Neural Control and Coordination

Given: The human brain is the central organ of the neural system enclosed within the skull.

Structure of the Human Brain:

The human brain is protected by the bony cranium (skull) and is covered by three meningeal layers: dura mater (outermost), arachnoid (middle), and pia mater (innermost). The brain is divided into three major regions:

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I. Forebrain:
It consists of:
- Cerebrum: The largest part of the brain. It is longitudinally divided into two cerebral hemispheres connected by the corpus callosum. The outer layer is the cerebral cortex (grey matter), which is highly folded into gyri (ridges) and sulci (grooves), increasing surface area. The inner region contains white matter. The cerebrum is divided into four lobes: frontal, parietal, temporal, and occipital. It controls voluntary movements, memory, intelligence, and sensory perception.
- Thalamus: Acts as a relay centre for sensory and motor signals to and from the cerebral cortex.
- Hypothalamus: Controls body temperature, hunger, thirst, sleep, and regulates the pituitary gland. It forms the floor of the diencephalon.
- Limbic System: Formed by inner parts of cerebral hemispheres and associated deep structures. It is concerned with olfaction, autonomic responses, sexual behaviour, emotional reactions, and motivation.

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II. Midbrain:
- Located between the forebrain and hindbrain.
- The dorsal portion has four rounded lobes called corpora quadrigemina (two superior and two inferior colliculi).
- It receives and integrates visual, tactile, and auditory inputs.
- The midbrain and hindbrain together form the brain stem.

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III. Hindbrain:
It consists of:
- Pons: Contains fibre tracts that interconnect different regions of the brain. It also helps in regulating respiration.
- Cerebellum: Has a highly convoluted surface. It integrates information from the semicircular canals of the ear and the auditory system. It coordinates voluntary movements, maintains posture and balance.
- Medulla Oblongata: Connects the brain to the spinal cord. It contains vital centres that control respiration, cardiovascular reflexes, and gastric secretions.

Conclusion: The brain is a highly complex organ that integrates and coordinates all body functions through its three major divisions.

(a) Central Neural System (CNS) vs. Peripheral Neural System (PNS):

| Feature | Central Neural System (CNS) | Peripheral Neural System (PNS) |
|---|---|---|
| Components | Brain and Spinal cord | Cranial nerves (12 pairs) and Spinal nerves (31 pairs) |
| Location | Located within the skull and vertebral column | Located outside the CNS, throughout the body |
| Protection | Protected by skull, vertebral column, and meninges | Not enclosed in bony structures |
| Function | Integrates and processes all sensory information; generates responses | Transmits impulses to and from the CNS |
| Divisions | Brain (forebrain, midbrain, hindbrain) and spinal cord | Somatic neural system and Autonomic neural system |
| Role | Acts as the main control centre | Acts as a communication network between CNS and body organs |

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(b) Resting Potential vs. Action Potential:

| Feature | Resting Potential | Action Potential |
|---|---|---|
| Definition | The electrical potential difference across the membrane of a neuron when it is NOT conducting an impulse | The electrical potential difference across the membrane when the neuron IS conducting an impulse |
| Value | Approximately $-70$ mV (inside negative relative to outside) | Rises to approximately $+30$ mV (inside becomes positive) |
| Ion distribution | $\text{Na}^+$ ions are more outside; $\text{K}^+$ ions are more inside | $\text{Na}^+$ ions rush inside (depolarisation), then $\text{K}^+$ ions rush outside (repolarisation) |
| Membrane state | Polarised — outer surface positive, inner surface negative | Depolarised — outer surface becomes negative, inner surface becomes positive |
| Stimulus | No stimulus; neuron at rest | Triggered by a threshold stimulus |
| Na$^+$/K$^+$ pump | Active — maintains ion gradient | Temporarily inactive during depolarisation |
| Nature | Stable, maintained state | Transient, propagated wave |

(a) Polarisation of the Membrane of a Nerve Fibre:

Concept: The resting nerve membrane is said to be in a polarised state.

