Chapter 10: The Nervous System
The nervous system, together with the endocrine glands, contributes to maintaining a stable internal environment (homeostasis) by controlling and coordinating the functions of the body’s other systems. For survival, the body must be able to sense changes in its environment and respond appropriately.
Specialized receptors collect information about these changes and transmit it to the central nervous system (CNS). After processing this information, the CNS sends commands to the muscles and glands, enabling the body to adapt its functions to internal and external variations.
The organs of the nervous system are divided into:
- The Central Nervous System (CNS), consisting of the brain and the spinal cord, which act as the primary centers of processing and control.
- The Peripheral Nervous System (PNS), consisting of the nerves that connect the CNS to the rest of the body, transmitting signals back and forth.
Structure and Function of Nerve Cells
The organs of the nervous system—namely the brain, spinal cord, and nerves—are composed of nervous tissue. This tissue contains two types of cells: neurons and neuroglial (glial) cells.
- Neurons are the structural and functional units of the nervous system. They have the unique ability to respond to specific changes in the environment, such as variations in temperature, pressure, light intensity, or pH.
- Neuroglial cells are far more numerous than neurons and serve supportive roles, providing nutrients, protection, and maintenance for neurons.
Neurons
Each neuron consists of a cell body and processes (extensions).
- The cell body contains the nucleus and organelles of the cell.
- The extensions are divided into:
- Dendrites – short, branched projections that receive signals.
- Axon (nerve fiber) – a single long projection that can reach up to one meter in length, ending in many branches with specialized tips called axon terminals.
Neurons vary in structure and function. They are classified into three main types:
- Sensory neurons – carry information from sensory receptors in the body to the spinal cord and brain.
- Motor neurons – transmit signals from the brain and spinal cord to effector organs such as muscles (causing contraction) and glands (causing secretion).
- Interneurons (association neurons) – located only in the brain and spinal cord, they connect sensory neurons to appropriate regions of the CNS or to motor neurons, allowing integration and coordination of responses.
💡 The human brain contains about 100 billion neurons, with their combined extensions reaching an estimated total length of 2 million kilometers. From around age 30, the number of neurons gradually decreases, and by age 75, brain weight may be reduced by up to 44% due to neuron loss.
Neuroglial Cells
The term “glia” comes from the Greek word for “glue.” These cells vary in shape and function, but all serve essential supportive roles:
- Supplying neurons with nutrients.
- Removing waste products.
- Providing insulation around axons to enhance the efficiency of electrical signal transmission.
Myelin
Myelin is a fatty substance that forms a sheath around many axons, creating what are known as myelinated fibers.
- Myelin greatly improves the speed and efficiency of nerve impulse conduction, particularly over long distances between the brain and extremities.
- Axons without myelin are called unmyelinated fibers.
- Myelin is organized into concentric layers formed by glial cells. It is interrupted at regular intervals by gaps called nodes of Ranvier.
This unique structure allows for saltatory conduction, in which the nerve impulse “jumps” from node to node. This process dramatically increases conduction speed, allowing impulses to travel at up to 130 meters per second.
A disorder called multiple sclerosis results from demyelination, where the protective myelin sheath is lost, severely impairing nerve function.
Resting Membrane Potential
A neuron at rest—that is, when it is not receiving a stimulus—shows a characteristic electrical charge difference across its membrane.
- On the outside of the membrane, there is a high concentration of sodium ions (Na⁺).
- On the inside, there is a high concentration of potassium ions (K⁺) and negatively charged ions (such as phosphates and sulfates).
This unequal distribution of ions is maintained by an active transport mechanism called the sodium–potassium pump (Na⁺/K⁺ pump), located in the cell membrane.
- For every three Na⁺ ions pumped out, the pump brings in two K⁺ ions.
- This continuous activity requires energy in the form of ATP (adenosine triphosphate).
- Because more positive ions leave than enter, the inside of the cell remains negatively charged compared to the outside.
