Topic 16- Nervous System Organization

Outcomes:

• compare and contrast animal nervous systems

• compare and contrast the types of glial cells

• compare and contrast the CNS and PNS

• sequence nervous system signaling pathways

• sequence brain development

• compare and contrast brain structures

I. Nervous System Overview

• Enables organisms to sense and react to the environment

• Increases survival and reproduction in changing conditions

• Higher organism complexity → more complex nervous systems

A. Evolutionary Examples

1. Cnidarians

   • Simplest animals with a nervous system

   • No central control → nerve net

2. Sea Stars (Echinoderms)

   • Central control present

   • Nerve ring signals muscles

3. Bilateria

   • Wide variety in nervous system structure

   • Platyhelminthes: simple CNS, brain, eyespots, ladder-type structure

   • Annelids/Arthropods: more complex brains and ganglia

4. Vertebrates

   • CNS: Brain and spinal cord

   • PNS: Nerves and ganglia

5. Nervous System vs. Lifestyle

   • Chiton: slow, simple nervous system

   • Squid: fast, complex nervous system

II. Glial Cells

A. Adult Glial Cells (Support CNS)

1. Microglia

   • Scavenge dead cells

   • Protect brain from microorganisms

2. Astrocytes

   • Assist blood and nutrient flow to neurons

   • Form the blood-brain barrier

3. Ependymal Cells

   • Produce, circulate, and support cerebrospinal fluid (CSF)

4. Myelin-producing Glia

   • Oligodendrocytes (CNS): myelinate multiple axons

   • Schwann cells (PNS): myelinate one axon each

     • Myelin: Insulates axons, speeds up signal conduction

B. Radial Glia

• Form tracks in the developing embryo for neuron migration

• May aid in synaptic plasticity in adults

• Develop into other glial cell types

III. Central Nervous System (CNS)

A. Brain

• Central control organ

Structure:

• Ventricles: fluid-filled cavities

• Grey matter (cell bodies) surrounds

• White matter (myelinated axons)

• Filled with cerebrospinal fluid (CSF)

B. Spinal Cord

Structure:

• Central canal with CSF

• Grey matter surrounds canal

• White matter surrounds grey

Functions:

1. Relay signals to/from brain

2. Interneurons connect to PNS

3. Reflexes:

   • Rapid, involuntary

   • Example: knee-jerk to prevent falling

IV. Peripheral Nervous System (PNS)

A. Sensory Receptors

• Detect visual, auditory, and other stimuli

• Transmit info to/from CNS

B. Nerves

1. Cranial Nerves

   • Originate in hindbrain

   • Serve head and upper body

2. Spinal Nerves

   • Originate in spinal cord

   • Serve entire body

C. Organization

1. Afferent Pathway (Sensory) – Arrives at CNS

• From receptors, not effectors

• Two types of input:

  • Visceral: unconscious (e.g., blood pressure)

  • Somatic: conscious (e.g., sight)

• Signal travels to interneurons for processing

2. Efferent Pathway (Motor) – Exits from CNS

• Sends signals to responders (effectors)

a) Motor System

• Targets: Skeletal muscle

• Includes voluntary and reflex actions

• No sensory receptors involved

b) Autonomic Nervous System

• Targets: Glands, heart, smooth muscle

• Controls involuntary functions:

  • Digestive

  • Cardiovascular

  • Excretory

  • Endocrine

• Divided into:

  • Enteric: digestive system

  • Parasympathetic: "rest and digest" : parachute

  • Sympathetic: "fight or flight"

V. Vertebrate Brain

A. Brain Development

• All vertebrates start with a neural tube

  • Posterior → spinal cord

  • Anterior → brain regions:

    • Forebrain

    • Midbrain

    • Hindbrain

• Relative sizes vary by species

• Size reflects functional importance

B. Brain Structure & Function

1. Grey Matter: Neuron cell bodies and dendrites

2. White Matter: Myelinated axons

3. Left & Right Hemispheres: Opposite perception and control

4. Corpus Callosum: Connects hemispheres (thick axon bundle)

C. Major Brain Regions

1. Cerebrum

• Two hemispheres + cortex

• Cerebral cortex (outer grey layer):

  • Voluntary movement

  • Learning

  • Emotion

  • Memory

  • Sensory processing

• Derived from the forebrain

2. Cerebellum

• Coordinates movement and balance

• Assists with learned motor skills

3. Diencephalon

• Thalamus: Routes sensory input to correct cortex area

• Hypothalamus: Regulates pituitary, hunger, thirst

4. Brainstem

• Midbrain: Routes sensory input

• Pons: Regulates respiration and sleep

• Medulla: Controls vital functions:

  • Breathing

  • Heartbeat

  • Blood pressure

  • Swallowing, coughing, vomiting