Explanation:
- In a resting (non-conducting) neuron, the neural membrane is selectively permeable to $\text{K}^+$ ions and nearly impermeable to $\text{Na}^+$ ions.
- The $\text{Na}^+$/$\text{K}^+$ pump actively transports 3 Na$^+$ ions out and 2 K$^+$ ions in per cycle, maintaining a concentration gradient.
- As a result:
- $\text{Na}^+$ concentration is high outside the membrane.
- $\text{K}^+$ concentration is high inside the membrane.
- Large negatively charged proteins are trapped inside.
- This creates a charge difference: the outer surface is positively charged and the inner surface is negatively charged.
- The resting membrane potential is approximately $-70$ mV (inside relative to outside).
- This state of charge separation across the membrane is called polarisation.

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(b) Depolarisation of the Membrane of a Nerve Fibre:

Concept: When a threshold stimulus is applied, the polarised membrane undergoes depolarisation.

Explanation:
- When a stimulus of sufficient intensity (threshold stimulus) is applied to a polarised nerve fibre, the membrane permeability to $\text{Na}^+$ ions increases drastically.
- Voltage-gated $\text{Na}^+$ channels open and $\text{Na}^+$ ions rapidly rush inside the membrane (down the concentration gradient).
- This causes the inner surface to become positively charged and the outer surface to become negatively charged at the stimulated point.
- The membrane potential reverses from $-70$ mV to approximately $+30$ mV.
- This reversal of polarity is called depolarisation.
- After depolarisation, $\text{Na}^+$ channels close and $\text{K}^+$ channels open; $\text{K}^+$ ions rush out, restoring the original polarity — this is called repolarisation.
- The wave of depolarisation followed by repolarisation travels along the axon as a nerve impulse (action potential).

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(c) Transmission of a Nerve Impulse Across a Chemical Synapse:

Concept: At a chemical synapse, neurotransmitters act as chemical messengers to transmit impulses from one neuron to the next.

Steps:
1. Arrival of impulse: The nerve impulse (action potential) travels along the axon and reaches the axon terminal (pre-synaptic terminal).
2. Depolarisation of pre-synaptic membrane: The action potential causes depolarisation of the pre-synaptic membrane.
3. Influx of Ca$^{2+}$ ions: Depolarisation opens voltage-gated $\text{Ca}^{2+}$ channels; $\text{Ca}^{2+}$ ions enter the pre-synaptic terminal.
4. Fusion of synaptic vesicles: The influx of $\text{Ca}^{2+}$ causes synaptic vesicles (containing neurotransmitters) to fuse with the pre-synaptic membrane.
5. Release of neurotransmitters: Neurotransmitters (e.g., acetylcholine) are released by exocytosis into the synaptic cleft.
6. Binding to receptors: Neurotransmitter molecules diffuse across the synaptic cleft and bind to specific receptor proteins on the post-synaptic membrane.
7. Generation of new impulse: Binding of neurotransmitters opens ion channels in the post-synaptic membrane, causing depolarisation and generation of a new action potential in the post-synaptic neuron.
8. Inactivation: The neurotransmitter is subsequently inactivated by specific enzymes (e.g., acetylcholinesterase breaks down acetylcholine) to stop continuous stimulation.

Conclusion: Thus, the nerve impulse is transmitted from one neuron to the next via chemical neurotransmitters across the synaptic cleft.

(a) Labelled Diagram of a Neuron:

*Note: Draw the diagram as described below in the exam.*

Description of Neuron diagram:
- Draw a large, roughly star-shaped cell body (soma/cyton) in the centre.
- Show the nucleus (large, round) inside the cell body.
- Show Nissl's granules (dark dots) in the cytoplasm.
- Draw several short, branched projections from the cell body — label them Dendrites.
- Draw one long projection from the cell body — label it Axon.
- At the base of the axon, show the Axon hillock.
- Show the axon covered by a sheath — label it Myelin sheath (Schwann cells).
- Show gaps between myelin segments — label them Nodes of Ranvier.
- Show the outer covering of the myelin sheath — label it Neurilemma.
- At the end of the axon, show branched endings — label them Axon terminals / Synaptic knobs.
- Label the entire structure: Myelinated Neuron.