Ion Gradients and Active Transport
The uneven distribution of ions is achieved and maintained through:
- Diffusion – ions moving down their concentration gradients.
- Electrical forces – movement due to differences in charge.
- Active transport – movement against concentration gradients, powered by ATP.
The Na⁺/K⁺ pump consumes about 25% of the cell’s total energy reserves, highlighting its importance in maintaining cellular balance (homeostasis).
Importance of Resting Potential
- The separation of charges across the membrane creates a voltage difference of about –70 millivolts (mV), known as the resting membrane potential.
- The negative value indicates that the inside of the neuron is electrically more negative compared to the outside.
- This potential is stable as long as the neuron does not receive a strong enough stimulus to trigger an action potential.
The constant activity of the sodium–potassium pump not only maintains the resting potential but also enables the cell to import essential nutrients such as glucose and amino acids via co-transport mechanisms, using the sodium gradient as a driving force.
Nerve Impulse
Changes in the environment act as stimuli that can alter the resting potential of a neuron.
- When a stimulus reaches a certain threshold level (which varies from neuron to neuron), the membrane’s permeability to sodium ions (Na⁺) suddenly increases for about 1 millisecond.
- Na⁺ rushes into the cell, making the inside of the membrane positively charged compared to the outside. This rapid reversal of polarity raises the membrane potential to about +50 mV.
- Immediately afterward, the membrane becomes more permeable to potassium ions (K⁺). K⁺ ions flow out of the cell, restoring the inside to a negative state, sometimes dropping slightly below the resting value of –70 mV.
- Finally, the Na⁺/K⁺ pump re-establishes the original ion distribution, returning the neuron to its resting potential (–70 mV).
This sequence of rapid electrical changes is called an action potential.
Propagation of the Impulse
- The action potential at one region of the membrane triggers the same change in the neighboring region.
- In this way, the action potential travels along the axon like a wave, forming what we call the nerve impulse.
- Stimuli that can trigger a nerve impulse include chemical, electrical, mechanical, and thermal signals.
- Stimuli below the threshold do not trigger an impulse.
Refractory Period
After an action potential, the neuron cannot immediately respond to another stimulus.
- This recovery period, lasting 0.5–2 milliseconds, is called the absolute refractory period.
- It ensures that each impulse is a distinct event and that the signal moves in one direction along the axon.
Synapses
Neurons communicate with other neurons or with effector organs (muscles or glands) through synapses. A synapse is the functional connection between the axon terminal of one neuron and the receptive surface of another neuron or target cell.
In most cases, transmission across synapses occurs chemically, via neurotransmitters. These are chemical substances stored in vesicles at the axon terminals and released when a nerve impulse arrives. The most common neurotransmitter in both the central and peripheral nervous systems is acetylcholine.
Structure of a Synapse
A synapse consists of three main parts:
- Presynaptic terminal – the axon endings of a neuron, containing mitochondria and vesicles filled with neurotransmitter.
- Synaptic cleft – a tiny gap (15–20 nm wide) separating the presynaptic and postsynaptic cells.
- Postsynaptic membrane – the receptive surface of the next neuron or effector organ, containing specific receptors for the neurotransmitter.
Transmission of a Nerve Impulse
When a nerve impulse reaches the presynaptic terminal:
- Neurotransmitters are released into the synaptic cleft.
- They diffuse across the cleft and bind to receptors on the postsynaptic membrane.
- If the neurotransmitter is excitatory, sodium channels open, Na⁺ rushes in, and a new action potential is triggered in the postsynaptic neuron.
Thus, the nerve signal is briefly converted from electrical → chemical → electrical again.
Neurotransmitters act only for a short time because they are either broken down by enzymes or reabsorbed into the presynaptic terminal for reuse.
Important: Synapses ensure the one-way direction of impulse transmission, always from the presynaptic neuron to the postsynaptic cell.
Clinical Note: Parkinson’s Disease
Parkinson’s disease is linked to reduced production of neurotransmitters in the brain, particularly dopamine, along with norepinephrine and serotonin. First described by James Parkinson in 1871, it usually appears in the 5th or 6th decade of life.
