Nervous System: Anatomy,Physiology, and Homeostasis

The Nervous System

• The nervous system's learning outcomes include: Identifying anatomy and physiology, understanding its role in homeostasis, physiological functions, and relating physiology to neurological medical conditions

Purpose of the Nervous System

• A complex and highly organized network of billions of neurons
• Controls the body through voluntary (somatic) and involuntary (autonomic) functions
• Its complex tasks include sensory, integrative, and motor functions

Homeostasis

• The nervous system is the body's main control system for homeostasis
• It monitors, responds to and regulates all systems within the human body

Central Nervous System (CNS)

• Consists of the brain and spinal cord
• The brain is divided into the hindbrain, midbrain, and forebrain
• The spinal cord is a column of nerves between the brain and peripheral nervous system

Peripheral Nervous System (PNS)

• PNS links the body to the outside world
• The Autonomic NS regulates involuntary bodily processes automatically
• The Somatic NS carries sensory information to the CNS

Sympathetic vs. Parasympathetic NS

• The Sympathetic NS prepares the body for action and stress
• This is known as "fight or flight"
• The Parasympathetic NS calms the body, helping to conserve energy

Brain Anatomy

• The brain is separated into right and left cerebral hemispheres
• Gyri are ridges on the brain's surface that increase surface area and process information
• Sulci are the grooves between gyri which also increase surface area
• The Corpus Callosum is a nerve fiber band that enables communication between hemispheres
• Ventricles are interconnected cavities with cerebrospinal fluid which also removes waste

Cerebrum/Cortex

• Brain anatomy includes the frontal, occipital, temporal and parietal lobes

Primitive Brain

• The brain stem and cerebellum are parts of the primitive brain

Limbic Brain

• Brain anatomy includes the thalamus, hypothalamus, amygdala, and hippocampus

Cerebral Lobes

• The frontal lobe handles planning, movement, executive functions, speech, language, and emotion
• The temporal lobe handles auditory perception, memory processing, and language comprehension
• The parietal lobe handles processing sensory information, spatial orientation and body awareness
• The occipital lobe receives and interprets visual information

Function of Cerebellum

• The cerebellum controls voluntary movements for smooth, precise actions
• The brain is also important for balance, posture, motor learning, and eye movements

The Brainstem

• The brainstem consists of the midbrain, pons, and medulla
• It regulates coordination of reflexes, balance, sensory, motor pathways, sleep, and wakefulness
• The thalamus relays sensory information to relevant parts of the brain
• The hypothalamus links to the endocrine system to maintain homeostasis

Protection of the Brain

• Physical protection for the brain includes the scalp, skull as well as meninges
• Meninges consist of the dura, arachnoid layer, and pia layer
• Venous sinuses (Superior Sagittal Sinus) are between the 2 layers of the dura
• Tiny blood vessels are adhered to the brain by the pia

Spinal Cord Function

• Has 31 pairs of nerves
• Each part of the cord connects to a body region via axons and spinal nerves

Autonomic Nervous System (PNS) functions

• This part of the PNS regulates involuntary bodily processes
• This controls cardiovascular, gastrointestinal functions, as well as respiratory functions
• Has subdivisions in the sympathetic (fight or flight) and parasympathetic (calming) systems.

Somatic Nervous System

• Controls skeletal muscle movement and processes sensory information from organs

Reflex Arcs

• Stimuli, sensory receptors, and sensory neurons are the basic components
• Also includes an interneuron, motor neuron, and effector neuron/organ
• Reflexes do not always involve the brain

Summary of Neural Pathways

• Pathway Connects the Brain to the Spinal Cord

Nervous tissue

• Comprises two cell types: Neurons, which are the structural and functional units and Neuroglia which is the nerve glue

Neurons

• Structural and functional units capable of generating and conducting electrical changes and nerve impulses

Neuroglia

• The connective tissue, glial cells, and nutritional supporters that wrap/surround delicate neurons, but do not conduct impulses

Neuron Structure

• Typically possess dendrites, a cell body (soma), an axon, and presynaptic terminals

Sensory Neurons

• Pass stimuli (light, heat, pressure, pain) from inside and outside the body to the CNS

Motor Neurons

• Pass instructions from the CNS (muscles/glands)

Association or Interneuron Neurons

• Connect the sensory and motor neurons

Dendrites

• Play a role in integrating information and action potentials

Function of cell body

• Integrates synaptic information, transmitting it to other cells via the axon

Axons

• Carry electrical signals from the cell body to axon terminals

Key points about Axons

• Transmit signals through neurotransmitters, and speed dependent on myelin presence
• Myelinated axons are associated with white matter
• Enables the coordination of complex actions and responses

Myelination

• Schwann cells in the PNS and oligodendrocytes in the CNS form myelin sheaths
• Myelin is in segments with unmyelinated gaps (Nodes of Ranvier)
• This electrically insulates the axons, increases impulse conduction speed, and is called saltatory conduction

What are nerves

• An axon is a nerve fiber, and a nerve is a bundle of axons of different sizes
• A bundle of nerves is called a tract
• A nerve may have sensory fibers or motor fibers
• Cranial nerves exit the skull through holes (foramen)
• Spinal nerves leave the spinal canal through gaps between vertebrae

Function of Axon Terminals

• These divide into fine processes with branches interacting with the dendritic tree of neurons
• Tips of some axon terminals are Synaptic Clefts

Action Potential

• Message/information being sent down the axon of a neuron

Process of Action Potential

• Begins when a stimulus results in a sudden influx of sodium ions
• Resting potential= -70mV; An action potential= triggered if there is a change of 15 mV
• Sodium channels open, causing rapid depolarization where sodium flows into cells
• Sodium channels then close and potassium channels open; potassium flows out
• Potassium returns sodium to outside and potassium to inside via a sodium potassium pump

Synapse Function

• Axon terminals contain neurotransmitters for transmissions
• Synaptic end bulbs then transmit the signal across the synaptic cleft
• Neurotransmitters are either destroyed or reabsorbed

What are neurotransmitters

• Chemicals carry signals between cells at the synapse
• There are more than 50 types
• May excite or inhibit a target cell
• Acetylcholine excites skeletal muscle but serotonin inhibits and plays a role in moods and sleep
• Dopamine affects the brain to have an excitatory or inhibitory effect

Alzheimer's Disease

• A common cause of dementia as well as a physical disease affecting the brain
• Plaques and tangles develop, and brain cells die
• Leads to fewer brain chemicals
• Can also cause symptoms based on which part of the brain has had nerve loss