Key labels: Cell body (Cyton), Nucleus, Nissl's granules, Dendrites, Axon hillock, Axon, Myelin sheath, Neurilemma, Node of Ranvier, Axon terminal (Synaptic knob).

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(b) Labelled Diagram of the Human Brain:

*Note: Draw a sagittal (longitudinal) section of the human brain as described below.*

Description of Brain diagram:
- Draw the large, dome-shaped Cerebrum at the top, showing gyri and sulci (folds and grooves).
- Show the Corpus callosum connecting the two hemispheres (as a thick band).
- Below and behind the cerebrum, draw the Cerebellum with its characteristic folded appearance.
- In the centre, show the Thalamus and below it the Hypothalamus.
- Show the Midbrain connecting forebrain and hindbrain, with Corpora quadrigemina on the dorsal side.
- Show Pons as a bulge on the ventral side of the brainstem.
- Show Medulla oblongata at the base, continuous with the Spinal cord.
- Label the Pituitary gland hanging from the hypothalamus.

Key labels: Cerebrum, Corpus callosum, Thalamus, Hypothalamus, Pituitary gland, Midbrain, Corpora quadrigemina, Pons, Cerebellum, Medulla oblongata, Spinal cord.

(a) Neural Coordination:

- Neural coordination is the process by which the neural system (nervous system) coordinates and integrates the functions of all organs and organ systems of the body.
- It involves the rapid transmission of electrical signals (nerve impulses) from one part of the body to another.
- The functional unit of the neural system is the neuron.
- Neural coordination helps in:
- Receiving stimuli from the environment (sensory input)
- Processing and integrating information (CNS)
- Sending appropriate responses to effector organs (motor output)
- It also regulates metabolic and homeostatic activities of the body.
- Neural coordination is fast but short-lived, unlike hormonal coordination.

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(b) Forebrain:

- The forebrain is the anterior-most and most developed part of the human brain.
- It consists of:
- Cerebrum: Largest part; divided into two hemispheres by a longitudinal fissure; connected by corpus callosum. Controls voluntary movements, memory, intelligence, speech, and sensory perception. The outer cortex is grey matter; inner region is white matter.
- Thalamus: Relay centre for sensory and motor signals; located below the cerebrum.
- Hypothalamus: Controls body temperature, hunger, thirst, sleep, and regulates the pituitary gland (master endocrine gland).
- Limbic System: Formed by inner parts of cerebral hemispheres; concerned with olfaction, autonomic responses, sexual behaviour, emotional reactions, and motivation.

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(c) Midbrain:

- The midbrain is located between the forebrain (diencephalon) and the hindbrain.
- The dorsal portion consists of four rounded lobes called corpora quadrigemina (two superior and two inferior colliculi).
- The superior colliculi are involved in visual reflexes; inferior colliculi are involved in auditory reflexes.
- The midbrain receives and integrates visual, tactile, and auditory inputs.
- It contains the cerebral aqueduct (Aqueduct of Sylvius) which connects the third and fourth ventricles.
- Along with the hindbrain, it forms the brain stem.
- It also contains the reticular activating system which is involved in maintaining consciousness and alertness.

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(d) Hindbrain:

- The hindbrain is the posterior part of the brain and consists of three parts:
1. Pons: A bulge on the ventral surface of the brainstem. Contains fibre tracts that interconnect different regions of the brain. Also helps in regulating respiration (pneumotaxic centre).
2. Cerebellum: Second largest part of the brain. Has a highly folded surface (folia). Integrates information from the semicircular canals of the ear and the auditory system. Coordinates voluntary movements, maintains posture, balance, and muscle tone.
3. Medulla Oblongata: The most posterior part; continuous with the spinal cord. Contains vital reflex centres that control respiration (respiratory rhythm centre), cardiovascular reflexes (cardiac centre), and gastric secretions.
- Together, pons and medulla form the brain stem along with the midbrain.

