Key Points

The neural system provides a point-to-point rapid coordination network, while the endocrine system provides chemical integration through hormones. Both systems jointly coordinate and integrate all organ activities to maintain homeostasis.

The neural system is composed of highly specialised cells called neurons, which can detect, receive, and transmit different kinds of stimuli. A neuron has three main parts: the cell body, dendrites, and an axon.

The human neural system is divided into the Central Neural System (CNS), which includes the brain and spinal cord, and the Peripheral Neural System (PNS), which consists of all nerves associated with the CNS.

Afferent (sensory) nerve fibres transmit impulses from tissues or organs to the CNS. Efferent (motor) nerve fibres transmit regulatory impulses from the CNS to peripheral tissues or organs.

The PNS has two main divisions: the somatic neural system, which controls skeletal muscles, and the autonomic neural system, which controls involuntary organs and smooth muscles. The autonomic system is further divided into sympathetic and parasympathetic systems.

A neuron consists of a cell body containing Nissl's granules, dendrites that transmit impulses towards the cell body, and a long axon that transmits impulses away from the cell body.

Myelinated nerve fibres are enveloped by Schwann cells forming a myelin sheath, with gaps called nodes of Ranvier that speed up impulse conduction. Unmyelinated fibres are also enclosed by Schwann cells but without the myelin sheath.

In a resting (non-conducting) neuron, the axonal membrane is polarized. It is more permeable to potassium ions ($K^+$) and nearly impermeable to sodium ions ($Na^+$), creating a positive charge outside and a negative charge inside.

The ionic gradients across the resting membrane are maintained by the sodium-potassium pump. This pump actively transports $3$ sodium ions ($Na^+$) outwards for every $2$ potassium ions ($K^+$) it moves into the cell.

When a stimulus is applied, the membrane becomes permeable to $Na^+$, causing a rapid influx of these ions. This reverses the polarity, making the inside positive and the outside negative, an event called depolarization or an action potential (nerve impulse).

A synapse is the junction where a nerve impulse is transmitted from a pre-synaptic neuron to a post-synaptic neuron. Synapses can be either electrical (rare) or chemical.

At a chemical synapse, an action potential triggers the release of neurotransmitters from the axon terminal into the synaptic cleft. These chemicals bind to receptors on the post-synaptic neuron, generating a new potential.

The human brain is divided into three major parts: the forebrain (cerebrum, thalamus, hypothalamus), the midbrain, and the hindbrain (pons, cerebellum, medulla oblongata).

The cerebrum is the largest part, divided into two cerebral hemispheres connected by the corpus callosum. The outer layer, the cerebral cortex (grey matter), is responsible for complex functions like memory and communication.

The thalamus is a major coordinating centre for sensory and motor signaling. The hypothalamus controls body temperature, the urge for eating and drinking, and secretes hormones.

The limbic system, which includes inner parts of the cerebral hemispheres and structures like the amygdala and hippocampus, is involved in regulating sexual behaviour, emotional reactions (fear, pleasure), and motivation.

The hindbrain includes the pons, which connects different brain regions; the cerebellum, which maintains balance and posture; and the medulla, which controls respiration, cardiovascular reflexes, and gastric secretions.

The brain stem is formed by the midbrain, pons, and medulla oblongata. It forms the connection between the brain and the spinal cord and controls many vital autonomic functions.