Unit 15: Human Nervous System — Short Questions

The basic elements of nervous coordination are: (1) Receptors, which detect stimuli and generate nerve impulses; (2) Sensory neurons, which carry impulses to the CNS; (3) Central nervous system (made of inter-neurons), which processes the information; (4) Motor neurons, which carry impulses away from the CNS; and (5) Effectors (muscles and glands), which produce the response.

Receptors are organs, tissues or cells that detect stimuli (changes in the external or internal environment) and act as transducers, generating nerve impulses in sensory neurons; examples include the eyes, ears, and skin. Effectors are the parts that produce a response on receiving a nerve impulse from the CNS via motor neurons; muscles (which contract) and glands (which secrete) act as effectors.

A message (nerve impulse) travels along the path: receptors → sensory neurons → central nervous system (inter-neurons) → motor neurons → effectors.

A nerve impulse is a wave of electrochemical changes that travels along the length of a neuron.

(1) Large, negatively charged fixed anions (proteins and nucleic acids) are more abundant inside the cell and cannot diffuse out. (2) The Na⁺–K⁺ pump actively moves 2 K⁺ in for every 3 Na⁺ pumped out, using ATP, making the inside more negative. (3) Some K⁺ ions continuously leak out through K⁺ gates of the membrane.

The human nervous system consists of the central nervous system (CNS: brain and spinal cord) and the peripheral nervous system (PNS: cranial and spinal nerves). The PNS forms sensory and motor pathways; the motor pathway is further divided into the somatic (voluntary) nervous system and the autonomic (involuntary) nervous system, the latter having sympathetic and parasympathetic divisions.

A narcotic drug is a type of opioid drug used for pain relief that has a high potential for addiction and abuse. Examples include morphine, codeine, and oxycodone.

Addictive drugs make presynaptic neurons release extra dopamine (producing pleasure/reward) and block its reabsorption, so dopamine accumulates in the synaptic cleft. The brain responds by reducing the number of dopamine receptors on the postsynaptic neuron — this is tolerance, and it means the user needs higher doses to get the same pleasurable effect, deepening addiction. When the drug is stopped abruptly, dopamine release returns to normal, but with fewer receptors the postsynaptic neuron is under-stimulated, producing withdrawal symptoms.

Stroke Causes — high blood pressure, smoking, diabetes, high cholesterol, obesity, family history. Symptoms — paralysis on the side opposite the affected brain area, aphasia, sudden trouble seeing/walking, dizziness, loss of balance, severe headache. Treatment — blood thinners, clot-dissolving or antihypertensive medications, surgery to remove a clot or repair a ruptured vessel, and rehabilitation. Meningitis: Causes — bacterial (most severe), viral (usually less severe), fungal (rare, in people with weak immunity), or parasitic infection of the meninges. Symptoms — neck stiffness, headache, fever; severe cases may cause paralysis or coma. Treatment — immediate antibiotics for bacterial meningitis, antiviral medication (viral meningitis is usually self-limiting), antifungal medication for fungal meningitis; vaccines prevent some bacterial forms.

Causes the exact cause is unknown, but risk factors include exposure to radiation, family history, and certain genetic mutations. Symptoms depend on the tumour's location and may include headaches, severe nerve pain, paralysis, seizures, and coma. Treatment includes surgery, radiation therapy, and chemotherapy.

Causes a combination of genetic, environmental, and lifestyle factors. Symptoms typically begin with mild memory loss and confusion and progress to more severe cognitive impairment, including difficulty with language, problem-solving, and completing familiar tasks. Treatment: there is no cure, but medicines are available to slow its progression and improve memory and cognitive function.

An axon is a single, long, membrane-bounded extension of the cell body that transmits impulses away from the cell body to the axon terminal. A dendrite is a (often branched) membrane-bounded extension that receives information from other neurons/cells, converts it into nerve impulses, and carries them toward the cell body.

Sensory neurons conduct impulses from receptors to the CNS; they are unipolar, with cell bodies grouped in ganglia outside the CNS. Inter-neurons (associative neurons) lie entirely within the CNS, receiving impulses from sensory neurons/other inter-neurons and passing them to motor neurons/other inter-neurons. Motor neurons conduct impulses away from the CNS to effectors; they are multipolar, with many dendrites, and their cell bodies lie within the CNS.

Myelinated neurons have a myelin sheath (produced by Schwann cells in the PNS or oligodendrocytes in the CNS) around their axon, allowing fast saltatory conduction (up to 120 m/s). Non-myelinated neurons lack this covering and conduct impulses much more slowly (1–3 m/s).

