Week 2: Structure of the Nervous System

[photo of Logan, credit www.borterwagner.com]

Week 2: Structure of the Nervous System, relates to pp. 36-73 in The Human Brain Book. I think the best 2 pages on overall structure are pp. 40-41, and the best neuron pictures and info are pp. 68-73.

The nervous system in all animals essentially serve the same basic functions - body control and registration of sensory input from the outside world. Yes, some body control is in response to input and some input leads to complex subjective experience so this is no simple task! But let's leave the "thinking" the brain associated with for later. How does the brain do this? The specialized function of neurons allows them to transmit signals between each other - the signals are chemicals called neurotranmitters, but the overall process is "electrochemical".

The hot-linked text below will take you to the relevant section of the GetBodySmart tutorial. These are optional, but I will use some for visual support and additional explanation.

The nervous system is made of literally TRILLIONS of TWO types of cells:
neurons - actively communicate by synaptic connections (electro-chemical process)

glial cells - support cells, protect and increase speed of communication (myelin sheath)110,000,000,000 neurons and about 50x that number of neuroglia. Also tend to be larger than neurons.
(50x110,000,000,000) + 110,000,000,000 = total cells in the NS.

5,610,000,000,000 or 5.6 trillion cells

Organ system hierarchy:

System/organ/tissue/cells/molecules/atoms

Example of the nervous system:

NS/brain, spinal cord?/nuclei (gray matter) and pathways (white matter) make up networks/neurons and glia/cytoplasmic parts and products of neurons, most unique and important: neurotransmitters...

[Cardiovascular/heart, vasculature? and blood?/smooth muscle and blood/muscle fibers and blood cells (white, red)]

Traditional 3 parts/DIVISONS of the Nervous System:

CNS = central nervous system (intensely protected by bone, spinal fluid, glia)
brain - the central processing unit
spinal cord - pathways going up and down, as well as reflexes ('u-turns', 2 neuron reflex)

PNS = peripheral n.s.; mostly enters or exits holes in the spinal cord or vertebrae
carries information from receptors in the body to the spinal and then UP to braincarries information coming DOWN spinal cord to effectors in the body (muscles, glands)specialized cranial nerves (12 total) enter or exit holes in the cranium, not the spinal cord

ANS = autonomic n.s.; functional system made up portions of both CNS & PNS, 'automated' (non-conscious) control of body:
Sympathetic branch: activation/arousal, flight or flight response, emotion
Parasympathetic branch: recuperation, relaxation, rejuvination

4th? "Somatic nervous system" - carries voluntary/conscious control and sensory information we perceive

Divisions of the Brain
No one system for this.... I enjoy the theoretical and integrative framework of the "triune brain" hypothesis by Paul MacLean. His "3 brains in 1", represents the three major evolutionary steps leading to the modern human brain structure and function; reptile brain, mammal brain, primate brain. JUST a model for understanding, not "real".

Major Divisions of the adult brain - this GetBodySmart tutorial goes top-down...

and the Human Brain Book (Carter) shuffles around.
Each division is made up of a number of specific neuroanatomical structures with specific functions.

Often, brain divisions are presented from the bottom-up... "lower" functions and structures first. Like triune brain. Good if you can learn to see from both perspectives as the NS provides both upward and downward messages. Any complex human behavior will involve sensory signals that go "up" and control signals that go "down".

brainstem - basic body functions (medulla oblonglata, pons, midbrain) - directly above spinal cord
cerebellum - motor coordination & motor learning, posture, means "little brain".

diencephalon - subcortical forebrain - emotion, motivation, awareness ("Limbic System" of thalamus, etc.)

telencephalon - 'forebrain' - cerebrum - cerebral cortex - outer layer, newest, conscious thought (cognition)

THE CORTEX will be described in greater detail as we address the functions of the four lobes in the coming weeks

Organizational Features of the Cerebrum (many will re-appear this quarter):
Right and left hemispheres of the cerebrum/telencephalon
Both symmetrical AND asymmetrical

Four lobes and their boundaries

White matter and gray matter

Primary or secondary cortex

Sensory mapping for senses, body mapping ("homunculus") for motor control

"Decussation", contra-lateral representation, or crossing-over of pathways from one side to the other