Biology Chapter 8: Nervous System

Functions of the Nervous System

• Receives sensory input through sensory receptors
• Integrates stimuli through the brain and spinal cord
• Controls muscles and glands
• Facilitates mental activity in the brain
• Maintains homeostasis

Main Divisions of the Nervous System

• Central Nervous System (CNS): consists of the brain and spinal cord
• Peripheral Nervous System (PNS): consists of all neurons outside the CNS

Neuron Characteristics

• Receive input, process input, and produce a response
• Require oxygen and glucose
• Have three main parts: dendrite, cell body, and axon

Neuron Structures

• Dendrite: receives stimulus from other neurons or sensory receptors
• Cell body: processes stimulus and contains a nucleus
• Axon: transmits stimulus to a gland, muscle, organ, or other neuron

Myelin Sheath

• A fatty, protective wrapping around axons
• Excellent insulator
• Has gaps called nodes of Ranvier where action potentials develop
• Allows for saltatory conduction, where action potentials "jump" from node to node

Action Potential Conduction

• Action potentials are generated when the inside of the membrane becomes positive and the outside becomes negative
• Depolarization occurs when the inside of the membrane becomes positive, causing the generation of a new action potential
• Local currents are generated, causing the action potential to propagate along the axon

Types of Neurons

• Multipolar: has many dendrites and a single axon (e.g. CNS and most motor neurons)
• Bipolar: has one dendrite and one axon (e.g. eye and nasal cavity)
• Pseudo-unipolar: has one axon and no dendrites (e.g. sensory neurons)

Neuroglia Characteristics

• Supporting cells for neurons
• More numerous than neurons
• Can divide to produce more cells
• There are 5 types: astrocytes, ependymal cells, microglia, oligodendrocytes, and Schwann cells

Types of Neuroglia

• Astrocytes: star-shaped, most abundant, and form the blood-brain barrier
• Ependymal cells: produce and circulate cerebrospinal fluid (CSF)
• Microglia: help remove bacteria and cell debris from the CNS
• Oligodendrocytes: produce myelin sheath in CNS
• Schwann cells: produce myelin sheath in PNS

Organization of Nervous Tissue

• Gray matter: collection of dendrites and cell bodies
• White matter: collection of axons and their myelin sheath
• CNS: oligodendrocytes produce myelin sheath, while PNS: Schwann cells produce myelin sheath

Electrical Signals and Neural Pathways

• Resting membrane potential: outside of cell is more positive (Na+), inside of cell is more negative (K+)
• Leak ion channels: always open, K+ channels
• Gated ion channels: closed until opened by specific signal, Na+ channels

Action Potentials

• "Electricity" that causes depolarization and repolarization
• Change resting membrane potential by activating gated ion channels
• Local current: movement of Na+ which causes inside of cell to be more positive (depolarize)

Synapse

• Where an axon attaches to a muscle, gland, organ, or other neuron
• Involved with release of neurotransmitters
• Example: neuromuscular junction

Reflexes

• Involuntary response to a stimulus
• Reflex arc: path reflex travels
• Components of reflex arc:
  • 1. Sensory receptors: pick up stimulus
  • 2. Sensory (afferent) neurons: send stimulus to interneurons in spinal cord
  • 3. Interneurons (Association) neuron: process stimulus
  • 4. Efferent (motor) neurons: send response to effector
  • 5. Effector: muscle, gland, or organ

Neuronal Pathways

• Converging: two or more neurons synapse with the same neuron
• Diverging: axon from one neuron divides and synapses with more than one neuron

Central Nervous System

• Consists of brain and spinal cord

• Spinal cord: extends from foramen magnum to 2nd lumbar vertebra, protected by vertebral column, and allows for movement

• Gray and white matter in spinal cord: gray matter is in the center, looks like letter H or a butterfly, and white matter is outside, contains myelinated fibers### Gray Matter in Spinal Cord

• Posterior horns contain axons that synapse with interneurons

• Anterior horns contain somatic neurons

• Lateral horns contain autonomic neurons

• Central canal is a fluid-filled space in the center of the cord

Spinal Nerves

• Arise along the spinal cord from the union of dorsal and ventral roots
• Contain axons of sensory and somatic neurons
• Located between vertebrae
• Categorized by region of the vertebral column from which they emerge (e.g., C for cervical)
• There are 31 pairs of spinal nerves
• Organized into 3 plexuses: cervical, brachial, and lumbosacral

Plexuses

• Cervical plexus: spinal nerves C1-4, innervates muscles attached to the hyoid bone and neck, and contains the phrenic nerve which innervates the diaphragm
• Brachial plexus: originates from spinal nerves C5-T1, supplies nerves to the upper limb, shoulder, and hand
• Lumbosacral plexus: originates from spinal nerves L1-S4, supplies nerves to the lower limbs

Cerebrospinal Fluid

• Fluid that bathes the brain and spinal cord
• Provides a protective cushion around the CNS
• Produced in the choroid plexus of the brain

Brainstem

• Components: medulla oblongata, pons, and midbrain
• Medulla oblongata: regulates heart rate, blood vessel diameter, breathing, swallowing, vomiting, hiccupping, coughing, sneezing, and balance
• Pons: regulates breathing, chewing, salivation, and swallowing
• Midbrain: coordinates eye movement, pupil diameter, and turning head toward noise

Reticular Formation

• Location: scattered throughout the brainstem
• Function: regulates cyclical motor function, respiration, walking, chewing, arousing and maintaining consciousness, and regulates sleep-wake cycle