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(e) Synapse:

- A synapse is the junction between two neurons where nerve impulses are transmitted from one neuron to the next.
- It is formed by the membranes of a pre-synaptic neuron (the neuron sending the signal) and a post-synaptic neuron (the neuron receiving the signal).
- Types of synapses:
1. Electrical synapse: Pre- and post-synaptic membranes are in very close proximity; impulse transmission is direct and faster.
2. Chemical synapse: Pre- and post-synaptic membranes are separated by a fluid-filled space called the synaptic cleft (~20 nm wide). Transmission occurs via chemical messengers called neurotransmitters (e.g., acetylcholine, dopamine).
- Structure: The pre-synaptic terminal contains synaptic vesicles filled with neurotransmitters. The post-synaptic membrane has specific receptor proteins.
- Synapses ensure unidirectional transmission of nerve impulses.

Given: Synaptic transmission is the process by which a nerve impulse is transmitted from one neuron to another across a synapse.

Mechanism of Synaptic Transmission (Chemical Synapse):

Step 1 – Arrival of Action Potential:
A nerve impulse (action potential) travels along the axon of the pre-synaptic neuron and reaches the axon terminal (pre-synaptic knob).

Step 2 – Depolarisation of Pre-synaptic Membrane:
The action potential causes depolarisation of the pre-synaptic membrane, making the inside positive.

Step 3 – Influx of Ca$^{2+}$ ions:
Depolarisation opens voltage-gated calcium ($\text{Ca}^{2+}$) channels in the pre-synaptic membrane. $\text{Ca}^{2+}$ ions flow into the pre-synaptic terminal from the extracellular fluid.

Step 4 – Fusion of Synaptic Vesicles:
The rise in intracellular $\text{Ca}^{2+}$ concentration causes synaptic vesicles (containing neurotransmitters) to move towards and fuse with the pre-synaptic membrane.

Step 5 – Release of Neurotransmitters:
Neurotransmitters (e.g., acetylcholine) are released into the synaptic cleft by the process of exocytosis.

Step 6 – Diffusion Across Synaptic Cleft:
The neurotransmitter molecules diffuse across the synaptic cleft (approximately 20 nm wide) towards the post-synaptic membrane.

Step 7 – Binding to Receptors:
Neurotransmitters bind to specific receptor proteins on the post-synaptic membrane. This binding is highly specific (lock and key mechanism).

Step 8 – Generation of New Impulse:
Binding of neurotransmitters causes opening of specific ion channels in the post-synaptic membrane:
- If $\text{Na}^+$ channels open → depolarisation → excitatory post-synaptic potential (EPSP) → new action potential is generated.
- If $\text{Cl}^-$ or $\text{K}^+$ channels open → hyperpolarisation → inhibitory post-synaptic potential (IPSP) → impulse is inhibited.

Step 9 – Inactivation of Neurotransmitter:
After transmission, the neurotransmitter is rapidly inactivated by specific enzymes:
- Acetylcholinesterase breaks down acetylcholine into acetate and choline.
- This prevents continuous stimulation of the post-synaptic membrane.
- The breakdown products are reabsorbed by the pre-synaptic terminal for recycling.

Key Features:
- Transmission is unidirectional (pre-synaptic → post-synaptic).
- There is a slight synaptic delay (~0.5 ms) due to the time taken for neurotransmitter release and diffusion.

Conclusion: Synaptic transmission converts an electrical signal into a chemical signal and back into an electrical signal, ensuring precise communication between neurons.

Given: The generation of action potential (nerve impulse) is fundamentally dependent on the movement of $\text{Na}^+$ ions across the neural membrane.

Role of Na$^+$ in the Generation of Action Potential:

1. Resting State (Polarised Membrane):
- In the resting neuron, the $\text{Na}^+$/$\text{K}^+$ pump actively transports 3 Na$^+$ ions out and 2 K$^+$ ions in per cycle.
- As a result, $\text{Na}^+$ concentration is high outside and low inside the membrane.
- The resting membrane potential is approximately $-70$ mV (inside negative).
- Voltage-gated $\text{Na}^+$ channels are closed at rest.

2. Application of Threshold Stimulus:
- When a stimulus of sufficient intensity (threshold) is applied, the membrane permeability to $\text{Na}^+$ increases.
- Voltage-gated Na$^+$ channels open rapidly.