Typical symptoms include:
- rhythmic muscle tremors,
- difficulty initiating voluntary movements,
- and slowness in executing movements.
Treatment often involves administering L-DOPA (L-3,4-dihydroxyphenylalanine), a precursor of dopamine. Inside the brain, L-DOPA is converted into dopamine, partially restoring neurotransmitter balance and reducing symptoms.
Peripheral Nervous System (PNS)
Nerves
Nerves are bundles of long dendrites and/or axons held together by connective tissue. These nerve fibers are wrapped in neuroglial cells, giving them a white, shiny appearance. The cell bodies of the neurons that form these nerves are located either in the central nervous system (CNS: brain and spinal cord) or in ganglia, which are clusters of neuronal cell bodies outside the CNS.
Depending on their function, nerves are classified as:
- Sensory nerves – composed of sensory neuron fibers.
- Motor nerves – composed of motor neuron axons.
- Mixed nerves – containing both sensory and motor fibers.
Humans have 12 pairs of cranial nerves, which can be sensory, motor, or mixed. These arise from the brain and mainly innervate the head and neck regions.
From the spinal cord, 31 pairs of spinal nerves emerge. All spinal nerves are mixed, formed by sensory and motor fibers, and they innervate the neck, trunk, and limbs.
Nerve Pathways and Reflexes
A nerve pathway is the route followed by nerve impulses within the nervous system.
- Pathways carrying impulses from the CNS to effectors are called motor (efferent) pathways.
- Pathways carrying impulses from the periphery to the CNS are called sensory (afferent) pathways.
The simplest nerve pathway is the reflex arc, usually consisting of a sensory neuron, one or more interneurons, and motor neurons. Interneurons act as the processing center for the stimulus.
Components of a Reflex Arc (Table 1)
| Component | Function |
|---|---|
| Receptor | Sensitive to a specific type of environmental change; generates nerve impulses. |
| Sensory neuron | Transmits the impulse from the receptor to the spinal cord. |
| Interneuron | Processing center; relays impulses from sensory neurons to motor neurons and to the brain. |
| Motor neuron | Transmits the impulse from the spinal cord to the effector. |
| Effector organ | Responds to the stimulus: muscles contract or glands secrete substances. |
Reflexes
Reflexes are automatic, involuntary responses to changes inside or outside the body. They allow for rapid reactions in emergencies and maintain balance and posture. Reflexes also help regulate homeostasis, controlling functions such as heart rate, breathing rate, and blood pressure.
Some reflexes involve the brain (e.g., blinking), while others do not (e.g., withdrawal of the hand from a hot object).
The knee-jerk reflex is a classic example:
- A tap on the patellar tendon stimulates sensory receptors in the quadriceps muscle.
- The sensory neuron carries the impulse to the spinal cord, where it synapses directly with a motor neuron.
- The motor neuron sends the impulse back to the quadriceps, causing contraction and extension of the leg.
This reflex helps maintain upright posture and balance.
Central Nervous System (CNS)
The Central Nervous System coordinates all functions of the body. It consists of the brain and the spinal cord (Fig. 10a). These organs are protected inside the cranial cavity and the vertebral canal, respectively. In addition, both the brain and the spinal cord are surrounded by three protective membranes called the meninges.
Between the two inner meninges lies the subarachnoid space, which contains cerebrospinal fluid (CSF). This fluid cushions against shocks, supports the brain and spinal cord, and contributes to their nourishment. CSF also circulates in the central canal of the spinal cord and in the four ventricles of the brain. These ventricles are interconnected cavities where CSF is produced and then flows into the central canal of the spinal cord.
Spinal Cord
The spinal cord is a slender, almost cylindrical column of nervous tissue housed within the vertebral canal. It begins at the level of the foramen magnum and extends down to approximately the level of the second lumbar vertebra. From the spinal cord emerge 31 pairs of spinal nerves.
