  • a Form of energy
  • An Electrical message, pressure
• These transform electricity for organs.
• Receptors for senses:
  • Have to be biological.
  • Responds only to stimulus.

Sensory Receptors

• Classification of Sensory Receptors
• Receptors near the external body surface
• Receptors in internal organs
• Detect muscle expansion
• Sense position
• Found in joints or tendons
• Are organized:
  • With thermal qualities
  • With lighting qualities

Chemoreception

• This sense can be
  • universal
  • unicellular
• Allows the organisms to sense certain materials, both good and bad.
• Orientation can be both toward and away
• Insects use sensory receptors in their sensory organs to find what they need.

Pheromones

• Many mammals release them.
• They are species specific
  • Communicate with one another
• Pheromones influence behavior and reproduction through territory, hierarcny etc.
• Releaser - Releases specific effects
• Primer - The reverse of a releaser.

Olfaction

• Most distance chemoreceptors have sensitivity
  • These animals often use a 'smell',
• This sense supports behaviors including:
• Mating, territory, finding food
• These can be found more in mammals
• Epithelium houses endings in nasal cavity
• Neurons lie with cilia
• Odors influence receptor activity
• This in turn will signal an organ used to understand the behavior at hand.

Odor Reception

• Sent to the relevant part of the brain.
• Affecting thoughts and feelings. emotions

Taste Receptors

• Restricted range:
  • Has a CNS used for the different senses at hand
• Can be found in;
• Oral cavity
• A tongue

Taste Bud

• Are clusters containing surrounding tissues.
• Cells are projected through an opening.
• Combines and reacts to certain locations on the receptors
• They are continuous as well and have short functions.
• The most likely way this occurs is by recognizing papillae on the human tongue

Taste Sensations

• The human can understand;
  • The key tastes
  • sweet
  • salty
• many can also detect bitterness
• Taste can occur when multiple inputs are activated
• This results from a specific type of function by any amount of different activation

Mechanoreception

• Senses mechanical functions of the body through receptors
• These activate functions
  • pressure is activated
  • Motion of the body occurs Touch Receptors
  • Senses by follicle activation

Pacinian Corpuscle

• Great for knowing what touch is
• Can be felt by skin or in body through certain capsules.
• Also activates receptor potential.
• Over more strength of the stimulant the more receptors can be opened
• Over time a corpuscle can adjust and create new body shape.

Pain Receptors

• Consists of free linings that conduct stimulus reactions.
• Activates smaller peptides
• This sends chain causes to be done about damage to an area.
• Pin pick test is a direct test of damage to the nervous system.

Lateral Line System

• Seen commonly among fish and amphibious creatures where water flow assists with vibrations and currents.
• A distant sensing touch.
• Neuromasts - Receptive cells often in aquatic fish only

Characteristics of the Lateral Line System

• Can be shown to be helpful for schooling behaviors.
• And can be used for electrical sensing

Sense of Hearing

• The detection of waves with the help of:
  • The ear for detecting messages
• Most creatures can not hear however.
• While certain crustaceans (Spiders and other bugs) can hear.
• They might also detect grasshoppers that detect danger or want mating partners that are on alert.

Vertebrate Ear

• These ears evolve.
• They allow for balance and orientation with two different locations;
  • Labyrinth in fish base has a pocket for hearing when sacs are full.
• And the inner pocket has the most control.

Human Ear

• The outer assists with the use of directional sounds.
  • There is canal for passing sound, like the inner ear as well There's also a third in the circle Auditory middle side canal 1- The middle region assists with pressure 2 The ear also has equal access into this region, where the inner functions

Inner Ear

• The inner type consists of three different regions which run:;
  • Tubular and in close proximity with their respective region.
  • And this causes a vestibular type reaction inside.
  • And that is only second with:
    • Tymphanic canal as well
      • That is more sensitive

Equilibrium

• How do creatures adapt?
• The inner parts move and orient themselves according to needs. Invertebrates - detect gravity
• low frequnecy waves with 'statocysts'
• Each sac is aligned with hair cells

Labyrinth

These provide an static environment for: - Statucysts that are alligned with balancing - Invertebrates

Canals - These functions to adjust and accommodate for change These can all occur independently to handle balance needs.

Photoreception

• This involves light sensitivity
  • Ranges are very wide:
• Simple bodies of various insects can take readings, complex vertebrates use it This involves compunding what can be read with their eyesights such as,

  • Arthropods - Eyes of bee's.

    • About 15000 units.

    Motion Detection
    

Insect Senses - In a very low result. - Can be a low color type.

• Ultraviolet lights can spot honey.
• With these skills, creatures can get into the right positions and head accordingly

Single-Lens Camera-Type Eye

• These simple eyes allow a small degree of function with very different types of sensing
• Used in some species.
• Light reacts on the sensitive front.
• Image reads at back of chamber.
• Sclera - Outer later and tough exterior.
• Choroid Middle part with high functions.
• Retina - Inner layering with sensitivities to light.

Retina

• Divided and comprised of these qualities;
• Most with pigment near;
  • Light reading abilities due to cones and rods. Rodes
• Rods provide color. Canes
• Can detect colors.

Color Vision

• The color will depend due to light stimulation via cone and rod reaction
• Nocturnal vision has more rods.
• While those out mainly during the day have more cones <