3. Depolarisation (Role of Na$^+$):
- $\text{Na}^+$ ions rush rapidly into the cell (inward flow) down the electrochemical gradient (both concentration gradient and electrical gradient favour inward movement).
- This influx of positive $\text{Na}^+$ ions makes the inside of the membrane positive and the outside negative.
- The membrane potential changes from $-70$ mV to approximately $+30$ mV.
- This reversal of polarity is called depolarisation.
- This constitutes the rising phase of the action potential.

4. Repolarisation:
- After a brief period, voltage-gated $\text{Na}^+$ channels close (inactivation).
- Simultaneously, voltage-gated $\text{K}^+$ channels open and $\text{K}^+$ ions rush out of the cell.
- This restores the negative resting potential — called repolarisation.

5. Propagation of Action Potential:
- The local depolarisation at one point creates a local circuit with the adjacent resting membrane.
- This causes $\text{Na}^+$ channels to open in the adjacent region, propagating the wave of depolarisation along the entire length of the axon.

Summary Equation of Events:
$$\text{Stimulus} \rightarrow \text{Na}^+ \text{ channels open} \rightarrow \text{Na}^+ \text{ influx} \rightarrow \text{Depolarisation} \rightarrow \text{Action Potential}$$

Conclusion: $\text{Na}^+$ ions play the central and critical role in the generation of action potential. The rapid influx of $\text{Na}^+$ through voltage-gated channels is the primary event that causes depolarisation and initiates the nerve impulse.

(a) Myelinated vs. Non-myelinated Axons:

| Feature | Myelinated Axons | Non-myelinated Axons |
|---|---|---|
| Myelin sheath | Present — axon is covered by a myelin sheath formed by Schwann cells | Absent — no myelin sheath |
| Neurilemma | Present (outer layer of Schwann cells) | Present |
| Nodes of Ranvier | Present — gaps between adjacent myelin segments | Absent |
| Conduction speed | Very fast (saltatory conduction — impulse jumps from node to node) | Slow (continuous conduction along entire membrane) |
| Diameter | Generally larger diameter | Generally smaller diameter |
| Location | Found in spinal nerves, cranial nerves (somatic nervous system) | Found in autonomic and somatic nervous systems (e.g., C-fibres) |
| Example | Motor neurons supplying skeletal muscles | Autonomic nerve fibres |

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(b) Dendrites vs. Axons:

| Feature | Dendrites | Axons |
|---|---|---|
| Number | Many (multiple) per neuron | Only one per neuron |
| Length | Short | Long (can be up to 1 metre) |
| Branching | Highly branched near cell body | Branches only at the terminal end (axon terminals) |
| Myelin sheath | Absent | May be present (myelinated) or absent (non-myelinated) |
| Direction of impulse | Carry impulse towards the cell body (afferent) | Carry impulse away from the cell body (efferent) |
| Nissl's granules | Present | Absent |
| Function | Receive signals from other neurons or sensory receptors | Transmit signals to other neurons, muscles, or glands |

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(c) Thalamus vs. Hypothalamus:

| Feature | Thalamus | Hypothalamus |
|---|---|---|
| Location | Located in the diencephalon, above the hypothalamus | Located below the thalamus, forms the floor of the diencephalon |
| Function | Acts as a relay centre for sensory and motor signals between the cerebral cortex and other brain regions | Controls body temperature, hunger, thirst, sleep, and regulates the pituitary gland |
| Role in homeostasis | Indirect role | Direct role — major homeostatic centre |
| Connection | Connected to cerebral cortex and other brain regions | Connected to pituitary gland (neuroendocrine link) |
| Sensory processing | Processes and relays all sensory information (except olfaction) | Involved in olfaction and autonomic regulation |

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(d) Cerebrum vs. Cerebellum:

| Feature | Cerebrum | Cerebellum |
|---|---|---|
| Location | Anterior and superior part of the brain (forebrain) | Posterior and inferior part of the brain (hindbrain) |
| Size | Largest part of the brain (~85% of brain weight) | Second largest part of the brain |
| Structure | Divided into two hemispheres by longitudinal fissure; connected by corpus callosum | Divided into two hemispheres; surface has folia (folds) |
| Cortex | Cerebral cortex (grey matter) is highly folded into gyri and sulci | Cerebellar cortex with characteristic folia |
| Functions | Controls voluntary movements, intelligence, memory, speech, sensory perception, consciousness | Coordinates voluntary movements, maintains posture, balance, and muscle tone |
| Damage effect | Loss of voluntary control, memory, speech | Loss of balance, uncoordinated movements (ataxia) |

(a) Most Developed Part of the Human Brain:

The Cerebrum is the most developed part of the human brain.