White matter is composed of myelinated axons and links regions of the CNS with each other. Grey matter contains cell bodies and non-myelinated axons/dendrites. In the brain, grey matter forms the outer cerebral cortex while white matter lies beneath it; in the spinal cord the arrangement is reversed — grey matter is the inner H-shaped zone and white matter is the outer zone.

Cranial nerves are 12 pairs that arise from the brain; some are sensory, some motor, and others mixed. Spinal nerves are 31 pairs that arise from the spinal cord; all spinal nerves are mixed, carrying both sensory and motor information.

Resting membrane potential (RMP) is the potential of a polarized, non-conducting neuron, about –70 mV, with the inside negative relative to the outside. Active membrane potential (action potential) occurs when the neuron is active/conducting an impulse, where the membrane potential reverses and the inside becomes positive relative to the outside (around +50 mV).

Polarization is the resting state of the neuron membrane, with the inside negative and outside positive (RMP, about –70 mV). Depolarization is the reversal of this polarity when Na⁺ gates open and Na⁺ rushes in, driving the membrane potential from –70 mV toward 0 and then to about +50 mV.

Repolarization is the return of the membrane toward a negative inside after depolarization, as Na⁺ gates close and K⁺ gates open, letting K⁺ diffuse out. Hyperpolarization is a brief overshoot in which, due to delayed closing of K⁺ gates, the membrane potential becomes slightly more negative than the resting potential; during this refractory period the neuron cannot generate another action potential.

The presynaptic neuron is the one whose axon transmits action potentials toward the synapse and releases neurotransmitter. The postsynaptic cell/neuron is on the other side of the synapse; it receives the neurotransmitter and, if excited, generates its own action potential.

A synaptic knob is a swollen structure at the axon terminal of the presynaptic neuron. Synaptic vesicles are small sacs inside the synaptic knob that store and are packed with neurotransmitter chemicals, and which fuse with the presynaptic membrane to release them.

The somatic nervous system consists of motor neurons that control skeletal muscles and is voluntary (muscles can be moved at will). The autonomic nervous system consists of motor neurons that control smooth muscles and glands, is involuntary, and regulates internal body conditions; it is divided into sympathetic and parasympathetic divisions.

Drug addiction is a disorder of uncontrollable drug desire and use despite its negative effects. Drug tolerance is the body's adaptive, decreasing sensitivity to a drug with continued use, so that higher doses are needed to achieve the same effect — tolerance leads to, and deepens, addiction.

Everyday experiences reveal all the major receptor types the chapter describes: seeing uses photoreceptors (rods and cones of the eye, which transduce light energy into nerve impulses); tasting food uses the chemoreceptors of taste buds; smelling food uses the olfactory chemoreceptors of the nasal epithelium; feeling a light touch (e.g., a feather) or vibration uses Meissner's and Pacinian corpuscles; feeling sustained pressure or stretching of skin uses Ruffini endings; feeling hair move in a breeze uses hair follicle receptors; touching something hot/cold uses thermoreceptors; and an injury such as a cut or burn stimulates nociceptors (pain receptors). Together these everyday experiences show that the body carries photoreceptors, chemoreceptors, mechanoreceptors, thermoreceptors, and nociceptors.

Playing the piano or writing requires continuous integration of sensory and motor information. The frontal lobe's motor areas plan and send precise impulses via multipolar motor neurons to specific skeletal muscles of the hand and fingers (somatic nervous system, voluntary). The parietal lobe provides awareness of hand/finger position and helps plan and execute the motor movements, while touch receptors (e.g., Meissner's and Pacinian corpuscles, Merkel cells) in the fingertips continuously feed back pressure and position information through sensory neurons. The cerebellum receives this sensory information from muscles, tendons, joints, eyes, and ears, and works with the cerebral cortex and brainstem to finely coordinate the rapid, ongoing contractions needed for such intricate movements and to control posture. Because the motor and sensory neurons involved are largely myelinated, saltatory conduction transmits these impulses very fast, allowing the split-second corrections needed for fluid, precise finger movements.

Computers process information much as the nervous system does: just as a receptor transduces a stimulus into an electrochemical nerve impulse that is sent through sensory neurons to be processed by inter-neurons in the CNS and acted on by motor neurons/effectors, a computer takes an input signal, processes it through logic circuits, and produces an output. The all-or-none, on/off nature of an action potential (depolarization producing an impulse, or no impulse at all) parallels the binary (0/1) logic of digital computers. The brain's vast network of about 100 billion neurons, each forming synapses with thousands of others, inspired the architecture of artificial neural networks used in modern computing/AI, where many simple interconnected units together perform complex learning and decision-making, much like the brain's cerebral cortex and association areas carry out higher mental functions. Even memory storage in computers echoes the hippocampus's role in consolidating information into long-term memory.