Diencephalon

• Located between the brainstem and cerebrum
• Components: thalamus, hypothalamus, and epithalamus
• Thalamus: influences moods and detects pain
• Epithalamus: involved in emotional and visceral response to odors
• Hypothalamus: controls body temperature, thirst, hunger, fear, rage, and sexual emotions

Cerebrum

• Characteristics: largest portion of the brain, divided into right and left hemispheres, and has four lobes: frontal, parietal, occipital, and temporal
• Cerebral cortex: surface of the cerebrum, composed of gray matter, controls thinking, communicating, remembering, understanding, and initiates involuntary movements
• Left hemisphere: controls right side of the body, responsible for math, analytic, and speech
• Right hemisphere: controls left side of the body, responsible for music, art, and abstract ideas
• Corpus callosum: connects the two hemispheres

Lobes of the Brain

• Frontal lobe: controls voluntary motor functions, aggression, moods, and smell
• Parietal lobe: evaluates sensory input such as touch, pain, pressure, temperature, and taste
• Occipital lobe: vision
• Temporal lobe: hearing, smell, and memory

Cerebellum

• Location: below the cerebrum
• Characteristics: means little brain, cortex is composed of gyri, sulci, and gray matter
• Functions: controls balance, muscle tone, and coordination of fine motor movement

Sensory Functions

• CNS constantly receives sensory input
• We are unaware of most sensory input
• Sensory input is vital for our survival and normal functions

Ascending Tracts

• Pathways in the brain and spinal cord
• Transmit information via action potentials from the periphery to the brain
• Each tract has a limited type of sensory input
• Tracts are named according to their origin and termination

Sensory Areas of the Cerebral Cortex

• Primary sensory areas: where ascending tracts project, where sensations are perceived
• Primary somatic sensory cortex: general sensory area, in the parietal lobe, receives sensory input such as pain, pressure, and temperature

Motor Areas of the Cerebral Cortex

• Primary motor cortex: frontal lobe, controls voluntary motor movement
• Premotor area: frontal lobe, where motor functions are organized before initiation
• Prefrontal area: motivation and foresight to plan and initiate movement

Descending Tracts

• Project from upper motor neurons in the cerebral cortex to lower motor neurons in the spinal cord and brainstem
• Control different types of movements

Basal Nuclei

• Group of functionally related nuclei
• Plan, organize, and coordinate motor movements and posture
• Corpus striatum: deep in the cerebrum
• Substantia nigra: in the midbrain

Speech

• Mainly in the left hemisphere
• Sensory speech (Wernicke's area): parietal lobe, where words are heard and comprehended
• Motor speech (Broca's area): frontal lobe, where words are formulated

Brain Waves and Consciousness

• Used to diagnose and determine treatment of brain disorders
• Electroencephalogram (EEG): electrodes placed on the scalp to record brain electrical activity
• Alpha waves: person is awake in a quiet state
• Beta waves: intense mental activity
• Delta waves: deep sleep
• Theta waves: in children

Memory

• Encoding: brief retention of sensory input received by the brain, also called sensory memory, lasts less than a second

• Consolidated: data that has been encoded, temporal lobe, short-term memory

• Storage: long-term memory, can last for a few minutes or permanently

• Retrieval: how often info is used

• Types of memory:

  • Short-term memory: info is retained for a few seconds or minutes, limited to 7 bits of info
  • Long-term memory: can last for a few minutes or permanently
  • Episodic memory: places or events
  • Learning: utilizing past memories### Protective Wrapping Around Brain and Spinal Cord

• Meningitis is an infection of the meninges, which can be caused by bacteria or viruses.

• The three types of meninges are:

  • Dura mater: the superficial and thickest layer
  • Arachnoid mater: the 2nd layer
  • Pia mater: the 3rd layer that covers the surface of the brain

Subarachnoid Space and Epidural Space

• The subarachnoid space is where cerebrospinal fluid sits.
• The epidural space is located in the vertebral column between the dura and vertebra, and is an injection site for epidural anesthesia.

Spinal Block and Spinal Tap

• A spinal block and spinal tap involve injecting anesthetic into the subarachnoid space or removing cerebrospinal fluid, which numbs the spinal nerves.

Ventricles

• Ventricles are cavities in the CNS that contain fluid.
• The fourth ventricle is located at the base of the cerebellum and is continuous with the central canal of the spinal cord.
• The lateral ventricle has three horns: anterior, posterior, and inferior.

Cranial Nerves

• There are 12 pairs of cranial nerves, named by Roman numerals.
• The two categories of cranial nerve functions are sensory and motor.

Peripheral Nervous System

• The peripheral nervous system consists of all neurons outside the brain and spinal cord.
• It collects input from different sources, relays input to the CNS, and performs action.

Divisions of Peripheral Nervous System

• The two main divisions of the peripheral nervous system are:
  • Afferent (Sensory): collects input from the periphery and sends it to the CNS
  • Efferent (Motor): carries processed input from the CNS to effector organs

Divisions of Efferent (Motor)

• The two divisions of the efferent system are:
  • Autonomic: response is automatic (involuntary), controls smooth and cardiac muscles and glands
  • Somatic: response is voluntary, controls skeletal muscles

Autonomic Nervous System

• The autonomic nervous system has two divisions:
  • Sympathetic: activated during times of stress, part of the fight or flight response
  • Parasympathetic: activated under normal conditions, involved in digestion, urine production, and other functions