Biology 30 - Nervous System Overview

Biology 30 - Nervous System

• The nervous system's main function is maintaining homeostasis (internal environment balance), including pH, body temperature, glucose levels, blood pressure, and water levels.
• The nervous system has four general functions: reception, conduction, interpretation and organization, and transmission.
• The nervous system is organized into two main divisions: central nervous system (CNS) and peripheral nervous system (PNS).
• The CNS includes the brain and spinal cord.
• The PNS contains cranial nerves (12 pairs) and spinal nerves (31 pairs), controlling the face, neck, shoulders, and emerging from the spinal cord.
• Dorsal root nerves carry sensory information to the CNS, and ventral root nerves carry motor information away from the CNS.
• The Neuron is the functional unit of the nervous system.
• Types of neurons:
  • Sensory (afferent): carry impulses from body receptors to the CNS
  • Motor (efferent): carry impulses away from the CNS to muscles and glands
  • Interneurons: reside entirely within the CNS (brain and spinal cord) connecting sensory and motor neurons.
• Neuroglia (glial cells): support interneurons, nourish neurons, remove waste, and protect.
• Neuron structure:
  • Cell body: contains nucleus and cytoplasm
  • Dendrites: finger-like projections receiving information
  • Axon: extension of cytoplasm transmitting impulses away from the cell body
• Some neurons also contain:
  • Myelin sheath: insulating fatty layer forming gaps called Nodes of Ranvier, speeding impulse conduction and regeneration. Schwann cells produce myelin.
• All nerves of the PNS are myelinated, but only some CNS nerves are. Myelinated nerves in the CNS are called white matter, while non-myelinated nerves are called grey matter.
• Reflex arc: automatic, quick, involuntary responses to internal or external stimuli, not immediately involving the brain. It allows for quicker reaction to harmful stimuli.
• Five components of a reflex arc:
  • Sensory receptor: senses something
  • Sensory neuron: sends signal to the CNS
  • Interneuron: registers the signal
  • Motor neuron: sends the response
  • Effector: carries out the action
• Action potential: nerve impulses are electrochemical. A sudden change in polarity (relative electrical potential) propagates down a nerve cell.
• Steps of an action potential:
  • Resting potential: (-70mV) - polarized state, sodium potassium pump maintains a negative charge inside.
  • Stimulation/depolarization: ( +40mV) - temporary reversal of charge. Threshold stimulus causes sodium (Na+) channels to open, Na+ rushes into the inside.
  • Repolarization: Sodium channels close, potassium (K+) channels open, K+ rushes out creating a negative charge in the inside again.
  • Refractory period: Resting potential restored (-70mV) to allow the neuron to fire again, involving pumping of ions back to their original concentration.
• The speed of nerve impulse (action potential) propagation can be increased by saltatory action (jumping from node to node). Threshold level is minimum depolarization (usually around +30mV) needed before enough Na+ gates open. All or none response means that if threshold isn't met, no action potential occurs.
• Intensity is determined by:
  • Number of neurons firing simultaneously.
  • Frequency of neuron firing.
  • Threshold level of neurons (lower threshold neurons fire more easily and are in sensitive areas).
• Synapse: The gap between neurons, electrical impulses cannot directly cross this gap so neurotransmitters are needed.
• Neurotransmitters: chemical messengers stored in synaptic vesicles of the axon. They are released and diffuse across the gap, binding to specialized receptors on the postsynaptic dendrite, continuing the wave of depolarization to the next neuron.
• Neurotransmitters are categorized as excitatory(cause depolarization by opening Na+ channels) or inhibitory (block Na+ channels and open K+ channels, causing hyperpolarization, thereby inhibiting action potentials)
• Summation: multiple neurons release NTs at the same time; the combined effect (summation) can result in a response—ONLY if there is adequate excitation to reach threshold.
• Multiple sclerosis: caused by destruction of the myelin sheath surrounding nerve axons leading to various symptoms.
• Diseases of the Nervous System:
  • Parkinson's: Dopamine deficiency caused by degeneration of dopamine-producing cells in the brain; wide-eyed, unblinking expression, involuntary tremor, muscle rigidity, shuffling gait. Caffeine may offer protection.
  • Alzheimer's: loss of memory, senility, deterioration of cells in the basal nuclei, presence of tangles and plaques, possibly due to acetylcholine malfunction, and linked to a gene on chromosome 21.
  • Schizophrenia: delusions, random thoughts, disjointed thoughts, sensory hallucinations, may be excessive activity of brain neurotransmitters such as dopamine.
  • Huntington's disease: a progressive deterioration of the nervous system that leads to writhing movements, insanity, and death; seems to be caused by malfunction of the inhibitory neurotransmitter GABA.
  • Depression: low affect, feeling blue, excessive sleep and eating patterns, seems to be linked to malfunctions in dopamine and serotonin, perhaps caused by excess of monoamine oxidase enzymes.
  • Stroke: interruption of blood flow to the brain leading to brain cell death.
  • Epilepsy: seizure disorder with unexplained electrical surges in the brain.
  • Chemical warfare (toxins): Nerve gas inhibits acetylcholinesterase, strychnine causes convulsions and spastic paralysis, latrotoxin.
• Brain structures:
  • Hindbrain: cerebellum (coordination, posture, coordinated muscle movement and balance), medulla oblongata (controls heartbeat, respiration, blood pressure, reflex centers for vomiting, sneezing, hiccupping, coughing and swallowing), pons (connects cerebrum with other parts of the brain, regulates breathing rate).
  • Midbrain: reflex center for head movements in response to visual stimuli; connects cerebrum.
  • Forebrain: thalamus (central relay station, directs sensory information), hypothalamus (produces hormones, controls thirst, hunger, pituitary hormones, aids in sleep, sexual arousal, emotions).
  • Cerebrum: largest part of the brain; responsible for intellect, memory, consciousness; has frontal lobe (voluntary movements, higher-level thinking), temporal lobe (hearing, short-term memory), parietal lobe (touch, temperature, pressure), occipital lobe (vision), olfactory lobe (smell), limbic system (emotions) and corpus callosum (connects two hemispheres).
• Pituitary Gland: master gland attached to hypothalamus.
• Imaging techniques: PET (positron emission tomography), SPECT (single photon emission computed tomography), MRI (magnetic resonance imaging), EEG (electroencephalography), CT (computed tomography).
• Peripheral Nervous System: includes cranial and spinal nerves, somatic NS (voluntary movements), and autonomic NS (involuntary functions).
• Somatic Nervous System controls voluntary muscles and sensory organs.
• Autonomic Nervous System has two parts: Sympathetic (fight or flight response) and parasympathetic (rest and digest).
• Spinal Cord Injuries: Damage to the spinal cord can result in different levels of paralysis, depending on the level of injury (C4-quadriplegia, L1-paraplegia).