At the cervical and lumbar regions, the spinal cord shows enlargements (Fig. 10b). These correspond to the origins of nerves that innervate the upper and lower limbs, respectively (see Fig. 7).
The spinal cord contains reflex centers and serves as the main communication pathway between the brain and the spinal nerves. In cross-section, the central region consists of gray matter, which resembles a butterfly with open wings (Fig. 11). The gray matter is made up mainly of neuron cell bodies, while the surrounding white matter consists of long axons. These axons connect the brain, via the spinal nerves, to the rest of the body.
Brain
Overview
- The brain is the largest, most complex part of the nervous system. It is made of neurons that receive, process, and transmit information.
- Specialized centers handle sensation, perception, motor control/coordination, and higher mental functions; other centers/pathways regulate visceral (autonomic) activity.
- Protective responses the CNS uses to “guard” a threatened region: pain, muscle inhibition, and range-of-motion limitation (shortening).
Anatomical divisions
- Cerebral hemispheres, brainstem, cerebellum.
Meningitis (clinical)
- Inflammation of meninges due to bacteria/viruses in CSF. Bacterial forms (e.g., Neisseria meningitidis, Streptococcus pneumoniae, Haemophilus influenzae type B) are less common but more dangerous.
- Spread via droplets/kissing; more frequent in neonates/children. Early diagnosis + antibiotics (for bacterial) → good outcomes.
Cerebrospinal fluid (CSF)
- Produced by specialized cells in brain ventricles; absorbed by arachnoid granulations → pressure stays stable.
- Obstruction (infection/tumor/thrombus) ↑ CSF pressure → potential neural injury. In infants, can cause hydrocephalus (enlarged skull).
- Lumbar puncture (between L3–L4) measures pressure and samples CSF (e.g., blood cells indicate CNS bleed).
Cerebral hemispheres
- External gray matter (cortex) over internal white matter (fiber tracts).
- Surface has gyri (ridges) and sulci/fissures. Longitudinal fissure separates left/right; corpus callosum connects them.
- Cortex is the only CNS region responsible for conscious functions.
Functional areas of cortex
- Motor areas (frontal lobe, especially precentral gyrus): command voluntary skeletal movements; somatotopic organization.
- Somatosensory cortex (anterior parietal lobe/postcentral gyrus): touch, pressure, temperature, pain; somatotopic organization.
- Occipital lobe: vision.
- Temporal lobe: hearing; also olfaction areas.
- Association (integrative) areas (>50% of cortex): memory, reasoning, language, judgment, emotions.
- Broca’s area: speech production (usually left frontal).
Lobes & key functions (quick reference)
- Frontal: voluntary motor control; executive functions (planning, problem-solving), behavior regulation.
- Parietal: general sensation (touch/pressure/temp/pain), taste; language comprehension, expression of thoughts/emotions.
- Temporal: hearing & smell; interpretation of sensory experiences, auditory memory.
- Occipital: vision; visual association (linking vision to other senses).
Alzheimer’s disease (clinical)
- Affects ~5% >65y, ~20% >80y; progressive dementia (memory loss, reasoning/communication decline).
- Findings: cortical neuron abnormalities; amyloid plaques (β-amyloid from APP); some familial forms (chromosome 21 links; Down syndrome increases risk).
Brainstem
- Connects hemispheres to spinal cord. Major parts: thalamus, hypothalamus, medulla oblongata.
- Thalamus: relay to cortex for sensory inputs.
- Hypothalamus: homeostasis center; controls pituitary (neuro–endocrine link), autonomic nervous system, and sleep regulation.
- Medulla: vital autonomic centers (respiration, cardiac function, blood pressure). Damage is fatal.
Cerebellum
- Two hemispheres joined by vermis; superficial gray cortex over white matter.
- Functions: coordination of voluntary movement, maintenance of muscle tone and balance.
- Inputs from vision, vestibular system, and tendon/muscle receptors.
Crossed (decussated) representation
- Generally, right brain ↔ left body for sensation and voluntary motor control (and vice versa).