Justification:
- The cerebrum constitutes approximately 85% of the total brain weight.
- It is the seat of intelligence, memory, consciousness, voluntary movements, speech, and sensory perception.
- The cerebral cortex is highly folded into gyri and sulci, greatly increasing the surface area for higher neural functions.
- It is divided into two cerebral hemispheres connected by the corpus callosum.
- The high degree of development of the cerebrum distinguishes humans from other animals and is responsible for complex cognitive abilities.

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(b) Part of CNS that Acts as a Master Clock:

The Hypothalamus acts as the master clock (biological clock) of the central neural system.

Justification:
- The hypothalamus contains the suprachiasmatic nucleus (SCN), which is considered the primary circadian pacemaker or master clock.
- It regulates circadian rhythms (24-hour biological cycles) including sleep-wake cycles, body temperature fluctuations, hormone secretion patterns, and metabolic activities.
- It receives light information from the retina and synchronises the body's internal clock with the external environment.
- It also controls the pituitary gland, thereby regulating hormonal rhythms throughout the body.

(a) Afferent Neurons vs. Efferent Neurons:

| Feature | Afferent Neurons | Efferent Neurons |
|---|---|---|
| Also called | Sensory neurons | Motor neurons |
| Direction of impulse | Carry impulses from sensory organs/receptors to the CNS (brain/spinal cord) | Carry impulses from the CNS to effector organs (muscles/glands) |
| Location of cell body | Cell body located in the dorsal root ganglion of spinal nerves | Cell body located in the ventral horn of the spinal cord or in the brain |
| Function | Transmit sensory information (pain, touch, temperature, etc.) to the CNS | Transmit motor commands from CNS to muscles and glands |
| Axon length | Generally have long peripheral processes | Generally have long axons reaching effectors |
| Example | Neurons carrying pain signals from skin to spinal cord | Neurons carrying signals from spinal cord to skeletal muscles |

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(b) Impulse Conduction in Myelinated vs. Unmyelinated Nerve Fibre:

| Feature | Myelinated Nerve Fibre | Unmyelinated Nerve Fibre |
|---|---|---|
| Type of conduction | Saltatory conduction — impulse jumps from one Node of Ranvier to the next | Continuous conduction — impulse travels along the entire length of the membrane |
| Speed | Very fast (up to 120 m/s) | Slow (0.5–2 m/s) |
| Energy expenditure | Less energy required (fewer ions need to be pumped back) | More energy required |
| Nodes of Ranvier | Present — act as sites of action potential generation | Absent |
| Mechanism | Depolarisation occurs only at nodes; current flows through the axoplasm between nodes | Depolarisation occurs at every point along the membrane sequentially |
| Efficiency | More efficient and faster | Less efficient and slower |
| Example | Motor neurons of somatic nervous system | Autonomic C-fibres, pain fibres |

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(f) Cranial Nerves vs. Spinal Nerves:

| Feature | Cranial Nerves | Spinal Nerves |
|---|---|---|
| Origin | Arise from the brain (mainly brain stem) | Arise from the spinal cord |
| Number | 12 pairs in humans | 31 pairs in humans |
| Numbering | Numbered I to XII (Roman numerals) | Numbered according to vertebral region (8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal) |
| Nature | May be sensory only, motor only, or mixed | All spinal nerves are mixed (both sensory and motor) |
| Foramen | Pass through foramina in the skull | Pass through intervertebral foramina |
| Distribution | Supply structures of the head, neck, and some thoracic/abdominal organs | Supply the trunk, limbs, and some neck regions |
| Examples | Olfactory (I), Optic (II), Oculomotor (III), Vagus (X) | Cervical nerves (C1–C8), Lumbar nerves (L1–L5) |