- Notable exception: cerebellar control is ipsilateral (each cerebellar hemisphere coordinates the same side of the body).
- Some muscle controls have special cases (e.g., sternocleidomastoid biomechanics).
Limbic system
- Interconnected regions (frontal/temporal areas, thalamus, hypothalamus, basal nuclei) mediating emotions (pleasure, pain, fear, anger, sadness) and survival-oriented behaviors.
Electroencephalogram (EEG)
- Surface electrodes record brain waves:
- α (alpha) 6–13 Hz, ~45 μV: relaxed wakefulness (eyes closed); disappear in sleep.
- β (beta) >13 Hz, lower amplitude: mental activity/attention.
- θ (theta) 4–7 Hz: common in children; in adults during early sleep/emotional stress.
- δ (delta) <4 Hz: deep sleep; REM (“paradoxical sleep”) shows bursts of activity, dreams, irregular HR/RR, rapid eye movements.
- Uses: epilepsy diagnosis, tumor suspicion, brain death confirmation (absence of activity).
Higher functions
Memory
- Short-term (minutes) can consolidate to long-term (structural/functional neural changes).
- Consolidation depends on stimulus salience (intense/repeated/very pleasant or unpleasant).
- Long-term stores are distributed (occipital/temporal for faces, words, images, sounds, etc.); recall integrates multiple cortical sites.
- Injury/disease can cause amnesia specific to affected regions (e.g., temporal lesions → auditory memory loss).
Learning
- Habituation: decreased response to inconsequential repeated stimulus.
- Sensitization: enhanced response after noxious/repeated stimulus.
- Associative learning: linking stimuli (e.g., lightning → thunder expectation).
- Perception/problem solving: using recalled experiences to resolve tasks.
Behavior
- Emerges from genetic factors and environment/learning.
- Innate (instinctive) components: reflexes, universal facial expressions.
- Learned components: modified via habituation/sensitization and higher learning to adapt behavior to context.
Autonomic Nervous System
What it is
- The ANS runs continuously and involuntarily, mainly via reflexes.
- Sensory (afferent) signals from skin & viscera → CNS centers (brainstem, spinal cord).
Motor (efferent) outflow → autonomic ganglia → effector organs (smooth & cardiac muscle, glands).
Processing within ganglia gives the ANS partial autonomy.
Divisions & overall roles
- Sympathetic (SNS): “fight/flight”—mobilizes in stress/emergencies.
- Parasympathetic (PSNS): “rest/digest”—dominant at rest; restores baseline after stress.
- When both innervate the same organ, actions are antagonistic (e.g., pupil: SNS dilates, PSNS constricts; heart rate: SNS ↑, PSNS ↓).
Central control (hierarchy)
- Medulla (brainstem): vital reflex centers (cardiac, respiratory, BP) receive visceral input and drive autonomic output.
- Hypothalamus: overall homeostasis—body temperature, hunger, thirst, water/salt balance—by steering ANS (and pituitary).
- Higher brain areas: shape emotional expression/behavior via ANS during affective states.
Functional effects — quick reference
| Target | Sympathetic (SNS) | Parasympathetic (PSNS) |
|---|---|---|
| Eye (pupil) | Dilates (mydriasis) | Constricts (miosis) |
| Heart | ↑ Rate & support for ↑ output | ↓ Rate; promotes rest state |
| Viscera/glands | Readies body for stress/emergency | Supports digestion, secretion, energy storage |
System summary (context)
- PNS: 12 cranial + 31 spinal nerve pairs; innervates skeletal muscles (somatic) and internal organs (autonomic).
- CNS: spinal cord (reflex centers; long tracts) + brain.
- Cerebral hemispheres: conscious functions (sensory interpretation, motor commands, higher cognition).
- Hypothalamus: homeostasis hub.
- Medulla: autonomic vital centers (respiration, cardiac, BP).
- Cerebellum: balance/coordination.
- Reflexes: automatic responses; some do not require cortical mediation (e.g., spinal reflexes).